The pathogen; mechanisms of attack and novel targets
John Mansfield, Marta de Torres, Ian Brown and Murray Grant, Division of Biology, Imperial College London
Perhaps the most remarkable discovery in recent years has been that bacteria are able to inject proteins into plant cells. Unravelling the role of the type three secretion system (T3SS) has allowed new perspectives to be developed on mechanisms of innate immunity and their suppression by pathogens. A clear link has been forged between plant and animal pathosystems. The role of effector proteins delivered through the T3SS in the induction of disease and activation of the hypersensitive reaction has provided new insights into the regulation of plant defences.
We have used delivery of proteins through a non-pathogen and also in planta expression to examine the impact of potential effectors on plant defence in Arabidopsis. Our focus is on the HopAB family including AvrPtoB. Using the RW60 strain of Pseudomonas syringae pv. phaseolicola we found that AvrPtoB suppresses basal resistance particularly in the absence of the flagellin (flg22) receptor FLS2. Expression of AvrPtoB in the plant suppressed defences induced by flg22 and also the elf20 peptide, but only if the effector was induced one or two hours before elicitor challenge. The timing of exposure of plant cells to elicitors and effectors has a clear influence on the outcome of interactions. Reduction in callose accumulation was observed and also a reprogramming of the defence transcriptome characterising basal resistance. Disease development and AvrPtoB-induced susceptibility, were associated with increases in abscisic acid (ABA) concentrations and ABA-induced gene expression. The potential and rather unexpected role of plant hormones such as ABA in rapidly modulating the leaf environment to favour pathogenesis will be discussed.