A Leucine-Rich Repeat Receptor-like Kinase TaBIR1 Contributes to Wheat Resistance against Puccinia striiformis f. sp. tritici.

Plant cell surface-localized receptor-like kinases (RLKs) recognize invading pathogens and transduce the immune signals inside host cells, subsequently triggering immune responses to fight off pathogen invasion. Nonetheless, our understanding of the role of RLKs in wheat resistance to the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst) remains limited. During the differentially expressed genes in Pst infected wheat leaves, a Leucine-repeat receptor-like kinase (LRR-RLK) gene TaBIR1 was significantly upregulated in the incompatible wheat-Pst interaction. qRT-PCR verified that TaBIR1 is induced at the early infection stage of Pst. The transient expression of TaBIR1-GFP protein in N. bentamiana cells and wheat mesophyll protoplasts revealed its plasma membrane location. The knockdown of TaBIR1 expression by VIGS (virus induced gene silencing) declined wheat resistance to stripe rust, resulting in reduced reactive oxygen species (ROS) production, callose deposition, and transcripts of pathogenesis-related genes TaPR1 and TaPR2, along with increased Pst infection area. Ectopic overexpression of TaBIR1 in N. benthamiana triggered constitutive immune responses with significant cell death, callose accumulation, and ROS production. Moreover, TaBIR1 triggered immunity is dependent on NbBAK1, the silencing of which significantly attenuated the defense response triggered by TaBIR1. TaBIR1 interacted with the NbBAK1 homologues in wheat, co-receptor TaSERK2 and TaSERK5, the transient expression of which could restore the impaired defense due to NbBAK1 silencing. Taken together, TaBIR1 is a cell surface RLK that contributes to wheat stripe rust resistance, probably as a positive regulator of plant immunity in a BAK1-dependent manner.

TaRac6 Is a Potential Susceptibility Factor by Regulating the ROS Burst Negatively in the Wheat-Puccinia striiformis f. sp. tritici Interaction.

Rac/Rop proteins play important roles in the regulation of cell growth and plant defense responses. However, the function of Rac/Rop proteins in wheat remains largely unknown. In this study, a small G protein gene, designated as TaRac6, was characterized from wheat (Triticum aestivum) in response to Puccinia striiformis f. sp. tritici (Pst) and was found to be highly homologous to the Rac proteins identified in other plant species. Transient expression analyses of the TaRac6-GFP fusion protein in Nicotiana benthamiana leaves showed that TaRac6 was localized in the whole cell. Furthermore, transient expression of TaRac6 inhibited Bax-triggered plant cell death (PCD) in N. benthamiana. Transcript accumulation of TaRac6 was increased at 24 h post-inoculation (hpi) in the compatible interaction between wheat and Pst, while it was not induced in an incompatible interaction. More importantly, silencing of TaRac6 by virus induced gene silencing (VIGS) enhanced the resistance of wheat (Suwon 11) to Pst (CYR31) by producing fewer uredinia. Histological observations revealed that the hypha growth of Pst was markedly inhibited along with more H2O2 generated in the TaRac6-silenced leaves in response to Pst. Moreover, transcript levels of TaCAT were significantly down-regulated, while those of TaSOD and TaNOX were significantly up-regulated. These results suggest that TaRac6 functions as a potential susceptibility factor, which negatively regulate the reactive oxygen species (ROS) burst in the wheat-Pst interaction.