Functional assignment to positively selected sites in the core type III effector RipG7 from Ralstonia solanacearum

The soil-borne pathogen Ralstonia solanacearum causes bacterial wilt in a broad range of plants. The main virulence determinants of R. solanacearum are the type III secretion system (T3SS) and its associated type III effectors (T3Es), translocated into the host cells. Of the conserved T3Es among R. solanacearum strains, the Fbox protein RipG7 is required for R. solanacearum pathogenesis on Medicago truncatula. In this work, we describe the natural ripG7 variability existing in the R. solanacearum species complex. We show that eight representative ripG7 orthologues have different contributions to pathogenicity on M. truncatula: only ripG7 from Asian or African strains can complement the absence of ripG7 in GMI1000 (Asian reference strain). Nonetheless, RipG7 proteins from American and Indonesian strains can still interact with M. truncatula SKP1-like/MSKa protein, essential for the function of RipG7 in virulence. This indicates that the absence of complementation is most likely a result of the variability in the leucine-rich repeat (LRR) domain of RipG7. We identified 11 sites under positive selection in the LRR domains of RipG7. By studying the functional impact of these 11 sites, we show the contribution of five positively selected sites for the function of RipG7CMR15 in M. truncatula colonization. This work reveals the genetic and functional variation of the essential core T3E RipG7 from R. solanacearum. This analysis is the first of its kind on an essential disease-controlling T3E, and sheds light on the co-evolutionary arms race between the bacterium and its hosts.

Fungal endopolygalacturonases are recognized as microbe-associated molecular patterns by the arabidopsis receptor-like protein RESPONSIVENESS TO BOTRYTIS POLYGALACTURONASES1

Plants perceive microbial invaders using pattern recognition receptors that recognize microbe-associated molecular patterns. In this study, we identified RESPONSIVENESS TO BOTRYTIS POLYGALACTURONASES1 (RBPG1), an Arabidopsis (Arabidopsis thaliana) leucine-rich repeat receptor-like protein, AtRLP42, that recognizes fungal endopolygalacturonases (PGs) and acts as a novel microbe-associated molecular pattern receptor. RBPG1 recognizes several PGs from the plant pathogen Botrytis cinerea as well as one from the saprotroph Aspergillus niger. Infiltration of B. cinerea PGs into Arabidopsis accession Columbia induced a necrotic response, whereas accession Brno (Br-0) showed no symptoms. A map-based cloning strategy, combined with comparative and functional genomics, led to the identification of the Columbia RBPG1 gene and showed that this gene is essential for the responsiveness of Arabidopsis to the PGs. Transformation of RBPG1 into accession Br-0 resulted in a gain of PG responsiveness. Transgenic Br-0 plants expressing RBPG1 were equally susceptible as the recipient Br-0 to the necrotroph B. cinerea and to the biotroph Hyaloperonospora arabidopsidis. Pretreating leaves of the transgenic plants with a PG resulted in increased resistance to H. arabidopsidis. Coimmunoprecipitation experiments demonstrated that RBPG1 and PG form a complex in Nicotiana benthamiana, which also involves the Arabidopsis leucine-rich repeat receptor-like protein SOBIR1 (for SUPPRESSOR OF BIR1). sobir1 mutant plants did not induce necrosis in response to PGs and were compromised in PG-induced resistance to H. arabidopsidis.

An atypical kinase under balancing selection confers broad-spectrum disease resistance in Arabidopsis

The failure of gene-for-gene resistance traits to provide durable and broad-spectrum resistance in an agricultural context has led to the search for genes underlying quantitative resistance in plants. Such genes have been identified in only a few cases, all for fungal or nematode resistance, and encode diverse molecular functions. However, an understanding of the molecular mechanisms of quantitative resistance variation to other enemies and the associated evolutionary forces shaping this variation remain largely unknown. We report the identification, map-based cloning and functional validation of QRX3 (RKS1, Resistance related KinaSe 1), conferring broad-spectrum resistance to Xanthomonas campestris (Xc), a devastating worldwide bacterial vascular pathogen of crucifers. RKS1 encodes an atypical kinase that mediates a quantitative resistance mechanism in plants by restricting bacterial spread from the infection site. Nested Genome-Wide Association mapping revealed a major locus corresponding to an allelic series at RKS1 at the species level. An association between variation in resistance and RKS1 transcription was found using various transgenic lines as well as in natural accessions, suggesting that regulation of RKS1 expression is a major component of quantitative resistance to Xc. The co-existence of long lived RKS1 haplotypes in A. thaliana is shared with a variety of genes involved in pathogen recognition, suggesting common selective pressures. The identification of RKS1 constitutes a starting point for deciphering the mechanisms underlying broad spectrum quantitative disease resistance that is effective against a devastating and vascular crop pathogen. Because putative RKS1 orthologous have been found in other Brassica species, RKS1 provides an exciting opportunity for plant breeders to improve resistance to black rot in crops.

During the evolution of plant-pathogen interactions, plants have evolved the capability to defend themselves from pathogen infection by different overlapping mechanisms. Disease resistance is constituted by an elaborate, multilayered system of defense. Among these responses, quantitative resistance is a prevalent form of resistance in crops and natural plant populations, for which the genetic and molecular bases remain largely unknown. Thus, identification of the genes underlying quantitative resistance constitutes a major challenge in plant breeding and evolutionary biology, and might have enormous practical implications for human health by increasing crop yield and quality. Our work contributes to understanding the molecular bases of quantitative resistance to the vascular pathogen Xanthomonas campestris (Xc), which is responsible for black rot, an important disease of crucifers worldwide. By multiple approaches, we demonstrate that RKS1 is a quantitative resistance gene in Arabidopsis thaliana conferring broad-spectrum resistance to Xc and that this resistance mechanism in plants is associated with regulation of RKS1 expression. We also provide evidence that RKS1 allelic variation is a major component of quantitative resistance to Xc at the species level. Finally, the long-lived polymorphism associated with RKS1 suggests that evolutionary stable broad-spectrum resistance to Xc may be achieved in natural populations of A. thaliana.

A polygalacturonase-inhibiting protein from grapevine reduces the symptoms of the endopolygalacturonase BcPG2 from Botrytis cinerea in Nicotiana benthamiana leaves without any evidence for in vitro interaction

Six endopolygalacturonases from Botrytis cinerea (BcPG1 to BcPG6) as well as mutated forms of BcPG1 and BcPG2 were expressed transiently in leaves of Nicotiana benthamiana using agroinfiltration. Expression of BcPG1, BcPG2, BcPG4, BcPG5, and mutant BcPG1-D203A caused symptoms, whereas BcPG3, BcPG6, and mutant BcPG2-D192A caused no symptoms. Expression of BcPG2 caused the most severe symptoms, including wilting and necrosis. BcPG2 previously has been shown to be essential for B. cinerea virulence. The in vivo effect of this enzyme and the inhibition by a polygalacturonase-inhibiting protein (PGIP) was examined by coexpressing Bcpg2 and the Vvpgipl gene from Vitis vinifera in N. benthamiana. Coinfiltration resulted in a substantial reduction of the symptoms inflicted by the activity of BcPG2 in planta, as evidenced by quantifying the variable chlorophyll fluorescence yield. In vitro, however, no interaction between pure VvPGIP1 and pure BcPG2 was detected. Specifically, VvPGIP1 neither inhibited BcPG2 activity nor altered the degradation profile of polygalacturonic acid by BcPG2. Furthermore, using surface plasmon resonance technology, no physical interaction between VvPGIP1 and BcPG2 was detected in vitro. The data suggest that the in planta environment provided a context to support the interaction between BcPG2 and VvPGIP1, leading to a reduction in symptom development, whereas neither of the in vitro assays detected any interaction between these proteins.

Functional analysis of Botrytis cinerea pectin methylesterase genes by PCR-based targeted mutagenesis: Bcpme1 and Bcpme2 are dispensable for virulence of strain B05.10

SUMMARY Botrytis cinerea is a necrotrophic pathogen that produces an array of enzymes capable of attacking the plant cell wall components. We have previously shown that growth of the fungus in planta is accompanied by the degradation of pectin and that endopolygalacturonase (Bcpg) genes are expressed during infection of different plant tissues. It was assumed that pectin demethylation by pectin methylesterases (PME) was essential for the subsequent depolymerization by BcPGs to occur efficiently. We report here on the functional analysis of two Bcpme genes in strain B05.10, using a gene-replacement approach. The method used for the generation of constructs for gene replacement in B. cinerea circumvents the need for cloning and yielded a high proportion of homologous recombinants. Mutants lacking both Bcpme genes are not affected in their growth on highly methylated pectin, nor did they show any reduction in virulence. The results suggest that B. cinerea strain B05.10 can efficiently degrade pectin without prior demethylation.

Necrotizing activity of five Botrytis cinerea endopolygalacturonases produced in Pichia pastoris

Five Botrytis cinerea endopolygalacturonase enzymes (BcPGs) were individually expressed in Pichia pastoris, purified to homogeneity and biochemically characterized. While the pH optima of the five enzymes were similar (approximately pH 4.5) the maximum activity of individual enzymes differed significantly. For hydrolysis of polygalacturonic acid (PGA), the V(max,app) ranged from 10 to 900 U mg(-1), while the K(m,app) ranged from 0.16 to 0.6 mg ml(-1). Although all BcPGs are true endopolygalacturonases, they apparently have different modes of action. PGA hydrolysis by BcPG1, BcPG2 and BcPG4 leads to the transient accumulation of oligomers with DP < 7, whereas PGA hydrolysis by BcPG3 and BcPG6 leads to the immediate accumulation of monomers and dimers. The necrotizing activity (NA) of all BcPGs was tested separately in tomato, broad bean and Arabidopsis thaliana. They showed different NAs on these plants. BcPG1 and BcPG2 possessed the strongest NA as tissue collapse was observed within 10 min after infiltration of broad bean leaves. The amino acid (aa) D192A substitution in the active site of BcPG2 not only abolished enzyme activity but also the NA, indicating that the NA is dependent on enzyme activity. Furthermore, deletion of the Bcpg2 gene in B. cinerea resulted in a strong reduction in virulence on tomato and broad bean. Primary lesion formation was delayed by approximately 24 h and the lesion expansion rate was reduced by 50-85%. These data indicate that BcPG2 is an important virulence factor for B. cinerea.