Effector-triggered immunity against Pseudomonas syringae pv. actinidiae in nonhost plants : thesis submitted to the Massey University for the degree of Doctor of Philosophy

Abstract

𝘗𝘴𝘦𝘶𝘥𝘰𝘮𝘰𝘯𝘢𝘴 𝘴𝘺𝘳𝘪𝘯𝘨𝘢𝘦 pv. 𝘢𝘤𝘵𝘪𝘯𝘪𝘥𝘪𝘢𝘦 (Psa) is a virulent and highly damaging pathogen causing bacterial canker in all currently commercially important cultivars of kiwifruit (𝘈𝘤𝘵𝘪𝘯𝘪𝘥𝘪𝘢 spp.). Arabidopsis and 𝘕𝘪𝘤𝘰𝘵𝘪𝘢𝘯𝘢 spp. plants, however, are nonhosts to 𝘗𝘴𝘢. In our course of investigating the various nonhost resistance mechanisms in play against 𝘗𝘴𝘢, we identified several sources of resistance against several 𝘗𝘴𝘢 strains as well as a possible novel virulence mechanism used by 𝘗𝘴𝘢 and 𝘏𝘺𝘢𝘭𝘰𝘱𝘦𝘳𝘰𝘯𝘰𝘴𝘱𝘰𝘳𝘢 𝘢𝘳𝘢𝘣𝘪𝘥𝘰𝘱𝘴𝘪𝘥𝘪𝘴 (Hpa), a b iotrophic pathogen of Arabidopsis.

Firstly, we discovered that the highly virulent strain, 𝘗𝘴𝘢 V13, triggers hypersensitive response (HR) in Arabidopsis in an accession-­‐specific manner and that HopZ5PsaV13, a member of the YopJ family of putative acetyltransferases, confers this bacterial avirulence. We also show that the immunity triggered by HopZ5 is independent from HR in the Arabidopsis accession Col-­‐0. Through mutagenesis, we show that key amino acid residues predicted for acetyltransferase activity are vital to HopZ5-­‐triggered immunity and HR, phenotypes reproduced in Nicotiana spp. Secondly, we identified multiple sources of avirulence for the kiwifruit low-­‐ virulence strain, 𝘗𝘴𝘢 LV5, in Arabidopsis and Nicotiana benthamiana, namely homologs of previously characterized effectors, HopAR1 and HopAB3, respectively. We additionally show that HopAB3 can trigger resistance in cultivated tomato putatively due to a novel recognition by a cultivated tomato homolog (SlPtoB) of the resistance gene Fen. Finally, we identified several nuclear-­‐localized effectors from 𝘗𝘴𝘢 and Hpa that interact with Arabidopsis WRKY transcription factors, different to WRKYs targeted by previously identified AvrRps4 and PopP2. We show that some WRKYs can trigger a cell death response in N. benthamiana when overexpressed and that coexpression of AvrRps4 or PopP2 is able to suppress this cell death response for the WRKYs they interact with. We show that this suppression is associated with suppression of transcriptional activation ability of the WRKY and 7 propose that this mechanism of transcription suppression may be utilized by other 𝘗𝘴𝘢 and Hpa effectors identified in this study.