WRKY22 and WRKY25 transcription factors are positive regulators of defense responses in Nicotiana benthamiana

Abstract

Plants defend themselves against pathogens using a two-layered immune system. Pattern-triggered immunity (PTI) can be activated upon recognition of epitopes from fagellin including fg22. Pseudomonas syringae pv. tomato (Pst) delivers efector proteins into the plant cell to promote host susceptibility. However, some plants express resistance (R) proteins that recognize specifc efectors leading to the activation of efector-triggered immunity (ETI). Resistant tomato lines such as Rio Grande-PtoR (RG-PtoR) recognize two Pst efectors, AvrPto and AvrPtoB, and activate ETI through the Pto/Prf protein complex. Using RNA-seq, we identifed two tomato WRKY transcription factor genes, SlWRKY22 and SlWRKY25, whose expression is increased during Pst-induced ETI. Silencing of the WRKY25/22 orthologous genes in Nicotiana benthamiana led to a delay in programmed cell death normally associated with AvrPto recognition or several non-bacterial efector/R protein pairs. An increase in disease symptoms was observed in silenced plants infltrated with Pseudomonas syringae pv. tabaci expressing AvrPto or HopQ1-1. Expression of both tomato WRKY genes is also induced upon treatment with fg22 and callose deposition and cell death suppression assays in WRKY25/22-silenced N. benthamiana plants supported their involvement in PTI. Our results reveal an important role for two WRKYs as positive regulators of plant immunity against bacterial and potentially non-bacterial pathogens.