From bacterial avirulence genes to effector functions via the hrp delivery system: an overview of 25 years of progress in our understanding of plant innate immunity

Abstract

Cloning the first avirulence (avr) gene has led not only to a deeper understanding of gene-for-gene interactions in plant disease, but also to fundamental insights into the suppression of basal defences against microbial attack. This article (focusing on Pseudomonas syringae) charts the development of ideas and research progress over the 25 years following the breakthrough achieved by Staskawicz and coworkers. Advances in gene cloning technology underpinned the identification of both avr and hrp genes, the latter being required for the activation of the defensive hypersensitive reaction (HR) and pathogenicity. The delivery of Avr proteins through the type III secretion machinery encoded by hrp gene clusters was demonstrated, and the activity of the proteins inside plant cells as elicitors of the HR was confirmed. Key roles for avr genes in pathogenic fitness have now been established. The rebranding of Avr proteins as effectors, proteins that suppress the HR and cell wall-based defences, has led to the ongoing search for their targets, and is generating new insights into the co-ordination of plant resistance against diverse microbes. Bioinformatics-led analysis of effector gene distribution in genomes has provided a remarkable view of the interchange of effectors and also their functional domains, as the arms race of attack and defence drives the evolution of microbial pathogenicity. The application of our accrued knowledge for the development of disease control strategies is considered.

Figure 1

The Phaseolus bean halo‐blight disease gene‐for‐gene matrix proposed by Taylor et al. (1996) to explain race specificity in the Phaseolus syringae pv. phaseolicola (Pph)–Phaseolus vulgaris interaction. The avr genes cloned by function from Pph include avrPphB, avrPphE and avrPphF, which correspond to avr genes 3, 2 and 1 indicated in the matrix. Stab inoculation assays for virulence in bean pods are also illustrated, comparing susceptible water‐soaked lesions with the hypersensitive resistance reaction. Despite the application of targeted cloning and selection strategies, the avr gene matching R4 has not been identified. Other avr genes recovered from Pph by the exchange of genomic libraries between pathovars and assays in soybean and pea include avrPphC and avrPphD, respectively (Arnold et al., 2001a; Yucel et al., 1994).

Figure 2

Immunogold localization of HrpZ (viewed as black electron‐dense dots) coating the HrpA pilus (top arrow) of Phaseolus syringae pv. tomato DC3000. A curved flagellum (bottom arrow) containing flagellin, which activates basal defences through the receptor FLS2 (Felix and Boller, 2009), is also shown, but is not associated with HrpZ. Image kindly provided by Ian Brown (see also Li et al., 2002).

Figure 3

Comparison of the plasmid‐borne pathogenicity island containing effector genes in Phaseolus syringae pv phaseolicola strains 1448A (race 6) and 1449B (race 7). The cosmid clone pAV520, used to detect virulence functions, contains the region from 1449B shaded in dark pink (Jackson et al., 1999). The avrPphF gene conferring avirulence on beans with R1‐based resistance to halo‐blight is absent from 1448A, probably because of a deletion of 9471 nucleotides flanked by transposon fragments. In 1448A, an insertion of 10 nucleotides also generates a deduced 218AA effector, A514 (HopAW1). Kindly provided by Rob Jackson (see also Joardar et al., 2005; Rivas et al., 2005).

Figure 4

Suppression of basal defences by virulent Xanthomonas campestris pv. vesicatoria in pepper. Mixed inocula of wild‐type and hrp mutant bacteria were infiltrated into leaf mesophyll tissue. The hrp mutant bacteria were tagged with AvrBs3 and identified by immunocytochemistry. The hrp mutants (large arrow) have triggered papilla development (white asterisk) in one cell, but no papilla has formed in the cell in contact with the colony containing wild‐type X. campestris pv. vesicatoria, in which bacteria are embedded in extracellular polysaccharides (small arrow). It is noticeable, however, that chloroplasts have accumulated next to the wild‐type colony. Image kindly provided by Mansureh Keshavarzi; for further details see Keshavarzi et al. (2004).