Abstract:
The hemibiotrophic plant pathogen Phytophthora cinnamomi Rands is the most
devastating pathogen of avocado (Persea americana Mill.) and, as such, causes
significant annual losses in the industry. Although the molecular basis of P. cinnamomi
resistance in avocado and P. cinnamomi virulence determinants have been the subject of
recent research, none have yet attempted to compare the transcriptomic responses of
both pathogen and host during their interaction. In the current study, the transcriptomes
of both avocado and P. cinnamomi were explored by dual RNA sequencing. The basis
for partial resistance was sought by the inclusion of both susceptible (R0.12) and partially
resistant (Dusa®) rootstocks sampled at early (6, 12 and 24 hours post-inoculation,
hpi) and late time-points (120 hpi). Substantial differences were noted in the number
of differentially expressed genes found in Dusa® and R0.12, specifically at 12 and
24 hpi. Here, the partially resistant rootstock perpetuated defense responses initiated
at 6 hpi, while the susceptible rootstock abruptly reversed course. Instead, gene
ontology enrichment confirmed that R0.12 activated pathways related to growth and
development, essentially rendering its response at 12 and 24 hpi no different from
that of the mock-inoculated controls. As expected, several classes of P. cinnamomi
effector genes were differentially expressed in both Dusa® and R0.12. However, their
expression differed between rootstocks, indicating that P. cinnamomi might alter the
expression of its effector arsenal based on the rootstock. Based on some of the
observed differences, several P. cinnamomi effectors were highlighted as potential
candidates for further research. Similarly, the receptor-like kinase (RLK) and apoplastic
protease coding genes in avocado were investigated, focusing on their potential role in
differing rootstock responses. This study suggests that the basis of partial resistance in Dusa® is predicated on its ability to respond appropriately during the early stages
following P. cinnamomi inoculation, and that important components of the first line of
inducible defense, apoplastic proteases and RLKs, are likely to be important to the
observed outcome.
