SlBIR3 Negatively Regulates PAMP Responses and Cell Death in Tomato

Bri1-associated kinase 1 (BAK1)-interacting receptor-like kinase (BIR) proteins have been shown to play important roles in regulating growth and development, pathogen associated molecular pattern (PAMP)-triggered immunity (PTI) responses, and cell death in the model plant, . We identified four BIR family members in tomato ( ), including , an ortholog of from . is predicted to encode a membrane localized non-arginine-aspartate (non-RD) kinase that, based on protein sequence, does not have autophosphorylation activity but that can be phosphorylated in vivo. We established that SlBIR3 interacts with SlBAK1 and AtBAK1 using yeast two-hybrid assays and co-immunoprecipitation and maltose-binding protein pull down assays. We observed that overexpression in tomato (cv. micro-tom) and has weak effect on growth and development through brassinosteroid (BR) signaling. overexpression in suppressed flg22-induced defense responses, but did not affect infection with the bacterial pathogen ( DC3000). This result was confirmed using virus-induced gene silencing (VIGS) in tomato in conjunction with DC3000 infection. Overexpression of in tomato (cv. micro-tom) and resulted in enhanced susceptibility to the necrotrophic fungus . In addition, co-silencing with or using VIGS and the tobacco rattle virus (TRV)-RNA2 vector containing fragments of both the and genes induced spontaneous cell death, indicating a cooperation between the two proteins in this process. In conclusion, our study revealed that is the ortholog of and that it participates in BR, PTI, and cell death signaling pathways.