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

Melissa Joubert,Noëlani Van Den Berg,Juanita Engelbrecht,Robert Backer,Kandasamy Ulaganathan

doi:10.1371/journal.pone.0254645

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.

Highlights

Phytophthora cinnamomi Rands is a soil-borne, hemibiotrophic, plant-pathogenic oomycete
This study aimed to identify candidate RxLR effectors in P. cinnamomi that play a role in establishment of Phytophthora root rot (PRR) in avocado, by focusing on the discovery of putative RxLR genes which were upregulated during infection of a susceptible avocado rootstock
A library of putative P. cinnamomi RxLR effector genes was constructed using available sequence lists of candidates predicted in recent literature [1,23,24,25] as well as a set of 192 candidate RxLRs predicted in this study using a bioinformatics pipeline adapted from Win et al [22]

Summary

Introduction

Phytophthora cinnamomi Rands is a soil-borne, hemibiotrophic, plant-pathogenic oomycete. It is globally distributed and affects at least 5000 plant species, including economically important crops such as avocado, macadamia, peach and chestnut. P. cinnamomi typically infects the roots of host plants causing root rot, as well as stem cankers and dieback of shoots [1]. The wide host range of the pathogen has led to devastating impacts on biodiversity in natural ecosystems in Europe and Australia, where P. cinnamomi infection poses a major threat to forests and natural flora [2,3,4,5]. The oomycete causes Phytophthora root rot (PRR) of avocado, which has had a devastating impact on this crop [6,7,8,9].

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