*3.2.2. Perception of Fusarium oxysporum f. sp. lycopersici*

Three I (immunity) genes have been identified in tomato [184] in a resistance response to *Fusarium oxysporum* f. sp. *lycopersici* (*Fol*) that involves callose deposition, accumulation of phenolics and formation of tyloses and gels [185]. The *I-2* gene encodes an intracellular CC-NB-LRR receptor protein that perceives *Fol* effector protein Avr2 (secreted in xylem 3; Six3) [186]. Avr2 is under the control of transcription factor Sge1 and is highly expressed in roots and xylem vessels [187]. Avr2 forms homodimers and requires nuclear localization to trigger I-2-mediated cell death [188], which can be strongly suppressed by *Fol* effector Six6 [189]. It has recently been shown that Six5 also contributes to the virulence of *Fol* in tomato plants that Six5 and Avr2 can interact and are together required for *I-2*-mediated resistance [190].

#### *3.2.3. Perception of Xanthomonas oryzae pv. oryzae*

The rice LRR RLK Xa21 that provides resistance to *Xanthomonas oryzae* pv. *oryzae* (*Xoo*) [191] recognizes sulphated peptide from the N-terminal part of the secreted quorum-sensing (QS) signal molecule activator of Xa21 (Ax21) [192,193]. In the absence of a signal, Xa21 associates with XB24, a protein with ATPase activity that enhances Xa21 autophosphorylation of Ser/Thr residues [194]. On Ax21 recognition, XB24 is released and Xa21 becomes activated to induce rice defence responses [193]. Subsequently, a protein phosphatase 2C (XB15) specifically interacts with activated Xa21, leading to dephosphorylation and inactivation of Xa21 [195]. In addition, several other proteins (e.g. RING finger ubiquitin ligase XB3 [196] and plant-specific ankyrin-repeat protein XB25 [197]) are associated with Xa21 and required for Xa21 accumu‐ lation and resistance to *Xoo* [193]. Moreover, Xa21 binds XB10, a WRKY62 transcription factor. When overexpressed, XB10 suppresses the activation of defence-related genes *OsPR1* and *OsPR10* and acts as a negative regulator of basal and Xa21-mediated immunity [198]. Xa21 also interacts with the endoplasmic reticulum (ER) chaperone BiP3, which regulates its stability and processing [199].

#### *3.2.4. Perception of Ralstonia solanacearum*

A pair of *Arabidopsis thaliana* TIR-NB-LRR proteins, RRS1 and RPS4, function together in disease resistance against *Colletotrichum higginsianum*, *Pseudomonas syringae* pv.*tomato* and *Ralstonia solanacearum* [200–202]. RRS1 and RPS4 proteins form an inactive heterodimer complex through the SH motif in their TIR domains [5,121]. RRS1 protein recognizes and, through its C-terminal WRKY domain, directly binds *R. solanacearum* effector PopP2 [203– 205]. This leads to disruption of RRS1/RPS4 TIR heterodimer (but not full-length heterocomplex), allowing the formation of signalling active RPS4 TIR homodimer. PopP2 interacts with other WRKY domain-containing proteins and acetylates lysines to block DNA binding, suggesting that PopP2 interferes with WRKY TF-dependent defence [205,206].

#### **3.3. Induced defence responses to vascular wilt pathogens**

Ve1 forms heterodimers with a tomato orthologue of the *Arabidopsis* RLK suppressor of BIR1-1/ evershed (SOBIR1/EVR) in the absence of Ave1 [181]. However, Ve1-mediated signalling also requires other critical signalling components, such as SERK1 and SERK3/BAK1, to establish *Verticillium* resistance in tomato and *Arabidopsis* [177,178,182]. Additionally, Ve1-mediated signalling depends on ER-QC-assisted folding mediated by ER-resident chaperones HSP70

Three I (immunity) genes have been identified in tomato [184] in a resistance response to *Fusarium oxysporum* f. sp. *lycopersici* (*Fol*) that involves callose deposition, accumulation of phenolics and formation of tyloses and gels [185]. The *I-2* gene encodes an intracellular CC-NB-LRR receptor protein that perceives *Fol* effector protein Avr2 (secreted in xylem 3; Six3) [186]. Avr2 is under the control of transcription factor Sge1 and is highly expressed in roots and xylem vessels [187]. Avr2 forms homodimers and requires nuclear localization to trigger I-2-mediated cell death [188], which can be strongly suppressed by *Fol* effector Six6 [189]. It has recently been shown that Six5 also contributes to the virulence of *Fol* in tomato plants that Six5 and Avr2 can interact and are together required for *I-2*-mediated resistance [190].

The rice LRR RLK Xa21 that provides resistance to *Xanthomonas oryzae* pv. *oryzae* (*Xoo*) [191] recognizes sulphated peptide from the N-terminal part of the secreted quorum-sensing (QS) signal molecule activator of Xa21 (Ax21) [192,193]. In the absence of a signal, Xa21 associates with XB24, a protein with ATPase activity that enhances Xa21 autophosphorylation of Ser/Thr residues [194]. On Ax21 recognition, XB24 is released and Xa21 becomes activated to induce rice defence responses [193]. Subsequently, a protein phosphatase 2C (XB15) specifically interacts with activated Xa21, leading to dephosphorylation and inactivation of Xa21 [195]. In addition, several other proteins (e.g. RING finger ubiquitin ligase XB3 [196] and plant-specific ankyrin-repeat protein XB25 [197]) are associated with Xa21 and required for Xa21 accumu‐ lation and resistance to *Xoo* [193]. Moreover, Xa21 binds XB10, a WRKY62 transcription factor. When overexpressed, XB10 suppresses the activation of defence-related genes *OsPR1* and *OsPR10* and acts as a negative regulator of basal and Xa21-mediated immunity [198]. Xa21 also interacts with the endoplasmic reticulum (ER) chaperone BiP3, which regulates its stability

A pair of *Arabidopsis thaliana* TIR-NB-LRR proteins, RRS1 and RPS4, function together in disease resistance against *Colletotrichum higginsianum*, *Pseudomonas syringae* pv.*tomato* and *Ralstonia solanacearum* [200–202]. RRS1 and RPS4 proteins form an inactive heterodimer complex through the SH motif in their TIR domains [5,121]. RRS1 protein recognizes and, through its C-terminal WRKY domain, directly binds *R. solanacearum* effector PopP2 [203– 205]. This leads to disruption of RRS1/RPS4 TIR heterodimer (but not full-length heterocomplex), allowing the formation of signalling active RPS4 TIR homodimer. PopP2 interacts

binding proteins (BiPs) and lectin-type calreticulins (CRTs) [183].

*3.2.2. Perception of Fusarium oxysporum f. sp. lycopersici*

422 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

*3.2.3. Perception of Xanthomonas oryzae pv. oryzae*

and processing [199].

*3.2.4. Perception of Ralstonia solanacearum*

Recognition of vascular wilt pathogens by plant immune receptor complexes activates defence responses in the xylem vessels. Physical defence responses that confine patho‐ gens from further spread comprise the formation of tyloses, accumulation of pectin-rich gels and gums, vascular coating and callose and secondary cell wall deposition [20,21]. An interesting adaptation to vascular wilt infection is vein clearing, a tissue-specific develop‐ mental programme leading to the formation of new xylem elements [207]. Furthermore, significant metabolic changes have been reported in response to xylem infection and involve the induction of pathogenesis-related (PR) proteins, peroxidases, proteases as well as the production of antimicrobial secondary metabolites such as phytoalexins, sulphur-contain‐ ing compounds and phenolic compounds [20,21].

Studies of defence signalling in response to root pathogens have so far mainly focused on the leaves and have provided evidence that defence mechanisms involve similar signalling pathways (Ca2+-signalling, induction of ROS and MAPK cascades, modulation of phytohor‐ mone signalling) [20]. Moreover, plant microarray and RNASeq studies have revealed that the interaction between vascular wilt pathogens and host plants involves transcriptional reprog‐ ramming of hundreds of genes [208–211]. Interestingly, in an incompatible interaction, only modest changes in gene and protein expression have been reported [210,212–214] and most of the differentially expressed genes have been repressed in roots rather than in leaves [211]. Moreover, genes implicated in photorespiration, hypoxia, glycoxylate metabolism and auxin signalling show inverse regulation on infection with the foliar pathogen *Cladosporium fulvum* or root pathogen *Verticillium dahliae* [210].

Genome-wide analyses on transcriptional and proteomic levels, together with functional characterization of individual genes, have revealed a convergence of signalling pathways in response to individual pathogens, in mostly controlled conditions. In the field, plants are simultaneously challenged by multiple stress factors, both biotic and abiotic. Even though signalling components of plant regulatory networks are partly shared in both and point to general stress response mechanisms, there is evidence of specific responses to combined stresses that are controlled by different signalling pathways and such studies may provide additional candidates for crop protection breeding [14–16,18].
