On the move: induced resistance in monocots

Abstract

Although plants possess an arsenal of constitutive defences such as structural barriers and preformed antimicrobial defences, many attackers are able to overcome the pre-existing defence layers. In response, a range of inducible plant defences is set up to battle these pathogens. These mechanisms, commonly integrated as induced resistance (IR), control pathogens and pests by the activation of specific defence pathways. IR mechanisms have been extensively studied in the Dicotyledoneae, whereas knowledge of IR in monocotyledonous plants, including the globally important graminaceous crop plants, is elusive. Considering the potential of IR for sustainable agriculture and the recent advances in monocot genomics and biotechnology, IR in monocots is an emerging research field. In the following, current facts and trends concerning basal immunity, and systemic acquired/induced systemic resistance in the defence of monocots against pathogens and herbivores will be summarized.

Crops, inducible defence, plant immunity, systemic resistance

Introduction

Plants are continuously confronted with an armada of different pathogens and pests. These potential attackers utilize diverse tactics to clash with the plant defensive system. Bacteria can invade plants through natural openings such as stomata or wounds, pathogenic fungi can violently break cell walls to enter the host cell (Fig. 1), and insect herbivores employ enzymes to attenuate plant toxins. Moreover, pathogens are able to manipulate plant immunity by delivering effector molecules that are hijacking the defence pathways. Nonetheless, only a few pathogens successfully infect a specific plant species, although plants, unlike animals, do not possess specialized and mobile defender cells. Thus, the self- protection plants have developed throughout the evolutionary arms race with their attackers has to be highly intricate and efficient to help in surviving the diverse biological stress situations.

Fig. 1.

Snapshot of IR mechanisms in monocots. (A) Molecular mechanisms of pattern-triggered immunity (PTI). The bacterial MAMP (microbe-associated molecular patterns) flagellin is recognized by FSL2, a PRR consisting of an extracellular LRR and cytoplasmic kinase (K) domain. The MAMP chitin is sensed by the LysM PRRs CEBiP and CERK1. MAMP-signalling activates MAPK cascades, which regulate transcription factors (TFs) driving the expression of defence genes. HAMPs (herbivore-associated molecular patterns) and damage-associated molecular patterns (DAMPs) are also triggering PTI. In maize, the HAMP volicitin is recognized by an unknown receptor, and the DAMP ZmPep1 functions as endogenous signal regulating jasmonic acid (JA)- and ethylene (ET)-dependent pathways during pathogen attack. (B) Effector-triggered immunity (ETI) mediated by NBS-LRR (nucleotide-binding leucine rich repeat) proteins. Pathogens employ effectors (represented by stars) to suppress PTI. Such effectors are contained by NBS-LRR proteins. Monocot NBS-LRR proteins usually have coiled-coil (CC) or serine/threonine protein kinase (S/TPK) domains and are localized in both the cytoplasm and nucleus. NBS-LRR proteins are folded into an active form by the heat shock protein 90 (HSP90). They interact directly with effectors and also regulate WRKY transcription factors. (C) Induced systemic resistance (ISR) following root infection by beneficial soil-borne microbes: examples of organisms triggering ISR in monocots. (D) Systemic acquired resistance (SAR). Mobile signals travel from attacked tissues to distant organs where systemic resistance responses are induced. In rice, the SAR key player NPR1 down-regulates genes. SA suppresses the abscisic acid (ABA) pathway. Expressing the SA-degrading enzyme NahG in rice reduces pathogen resistance. SAR can also be triggered in monocots by the application of SAR inducers such as BTH (S-methyl benzo-1,2,3-thiadiazole-7-carbothioate), INA (2,6-dichloroisonicotinic acid), BIT (1,2-benzisothiazole-1,1-dioxide) or NCI (N-cyanomethyl-2-chloroisonicotinamide). Image of the rhizobacteria P. fluorescens CHA0: courtesy of P Kupferschmied and C Keel, University of Lausanne.

In order to defend themselves, plants are armed with constitutive, pre-existing defences such as cell wall barriers or pre-formed and stored antimicrobial toxins. In such cases where attackers are able to overcome the constitutive defence layers, they face an arsenal of inducible defences (Fig. 1; Pieterse et al., 2009; Spoel and Dong, 2012). During an initial phase, plant cells exert a so-called ‘innate immunity’. In a first branch of this immunity, pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) such as chitin or flagellin are recognized by membrane-localized pattern-recognition receptors (PRRs) (Zipfel, 2009). The perception of MAMPs by PRRs leads to the activation of multiple downstream defence signalling events. The second branch of the plant innate immune system acts mostly in the cytoplasm; NB-LRR (nucleotide-binding leucine rich repeat) proteins, which are encoded by plant resistance (R) genes, recognize pathogen-derived avirulence (Avr) proteins. These effector proteins help pathogens to overcome PAMP- or pattern-triggered immunity (PTI; Jones and Dangl, 2006). The recognition and attenuation of Avr proteins by plant R-proteins results in effector-triggered immunity (ETI), which is usually manifest in a hypersensitive response (HR; Greenberg and Yao, 2004).

PTI and ETI alleviate pathogen and pest attacks by inducing downstream responses that can result in a local and systemic induced resistance. Locally, these inducible defences consist of cell wall reinforcements through callose apposition and lignification, the production of secondary antimicrobial compounds, and the accumulation of pathogenesis-related (PR) proteins. Moreover, the attacked tissue is able to generate long-distance mobile alarm signals that are inducing systemic resistance in non-colonized organs (Shah, 2009). The systemic expression of defence in distal tissues can be observed upon infection with pathogens and is referred to as systemic acquired resistance (SAR). Resistance expressed following root colonization by non-pathogenic soil microbes is known as induced systemic resistance (ISR). SAR is predominantly effective against biotrophic pathogens (Vlot et al., 2008), whereas ISR is mainly counteracting necrotrophic pathogens and pests (Van Loon, 2007). Commonly, the inducible defence networks are regulated pivotally by phytohormones, which serve as specific chemical signals induced in response to particular attackers (Balmer and Mauch-Mani, 2012).

The vast majority of knowledge has been gathered from dicots such as cucumber, tobacco, and Arabidopsis. The knowledge about monocots remains elusive (Kogel and Langen, 2005). Monocots are a large group of about 59 300 species, amongst them the largest family is represented by orchids (Orchidaceae), followed by Poaceae, which include economically important plants such as rice, wheat, maize, sugarcane, and bamboo. Originating from a common angiosperm ancestor and going through an intimate co-evolution with plant pathogens, monocots and dicots are assumed to share most of the immune pathways. Here, we present the current knowledge of local and systemic IR mechanisms in monocots.

Pattern-triggered immunity (PTI): a stealth mission for pathogens?

Pathogens cannot sneak in: upon contact with invaders, plant cells use the first branch of their innate immune system by perceiving conserved microbial structures and peptides with the help of plasma membrane-localized PRRs (Fig. 1; Zipfel, 2009; Tsuda and Katagiri, 2010). In Arabidopsis, the best case study of this immune reaction is represented by the receptor-like kinase flagellin insensitive 2 (FLS2), which recognizes amino acids derived from bacterial flagellin. FLS2 interacts with BAK1, the brassinosteroid receptor BRI1-associated receptor kinase 1, to activate downstream defence responses (Chinchilla et al., 2007). Amongst monocots, various PRRs have been identified over the past few years (Table 1), notably in the model monocot rice (Oryza sativa; Chen and Ronald, 2011). FSL2 homologues are found in all higher plants, and the rice homologue OsFLS2 has been demonstrated to act as a functional flagellin receptor (Takai et al., 2008). Moreover, a variety of different MAMPs have been shown to be active in rice, including bacterial lipopolysaccharides (LPS; Desaki et al., 2006) and chitin (Kishimoto et al., 2010). In rice, chitin is perceived by the plasma membrane glycoprotein CEBiP, which forms a dimer with the chitin elicitor receptor kinase 1 (CERK1, also known as Lys-M-RLK1; Shimizu et al., 2010). As for Arabidopsis, chitin reception in rice then triggers the generation of reactive oxygen species (ROS) and the expression of PR genes. The best-studied example of PTI in monocots is the Xa21-mediated disease resistance in rice. Xa21 encodes a receptor exhibiting an extracellular LRR domain, as well as an intracellular non-RD (non-arginine–aspartate) domain. XA21 perceives the 194-amino acid bacterial protein Ax21, which is conserved in all known Xanthomonas strains (Lee et al., 2009). As for OsFLS2, XA21 induces downstream defence mechanisms by activating MAPK cascades, thereby actuating transcription factors, triggering the expression of PR genes and the development of HR (Tena et al., 2011). Xa21 homologues have been found in Brachypodium, sorghum, and maize (Tan et al., 2012). Several other non-RD receptor kinases have been identified in monocots. In rice, the B-lectin receptor kinase Pi-d2 confers resistance against Magnaporthe grisea (Chen et al., 2006). In addition to PAMPs and MAMPs, so-called damage-associated molecular patterns (DAMPs) are also recognized during pathogen attack. Known DAMPs are polysaccharides released from plant cell walls, or endogenous peptides such as the 23-amino acid peptide AtPep1 in Arabidopsis. Recently, the maize ZmPep1 peptide has been identified as an orthologue of AtPep1 (Huffaker et al., 2011), suggesting a similar role of DAMPs in monocots and dicots. In conclusion, PTI mechanisms are highly conserved in both monocots and dicots, although some PRRs such as EFR, the Arabidopsis receptor of bacterial EF-TU (elongation factor unstable), are not found in monocots (Boller and He, 2009). Nevertheless, the fact that rice encodes a higher variety of non-RD domain receptor kinases than Arabidopsis (Dardick and Ronald, 2006) indicates that, although PTI signalling is conserved in all angiosperms, both monocots and dicots underwent particular evolutionary adaptations.

Table 1.

Selected monocot sensors recognizing conserved molecular patterns

Plant species	Protein name	Molecular pattern	Pathogen	Reference
Rice	CEBiP	Chitin	Magnaporthe grisea	Shimizu et al., 2010
	OsFLS2	Flagellin	Pseudomonas avenae Acidovorax avenae	Takai et al., 2008
	Pi-d2	Unknown	M. grisea	Chen et al., 2006
	XA21	Sulphated Ax21	Xanthomonas spp.	Lee et al., 2009
Barley	HvCEBiP	Chitin	M. oryzae	Tanaka et al., 2010
Wheat	WKS1 (Yr36)	Unknown	Puccinia striiformis	Fu et al., 2009
Maize	Unknown	ZmPep1	(Endogenous elicitor)	Huffaker et al., 2011

Effector-triggered immunity (ETI): Special Forces striking back

Once detected by plant cells and facing PTI-triggered defences, successful pathogens are able to perturb the first inducible defence lines (Jones and Dangl, 2006). Bacteria, fungi, and oomycetes are delivering effectors behind enemy lines to suppress PTI. There, these effectors manipulate host cellular mechanisms to favour subsequent invasion steps. Examples of such effectors are AvrPtoB and AvrPto, effectors from Pseudomonas syringae strains targeting the kinase domains of EFR, FLS2, and BAK1 (Boller and He, 2009). In contrast to bacterial effectors, eukaryotic pathogen effectors are less well studied. The oomycete Hyaloperonospora arabidopsidis produces ATR1 and ATR13 effectors (Sohn et al., 2007), and the fungus Blumeria graminis f.sp. hordei delivers AVRK and AVRA10 proteins into barley cells (Ellis et al., 2007). Pathogen effectors are able to render a plant susceptible, thus being a serious threat for plant survival. However, plants are promptly counterstriking by sending in recon troops that recognize effectors, thus triggering ETI (Fig. 1; Table 2). These recon troops are mostly NB-LRR proteins encoded by resistance (R) genes (Elmore et al., 2011). NB-LRR proteins usually exhibit an N-terminal TIR (Toll/Interleukin-1 Receptor) domain or coiled-coil (CC) motif. Activation of NB-LRRs induces local and systemic defence signalling involving hormonal networks, ROS-generation, and gene expression adaptations by WRKY and TGA transcription factors (Jones and Dangl, 2006). According to the guard hypothesis, some R genes can directly recognize pathogen molecules (effectors), while other R genes indirectly recognize metabolic perturbations due to the presence of the pathogen (Jones and Dangl 2006). In cereals, the prevailing situation seems to consist of direct surveillance as, in most cases, a direct interaction between the resistance gene and the corresponding effector is the rule (Table 2).

NB-LRR encoding genes represent one of the largest and widely conserved gene families in plants, with over one-hundred family members for the majority of sequenced plants (Jones and Dangl, 2006), including monocots and dicots. Despite the extensive knowledge of NB-LRRs in monocots, their elucidation has been mainly limited to rice and, more recently, to wheat and sorghum. Compared with dicots, monocot genomes encode higher numbers of CC-NB-LRRs (Martin et al., 2011). Intriguingly, genes coding for TIR-NB-LRRs homologues are rare in monocots (Kim et al., 2012). The majority of described rice NB-LRRs is promoting resistance to M. grisea, such as Pita, Pib, Piz-t, Pikm, and Pit (reviewed in Chen and Ronald, 2011). Bph14 confers resistance to the brown planthopper (Du et al., 2009), and XA1 mediates resistance against Xanthomonas oryzae (Yoshimura et al., 1998). Despite the large number of rice NB-LRRs, most of their target effectors are unknown. Only four M. grisea effectors are described, AvrPiz-t (Shang et al., 2009), AvrPita (Jia et al., 2000), AvrPia and AvrPik/km/kp (Qu et al., 2006). AvrPita is recognized by the rice NBS-LRR protein Pita; direct binding of Pita to AvrPita induces cell death that retards the spread of M. grisea on rice (Jia et al., 2000). Other R-genes conferring resistance to Xanthomonas oryzae pv. oryzae in rice do not exhibit NBS or LRR domains, such as xa13 and Os11N3 (Antony et al., 2010). Xa13, a recessive allele belonging to the NODULIN3 (N3) gene family, triggers immunity by recognizing the Xanthomonas effectors AvrXA7. In turn, the type III effector AvrXa7 drives the expression of the rice susceptibility gene OS-8N3, which defeats Xa13 and induces effector-triggered susceptibility (ETS; Antony et al., 2010). The extensive synteny between the genomes of several major cereal species and the high colinearity between large portions of these genomes facilitates synteny-based positional cloning. The availability of detailed rice (International Rice Genome Sequencing Project, 2005) and, recently, barley genomic data as well (Mayer et al., 2011) will allow the identification of genes playing a crucial role in IR in major cereal species and, hopefully, lay the basis for genomics-based breeding strategies for defence in these plants.

In other monocot species, NB-LRRs are less explored. Nonetheless, in the genomes of Brachypodium distachyon, Sorghum bicolor, and Zea mays, conserved NB-LRR-encoding genes were identified (Kim et al., 2012). In sorghum, a CC-NB-LRR encoding gene cluster that confers resistance to Setosphaeria turcica has recently been discovered (Martin et al., 2011). The corresponding resistance gene has been found to be conserved in maize, rice, foxtail millet, and in Brachypodium distachyon. In addition, the NB-LRR encoding R genes Cs1A and Cs2A were shown to mediate the resistance of sorghum against Colletotrichum sublineolum (Biruma et al., 2012). In wheat, the recently identified CC-NB-LRR protein TmMla1 functions in resistance against Blumeria graminis f.sp. hordei (Jordan et al., 2011). Wild wheat (Triticum turgidum L. ssp. dicoccoides) possesses the Yr36 gene, which encodes a kinase and putative START lipid-binding domain and confers resistance to Puccinia striiformis (Fu et al., 2009). In barley, the CC-NB-LRR-type gene Rdg2a has been discovered to confer resistance to Pyrenophora graminea (Bulgarelli et al., 2010). Another barley gene, Rpg1, regulates resistance against Puccinia graminis f.sp. tritici (Brueggeman et al., 2002). RPG1 interacts with two effector proteins from urediniospores, one of them is characterized as a vacuolar protein sorting-associated protein (VPS9). This leads to rapid phosphorylation followed by the degradation of RPG1. The resulting HR then confers resistance to the rust fungus (Nirmala et al., 2011). Thus far, ETI-mechanisms in monocots and dicots are highly conserved.

Table 2.

Selected monocot proteins recognizing pathogen effectors

Plant Species	Protein name	Effector	Pathogen	Reference
Rice	Bph14	Unknown	Brown planthopper	Du et al., 2009
	Os11N3	AvrXA7	Xanthomonas spp.	Antony et al., 2010
	Pita	AvrPita1	Magnaporthe grisea	Jia et al., 2000
	Piz-t	AvrPiz-t	M. grisea	Li et al., 2009
	XA27	AvrXA27	Xanthomonas spp.	Gu et al. 2005
Barley	RDG2A	Unknown	Pyrenophora graminea	Bulgarelli et al., 2010
	RPG1	Urediniospore effectors (protein with a fibronectin type III susceptibility domain; vacuolar protein sorting associated protein 9)	Puccinia graminis	Brueggeman et al., 2002
Wheat	TmMla1	Unknown	Blumeris graminis f.sp. hordei	Jordan et al., 2011
	Tsn1	ToxA	Stagonospora nodorum	Faris et al. 2010
Sorghum	Cs1A & Cs2A	Unknown	Colletotrichum sublineolum	Biruma et al., 2012

Systemic acquired resistance: a defence in depth in monocots?

Upon locally induced defence, plants employ an intricate defence mechanism that activates resistance responses in not-yet-attacked tissues. In the case of a local challenge by leaf pathogens, mobile alarm signals are sent to distal leaves to induce a systemic resistance against a broad range of subsequent attackers. This mechanism is known as SAR (Shah, 2009). SAR has been extensively studied in the two dicot models tobacco and Arabidopsis, leading to the identification of specific molecular components and of a set of mobile defence signals (Vlot et al., 2008). Salicylic acid (SA) has been found to be the main chemical regulator of SAR. SA exerts its canonical action on NPR1 (non-expressor of PR genes, also known as NIM1). Originally, npr1 was discovered as recessive mutation conferring a SAR^- phenotype (Cao et al., 1997). Now it is known that NPR1 is a transcription factor activator that is present in the cytosol in an oligomeric form. SA accumulation leads to its constitutive monomerization. As a monomer, NPR1 enters the nucleus to interact with transcription factors (Mou et al., 2003), triggering extensive changes in the defence gene transcriptome (Maleck et al., 2001). Novel evidence shows that two paralogues of NPR1, NPR3 and NPR4, are SA receptors with different binding affinities to SA. They regulate NPR1 stability and activity depending on the SA level in the cell. In unchallenged plants, NPR4 mediates the degradation of most of the NPR1. When a pathogen triggers ETI, a gradient of SA builds up from the local to the systemic part and the elevated SA levels trigger an HR. Further expansion of cell death is then restricted through NPR3/NPR1 interactions in the cells adjacent to the HR (Fu et al., 2012). Prior activation of defence genes in distal tissues renders them more resistant against future attacks. A common marker of SAR in dicots is the up-regulation of PR genes such as PR1 and PR5.

For an effective SAR reaction, mobile alarm signal(s) have to be sent from locally infested leaves to distant tissues. In Arabidopsis, several mobile SAR signals have been discovered, such as glycerol-3-phospate (G3P; Chanda et al., 2011), azelaic acid (Jung et al., 2009), and the volatile methyl salicylate (MeSA) (Park et al., 2007). Recent findings also propose dehydroabietinal (DA), a diterpenoid aldehyde, as the SAR-signal in Arabidopsis (Chaturvedi et al., 2012). The known SAR signals are generally controversial as they are highly conditional, depending on the experimental systems. This abundance of different signals could be considered as a safety mechanisms to prevent accidental activation of the cost-intensive immune response. Through cross-interaction between signals or even requirement of parallel activation, an appropriate induction of IR for a given specific situation might be achieved (Dempsey and Klessig, 2012). SAR can also be induced by the application of various synthetic chemical compounds such as INA (2,6-dichloroisonicotinic acid; Métraux et al., 1990), BTH (S-methyl benzo-1,2,3-thiadiazole-7-carbothioate; Görlach et al., 1996), probenazole (3-allyloxy-1,2-benziso-thiazole-1,1-dioxide; Nakashita et al., 2002,a), BIT (1,2-benzisothiazole-1,1-dioxide; Yoshioka et al., 2001) NCI (N-cyanomethyl-2-chloroisonicotinamide; Nakashita et al. 2002b) or tiadinil (3’-chloro-4,4’-dimethyl-1,2,3-thiadiazole-5-carboxanilide; Yasuda et al., 2004).

Compared with dicots, the knowledge of SAR in monocots is scarce. NPR1, the master regulator of SAR in dicots, has been confirmed for all monocots where genomic data is available (Kogel and Langen, 2005). In rice, over-expression of both AtNPR1 (Chern et al., 2001) and the endogenous homologue OsNH1 (Chern et al., 2005) resulted in an enhanced resistance to Xanthomonas oryzae pv. oryzae. Transcriptomic analysis of OsNPR1 knockdown and over-expressing rice lines showed that OsNPR1 is dominantly involved in the down-regulation of genes, and in the SA-mediated suppression of abscisic acid (ABA)-responsive genes (Sugano et al., 2010). Chemical SAR inducers were also found to be active in monocots, such as BTH and INA in maize (Morris et al., 1998), BTH in wheat (Görlach et al., 1996), and INA in barley (Kogel et al., 1994). Similarly to Arabidopsis, BTH-treatment of maize triggers the expression of PR proteins such as PR1 and PR5 (Morris et al., 1998). Monocot and dicot PR protein sequences were found to share extensive similarities. However, when performing an unrooted phylogenetic tree analysis using PR1 homologues from different species, dicot PR1 genes grouped together in a cluster distant from monocot sequences (Lu et al., 2011a). Thus, PR1 probably underwent the main diversifications after the monocot–dicot separation. Other resistance inducers in addition are described for monocots, such as the effect of probenazole in rice (Umemura et al., 2009). Probenazole strongly up-regulates OsSGT1, which encodes an UDP-glucose:SA glucosyltransferase. OsSGT1 is believed to support rice defence mechanisms by converting free SA to conjugated SA-O-β-glucoside (SAG) which, in turn, can be converted back into SA when needed. SA-levels itself were not found to be altered upon probenazole-treatment, suggesting an exquisite role of SAG during SAR in rice (Umemura et al., 2009). In barley induced with INA, the situation presents itself differently: here, defence reactions against Blumeria graminis f.sp. hordei neither depend on, nor induce SA accumulation (Hückelhoven et al., 1999). In contrast to dicots, the role of SA during SAR in monocots has yet to be elucidated. Rice contains high endogenous levels of SA (Silverman et al., 1995), and pathogen infection does not up-regulate these levels. However, transgenic rice plants expressing the SA-degrading enzyme salicylate hydroxylase (NahG) exhibit a diminished resistance against Magnaporthe grisea (Yang et al., 2004), although PR gene expression profiles were found to be unaltered. The role of SA in other monocot models is less studied. Some reports on wheat and barley showed a ‘local acquired resistance” (LAR) where a first fungal inoculation on a leaf makes a second attack on the same leaf less efficient (Thordal-Christensen and Smedegaard-Petersen, 1988; Jørgensen et al., 1998). In both studies, SA levels were found to be unaffected. Nevertheless, a recent study of P. syringae pv. tomato-induced LAR in barley demonstrated similarities between gene expression profiles during LAR in barley and SAR in Arabidopsis (Colebrook et al., 2012).

Although general chemical and molecular SAR players such as NPR1, PR genes and transcription factors are conserved in monocots and dicots, only a few reports describe biological SAR phenomena in monocots. Infection of rice by P. syringae pv. syringae leads to a systemic resistance against M. grisea (Smith and Métraux, 1991). In wheat, SAR against stem and leaf rust has been noted (Barna et al., 1998). Nevertheless, these SAR phenomena are highly conditional, corroborated by the lack of reproducibility by other laboratories (Kogel and Langen, 2005). However, the intricate signalling process during SAR is highly conditional, depending on multiple factors such as type of attackers, age of plant, and growth conditions. In Arabidopsis, MeSA is not required for SAR when plants are exposed for more than 3.5h to light after a primary pathogen infection (Liu et al., 2011). Strong light conditions trigger SAR in Arabidopsis upon P. syringae pv. maculicola infection without the accumulation of either SA or PR1 in systemic leaves (Zeier et al., 2004). Hence, particular molecular or chemical SAR factors have to be specifically determined for a given pathosystem, which might, in turn, explain the discrepant mode of action of certain SAR regulators between dicots and monocots.

Induced systemic resistance: support from underground alliances

Colonization of plant roots by some soil microbes, such as plant growth-promoting rhizobacteria (PGPR) or endophytic fungi (PGPF), can directly stimulate plant growth by improving nutrient uptake or photosynthesis (Spaepen et al., 2009; Trillas and Segarra, 2009) or indirectly by suppressing soil-borne pathogens through the production of antibiotic compounds (De Vleesschauwer and Höfte, 2009). Moreover, these beneficial microorganisms can also indirectly reduce plant disease through an induction of a systemic resistance, named ISR. ISR confers a resistance against a wide spectrum of attackers, mostly necrotrophic pathogens and pests (Van Wees et al., 2008; Pineda et al., 2010). Similarly, mycorrhizae have been reported to induce plant resistance in a way resembling that of ISR (reviewed by Pozo and Azcón-Aguilar, 2007). Various beneficial microorganisms are known to induce ISR in monocots. In cereals, endophytic fungi, PGPR or mycorrhizae are reported to induce resistance against pathogens and insect herbivores (Table 3). The potential resistance induced by PGPR in monocots depends on the host-PGPR combination and on the type of attacker. P. aeruginosa 7NSK2 and Serratia plymuthica IC1270 induce resistance against Magnaporthe oryzae in rice, but they enhance disease severity caused by Rhizoctonia solani (De Vleesschauwer et al., 2006, 2009). However, some pseudomonads induce resistance of rice against R. solani (Table 3). Induction of resistance by a specific strain of PGPR is not restricted to only one plant species: for example, P. aeruginosa 7NSK2 triggers ISR in rice (De Vleesschauwer et al., 2006) and wheat (Muyanga et al., 2005). Application of a PGPR mixture enhances the efficacy of resistance induction compared with the use of individual strains in both dicots (De Boer et al., 2003) and monocots (Lucas et al., 2009).

Diverse microbial molecules have been identified as ISR elicitors in monocots. Exopolysaccharides produced by Pantoea agglomerans induce defence responses in wheat cells by triggering an increased accumulation of hydrogen peroxide and an augmented peroxidase activity (Ortmann and Moerschbacher, 2006). Siderophores and antibiotics produced by Pseudomonas strains, such as pseudobactins and pyocyanin, are important defence elicitors in rice against M. oryzae (De Vleesschauwer et al., 2008; De Vleesschauwer and Höfte, 2009). In contrast to tomato and bean, pyocyanin was shown to be the only component compulsory for triggering ISR in rice. Certain fungal endophytes have also been shown to trigger IR. A beneficial Penicillium primes Arabidopsis for defence against P. syringae (Hossain et al., 2008) and Glomus mossae protects tomatoes from infection by Phytophthora (Pozo et al., 2002). Trichoderma virens, an endophytic fungus that triggers ISR in maize, has been shown to facilitate resistance via the release of a proteinaceous elicitor (Djonovic et al., 2007). Piriformospora indica induces IR in both dicots and monocots but is probably best-known for this effect on barley. Here, it was shown to induce resistance without having to rely on the classical defence pathways involving SA, JA or ET (Waller et al., 2005). A barley leaf transcriptome and metabolite analysis revealed that P. indica-induced plants over-expressed a small set of defence-related genes including transcripts coding for PR and heat-shock proteins (Molitor et al., 2011). In creeping bentgrass (Agrostis stolonifera) which is closely related to cereals, treatment with (2R, 3R)-butanediol, a bacterial-derived volatile, induces resistance against Microdochium nivale (Cortes-Barco et al., 2010). Rhizobacteria can also produce hormones that manipulate phytohormone pathways. SA produced by P. aeruginosa strains triggers peroxidase accumulation in rice leading to an increase in resistance to R. solani (Saikia et al., 2006).

Some N-acyl homoserine lactones (AHL) controlling quorum sensing in bacteria (Miller and Bassler, 2001) also have the capacity to induce resistance. AHLs from Serratia liquefaciens and P. putida induce resistance against Alternaria in tomato (Schuhegger et al., 2006). Intriguingly, P. indica is closely associated with an endobacterium, Rhizobium radiobacter (Sharma et al., 2008), that produces a series of AHLs. Application of these AHLs to barley induces resistance against powdery mildew (Sharma et al., 2008). This raises the question as to whether the observed IR capacity of P. indica might not actually be due to the presence of the endophytic bacteria.

The efficacy of ISR in monocots against necrotrophic pathogens has been demonstrated repeatedly but only in a few cases, the involved defence signalling pathway has been investigated. ISR induced by P. fluorescens WCS374r against M. oryzae in rice depends on a jasmonic acid (JA)/ethylene (ET)-modulated signal but is independent from SA-signalling (De Vleesschauwer et al., 2008). Involvement of JA-signalling in ISR was also shown in maize (Djonovic et al., 2007; Song et al., 2011) and barley (Petti et al., 2010). Interestingly, ISR triggered by T. virens in maize also seems to be associated with the priming of genes involved in the production of volatile compounds called green leaf volatiles (GLV) (Djonovic et al., 2007). Several defence-related genes involved in SA- and JA-dependent pathways are strongly induced when mycorrhized maize plants are challenged with R. solani (Song et al., 2011). ISR in monocots is mostly linked to JA-dependent defences. However, some PGPR or PGPF induced an SA-dependent pathway effective against biotrophic pathogens (Muyanga et al., 2005; Molitor et al., 2011).

Overall, recent studies on ISR triggered by PGPR, PGPF or mycorrhiza in monocots and more specifically in cereals tend to point to common mechanisms with dicotyledonous plants.

Table 3.

Examples of established cereal ISR pathosystems

Plant species	Beneficial microorganisms	Plant attackers	References
Rice	Pseudomonas fluorescens PF1	Cnaphalocrocis medinalis	Radja Commarea et al., 2002
	P. fluorescens FP7
	Pseudomonas fluorescens PF1	Rhizoctonia solani	Radjacommarea et al., 2004
	P. fluorescens Pf1, TDK1, PY15	Cnaphalocrocis medinalis	Saravanakumar et al., 2007
	P. fluorescens WCS374r	Magnaporthe oryzae	De Vleesschauwer et al., 2008
	P. fluorescens Aur6 Chryseobacterium balustinum Aur9	Magnaporthe oryzae	Lucas et al., 2009
	P. aeruginosa	Rhizoctonia solani	Saikia et al., 2006
	P. aeruginosa 7NSK2	Magnaporthe oryzae Rhizoctonia solani	De Vleesschauwer et al., 2006
	Bacillus pumilus SE34 Bacillus subtilis GB03	Xanthomonas oryzae pv. oryzae	Chithrashree et al., 2011
	Serratia plymuthica IC1270	Magnaporthe oryzae Cochliobolus myiabeanus Rhizoctonia solani	De Vleesschauwer et al., 2009
Maize	Trichoderma virens T22	Colletotrichum graminicola	Djonovic et al., 2007
	Bacillus cereus C1L	Cochliobolus heterostrophus	Huang et al., 2010
	Glomus mosseae	Rhizoctonia solani	Song et al., 2011
Wheat	P. fluorescens CHA0	Fusarium graminearum	Henkes et al., 2011
	P. fluorescens CHA0	Gaeumannomyces graminis var. tritici	Sari et al., 2008
	P. fluorescens MKB158	Fusarium graminearum	Petti et al., 2008
	P. aeruginosa 7NSK2	Blumeria graminis Cochliobolus sativus	Muyanga et al., 2005
	Chaetomium globosum	Pyrenophora tritici-repentis	Istifadah and McGee, 2006
	Fungal endophytes	Puccinia recondite f.sp. tritici	Dingle and McGee, 2003
Barley	Piriformospora indica	Blumeria graminis f.sp. hordei	Molitor et al., 2011
	P. fluorescens MKB158	Fusarium graminearum	Petti et al., 2010
	Fusarium oxysporum f.sp. radicis-lycopersici	Blumeria graminis f.sp. hordei	Nelson, 2005
Pearl millet	B. pumilus INR7	Sclerospora graminicola	Raj et al., 2003
	B. pumilus SE34
	B. subtilis GB03
	P. fluorescens UOM SAR 14	Sclerospora graminicola	Raj et al., 2004
Sorghum	B. cereus KBS2-6	Pythium utlimum	Itris et al., 2008
	B. cereus KFP9-A
	Serratia marcescens KBS9-R

Induced resistance against insect herbivores: protection against air-borne assaults

Plants are confronted with a wide variety of insect herbivore attacks. To counteract these attacks promptly and specifically by inducing defence mechanisms, plants recognize molecules originating either from wounding damage or from compounds derived from the herbivore itself, such as oral secretions (OS) and oviposition fluids. These elicitors, called herbivore associated molecular patterns (HAMPs), have been found in several monocot pathosystems. Volicitin, a hydroxyl fatty acid-amino acid conjugate found in Spodoptera exigua OS, induces volatile emission in maize (Alborn et al., 1997) and caeliferins from Schistocerca americana OS trigger IR in maize (Alborn et al., 2007). Plant perception of HAMPs is widely elusive, but similarities to MAMP-recognition have been proposed (Bonaventure et al., 2011). In maize, volicitin is perceived by a plasma membrane protein (Truitt et al., 2004), which is so far the only known HAMP-receptor in monocots.

Upon perception of an herbivore, IR mechanisms are mediated by different defence-related hormones. Plant-induced defences against phloem-feeding herbivores seem to share a common plant reaction to biotrophic pathogens by activating SA-dependent pathways associated with the production of PR proteins (Alagar et al., 2010) and callose deposition at the feeding site (Hao et al., 2008). In rice, defence induced by an attack of the phloem-feeding brown planthopper is mediated by a SA-related signalling and is associated with an accumulation of PR proteins and an HR (Zhou et al., 2009). In resistant wheat cultivars, but not in susceptible ones, infestation by gall insects induces changes in SA levels (Tooker and De Moraes, 2011). By contrast, plants induce JA and ET-dependent pathways against chewing herbivores. In maize, JA and ET are important in plant defence against S. frugiperda (Shivaji et al., 2010; Harfouche et al., 2006). JA was also shown to have an important role in IR of wheat against pests (El-Wakeil et al., 2010). In rice, the JA-dependent pathway induces resistance against insect herbivores and suppression of JA activity results in an improved larval performance of the striped stem borer and leaf folder (Zhou et al., 2009). Ethylene is another key player in fending off herbivores. ET emission induced by elicitors of S. frugiperda OS influences the expression of direct defences such as defence proteins and secondary metabolites (Harfouche et al., 2006). In rice, the ethylene responsive factor ERF3 mediates between SA, JA, and ET pathways and thus orchestrates the response to chewing or phloem-feeding insects (Lu et al., 2011b).

After herbivore attack, plants can induce defences that will directly act against insect herbivore. The maize insect resistance 1-cysteine protease (Mir1-CP) content increases in roots and leaves in response to larvae feeding on leaves, conferring a systemic induction of plant defence against herbivores (Lopez et al., 2007). Trypsin proteinase inhibitors are important defence compounds against herbivores such as the striped stem borer and leaf folder in rice (Wang et al., 2011; Zhou et al., 2011). Secondary metabolites, such as the hydroxamic acids in cereals, can also have a direct negative effect on insect herbivores (Chen, 2008). Direct local defence can enhance direct plant defence systemically. Infestation of rice plants with S. frugiperda, for example, increases resistance against a subsequent attack by the rice water weevil (Hamm et al., 2010). Similarly, root infestation of maize by Diabrotica virgifera virgifera induces resistance in the leaves against S. littoralis and the necrotrophic pathogen Setosphaeria turcica (Erb et al., 2009). This illustrates that an induction of below-ground defences can induce above-ground resistance in maize.

Many plants respond to insect herbivory or wounding by emitting blends of volatile organic compounds (VOCs). VOCs release is an important cue for systemic defence signalling within an attacked plant as well as for plant–plant communication. Exposure of a maize plant to VOCs from infested plants primes the defence response against the generalist S. littoralis (Ton et al., 2007). Green leaf volatiles (GLVs), specific VOCs emitted by plants upon wounding damages, can also activate defence mechanisms in neighbouring intact plants (Ruther and Furstenau, 2005).

Induced resistance (IR) in non-cereal monocots: the last bastion

Because of their economic importance, most of the research on IR in monocots has been conducted on cereals. Nevertheless, IR such as SAR and ISR can also be found in non-cereal monocots. In Lilium formonasum, a previous infection with Botrytis elliptica suppresses a secondary infection with the same pathogen in systemic tissues (Lu et al., 2007). Classical synthetic chemical SAR inducers have been reported in diverse non-cereal monocot systems. L. formosanum can be protected against B. elliptica by probenazole. Here, resistance is associated with a stomatal closure and increased callose deposition (Lu et al., 2007). SA-treatment primes callose accumulation in onion, which confers enhanced resistance to downy mildew (Polyakovskiy and Dmitriev, 2011). BTH enhances plant defence in banana against Colletotrichum musae via a higher chitinase defence gene expression (Ma et al., 2009). Curcuma (Radhakrishnan et al., 2011) and sugarcane (Ramesh Sundar et al., 2006) were also protected by BTH treatment against Pythium aphanidermatum and Colletotrichum falcatum, respectively. Functional ISR has also been reported in non-cereal monocots, here mostly against necrotrophic fungal pathogens. For example, Bacillus cereus C1L was efficient in eliciting ISR in Lilium formonasum against Botrytis elliptica (Liu et al., 2008). In banana plants, a combination of the rhizobacteria Pseudomonas fluorescens CHA0 and chitin induces systemic resistance against banana bunch top virus (Kavino et al., 2008). A mixture of several PGPRs seems to have an increased positive effect compared with a single strain use on resistance in gladiolus (Shanmugam et al., 2011) and in banana (Sangeetha et al., 2010). ISR induced by a hypoagressive isolate of Fusarium oxysporum in date palm against Fusarium oxysporum f.sp. albedinis is characterized by a primed reaction of the plant with a faster induction of peroxidase activity and a higher amount of phenolics (El Hassni et al., 2004).

Conclusion

Historically, the majority of research on IR has been performed in dicot model plants. Recent advances in monocot genomics, however, are helping to identify the key components of IR signalling. Further improvements in monocot biotechnology such as plant transformation methods will provide a more profound insight into IR mechanisms. Moreover, a variety of cereal and non-cereal IR model systems are now well established, making IR in monocots a research field ready to move forward. Novel insights into the functioning of IR in monocots are expected to have a positive impact on sustainability in modern agriculture.

Acknowledgements

The authors were supported by the National Centre of Competence in Research (NCCR) ‘Plant Survival’ and by SNF Grant 31003A-120197, both research programs of the Swiss National Science Foundation. We apologize to our colleagues who could not be cited in this review due to space restrictions.

Abbreviations:

Abbreviations:
 
• ABA

  abscisic acid

 
• AHL

  N-acyl homoserine lactone

 
• Avr

  avirulence

 
• BABA

  β-aminobutyric acid

 
• DAMP

  damage-associated molecular pattern

 
• DIMBOA

  2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one

 
• ET

  ethylene

 
• ETI

  effector-triggered immunity

 
• GLV

  green leaf volatiles

 
• HAMP

  herbivore-associated molecular pattern

 
• HR

  hypersensitive response

 
• IR

  induced resistance

 
• ISR

  induced systemic resistance

 
• JA

  jasmonic acid

 
• LAR

  local acquired resistance

 
• LPS

  bacterial lipopolysaccharides

 
• MAMP

  microbe-associated molecular pattern

 
• MeJA

  methyl jasmonate

 
• MeSA

  methyl salicylate

 
• NB-LRR

  nucleotide-binding leucine rich repeat

 
• PAMP

  pathogen-associated molecular pattern

 
• PGPF

  plant growth promoting endophytic fungi

 
• PGPR

  plant growth promoting rhizobacteria

 
• PR

  pathogenesis-related

 
• PRR

  pattern-recognition receptor

 
• PTI

  pattern-triggered immunity

 
• R gene

  resistance gene

 
• ROS

  reactive oxygen species

 
• SA

  salicylic acid

 
• SAR

  systemic acquired resistance

 
• VOC

  volatile organic compound