https://orcid.org/0000-0003-2878-2027
Abstract
Plants are exposed to a variety of abiotic stresses; these stresses have profound effects on plant growth, survival, and productivity. Tolerance and adaptation to stress require sophisticated stress sensing, signaling, and various regulatory mechanisms. The plant hormone auxin is a key regulator of plant growth and development, playing pivotal roles in the integration of abiotic stress signals and control of downstream stress responses. In this review, we summarize and discuss recent advances in understanding the intersection of auxin and abiotic stress in plants, with a focus on temperature, salt, and drought stresses. We also explore the roles of auxin in stress tolerance and opportunities arising for agricultural applications.
Introduction
Plants cope with constantly changing environments that are often stressful or unfavorable for their growth. Thus, they have evolved the capacity to survive under wide-ranging abiotic stresses. However, the frequency and severity of these encountered stresses are increasing because of climate change, with deleterious effects on plant growth and crop productivity (reviewed in Hamann et al., 2021). Commonly encountered abiotic stressors include heat, salt, drought, cold, nutrient deficiency, and toxic metals (reviewed in Zhu, 2016). Amongst these abiotic stresses, extreme temperatures, salt, and drought are environmental factors that hinder plant growth, geographical distribution, and crop productivity (reviewed in Waadt et al., 2022). Plant survival under these environmental stresses requires sophisticated regulatory pathways that facilitate withstanding adverse growth conditions. Genetic, biochemical, and molecular studies have identified numerous factors that regulate abiotic stress responses, involving stress sensing, transcription, and signal transduction (reviewed in Zhang et al., 2022). Amongst these factors, many plant hormones mediate abiotic stress adaptation (reviewed in Waadt, 2020); in this review, we focus on the roles of the phytohormone auxin in abiotic stress.
Auxin is a master regulator of plant growth and development through its control of cell division, elongation, and differentiation. Fundamental auxin responses are divided into three major pathways: metabolism, transport, and signal transduction. Recent work on the biochemical, genetic, and structural aspects of auxin has resulted in an increased understanding of the interplay of auxin–stress responses. In this review, we discuss recent progress in our understanding of the roles of auxin in abiotic stress, especially drought, salt, and extreme temperatures.
Brief overview of auxin
The name ‘auxin’ is derived from the Greek word ‘auxein’ meaning ‘to grow or to expand’, which indicates that the critical effects of auxin are on plant growth (reviewed in Abel and Theologis, 2010). Over a century of studies have revealed that the regulation of plant growth and development is largely executed via coordination of auxin metabolism, auxin translocation, and auxin response (reviewed in Enders and Strader, 2015). Auxin biosynthesis and function are involved in response to abiotic stresses, such as heat, salt, drought, and cold.
Auxin metabolism
Indole-3-acetic acid (IAA) is the best-studied naturally occurring active auxin in plants. Research on auxin metabolism has established that the aromatic amino acid l-tryptophan (Trp) is the central precursor for IAA biosynthesis in plants (reviewed in Enders and Strader, 2015). Several decades of studies indicate that auxin levels are regulated by its biosynthesis and inactivation (Fig. 1), which are extensively involved in the heat, salt, drought, and cold stresses (reviewed in Casanova-Saez et al., 2021).
The central pathways for auxin biosynthesis, inactivation, and transport in plants. (A) The primary routes for auxin biosynthesis and inactivation. Indole-3-acetic acid (IAA), as a naturally occurring active auxin, is synthesized from the main precursor l-tryptophan (l-Trp). The Trp-dependent pathways comprise the indole-3-acetaldoxime (IAOx), the indole-3-acetamide (IAM), and the indole-3-pyruvic acid (IPyA) pathways. Amongst these pathways, the IPyA route is considered to be the main contributor to IAA synthesis. In addition to auxin biosynthesis, auxin levels are also regulated by its inactivation through conjugation and degradation. There are three reversible auxin conjugates: ester-linked IAA, amide-linked IAA, and methyl IAA. Irreversible auxin degradation is through the oxidation of IAA to 2-oxindole-3-acetic acid (oxIAA) by the dioxygenase for auxin oxidation (DAO). (B) The intercellular auxin distribution is established by the influx and efflux carriers. AUXIN RESISTANT1/LIKE AUX1 (AUX1/ LAX) transporters facilitate the influx of IAA. PIN-FORMED (PIN) and ATP-BINDING CASSETTE SUBFAMILY B (ABCB) protein families facilitate the efflux of IAA. The long canonical PIN proteins, PIN1, PIN2, PIN3, PIN4, and PIN7, are polarly localized to the plasma membrane (PM). The short non-canonical PIN proteins, PIN5, PIN6, and PIN8, localize to the endoplasmic reticulum (ER) membrane. PIN-LIKES (PILS) proteins are ER localized and contribute to intracellular auxin transport. In addition, the transporters of auxin precursor indole 3-butyric acid (IBA), that is converted to IAA by a β-oxidation in peroxisome, mediates auxin metabolism.
Auxin biosynthesis
Generating auxin from Trp is considered the main route of IAA biosynthesis (reviewed in Zhao, 2018). Several Trp-dependent auxin biosynthetic routes have been proposed, including the IAOx (indole-3-acetaldoxime), IAM (indole-3-acetamide), and IPyA (indole-3-pyruvic acid) pathways (reviewed in Casanova-Saez et al., 2021). Whereas the IAOx and IAM pathways are not fully understood, the IPyA pathway has been established as the main contributor to IAA synthesis and is the only completely described pathway to date (reviewed in Cao et al., 2019). The IPyA pathway consists of a two-step reaction: the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) family of enzymes converts Trp to IPyA, and then the YUCCA (YUC) family of enzymes converts IPyA to IAA (reviewed in Casanova-Saez et al., 2021). Trp is deaminated into IPyA by TAA1 and TAA1-RELATED proteins (TARs). IPyA is then decarboxylated in an irreversible reaction catalyzed by the YUC family of flavin-containing monooxygenases to produce IAA (reviewed in Cao et al., 2019). TAA1 and YUC homologs are found across the plant kingdom, and the role of the TAA/YUC pathway is fully supported by genetic and biochemical evidence, suggesting that the Trp-dependent IPyA pathway is a universal route for IAA biosynthesis in plants (reviewed in Morffy and Strader, 2020).
Auxin inactivation
In addition to the local auxin biosynthesis, auxin levels are modulated by metabolic inactivation through conjugation and degradation. Reversible formation of auxin conjugates provides a mechanism for rapid regulation of active auxin levels. Three major auxin storage forms have been identified, namely ester-linked IAA, amide-linked IAA, and methyl IAA (reviewed in Casanova-Saez et al., 2021). IAA-glucose (IAA-Glc) is the predominant ester-linked auxin conjugate (Pencik et al., 2018; Brunoni et al., 2020). Amide linkages are formed when auxin is conjugated by amino acids and small peptides. IAA–amino acids are major amide conjugates whose formation is catalyzed by products of the GRETCHEN HAGEN3 (GH3) family genes (Staswick et al., 2005; Ludwig-Muller et al., 2009). Moreover, IAA can also be converted to the methyl ester form MeIAA, which is mediated by IAA CARBOXYMETHYLTRANSFERASE1 (IAMT1) (Zubieta et al., 2003; Qin et al., 2005). In addition to inactivation of auxin by conjugation, auxin degradation also regulates IAA levels, frequently through the irreversible oxidation of IAA to 2-oxindole-3-acetic acid (oxIAA) (Kubes et al., 2012; Pencik et al., 2013). The DIOXYGENASE FOR AUXIN OXIDATION (DAO) gene product catalyzes the conversion of IAA to oxIAA (Zhao et al., 2013). IAA levels can also be regulated by the chain-lengthened precursor indole-3-butyric acid (IBA), which is converted into active IAA through a β-oxidation process (reviewed in Damodaran and Strader, 2019).
Auxin transport
Auxin forms local concentration minima, maxima, or gradients between cells to drive developmental events (Fig. 1). Auxin concentration gradients are established by a directional cell-to-cell transport (reviewed in Hammes et al., 2022). Plasma membrane-based influx and efflux carriers control intercellular auxin distribution (reviewed in Sauer and Kleine-Vehn, 2019). Members of the AUXIN RESISTANT1/LIKE AUX1 (AUX1/LAX) transporters facilitate the influx of IAA, whereas members of the PIN-FORMED (PIN) and ATP-BINDING CASSETTE SUBFAMILY B (ABCB) protein families facilitate the efflux of IAA. In parallel, transporters of the auxin precursor IBA can fine-tune auxin levels to affect plant development (reviewed in Damodaran and Strader, 2019; Michniewicz et al., 2019). Recent evidence suggests that auxin transport from local biosynthesis and storage forms plays essential roles during heat, salt, drought, and cold stresses.
Auxin influx
To date, four auxin influx carriers with specific functions have been described in Arabidopsis, namely AUX1, LAX1, LAX2, and LAX3 (reviewed in Swarup and Bhosale, 2019). The diversity of mutant phenotypes displayed in mutants defective in AUX/LAX family members reveals distinct function for these auxin carriers (Bennett et al., 1996; Bainbridge et al., 2008; Swarup et al., 2008; Péret et al., 2012; Moreno-Piovano et al., 2017). Altogether, these AUX1/LAX influx carriers play critical roles for auxin transport to regulate plant development.
Auxin efflux
PIN and ABCB family members are auxin efflux carriers. In Arabidopsis, there are eight PINs with conserved N- and C-terminal domains and less conserved hydrophilic loops (Nodzynski et al., 2016). The length of hydrophilic loops divides PINs into two classes, ‘long’ canonical PINs and ‘short’ or ‘intermediate’ non-canonical PINs (reviewed in Bogaert et al., 2022). Long canonical PIN proteins include PIN1, PIN2, PIN3, PIN4, and PIN7, which are polarly localized to the plasma membrane (Adamowski and Friml, 2015). Canonical PINs are polarly distributed in different cell types (Adamowski and Friml, 2015; reviewed in Hammes et al., 2022). Canonical PINs are necessary to establish differential distribution of auxin to regulate aspects of plant growth and stress responses. In parallel, ABCB protein transporters also mediate auxin efflux and play roles in plant development (Chen et al., 2023; reviewed in Hao et al., 2020; Hammes et al., 2022).
Auxin signal transduction
Over the past three decades, extensive genetic and biochemical studies have elucidated the mechanisms of auxin signaling. The major nuclear mechanism for auxin-mediated gene expression is based on the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEINS (TIR1/AFBs) pathway. Additionally, the non-canonical transmembrane kinase (TMK) and extranuclear TIR1/AFBs pathway also mediate auxin signal transduction. The auxin signal pathway is essential to heat, salt, drought, and cold responses in plants, as described below.
The canonical TIR1/AFB–AUX/IAA pathway
The TIR1/AFB auxin signal transduction pathway (Fig. 2) involves three major components: the SCFTIR1/AFB receptors complex, Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) repressor proteins, and AUXIN RESPONSE FACTOR (ARF) transcription factors (reviewed in Morffy and Strader, 2022). The TIR1/AFBs are F-box proteins that are incorporated into a SKP1–CUL1–F-box (SCF)-type E3 ubiquitin ligase (Pickart, 2001; Gagne et al., 2002). There are six members of the TIR1/AFB family, with distinct and overlapping functions (Parry et al., 2009; reviewed in Salehin et al., 2015). Aux/IAA proteins are transcriptional repressors, which have three major domains: the DI that encodes the repressive domain, DII that encodes the degron, and a type I/II Phox and Bem1 (PB1) domain that mediates the interactions between Aux/IAAs and ARFs (reviewed in Morffy and Strader, 2022). ARF transcription factors tightly control auxin responses by either activating or repressing target genes (reviewed in Powers and Strader, 2020). ARFs are divided into three conserved evolutionary classes: class A are considered to be transcriptional activators whereas class B/C are thought to be transcriptional repressors (Okushima et al., 2005; Finet et al., 2013; Mutte et al., 2018). Most ARF proteins contain three regions: an N-terminal B3-type DNA-binding domain (DBD), an intrinsically disordered variable middle region (MR) that functions as either a transcriptional activator or repressor, and a C-terminal PB1 protein–protein interaction domain (Roosjen et al., 2018). ARF proteins are regulated by the 26S proteasome (Li et al., 2004; Salmon et al., 2008; Lakehal et al., 2019; Jing et al., 2022). The latest discovery of the F-box protein AUXIN RESPONSE FACTOR F-BOX1 (AFF1) that regulates ARF accumulation and nucleo-cytoplasmic partitioning, provides new insights into auxin signal transduction (Jing et al., 2022; Gorska et al., 2023).
The central pathways for auxin signal transduction. Nuclear auxin signal transduction is mediated by the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEINS (TIR1/AFBs) pathway, which includes the AUXIN RESPONSE FACTOR (ARF) transcription factors and Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) repressor proteins. The E3 ubiquitin ligase TIR1/AFBs are complexed with the S PHASE KINASE-ASSOCIATED PROTEIN1 (SKP1) (or ASK1 in plants), CULLIN1 (CUL1), and RING BOX1 (RBX1), forming an SCFTIR1/AFB complex. Aux/IAAs interact with ARFs to prevent auxin signaling under low levels of auxin. As the auxin levels increase, auxin induces Aux/IAA binding to the SCFTIR1/AFB complex, causing the ubiquitination and degradation of Aux/IAA proteins through the 26S proteasome. The degradation of Aux/IAA proteins relieves ARF repression to allow ARF-mediated transcription to proceed. In parallel, the E3 ubiquitin ligase AUXIN RESPONSE FACTOR F-BOX1 (AFF1) facilitates the ubiquitylation of ARF7 and ARF19 for degradation through the 26S proteasome, regulating the auxin signaling transduction. Additionally, the transmembrane kinase (TMK) proteins localized in the plasma membrane mediate cell surface auxin signal transduction by binding to the non-canonical IAA32 and IAA34. The auxin-binding protein 1 (ABP1) also medicates auxin signaling through the TMK1-based cell surface pathway.
Under low auxin conditions, Aux/IAAs directly interact with ARFs via the PB1 domain and prevent transcription of auxin-responsive genes. As auxin levels increase, auxin acts as a ‘molecular glue’ to stabilize and promote the formation of a TIR1/AFB and Aux/IAA co-receptor complex. The formation of the SCFTIR1–Aux/IAA co-receptor allows ubiquitylation of Aux/IAAs, which are subsequently subjected to degradation through the 26S proteasome. Aux/IAA degradation relieves ARF repression, allowing the ARFs to activate transcription (reviewed in Morffy and Strader, 2022).
The non-canonical TMK and extranuclear TIR1/AFB pathway
In addition to the SCFTIR1–Aux/IAA nuclear auxin signaling pathway, TMK proteins also mediate transcription-independent cell surface auxin signaling (reviewed in Ang and Ostergaard, 2023). Moreover, the cytoplasmic-localized AFB1 activates auxin response independent of nuclear gene transcription (Fendrych et al., 2018; Li et al., 2021; Serre et al., 2021). These non-canonical auxin signaling pathways are required for rapid auxin-induced response, but have not yet been implicated in the abiotic stress response.
Auxin roles in elevated temperature
Elevated temperatures causing heat stress affect plant growth and development, and globally result in the reduction of crop yields (reviewed in Ding et al., 2020). A particular response to mildly elevated temperature is known as thermomorphogensis (reviewed in Casal and Balasubramanian, 2019). The morphological changes include many auxin-related phenotypes such as elongated hypocotyls, increased petiole angle and length, and hyponastic growth (reviewed in Casal and Balasubramanian, 2019). When temperatures increase further, heat stress resistance mechanisms enable the plant to survive and grow under these conditions (reviewed in Ding et al., 2020). Plants have evolved various efficient mechanisms for sensing and responding to high temperature, which involve the activation of phytohormone response and signal transduction (reviewed in Casal and Balasubramanian, 2019). Here, we review recent research progress of auxin guiding plant response to heat stress.
Auxin drives morphological changes that occur in response to elevated ambient temperature, known as thermomorphogensis (reviewed in Casal and Balasubramanian, 2019). Increased auxin levels is a cornerstone of thermomorphogenesis. PHYTOCHROME INTERACTING FACTOR 4 (PIF4), encoding a crucial thermomorphogenesis regulator, directly drives the expression of the auxin biosynthesis gene YUC8 at 28 °C in Arabidopsis (Sun et al., 2012). The yuc8 mutant shows a decrease in hypocotyl elongation at higher temperatures in Arabidopsis, demonstrating the impact of auxin synthesis (Sun et al., 2012). Additionally, transcript levels of auxin biosynthesis genes TAA1 and CYP79B are diminished at higher temperatures in the pif4 mutant (Franklin et al., 2011), suggesting direct regulation of multiple auxin biosynthesis steps by heat.
Auxin transport also impacts thermomorphogenesis. The polar auxin inhibitor 1-naphthylphthalamic acid (NPA) inhibits thermal response at higher temperatures (Stavang et al., 2009). Further, PIN1 and PIN2 reporter lines demonstrate decreased fluorescence at elevated temperatures (Parveen and Rahman, 2021). Further work will be required to determine direct roles of regulating auxin transport in response to heat stress.
In addition to auxin biosynthesis and transport, auxin signaling is crucial for thermomorphogenesis. The auxin signaling mutants tir1-1 (Gray et al., 1998), axr1-12 (Gray et al., 1998), and shy2-2/iaa3 (Sun et al., 2012) display diminished hypocotyl elongation in response to elevated temperature. Furthermore, the stabilization of TIR1 at higher temperatures via HSP90 bolsters the auxin response (Wang et al., 2016). The auxin-regulated family genes, SMALL AUXIN UPREGULATED RNA (SAUR), have increased expression downstream of PIF4 at elevated temperatures (Franklin et al., 2011). Together, these results demonstrate that not only the presence of cellular IAA, but also the ability to discern and respond to IAA is necessary for thermomorphogenesis.
Whereas thermomorphogenesis involves an increased auxin production and concomitant signaling, acute heat stress results in a disruption of auxin response and is responsible for agricultural disruption on a global scale (Lobell and Field, 2007). In particular, plants are vulnerable to heat stress during the development of pollen and male reproductive organs which is directly linked to diminished seed yield and quality (reviewed in Chaturvedi et al., 2021). A growing body of literature demonstrates auxin biosynthesis, production, and transport guide anther and pollen development (Cheng et al., 2006; Ru et al., 2006; Wu et al., 2006; Dal Bosco et al., 2012; Ding et al., 2012; Cecchetti et al., 2015; Yao et al., 2018). Critically, heat stress disrupts auxin processes in crop species and is likely to be a contributing factor to diminished pollen viability. In maize, it is estimated that every day with temperatures >30 °C results in a 1% loss in yield (Lobell and Field, 2007; Lobell et al., 2011). Thus, understanding the interplay of auxin and heat stress across agricultural plants is a challenge and an opportunity for agricultural advancement in the face of a changing global climate.
Auxin biosynthesis is critical to pollen formation at multiple stages. For example, the Arabidopsis yuc2 yuc6 auxin biosynthesis mutant fails to produce viable pollen due to arrested development early during gametogenesis (Cheng et al., 2006; Yao et al., 2018). Auxin biosynthesis is also important during the production of male reproductive structures in crop species and is disrupted at high temperatures. During heat stress, endogenous auxin levels and transcript levels of auxin biosynthesis genes are decreased in Arabidopsis, barley anthers, and rice spikelets, suggesting negative effects of warm temperatures on auxin levels (Sakata et al., 2010; Du et al., 2013a; Min et al., 2014; Wu et al., 2016; Sharma et al., 2018). Decreased auxin levels correspond to decreased transcript accumulation of auxin biosynthesis genes YUC6, YUC8, and TAA1 (Sakata et al., 2010; Sharma et al., 2018). Interestingly, the application of exogenous auxin during heat stress has been found to mitigate damaging impacts of heat shock on pollen viability, sterility, and overall grain yield in multiple crop species including wheat, rice, and barley (Sakata et al., 2010; Sharma et al., 2018; Abeysingha et al., 2021). Together, these results suggest that the availability of free IAA plays an important role in mitigating heat stress during pollination.
Auxin signaling also guides pollen development and is differentially regulated during heat stress. In Arabidopsis, arf17 knockout mutants display male sterility, have decreased DR5 signal within the anthers, and have disrupted callose deposition surrounding pollen tetrads (Yang et al., 2013). Further, arf6 arf8 double mutants are sterile and contain indehiscent anthers (Nagpal et al., 2005; Ru et al., 2006; Wu et al., 2006). Interestingly, ARF13/ARF17 and ARF6/ARF8 are regulated by miRNA160 and miR167, respectively (Wu et al., 2006). ARF regulation by miRNAs in response to heat stress translates to crop species, including barley and cotton (Kruszka et al., 2014). In cotton (Gossypium hirsutum), comparative studies between heat-insensitive strain 84021 and heat-sensitive H05 demonstrated that decreased miR160 levels and concomitant increased levels of its target ARF17 are found in the heat-insensitive strain 84021 compared with the heat-sensitive strain H05 (Ding et al., 2017). Further, overexpressing miRNA160 in the heat-insensitive 84021 resulted in anther indehiscence resembling the heat-sensitive strain H05. In contrast to rice and barley, exogenous application of auxin to cotton exacerbated the impacts of long-term heat stress on sterility. Further studies should explore the impacts of exogenous auxin across multiple species to understand the described opposing roles for auxin in this process. Further, it appears that the role of miRNA regulation in response to heat stress appears to be conserved in plants and could provide a potential area of research for agricultural advancement (Wu et al., 2006; Kruszka et al., 2014; J. Zhao et al., 2016; Ding et al., 2017).
Auxin further regulates anther development in Arabidopsis (Cecchetti et al., 2008). The auxin transporters PIN5 and PIN8 are present in the endoplasmic reticulum of developing pollen grain (Dal Bosco et al., 2012; Ding et al., 2012); these two PINs act antagonistically, as the pin5pin8 double mutant rescues the aberrant pollen morphology found in each of the single mutants. Pollen viability in the double mutant also suggests that additional auxin transporters regulate this process (Ding et al., 2012). Other auxin efflux carriers, ABCB1 and ABCB19, also impact stamen development (Cecchetti et al., 2008). ABCB1 plays a major role in anther development; abcb1 mutants display anther indehiscence and defective pollen development. Whereas auxin guides anther development, little is known about the impact of temperature stress on auxin transport genes. While these auxin transporters impact male sterility, their intersection with heat stress warrants further exploration. Other transporters such as the phosphate transporter OsPT8 bolster thermotolerance through changes in auxin distribution in Nicotiana tabacum (Z. Song et al., 2019). These changes in thermotolerance are conferred along with increased transcript levels of YUC auxin biosynthesis genes, PIN1 and PIN2 auxin transport genes, and ARF1 and ARF2 auxin signaling genes (Z. Song et al., 2019). Transcriptome data from heat-resistant cultivars of cotton and rice (Min et al., 2014; Sharma et al., 2021) also reveal a correlation between heat stress changes in auxin transport and signal components and tolerance to heat stress. Whereas auxin transport is probably impacted at higher temperatures, more work should be done to target auxin transport genes involved in male sterility at higher temperatures.
Taken together, auxin biosynthesis, transport, and signaling guide plants’ response to heat stress (Table 1). Interestingly, the role of auxin at elevated temperatures appears to have two distinct effects. At mildly elevated temperatures, auxin synthesis and signaling increase to drive thermomorphogenesis. However, once past a critical temperature threshold, heat stress results in aberrant auxin signaling which threatens global food supplies by impacting pollen development (Lobell et al., 2011). Moving forward, it will be critical to understand which temperatures (and for what durations) impact pollen viability among agriculturally important crops. Additionally, the impact of exogenous auxin treatment to mitigate the impacts of heat stress needs to be better understood. Whereas exogenous auxin treatment of barley anther and spikelets improves yield, similar treatments hinder pollen viability in cotton. Further comparisons should be made to understand differences in development and phytohormone crosstalk which might reconcile these differences.
Plant genes of auxin metabolism, transport, and signaling involved in abiotic stress
Plant genes of auxin metabolism, transport, and signaling involved in abiotic stress
Auxin roles in salinity stress
Worldwide, soil salinity has increased dramatically in recent years, with a major impact on global ecosystems and agricultural production (reviewed in Zandalinas et al., 2021). In particular, temperature extremes have become more frequent and intense, resulting in the lack of sufficient water and salt accumulation in the soil (reviewed in Gamalero and Glick, 2022). The salinity-related cations Na+, Mg2+, and Ca2+, and the anions SO42−, Cl−, and HCO3− have deleterious effects on plant growth when in excess. Amongst these constituents, NaCl represents 50–80% of the total soluble salts in the soil (Joshi et al., 2022). Salt stress affects plants throughout their life cycle, from inhibiting seed germination to modulating plant growth and development at all stages. When plants are first exposed to high levels of salt, a decrease of plant growth rate occurs. Subsequently, following the continued uptake of salt, sodium ions are translocated and accumulated in plants, resulting in the generation of a number of reactive oxygen species (ROS), which in turn negatively affect the plant yield and quality (reviewed in Gamalero and Glick, 2022). There are three major effects of salt on plants: water deficit, ion toxicity, and ion imbalance. Plants use a range of mechanisms to deal with salt stress, including exclusion of sodium ions, modulation of root uptake and transport, synthesis of various antioxidative enzymes, and modulation of some plant hormone levels (reviewed in Gamalero and Glick, 2022). Amongst these plant hormones, auxin has been reported to play essential roles during exposure to high salinity.
Auxin biosynthesis and transport are regulated in response to salt stress in plants. Overexpression of auxin biosynthesis genes leads to increased salt tolerance in multiple species (Dunlap and Binzel, 1996; Kim et al., 2013; Ke et al., 2015; Yan et al., 2016). For example, the expression of auxin biosynthesis genes NIT1, NIT2, and YUC4 is significantly increased in response to NaCl treatment in Arabidopsis (Cackett et al., 2022). The YUC family of auxin biosynthesis genes are induced under salt stress in Cucumis sativus and potato (Kim et al., 2013; Yan et al., 2016). IAOx pathway genes CYP79B2 and CYP79B3 show induction under mild salt stress (Julkowska et al., 2017). In the moss Physcomitrium patens, loss of the auxin-conjugating GH3 enzymes results in tolerance to high salt concentrations(Koochak and Ludwig-Muller, 2021). These results suggest a connection between auxin biosynthesis and salt stress response. Further, auxin transport is impaired under salt stress (Ribba et al., 2020). PIN1, PIN3, and PIN7 expression is negatively regulated under salt stress, which impacts PIN protein abundance and consequently inhibits auxin transport (Liu et al., 2015; Fu et al., 2019). Expression of CcPIN genes is regulated by salt treatment in Chinese hickory (Yang et al., 2022). PIN1 plays a critical role in regulating plant epidermal cell development under salt and drought stress (Bawa et al., 2022). Moreover, PIN2 and AUX1 display altered subcellular localization under salt stress. PIN2 phosphorylation status changes via the activity of PINOID (PID) under salt stress (Wang et al., 2019). LAX3 transcript levels are induced by overexpression of WRKY3, which results in salt stress tolerance (Hichri et al., 2017). Finally, auxin transporters are crucial for producing the asymmetric distribution of auxin to drive bending away from salt in the growth substrate (Galvan-Ampudia et al., 2013; van den Berg et al., 2016), providing a means to escape salt stress by root systems. These clues point towards a role for auxin levels and distribution in salt stress response.
In addition to auxin biosynthesis and transport, auxin signal transduction is also involved in salt stress response. The tir1/afb2/afb3 mutants, defective in multiple TIR/AFB receptors, display hypersensitivity to NaCl treatment in the root meristem, suggesting that the slowing of root growth might be an adaptive mechanism for plants surviving under a salt environment (Iglesias et al., 2010; Yu et al., 2020). Contrasting data suggest that TIR1 and AFB2 are down-regulated by salt stress, indicating that decreasing auxin response may be involved in plant growth and adaptation to salt (Iglesias et al., 2014; Yu et al., 2020). Moreover, overexpression of the auxin receptor gene AFB3 (AUXIN F-BOX3) results in an increased resistance to salinity in Arabidopsis roots (Garrido-Vargas et al., 2020). Overexpression of MdIAA8, MdIAA9, and MdIAA25 in transgenic apple calli results in salt stress tolerance (Y. Li et al., 2022). Although there is limited information on the role of Aux/IAAs in response to salt stress, IAA17 is stabilized by the application of exogenous nitric oxide and is also accumulated under salt stress, suggesting the important role of IAA17 in stress signaling (Liu et al., 2015; Shi et al., 2017; Jing et al., 2023). In contrast to Aux/IAA proteins, ARF proteins have been characterized as important components to salt stress responses (Ribba et al., 2020). For example, salt response cis-elements in the ARF gene promoters along with the response elements for MYB and MYC, transcription factors involved in stress response, suggest direct transcriptional regulation (Bouzroud et al., 2018). In rice, OsARF11 and OsARF15 genes are differentially expressed under salt conditions (Jain and Khurana, 2009). Overexpression of sweet potato IbMP/ARF5 leads to an enhanced salt and drought tolerance in Arabidopsis (Kang et al., 2018). Several ARF transcripts are post-transcriptionally regulated by miRNAs during salt stress, suggesting a crucial role for ARF proteins in the integration of environmental signals (Kinoshita et al., 2012; Kang et al., 2018). The expression of ARF3.1, ARF3.2, and ARF4 is significantly inhibited by salt treatment in Populus spp. (He et al., 2018). Thus, multiple auxin signaling components alter salt stress response across species.
Taken together, auxin is important in response to salt stress (Table 1), modulating a complex balance of biosynthesis, transport, and signal transduction, tuning physiological changes in plant architecture and salt stress adaption.
Auxin roles in drought stress
As a major abiotic environmental stress, drought is related to water scarcity and access. Drought stress causes multiple physiological, metabolic, and biochemical changes that limit plant growth and thus reduce crop yields. To survive and maintain growth, plants adjust their morphological and physiological characteristics in response to drought. One strategy is to maintain physiological water balance through increasing water uptake via roots, reducing water loss by closing stomata, and adjusting osmotic potential within tissues (reviewed in Gupta et al., 2020). Many phytohormones play roles in drought stress tolerance (reviewed in Kim et al., 2022), with abscisic acid (ABA) being ascribed the largest and best described role. Here, we focus on the role of auxin in drought stress (Table 1), with its interaction with ABA highlighted.
Auxin biosynthesis and transport are implicated in drought stress response. Increasing auxin levels in different species by either overexpression of YUC family genes or application of auxin enhances drought resistance. In particular, overexpression of YUC6 and YUC7 enhances drought resistance in different species (Lee et al., 2012; Kim et al., 2013; Park et al., 2019). Similarly, yuc1yuc2yuc6 triple mutants with a lower IAA level show decreased drought resistance (Shi et al., 2014). Increasing endogenous auxin levels by overexpression of bacterial tryptophan-2-monooxygenase (iaaM) enhances drought tolerance, whereas auxin application increases ROS detoxification and the expression of ABA-related stress genes RAB18, RD22, RD29A, RD29B, DREB2A, and DREB2B in Arabidopsis, linking ABA and auxin in growth–stress trade-offs (Shi et al., 2014). In white clover, application of IAA during stress increased relative water and total chlorophyll content under drought conditions while also increasing ABA and jasmonic acid (JA) abundance (Zhang et al., 2020). PIN proteins OsPIN2 and OsPIN5b are up-regulated by drought stress in rice (Du et al., 2013a). In another example, overexpression of OsPIN3t improved survival rates under approximated drought conditions created with 20% polyethylene glycol (PEG) (Zhang et al., 2012). Further, GmLAX genes which are up-regulated during PEG and ABA treatment are thus thought to be involved in drought response in soybean (Yang et al., 2021). Each of these examples points to a role for auxin levels and distribution in drought tolerance across several species.
Auxin signaling components are also affected by drought response and play roles in stress tolerance. Recent studies have identified that many Aux/IAA proteins may act as a hub that integrates other pathways including ABA. Loss-of-function mutants of IAA5, IAA6, and IAA19 display decreased drought tolerance in Arabidopsis; these Aux/IAA genes are directly targeted by the stress factor DREB/CBF proteins (Salehin et al., 2019). Overexpression of OsIAA18 and OsIAA20 enhances rice tolerance against drought stress (Wang et al., 2021; A. Zhang et al., 2021). The wheat Aux/IAA gene TaIAA15-1A enhances plant tolerance against drought stress by regulating ABA-related genes (Su et al., 2023). SAUR proteins such as SAUR32 has also been implicated in ABA-mediated stomatal closure during drought stress (He et al., 2021).
The ARFs are also regulated by drought stress in both Arabidopsis and crop species such as tomato. Many tomato SIARF genes, including SlARF1, SlARF2A, SlARF2B, SlARF4, SlARF6A, SlARF6B, SlARF8A, SlARF9A and SlARF18, are induced under drought stress in tomato (Bouzroud et al., 2018). Interestingly ARF7A is repressed by stress in the shoot but induced by stress in the root (Bouzroud et al., 2018). Loss of SlARF4 activity by either artificial miRNA knockdown or Cas9 editing increases drought and salinity tolerance (Bouzroud et al., 2020; Chen et al., 2021). BdARF4 (Brachypodium distachyon), along with BdARF8 and BdARF17, are up-regulated during drought stress (S. Song et al., 2019). SbARF10, 16, and 21 are up-regulated in Sorghum bicolor (Wang et al., 2010). In rice, OsARF2, OsARF4, OsARF10, OsARF14, OsARF16, OsARF18, OsARF19, OsARF22, and OsARF23 are induced in leaves and roots exposed to drought (Wang et al., 2007). In soybean (Glycine max), nearly all ARF genes are induced by water deficit (Ha et al., 2013). Expressing IbARF5 (Ipomoea batatas) in Arabidopsis improves drought tolerance, probably by inducing ABA biosynthesis (Kang et al., 2018). In Actinidia chinensis (kiwifruit), AcARF1a, 19a, 26a, 28a, 28b, 5, and 7 are up-regulated after drought (Su et al., 2021). These results implicate ARF proteins in drought stress, in particular SlARF4 and IbARF5 as mutants of or overexpression of these genes can increase drought tolerance (Kang et al., 2018; Bouzroud et al., 2020; Chen et al., 2021).
In most cases, changes in auxin levels or signaling activation affect drought tolerance through reduced ROS production and/or increasing ABA levels (Shi et al., 2014; Park et al., 2019; Wang et al., 2021; A. Zhang et al., 2021; Su et al., 2023). This connection is not surprising, as there are many points of auxin–ABA crosstalk in growth and development (Emenecker and Strader, 2020). A better understanding of auxin and ABA interactions in drought stress would yield insight into coordination of growth and stress response.
Much of the work connecting auxin to drought stress response has been done in crop species. Detailed molecular analysis of auxin components that are altered during drought stress is needed to better understand auxin’s role in drought tolerance and to possibly provide tools to enhance drought tolerance. For example, for the ARF and Aux/IAAs, only down-regulation of SlARF4 and overexpression of IbARF5 in Arabidopsis have been shown to alter drought tolerance in addition to reducing TaIAA15-1A in wheat. The overexpression of AtYUC6 in potato increases drought tolerance, but reduced tuber yield, highlighting the difficultly in optimizing growth–stress trade-offs in crop species by modulating the auxin pathway (Kim et al., 2013).
Auxin roles in cold stress
Cold stress deleteriously impacts plant survival, growth, and productivity, and limits geographical distribution of various plant species (reviewed in Kidokoro et al., 2022). Plants are frequently exposed to cold stress, including both chilling (0–12 °C) and freezing (<0 °C) stresses (reviewed in Ding et al., 2020). Chilling temperatures inhibit plant growth, but freezing temperatures destroy cell membranes, leading to cell death (reviewed in Kidokoro et al., 2022). Plants have evolved diverse systems to adapt and survive under low temperatures, referred to as cold acclimation. Cold sensors such as calcium channels and CHILLING TOLERANCE DIVERGENCE 1 (COLD1) perceive cold temperature (Ma et al., 2015; Zhang et al., 2019) whereas the C-repeat-binding factor (CBF)/dehydration-responsive element-binding factor (DREB1) activates cold-responsive (COR) genes for plant response to cold stress (Novillo et al., 2007; Jia et al., 2016; C. Zhao et al., 2016). Current studies indicate that CBF-dependent signaling pathways play a prominent role in cold stress (Shi et al., 2018). Additionally, many physiological and sophisticated molecular mechanisms exist during cold acclimation, including auxin response and its regulation.
Cold stress affects auxin levels differently depending on plant species, developmental stage, and physiological settings (Fukaki and Tasaka, 1999; Wyatt et al., 2002; Kosova et al., 2012; Majlath et al., 2012; Maruyama et al., 2014). Cold stress inhibits the gravitropic growth of stems and roots in Arabidopsis and rice (Fukaki and Tasaka, 1999; Wyatt et al., 2002). Application of auxin analogs induces the accumulation of freeze-protective metabolites and soluble sugars during cold stress in Brassica napus (Gaveliene et al., 2013). Auxin levels increase under low temperatures in wheat (Triticum aestivum L.) (Wang et al., 2023). Many auxin-related genes are significantly affected under cold stress in Arabidopsis and rice, including auxin efflux carrier genes, ARF transcription factor genes, and transcription repressor Aux/IAA genes (Hannah et al., 2005; Jain and Khurana, 2009). Moreover, cold treatment increases the expression of the YUC family members and reduces that of the OsGH3 family gene in rice (Du et al., 2013a). Overexpression of SgGH3.1 alters the sensitivity to exogenous IAA, induces AtCBF1–AtCBF3 gene expression, and enhances the chilling tolerances in Arabidopsis (Jiang et al., 2021). Cold stress inhibits root elongation and gravity response by inhibiting the intracellular trafficking of auxin efflux carriers PIN2 and PIN3 (Shibasaki et al., 2009). Further studies indicate that cold stress affects the polar transport of auxin by modulating intracellular cycling of PIN2 (Ashraf and Rahman, 2019). Moreover, mutants defective in the auxin transcriptional repressor IAA14 display a hypersensitive response under cold stress in Arabidopsis (Aslam et al., 2020). Overexpression of the auxin transcription factor gene CsARF5 enhances the chilling tolerance in cucumber, suggesting an important role in cold stress (X. Zhang et al., 2021).
Taken together, these findings suggest a link between cold stress and auxin response in regulation of plant growth and development (Table 1). However, compared with our understanding of auxin roles under elevated temperatures, auxin roles under cold stress are not well understood.
Conclusion and perspectives
Plants grown in the natural environment are subjected to a range of abiotic stresses, which affect plant growth and development. Abiotic stress factors that are particularly detrimental to plants include extremes of temperature, drought, and high salinity. To survive and maintain growth, plants have evolved diverse systems at the molecular, cellular, and physiological levels for adapting and surviving under adverse growth conditions. The phytohormone auxin plays critical roles in regulation of abiotic stresses (Table 1), although these roles are better defined for some stresses than for others.
Altering auxin levels, either through its application or by overexpressing auxin biosynthesis genes, can mitigate the impact of drought, heat, salinity, and cold stress, but mechanistic insight into how this occurs is largely lacking. This connection between auxin and stress tolerance, while not fully elucidated, provides a potential research direction for improving stress tolerance in agricultural settings. For example, defects in SlARF4 result in increased drought and salinity tolerance (Bouzroud et al., 2020; Chen et al., 2021), a feature that could be leveraged in tomato, and perhaps other crops, to engineer resilience. Additionally, a better understanding of auxin and other hormone crosstalk interactions may provide tools for crop improvement. Many points of interaction between auxin and ABA, and JA have been uncovered (Emenecker and Strader, 2020). Understanding how these known interactions regulate growth during stress would provide more mechanistic insight into auxin’s role in abiotic stress.
Further, insights into auxin and stress-protective metabolites may prove fruitful in increasing stress tolerance in crop species. Glucosinolates (GLSs) are secondary metabolites found in Brassicaceae that are involved in resilience to herbivore and pathogen attack and have also been found to regulate growth (Katz et al., 2015). The loss of Aux/IAA 5, 6, and 9 decreases GLS content during drought stress and leads to a defect in stomatal regulation (Salehin et al., 2019). Glucosinolates are but one example of downstream molecules activated by auxin to buffer resilience in response to stress, and more studies are needed to fully uncover connections between auxin, secondary metabolites, and stress response. Additional potential agricultural applications for auxin in abiotic stress response include the following:
Identification of downstream auxin factors which bolster stress responses in stress-tolerant cultivars using multi-omic studies between established stress-resistant and -sensitive crop varieties could identify new targets for breeding (Sharma et al., 2021; Da Ros et al., 2023).
Validation of stress-related secondary metabolites which are activated by auxin-related stress responses in crop species to identify effectors of auxin effects on stress response. For example, auxin signaling drives the expression of freeze-protective metabolites in cold stress (Gaveliene et al., 2013), and downstream application of exogenous GLSs facilitates the response to drought stress.
Using nuanced ways such as synthetic biology to alter auxin biosynthesis, signaling, and transport during developmental stages when plants are vulnerable to abiotic stress (i.e. pollen development and seedling establishment) could be a tool to regulate plant growth during stressful conditions (Brophy, 2022).
Modulation of miRNAs (i.e. MiR160 and 167) to alter ARF abundance during abiotic stress (Wu et al., 2006; Kruszka et al., 2014; Ding et al., 2017) could be used in crop species to engineer stress resilience.
Integration of new technologies such as CRISPR to edit auxin-related genes in crop species (Chen et al., 2021) and/or use of single-cell and tissue-specific approaches to identify and modify processes regulating abiotic stress response in vulnerable tissues of crop species (Mesihovic et al., 2016; Nelms and Walbot, 2019; Chaturvedi et al., 2021; X. Li et al., 2022).
The study of auxin modulation under abiotic stresses is growing. However, it is crucial to understand the mechanisms of auxin involved not only under single stress conditions, but also under multiple stresses. Many gaps remain in our understanding of auxin in responses to abiotic stress, particularly in stress sensing, early signaling, and post-translational regulation. Thus, more work is needed to explore the role of auxin in abiotic stress, providing detailed mechanisms for improving crop productivity and agricultural applications.
Author contributions
HJ, EGW, KSF, and LCS wrote the manuscript.
Conflict of interest
LCS is on the Science Advisory Board for Prose Foods.
Funding
This work was supported by the National Institutes of Health R35 GM136338 (to LCS).
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