When two negatives make a positive: the favorable impact of the combination of abiotic stress and pathogen infection on plants

Abstract

Combined abiotic and biotic stresses modify plant defense signaling, leading to either the activation or suppression of defense responses. Although the majority of combined abiotic and biotic stresses reduce plant fitness, certain abiotic stresses reduce the severity of pathogen infection in plants. Remarkably, certain pathogens also improve the tolerance of some plants to a few abiotic stresses. While considerable research focuses on the detrimental impact of combined stresses on plants, the upside of combined stress remains hidden. This review succinctly discusses the interactions between abiotic stresses and pathogen infection that benefit plant fitness. Various factors that govern the positive influence of combined abiotic stress and pathogen infection on plant performance are also discussed. In addition, we provide a brief overview of the role of pathogens, mainly viruses, in improving plant responses to abiotic stresses. We further highlight the critical nodes in defense signaling that guide plant responses during abiotic stress towards enhanced resistance to pathogens. Studies on antagonistic interactions between abiotic and biotic stressors can uncover candidates in host plant defense that may shield plants from combined stresses.

Abiotic stress, combined stress, drought, multi-stress interaction, pathogen, plant defense, positive impact, salinity, temperature

Introduction

Plants grow in complex and dynamic environments, encompassing continuously evolving interactions between various stresses. Although abiotic stresses in most cases predispose plants to various devastating diseases (Bostock et al., 2014; Sinha et al., 2019; Rai et al., 2022), the outcome of abiotic and biotic stress interactions can sometimes be favorable for the plant’s performance under combined stress conditions. We have termed such stress interactions ‘positive’ combined stresses, defined as stress conditions wherein the net impact of the interaction between the stresses is more favorable to the plants as compared with the individual effect of one or both of the stresses. Combined abiotic stress and pathogen infection can positively affect plants in three ways (Supplementary Fig. S1). Firstly, some abiotic stresses may enhance plant immunity, thereby protecting plants against a few pathogens (Melotto et al., 2017; Yang et al., 2019). Secondly, abiotic stress may affect pathogens directly by weakening pathogen virulence, transmission, or spread outside the plant interface (Desaint et al., 2021; Zarattini et al., 2021). Thirdly, certain pathogens may enhance the tolerance of some plants to abiotic stress or reduce the abiotic stress effects (Xu et al., 2008; Gorovits et al., 2019; Tuang et al., 2020). Factors influencing the net impact of combined stress on plants include the intensity, duration, and order of the stresses; plant age; and genetic makeup (Kissoudis et al., 2016; Pandey et al., 2017; Bai et al., 2018). More such studies are compiled in a resource Stress Combinations and their Interactions in Plants Database (http://www.nipgr.ac.in/scipdb.php; Priya et al., 2023). These factors are also responsible for turning on key switches that amplify plant defense against pathogens and steer the overall effect of a stress combination in favor of the plants (Box 1).

Box 1.

Factors governing the impact of stress combinations on plants: reflections on studies indicating a positive effect of abiotic stresses on pathogen defense

Due to the complexity of shared and unique responses triggered under combined stress, the overall impact of the stress combination cannot be easily predicted (Pandey et al., 2015, 2017). However, some characteristic factors (presented in the figure below) are crucial for governing the overall plant response to the stress combinations. Case studies exemplifying the importance of each of the factors in governing the impact of stress combinations are listed in Supplementary Table S3.

Here, we enumerate the possible ways in which abiotic stress negatively affects pathogen interaction with plants, thereby enhancing plant protection. We also review how certain pathogens boost plant abiotic stress tolerance. Although most reports on combined abiotic and biotic stress indicate their negative effect on plants, studying the positive impact of combined stress is also important, as highlighted by some key studies (Mittler, 2006; Mittler and Blumwald, 2010) (Box 1). Learning from positive combined stresses could reveal pathogen vulnerabilities and plant defense mechanisms that can be exploited to improve plant protection in field conditions. Furthermore, a clear understanding of positive combined stresses can be utilized to tailor phyllosphere and rhizosphere environments conducive to maximizing plant growth, development, and productivity.

Abiotic stresses as ‘eustressors’ in plant pathogen defense

Reviewing 262 research papers on combined abiotic and pathogen stresses showed that 38.55% corresponded to positive stress combinations (Supplementary Dataset S1), wherein the abiotic stress acts as a ‘eustress’ and protects plants from pathogen infection (Supplementary Table S1). Similarly, pathogen infection can also mitigate abiotic stress effects in some cases and improve plant performance (measured as maintenance of high leaf water content and increased shoot weight and leaf area; Supplementary Fig. S2). To provide a glimpse into how pathogen infection changes under abiotic stress conditions, we have limited our discussion to studies on the effect of three major abiotic stresses—drought, temperature, and salinity—on pathogens and their interactions with plants. Among the 101 studies discussing the impact of drought, temperature extremes, and salt on pathogen infection, 44 studies indicated reduced disease (reduction in pathogen load and disease index) under abiotic stresses (Fig. 1; Supplementary Dataset S1). It is noteworthy that the protection conferred by abiotic stress is confined to a particular pathogen during a specific stage of plants unless proven otherwise. The effect of environmental extremes and global warming on the occurrence and distribution of pathogens has been extensively reviewed (Velásquez et al., 2018). Although pathogens show better adaptability to environmental extremes than plants do, some abiotic stresses often cause significant reductions in inoculum levels (Supplementary Fig. S3). Abiotic stresses may protect plants against pathogens at various stages of plant–pathogen interactions (Box 1) in one or more of the following ways, namely (i) reducing the number of pathogens and their transmission in the rhizosphere and phyllosphere, (ii) turning off the pathogen virulence switch outside the plant, and (iii) turning off pathogen multiplication and growth inside the plant due to enhanced plant immunity.

Fig. 1.

Illustration of literature-based analysis of the effect of abiotic stresses on plant–pathogen interaction. The sunburst diagram depicts the positive effect of abiotic stress (drought, high temperature, and salt) in reducing pathogen-induced disease incidence under combined stresses in 44 studies. Different colors indicate stress combinations. The innermost layer represents the name of the stress combination; middle layer, the name of the plant species; and outer layer, stress treatments with calculated parameter values. The bars represent the percentage change in disease incidence and were calculated as changes in parameter values under combined stress compared with pathogen stress (for raw values and calculation, see Supplementary Dataset S1). The number in parentheses below each stress combination indicates the number of positive studies reported among the total number of articles published so far. *The order of stress imposed is important in the study; reversing the order will change the outcome. All the stress combinations are listed according to the order of stress imposed in the article. The arrow mark shows the direction of the radial tree read. HT, high temperature; LT, low temperature; S, salt; D, drought; HLT, high/low temperature; B, bacteria; F, fungus; V, virus; PRV, papaya ringspot virus; MNSV, melon necrotic spot virus; B.cin, Botrytis cinerea; S.rol, Sclerotium rolfsii; X.com, Xanthomonas campestris; P.syr, Pseudomonas syringae; U.may, Ustilago maydis; F.spp, Fusarium spp.; A.sol, Alternaria solani; R.sol, Rhizoctonia solani; P.str, Puccinia striiformis; A.rab, Ascochyta rabiei; B.fab, Botrytis fabae; A.len, Ascochyta lentis; F.oxy, Fusarium oxysporum f. sp. cubense; P.tri, Pyrenophora tritici-repentis; P.pac, Phakopsora pachyrhizi. DF, drought and fungus; DB, drought and bacteria; SF, salt and fungus; LTF, low temperature and fungus; HTF, high temperature and fungus; HLTV, high light and fungus; DI, disease incidence.

Below, we discuss the cases exemplifying the above-mentioned ways in which salt, drought, and temperature extremes act as eustressors during plant–pathogen interactions.

Salt stress

Among the three stressors, high salt concentrations predominantly inhibit the growth and disrupt the life cycle of some pathogens. Therefore, salt treatment has been used as an alternative strategy for controlling various fungal diseases (Deliopoulos et al., 2010). Salt treatment reduces the production of infectious spores in Fusarium oxysporum f. sp. elaeidis (causal agent of vascular wilt in oil palm), although it increases the pathogen germination rate (Dossa et al., 2019). High salt concentrations in the soil also adversely impact fungal load by increasing the population of symbiotic bacteria. For example, treatment of potted asparagus plants with 1% NaCl increases the density of manganese-reducing bacteria 3-fold in the rhizosphere, concurrent with the decrease in root rot disease severity (Elmer, 2003). Salt stress also reprograms the expression of pathogenicity-determining genes. For example, salt stress affects the virulence of Xanthomonas citri f. sp. citri (causal agent of bacterial canker) and impairs its motility by down-regulating the expression of flagellum-associated genes (Barcarolo et al., 2019).

Furthermore, ion accumulation in host intracellular spaces due to salt stress is known to decrease bacterial colonization inside the plant. For example, exposure of Arabidopsis to 300 mM NaCl decreased the number of pathogenic lesions caused by Pseudomonas syringae pv. maculicola DG3 (Yang et al., 2019). Similarly, high salt stress inhibited P. syringae pv. maculicola growth inside Arabidopsis (Singh et al., 2014). In some instances, salt stress also modulates the composition of root exudates, contributing to reduced pathogen infection. For example, salinity-induced reductions in the levels of malic acid and other organic acids in root exudates decrease F. oxysporum f. sp. asparagi (causal agent of root rot of asparagus) infection on asparagus roots (Elmer, 2003).

Drought stress

Early reports on the interaction between Blumeria graminis (causal agent of powdery mildew) and barley showed that dry soil conditions inhibit fungal growth, especially the emergence of secondary hyphae on the upper leaves of plants (Ayres and Woolacott, 1980). Drought suppresses fungal diseases caused by pathogens dispersed by rain splashes (Markell et al., 2008). Drought also affects the plant–insect–vector relationship and reduces the transmission of some circulative viruses associated with insect vectors. For example, the transmission of turnip mosaic virus in Arabidopsis decreases under severe drought (Yvon et al., 2017). Similarly, soybean mosaic virus transmission decreases significantly in soybean plants subjected to drought stress (Nachappa et al., 2016).

Although osmotic stress has been shown to reduce mycelial growth (Liu et al., 2021), evidence for drought-induced suppression of pathogen virulence outside the plant interface has not been reported. However, drought stress is known to inhibit the multiplication of P. syringae and Ralstonia solanacearum (causal agent of bacterial wilt in many crops) inside Arabidopsis and chickpea, respectively (Gupta et al., 2016; Sinha et al., 2016). Furthermore, restricting water flow to the site of infection inhibits P. syringae infection in Arabidopsis due to the drought-mediated down-regulation of type III secretion system (T3SS)-associated genes, which decreases the pathogen’s ability to colonize the host (Freeman et al., 2013). The effect of water-limiting conditions on P. syringae T3SS has been shown in in vitro studies but has not been studied during its association with plants. It would be interesting to study the effect of drought on pathogen virulence inside the plant by a co-transcriptomic study to explore the mechanistic details. Drought stress reduced tomato spotted wilt virus (causal agent of tomato spotted wilt) infection in tomatoes by inhibiting virus particle translocation and systemic spread (Córdoba et al., 1991). Similar effects of water deficit on systemic spread are reported for cauliflower mosaic virus (causal agent of cauliflower mosaic) in Arabidopsis (Bergès et al., 2018).

Temperature extremes

Temperature influences the growth and sporulation of pathogenic fungi such as Botrytis fabae (causal agent of chocolate spot disease), Phakopsora pachyrhizi (causal agent of soybean rust), and Mycosphaerella pinodes (causal agent of blight in peas) (Roger et al., 1999; Bonde et al., 2012; Benzohra et al., 2017). For example, high-temperature treatments (increase by ≥4 °C) inhibit uredinospore production in P. pachyrhizi (Bonde et al., 2012), which significantly reduces lesion development in soybean plants. Similarly, temperature extremes (>20 °C) decrease the formation of M. pinodes pycnidia in pea (Roger et al., 1999). The inhibitory effect of abiotic stresses on pathogen multiplication is highly pathogen specific, calling for a careful investigation of the stages of the pathogen life cycle vulnerable to abiotic stressors. Knowledge of the mechanism of abiotic stress-mediated inhibition of pathogen growth can help in designing strategies to curb the number of pathogen propagules in the rhizosphere and phyllosphere.

Temperature also plays a role in quorum sensing. For example, high temperature reduces the virulence of Pectobacterium carotovorum (causal agent of soft rot disease) by inhibiting the secretion of the quorum-sensing molecule N-(3-oxohexanoyl)-l-homoserine lactone (Saha et al., 2015). A similar high-temperature-mediated reduction in the production of toxins such as phaseolotoxin and coronatine occurs in P. syringae pv. phaseolicola (causal agent of bacterial blight) and P. syringae pv. coronafaciens, respectively (Nüske and Fritsche, 1989; Ullrich et al., 1995).

High temperatures also inhibit the in planta multiplication and accumulation of viruses such as soybean yellow mottle mosaic virus in legumes (Nagamani et al., 2020) and cotton leaf crumple virus in cotton (Tuttle et al., 2008). Temperature-mediated enhancement of plant immunity has also been reported (Ge et al., 1998; Cohen et al., 2017; Onaga et al., 2017; Feng et al., 2018). The induction of specific disease resistance by both pre- and post-infection cold stress has been reported in the grapevine–Erysiphe necator pathosystem (Moyer et al., 2010, 2016; Weldon et al., 2020). Low temperature down-regulates the photosynthetic machinery in grapevines, which translates to a low nutrient supply for the pathogen. In addition, cold hardening, which confers freezing/cold tolerance to some plants, is also associated with resistance to certain pathogens (Gaudet et al., 2011; Bahmani et al., 2020). In the chickpea–Ascochyta rabiei interaction, cold acclimation improves the antioxidative defense of the plants, conferring resistance to Ascochyta blight (Bahmani et al., 2020).

Role of pathogens in improving plant response to abiotic stresses

Some pathogens (primarily viruses and bacteria) improve abiotic stress tolerance of plants by manipulating host physiological (accumulation of osmoregulators), anatomical (de novo xylem formation), and molecular [up-regulation of genes encoding heat shock factor (HSF) proteins] responses (Fig. 2; Box 2). Viruses such as plum pox virus (PPV) and potato virus X (PVX) are known to provide drought tolerance to plants (Fig. 2). Several RNA viruses, including cucumber mosaic virus (CMV), brome mosaic virus (BMV), tobacco mosaic virus (TMV), and tobacco rattle virus (TRV), effectively delay the onset of drought-induced shoot wilting in beet, pepper, watermelon, squash, rice, and tobacco (Xu et al., 2008). Similarly, CMV infection relieves Arabidopsis plants from drought stress effects (Westwood et al., 2013). Viruses also confer tolerance to heat, cold, high light, and oxidative stress (Fig. 2). For example, tomato yellow leaf curl virus (TYLCV) alleviates heat stress effects in tomato by suppressing the cell death response (Gorovits et al., 2019). Another such example is the Curvularia thermal tolerance virus (CThTV)–Curvularia protuberata (fungal endophyte)–Dichanthelium lanuginosum association, wherein CThTV confers heat tolerance to both the endophytic fungus and the host plant (Márquez et al., 2007). Besides facilitating drought tolerance, TRV infection imparts cold tolerance and enhances silique production in Arabidopsis under low-temperature conditions (Fig. 2). On a similar note, PVX infection confers cross-tolerance to UV-mediated oxidative stress in tobacco (Shabala et al., 2011).

Box 2.

Modulation of host tolerance by pathogens to their advantage

Abiotic stress tolerance conferred to the host by a pathogen can be the pathogen’s ‘selfish’ strategy to maximize its survival in the host. Osmoregulators induced under viral infection that confer protection against the abiotic stresses also play a role in viral processes such as nucleic acid packaging, maintaining virion stability, and binding to host cells (Firpo and Mounce, 2020). Meanwhile, osmoregulators such as polyamines are also involved in defense against viruses (Fernández-Calvino et al., 2014).

The interaction between a plant and virus under abiotic stress conditions requires a complicated defense and counter-defense network involving several plant metabolites with different roles. For example, putrescine accumulation in TRV-infected Arabidopsis plants confers freezing tolerance and inhibits viral multiplication (Fernández-Calvino et al., 2014). Further detailed understanding of the in planta dynamics of the plant–virus interactome will tease out other unknown elements of plant defense. Moreover, abiotic stress-driven conditional mutualism has been observed in viruses. Evidence showing the role of viral pathogenesis factors in conferring host tolerance to stress conditions indicates a possible switch from a pathogenic to a mutualistic lifestyle under adverse conditions (González et al., 2020). This phenomenon is also seen in fungi. For example, Colletotrichum tofieldiae, a fungal endophyte, facilitates Arabidopsis growth under phosphate-deficient conditions (by enabling phosphorus transfer to the plant). Phosphate starvation in the host regulates the level of indole glucosinolates, the deficiency of which can convert the endophyte to a pathogen (Hiruma et al., 2016). Thus, the host environment can modulate its association with pathogens.

Fig. 2.

Pathogen stress-mediated alleviation of abiotic stress effects in plants. Selected examples of the positive effect of pathogens on improving plant defense against abiotic stresses. Lines connecting the pathogens to the plant response are indicated by different colors to differentially indicate a specific abiotic stress (brown, drought; orange, heat; blue, cold; purple, oxidative stress). Viruses confer tolerance to drought, heat, cold, high light, and oxidative stresses in various plants (González et al., 2020). Specifically, infection with viruses such as BMV, CMV, TMV, and TRV delays the appearance of wilting symptoms under drought stress by inducing the accumulation of osmolytes such as proline, trehalose, and putrescine (Xu et al., 2008). Further, PPV infection reduces the rate of transpiration in Nicotiana benthamiana, thus alleviating the effect of drought stress. The improved drought tolerance is accompanied by enhanced accumulation of SA (Aguilar et al., 2017). Under heat stress, TYLCV protects tomato plants from heat-induced cell death by inactivating 26S proteasome degradation and HSFA2 signaling (Moshe et al., 2016). PVX infection reduces UV-C- and H₂O₂-induced damage to chloroplasts and leads to better regulation of increased cytosolic-free calcium levels (Shabala et al., 2011). The bacterium Pseudomonas syringae alleviates heat and cold stress-induced injury in Arabidopsis by inducing the expression of HSPs and CBFs (Tuang et al., 2020). BMV, brome mosaic virus; CMV, cucumber mosaic virus; TMV, tobacco mosaic virus; TRV, tobacco rattle virus; TYLCV, tomato yellow leaf curl virus; PVX, potato virus X; PPV, plum pox virus; Pst, Pseudomonas syringae pv. tomato. Pro, proline; Tre, trehalose; Put, putrescine; SA, salicylic acid; HSFA2; heat shock factor A2; HSP, heat shock protein; CBF, C repeat-binding factor; ICE1, inducer of CBF expression.

Many plant viruses manipulate host metabolism to enhance drought tolerance by inducing the accumulation of osmoprotectants (e.g. proline, putrescine, and trehalose) and antioxidants (e.g. ascorbic acid, tocopherol, and anthocyanins) (Xu et al., 2008) (Fig. 2; Box 2). TRV infection under cold stress induces a higher accumulation of putrescine, which improves plant performance under such stress (Fernández-Calvino et al., 2014). PPV–PVX-mediated drought tolerance correlates with reduced host transpiration (Aguilar et al., 2017). CMV-2b protein, a suppressor of host post-transcriptional gene signaling against the virus, and the virulence protein P25 of PVX are actively involved in equipping Arabidopsis with drought tolerance (Westwood et al., 2013; Aguilar et al., 2017). Strikingly, TYLCV-mediated heat stress protection has been attributed to the down-regulation of the expression of heat-inducible genes such as heat shock protein (HSP) 90 (HSP90) and SGT1, and the inhibition of the ubiquitin–26S proteasome degradation system and heat shock transcription factor A2 (HSFA2)-regulated downstream signaling in tomato (Moshe et al., 2016; Gorovits et al., 2019). PVX infection imparts better regulation of calcium levels under UV-C-mediated oxidative stress by up-regulating two Ca²⁺ efflux channels: the plasma membrane Ca²⁺/H⁺ exchanger and P2A and P2B Ca²⁺ ATPases (Shabala et al., 2011).

Infection with pathogenic bacteria also confers abiotic stress tolerance to host plants. For example, P. syringae pv. tomato DC3000-infected Arabidopsis plants are more tolerant to heat and cold stress (Tuang et al., 2020). This tolerance accompanies salicylic acid (SA)-mediated increased expression of cold-responsive genes such as inducer of CBF expression 1 and C-repeat-binding factors (CBFs). Similarly, heat tolerance is attributed to the increased expression of HSPs and HSFs (Tuang et al., 2020) (Fig. 2).

Fungal endophytes are well known to confer tolerance to abiotic stress in plants (Singh et al., 2011), but evidence suggesting the involvement of pathogenic fungi in imparting tolerance to abiotic stresses is sparse. One notable example is of Verticillium longisporum, which imparts drought tolerance by inducing the de novo formation of xylem cells under drought stress (Reusche et al., 2014).

Critical regulators of plant defense in positive combined stress studies

To identify the regulators of plant defense in positive combined stress studies wherein abiotic stresses help in alleviating pathogen infection, we compared the shared molecular changes incited by multiple single-stress conditions. A meta-analysis of drought, heat, salt, and virus infection single-stress transcriptome studies (Supplementary Table S2) in Arabidopsis revealed the shared genes involved in defense against different biotic and abiotic stresses (Fig. 3; Supplementary Fig. S4). The analysis indicated the enrichment of pathways involved in protein processing, plant–pathogen interaction, mitogen-activated protein kinase (MAPK) signaling, plant hormone signal transduction, and secondary metabolite synthesis (Supplementary Dataset S1). These pathways were also enriched under combined drought and A. rabiei infection stress (Patil et al., 2021, Preprint) (Fig. 4; Supplementary Fig. S5).

Fig. 3.

Pathway enrichment analysis of shared up-regulated genes identified by a meta-analysis of single-stress transcriptome studies in Arabidopsis. Transcriptome datasets for single stresses [drought (GSE76827), high temperature, salt (GSE147962), and CMV (E-GEOD-37921)] in A. thaliana were compiled, curated, and analyzed. To decipher the core set of stress-responsive genes, a meta-analysis using Fisher’s approach was employed to combine the multiple studies into one single meta-study. (A) Enriched pathway terms obtained from the KEGG database visualized as a barplot. The length of the bar represents the enrichment ratio which is calculated as ‘input gene number’/‘background gene number’. The color of the bar represents different clusters of related pathways. (B) Network representation of key pathways associated with pathogen and abiotic stress tolerance, showing the enrichment of seven main pathway clusters grouped into broader categories based on KEGG pathway classification: lipid metabolism, carbohydrate metabolism, amino acid metabolism, biosynthesis of plant hormones, sugar and hormone signaling, secondary metabolite biosynthesis, and glycan metabolism. V-shaped nodes are key pathway clusters (names indicated). Specific pathways related to these clusters are indicated in similar colored boxes. Nodes in the circle are genes mapped to the corresponding pathways. The color of nodes depicts different enriched pathways and their corresponding associated genes in soft orange. The network is visualized with Cytoscape (v3.8.2) with the ‘degree sorted circle’ layout.

Fig. 4.

Pathway enrichment analysis of up-regulated combined stress genes from the drought and Ascochyta rabiei transcriptome in chickpea. ClueGO v2.5.8 and CluePedia v1.5.8 plugins were used to perform and comprehensively visualize functionally grouped pathway terms of differentially expressed up-regulated genes under combined drought and A. rabiei in chickpea. Functionally grouped terms as nodes linked based on their kappa score level (pathway network connectivity) (≥0.4), where only the label of the most significant terms per group is represented as an enriched pathway and the associated genes are depicted in the form of a network. The network was visualized using Cytoscape (v3.8.2) in the ‘Perfuse force-directed’ layout, and edge bundling was done for clarity. Major pathways are indicated in diamond shapes, while genes mapped onto them are highlighted in circles. Node size represents the significance of the term (i.e. the largest term is the most significant one). Different colors indicate distinct pathways, and their associated genes are colored similarly. Shared genes between pathways are depicted as multicolored pies. The top 10 pathways enriched under drought and pathogen combined stress treatments include the citrate cycle (TCA cycle); phenylpropanoid biosynthesis; phenylalanine, tyrosine, and tryptophan biosynthesis; plant–pathogen interaction; MAPK signaling pathway; oxidative phosphorylation; propanoate metabolism; phenylalanine metabolism; N-glycan biosynthesis; and protein export.

Both the analyses indicated the enrichment of genes involved in starch and sucrose metabolism (Figs 3, 4). Genes encoding glycosidases such as ATBFRUCT1, BGLU25, KOR2, and beta-glucosidase 12-like were up-regulated under the four individual stresses and the combined stress (Fig. 3). The role of sugar metabolism and transport in plant–pathogen interactions has been extensively described (Liu et al., 2022). Limited sugar supply is a chief constraint that reduces pathogen infection and survival in the phyllosphere. Sugars play a role in enhancing plant immunity by stimulating the synthesis of resistance (R) proteins (Morkunas and Ratajczak, 2014). Interestingly, SWEET4 was found to be related to sugar accumulation during the infection of Botrytis cinerea in Vitis vinifera (Chong et al., 2014), and SWEET10 was found to confer resistance to F. oxysporum in Ipomoea batatas (Li et al., 2017). Thus, sugar sensing, transport, and signaling can play important roles in plant defense under combined stress.

The proline metabolism pathway plays a crucial role in defense against individual and combined stresses (Rizhsky et al., 2004). Proline accumulation helps plants defend against various abiotic stresses, but the down-regulation of proline levels, and presumably its metabolism, has been shown to be important in defending tomatoes against multifactorial stress conditions (Pascual et al., 2023). It is noteworthy to mention that the role of proline metabolism in plant–pathogen interactions is complex and is just beginning to be understood (Alvarez et al., 2022). Additionally, proline metabolism has been found to be significant in defending plants against combined drought and A. rabiei infection (Patil et al., 2021, Preprint). Genes such as those encoding pyrroline-5-carboxylate reductase (P5CR), proline dehydrogenase (ProDH), and arginine decarboxylase 1 (ADC1) can serve as essential regulators of defense under stress combinations. Osmoregulators confer tolerance to many abiotic stresses and participate in biotic stress signaling (Liu et al., 2019). For instance, putrescine is involved in virus-mediated cold tolerance in Arabidopsis, and Arabidopsis mutants with defects in putrescine biosynthesis are hypersusceptible to TRV infection (Fernández-Calvino et al., 2014). Polyamine accumulation is also associated with the hypersensitive response during TMV infection in tobacco (Torrigiani et al., 1997). Because polyamine metabolism is essential for both plants and pathogens, its regulation under abiotic stress can determine the outcome of plant–pathogen interactions during abiotic stresses (Box 2).

Reactive oxygen species (ROS) scavenging is another key defense found to be activated under individual and combined abiotic and biotic stresses. The meta-analysis shows that genes encoding enzymes of the ascorbate–glutathione cycle, such as ascorbate peroxidase 5 (APX5), glutathione peroxidase 6 (GPX6), glutathione-S-transferase 6 (GSTF6), GSTu4, monodehydroascorbate reductase (MDHAR), and superoxide dismutase (SOD), are differentially up-regulated under individual and combined stress (Fig. 3). Peroxidases were earlier found to be associated with resistance against A. rabiei in the resistant chickpea cultivar Sultano (Angelini et al., 1993). Further, the NaCl-mediated inhibition of B. cinerea infection in the halophyte Mesembryanthemum crystallinum was attributed to the enhanced activities of antioxidant enzymes such as ascorbate peroxidase (Kuźniak et al., 2011). Drought-mediated resistance to P. syringae pv. tabaci in Nicotiana benthamiana plants has also been correlated with enhanced levels of ROS (Ramegowda et al., 2013).

Interestingly, the enrichment of genes of MAPK (MPK3, 4, and 6) and calcium-dependent protein kinase (CDPK1 and CPK28) signaling was observed in the single-stress meta-analysis, reflecting complex crosstalk between various abiotic and biotic stress signaling pathways (Fig. 3). Furthermore, the meta-analysis showed the up-regulation of genes involved in abscisic acid (ABA) signaling, such as ABA-responsive element-binding factor 4 (ABF4) and ABA receptor (PYL5) (Fig. 3). Studies have shown that ABA is prominently involved in conferring tolerance to pathogens under drought conditions. For example, drought-mediated resistance to Plasmopara viticola (causal agent of downy mildew) in V. vinifera was attributed to the accumulation of callose and the phytoalexin stilbene induced by increased ABA levels (Heyman et al., 2021). In combined stress studies, ABA is prominently involved in enhanced plant resistance to pathogen infection. ABA per se enhances resistance to Cochliobolus miyabeanus (causal agent of brown spot) in rice (de Vleesschauwer et al., 2010), Alternaria solani (causal agent of early blight) in tomato (Song et al., 2011), and B. graminis (causal agent of powdery mildew) in barley (Wiese et al., 2005). Another manifestation of drought-induced elevation in ABA levels is stomatal closure, and this physiological response is also related to enhanced resistance against pathogens such as P. syringae DC3000 and Melampsora apocyni (causal agent of rust) in Arabidopsis and luobuma plants (Melotto et al., 2006, 2017; Gao et al., 2018). However, under drought and P. syringae pv. tomato infection, SA and jasmonic acid (JA) were reported to have a dominant role in enhancing plant defense against the bacterium (Gupta et al., 2017). Interestingly a member of the JA signaling pathway, JAZ12, was found to be up-regulated under drought, heat, salt, and virus infection in the meta-analysis. Thus, the involvement of a specific hormone depends on factors such as the plant pathosystem and stress type and intensity, and it will be worthwhile to dissect the roles of ABA, SA, and JA in plant defense under combined abiotic and biotic stress on a case-specific basis.

Positive stress combinations and their role in crop improvement in complex environments: perspectives

Recently, there has been extensive research on molecular changes induced in plants by several abiotic–biotic stress combinations, mainly focusing on combinations that negatively impact plants. The studies have unraveled different susceptibility factors induced under abiotic stresses (Choudhary and Senthil‐Kumar, 2022; Irulappan et al., 2022). Meanwhile, positive combined stress studies have highlighted the extraordinary phenomenon of plant defense that neutralizes the effect of one or both of the individual stresses. Transcriptomic studies have unveiled the molecular events underlying the key defense mechanisms of plants, and there are several points to contemplate. For example, some abiotic stresses can prime plants against certain pathogen infections, which indicates the possibility of ‘immunizing’ plants using abiotic stress to protect them from pathogen infection. However, whether the ‘priming’ can confer broad-scale protection against pathogens needs to be tested. Nevertheless, traditional and conventional farming practices such as brine application to wheat seeds to curb smut infection (caused by Tilletia caries) (Deliopoulos et al., 2010) should be revisited in the new light of integretomics. Furthermore, since viruses are known to confer drought tolerance, inactivated viruses unable to cause disease can be used to prime plants against abiotic stresses (Gorovits et al., 2019).

Identification of critical regulators of plant responses in positive combined stress studies will highlight key candidates that can be utilized to enhance plant defense against combined abiotic stress and pathogen infection (Fig. 5). For instance, one effective plant defense strategy against pathogens is the inhibition of pathogen entry through the induction of physical barriers such as stomatal closure and callose deposition (Zandalinas and Mittler, 2022). Interestingly, stomatal opening and closure are traits that can be variably affected by different stresses, thereby influencing the outcome of stress combinations. Abiotic stress conditions such as drought induce stomatal closure, thereby preventing pathogen entry. However, the response changes in the presence of additional stressors such as heat or salinity, which have different effects on stomatal movement (heat causes opening, while salinity causes closure of stomata; as extensively reviewed in Zandalinas and Mittler, 2022).

Fig. 5.

Overview of the known and predicted mechanisms underlying the abiotic stress-mediated suppression of some pathogen infections. Brown, gray, yellow, and blue colors indicate salt, drought, heat, and cold stress effects, respectively. The figure shows the genes (predicted* and validated^# highlighted in green), pathways (highlighted in pink), and the corresponding physiological processes (highlighted in blue) that inhibit plant pathogens under abiotic stress conditions. The innermost square (outlined in green) represents the plant-specific responses. The outer square (outlined in purple) represents the net outcome of the responses leading to reduced pathogen infection. ¹Salt stress-induced deficiency in the organic acid content of root exudates decreases fungal infection (Elmer, 2003). ²Ion accumulation due to salt stress in intracellular spaces prevents bacterial colonization (Yang et al., 2019). ³Drought inhibits in planta pathogen multiplication (Gupta et al. 2016). ⁴Drought activates reactive oxygen species (ROS) defense mediated by proline metabolism (Patil et al., 2021). ⁵Drought stress leads to enhanced ROS production and confers resistance against P. syringae pv. tabaci in N. benthamiana plants (Ramegowda et al., 2013). ⁶Drought stress reduces the insect vector-mediated transmission of circulative viral pathogens (Yvon et al., 2017). ⁷Heat stress-induced plant resistance against fungal pathogens is conferred by genes such as Yr18, Yr29, Yr24, Yr26, and Yr39. ⁸Arabidopsis mutants defective in putrescine biosynthesis are hypersusceptible to TRV infection (Fernández-Calvino et al., 2014). NOP, nitrogenous osmoprotectants such as proline and putrescine; ABF4, abscisic acid-responsive element-binding factor 4; ABR17, ABA-responsive 17; PYL, pyrabactin resistance1-like; JAZ12, jasmonate-zim-domain 12; P5CR, pyrroline-5-carboxylate reductase; ProDH, proline dehydrogenase; ADC1, arginine decarboxylase 1; APX, ascorbate peroxidase; SOD, superoxide dismutase; GST, glutathione-S-transferase; CYP81E1, cytochrome P81E1 encoding isoflavone 2ʹ-hydroxylase.

In a recent study of note, continuous exposure to light was found to reduce P. syringae pv. tomato infection in Arabidopsis by preventing the pathogen’s ability to induce post-infection stomatal closure, thereby inhibiting apoplastic water accumulation. The post-infection stomatal opening was attributed to the induction of SA signaling, which counteracts ABA-mediated stomatal closure (Lajeunesse et al., 2023). Therefore, extensive investigations into the effects of different multifactorial stresses on the regulation of stomatal movement and associated physiological processes such as transpiration are warranted. Targeting ABA and/or SA signaling can induce responses that enhance physical barriers.

Restricting nutrient supply to pathogens by targeting sugar transporters can effectively inhibit pathogen growth under combined stresses. Similarly, focusing on signaling pathways involved in the metabolism of osmoregulators such as proline, putrescine, and trehalose can strengthen defense against combined stress (MacIntyre et al., 2022). Proline, in particular, has emerged as a critical metabolite that protects plants against various stresses. While proline accumulation is crucial for abiotic stress tolerance, its metabolism to P5C, leading to ROS production, is involved in conferring resistance to combined drought and A. rabiei infection (Patil et al., 2021, Preprint). Therefore, understanding the intricate regulation of the proline pathway under combined abiotic and biotic stresses is pertinent to identifying potential nodes of regulation affected by combined stresses and identifying candidates involved in imparting tolerance.

Exogenous application of trehalose, which is also induced under several abiotic stress conditions in plants, has been shown to increase resistance to R. solanacearum infection in tomatoes (MacIntyre et al., 2022). Another promising area to explore is the plant antioxidant defense, as many abiotic stresses induce ROS production, which can reduce pathogen multiplication (Ramegowda et al., 2013). Investigating the root secretome under combined abiotic and root pathogen infections can identify secondary metabolites and corresponding biosynthetic pathways that might help confer protection against pathogens. Root exometabolomics is a promising area to explore when studying the impact of abiotic stresses on root diseases (Sasse et al., 2018). Elmer (2003) demonstrated a negative correlation between the population of Mn-solubilizing bacteria and root rot disease severity under salt stress; therefore, deciphering the interactions among plants, pathogens, and inter-microbial exo-metabolites would help identify strategies to mitigate root diseases under combined stress.

Exploring only the plant side of plant–pathogen interactions provides an incomplete picture. Instead, co-transcriptomics or dual metabolomics studies of plants and pathogens can offer insights into ever-evolving plant–pathogen interactions. Furthermore, a critical area warranting extensive exploration is the effect of abiotic stresses on pathogen virulence outside the plant interface. Identifying abiotic stress intensities that weaken pathogen virulence but have mild effects on plants can guide the development of strategies for improving plant performance in the field. Moreover, it is crucial to explore the effects of stress combinations on the soil and leaf microbiome, as the intricate interplay between pathogens and the rhizosphere and phyllosphere microbiome significantly influences the vulnerability of plants to diverse diseases under complex stress combinations. This avenue of research holds great relevance in unraveling the complex dynamics that underlie plant health and disease resistance (Rivero et al., 2022).

Conclusion

Plant defense against stress combinations sways from positive to negative depending on various factors. The order and intensity of two or more stresses in combination decide the overall impact on the plant, reversing the consequences of combined stresses in some cases. In the case of positive abiotic and biotic stress combinations, abiotic stresses promote defense signaling against the pathogens. The defense regulation mechanisms include stomatal response, sugar transport, proline metabolism, and ABA signaling, and these can be modulated to improve plant performance under combined stresses. It is also crucial to consider the environmental effects directly impacting pathogens.

Research on the effects of abiotic stresses on pathogen virulence is in its infancy, and extensive investigations on the modulation of pathogen virulence outside and inside the plant under abiotic stress conditions are warranted. Pathogen-mediated alleviation of abiotic stress effects in plants and the switching of pathogen lifestyles under abiotic stress conditions are aspects that demand extensive investigation.

Supplementary data

The following supplementary data are available at JXB online.

Fig. S1. Illustration indicating the various effects of abiotic stresses on phytopathogens and plant defense.

Fig. S2. Illustration of literature-based stress combination analysis on the suppressive effect of pathogen stress on abiotic stress-induced damage.

Fig. S3. Schematic representation of the suppressive effect of abiotic stress on pathogens per se outside, at, and inside the plants studied to date.

Fig. S4. GO enrichment analysis of shared up-regulated genes identified by a meta-analysis of single-stress transcriptome studies in Arabidopsis.

Fig. S5. Pathway enrichment analysis of up-regulated combined stress genes from drought and Ascochyta rabiei transcriptome in chickpea.

Table S1. List of studies on positive combined stress related to drought, high temperatures, salinity, and pathogen infection.

Table S2. List of datasets of Arabidopsis single-stress studies included in the meta-analysis and corresponding number of DEGs identified at the P<0.05 significance level.

Table S3. List of positive combined stress studies that exemplify the factors affecting the impact of abiotic and biotic stress combinations.

Dataset S1. Details of data and resources used in this manuscript.

Acknowledgements

The authors are grateful to the DBT-eLibrary Consortium (DeLCON) for providing access to the e-resources.

Conflict of interest

The authors declare that there is no conflict of interest.

Funding

Projects in the MS-K lab are supported by core funding from the National Institute of Plant Genome Research and partly by SERB core grant (CRG/2019/005659). MP and Pi P acknowledge CSIR [No. 13 (9064-A)/2019-Pool] and CSIR [No. 13 (9106-A)/2020-Pool], respectively.