Expression of several Phytophthora cinnamomi putative RxLRs provides evidence for virulence roles in avocado

Abstract

Phytophthora cinnamomi is a plant pathogenic oomycete that causes Phytophthora root rot of avocado (PRR). Currently, there is a limited understanding of the molecular interactions underlying this disease. Other Phytophthora species employ an arsenal of effector proteins to manipulate host physiology, of which the RxLR effectors contribute to virulence by interfering with host immune responses. The aim of this study was to identify candidate RxLR effectors in P. cinnamomi that play a role in establishing PRR, and to infer possible functions for these effectors. We identified 61 candidate RxLR genes which were expressed during infection of a susceptible avocado rootstock. Several of these genes were present in multiple copies in the P. cinnamomi genome, suggesting that they may contribute to pathogen fitness. Phylogenetic analysis of the manually predicted RxLR protein sequences revealed 12 P. cinnamomi RxLRs that were related to characterised effectors in other Phytophthora spp., providing clues to their functions in planta. Expression profiles of nine more RxLRs point to possible virulence roles in avocado–highlighting a way forward for studies of this interaction. This study represents the first investigation of the expression of P. cinnamomi RxLR genes during the course of avocado infection, and puts forward a pipeline to pinpoint effector genes with potential as virulence determinants, providing a foundation for the future functional characterization of RxLRs that contribute to P. cinnamomi virulence in avocado.

Partially Resistant Avocado Rootstock Dusa® Shows Prolonged Upregulation of Nucleotide Binding-Leucine Rich Repeat Genes in Response to Phytophthora cinnamomi Infection

“…Two RGA2 NLRs (PC11g001210 and PC11g001240) showed much higher expression in Dusa® when compared to R0.12, in all samples ( Figure 5 ). As described earlier, the RxLR in P. cinnamomi with high similarity to a RGA2 protein counterpart, ipiO1, showed increased expression in R0.12 at 12 hpi, when compared to mycelia control samples ( Joubert et al, 2021 ). Since R0.12 RGA2 proteins are not upregulated at this timepoint, it may suggest that fewer of these RxLR effectors are recognized, resulting in a compromised HR.…”

“…RGA2 proteins elicit an immune response and confer resistance toward Phytophthora infestans in potato and tomato plants, after recognizing ipiO RxLR proteins ( Champouret et al, 2009 ). Recently, two P. cinnamomi RxLR proteins with high sequence similarity to P. infestans ipiO RxLRs were identified by Joubert et al (2021) . One of these RxLRs, PcinRxLR34a, was significantly upregulated in P. cinnamomi during infection of the susceptible rootstock R0.12, when compared to expression in mycelia.…”

“…Thus, investigating which cis -regulatory elements are shared between PaNLR genes would be of interest in future research. Moreover, P. cinnamomi RxLRs were shown to have increased expression levels at 12 and 24 hpi in R0.12 ( Joubert et al, 2021 ). Since some RxLRs suppress programed cell death, P. cinnamomi RxLRs could influence PaNLR expression and contribute to the results observed for R0.12 ( Dalio et al, 2018 ).…”

Comparative sub-cellular proteome analyses reveals metabolic differentiation and production of effector-like molecules in the dieback phytopathogen Phytophthora cinnamomi

“…The expression of α-and β-cinnamomin elicitins have been found in various cell types of P. cinnamomi, of which β-cinnamomin has been shown to play some role in virulence, however the infection process is a result of a number of effector proteins [34][35][36][37] . The expression of several predicted RXLR effectors has also been demonstrated in P. cinnamomi when inoculated onto avocado roots 38 . Effectors of P. cinnamomi have not been fully characterised and effectors at the protein level have not been identified.…”

Unraveling Plant Cell Death during Phytophthora Infection

“…Phytophthora RxLRs are cytoplasmic effectors with a modular architecture, including an N-terminal signal peptide for protein secretion, a conserved RxLR motif to facilitate translocation into host cells and a diverse C-terminal domain executing virulence activity (Figure 4) [64,[140][141][142]. The RxLR effector family is the largest class of translocated effectors and is specific to Phytophthora spp., with there being 560, 370, 390 and 238 RxLR-containing protein coding genes in the genomes of P. infestans, P. ramorum, P. sojae and P. cinnamomi, respectively [12,64,143]. These effectors localize to many subcellular organelles and structures, where they target a wide range of pathways throughout the plant cell [4,144].…”