Loss of function of bHLH transcription factor Nrd1 in tomato induces an arabinogalactan protein-encoding gene and enhances resistance to Pseudomonas syringae pv. tomato

Basic helix-loop-helix (bHLH) transcription factors constitute a superfamily in eukaryotes but their roles in plant immunity remain largely uncharacterized. We found that the transcript abundance in tomato leaves of one bHLH transcription factor-encoding gene, Nrd1 (negative regulator of resistance to DC3000 1), was significantly increased after treatment with the immunity-inducing flgII-28 peptide. Plants carrying a loss-of-function mutation in Nrd1 (Δnrd1) showed enhanced resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 although early pattern-triggered immunity responses such as generation of reactive oxygen species and activation of mitogen-activated protein kinases after treatment with flagellin-derived flg22 and flgII-28 peptides were unaltered compared to wild-type plants. An RNA-Seq analysis identified a gene, Agp1, whose expression is strongly suppressed in an Nrd1-dependent manner. Agp1 encodes an arabinogalactan protein and overexpression of the Agp1 gene in Nicotiana benthamiana led to ~10-fold less Pst growth compared to the control. These results suggest that the Nrd1 protein promotes tomato susceptibility to Pst by suppressing the defense gene Agp1. RNA-Seq also revealed that loss of Nrd1 function has no effect on the transcript abundance of immunity-associated genes including Bti9, Core, Fls2, Fls3 and Wak1 upon Pst inoculation, suggesting that the enhanced immunity observed in the Δnrd1 mutants is due to the activation of key PRR signaling components as well as loss of Nrd1-regulated suppression of Agp1.