Mai1 Protein Acts Between Host Recognition of Pathogen Effectors and Mitogen-Activated Protein Kinase Signaling

Abstract

The molecular mechanisms acting between host recognition of pathogen effectors by nucleotide-binding leucine-rich repeat receptor (NLR) proteins and mitogen-activated protein kinase (MAPK) signaling cascades are unknown. MAPKKKα (M3Kα) activates MAPK signaling leading to programmed cell death (PCD) associated with NLR-triggered immunity. We identified a tomato M3Kα-interacting protein, SlMai1, that has 80% amino acid identity with <i>Arabidopsis</i> brassinosteroid kinase 1 (AtBsk1). SlMai1 has a protein kinase domain and a C-terminal tetratricopeptide repeat domain that interacts with the kinase domain of M3Kα. Virus-induced gene silencing of <i>Mai1</i> homologs in <i>Nicotiana benthamiana</i> increased susceptibility to <i>Pseudomonas syringae</i> and compromised PCD induced by four NLR proteins. PCD was restored by expression of a synthetic <i>SlMai1</i> gene that resists silencing. Expression of AtBsk1 did not restore PCD in <i>Mai1</i>-silenced plants, suggesting SlMai1 is functionally divergent from AtBsk1. PCD caused by overexpression of M3Kα or MKK2 was unaffected by <i>Mai1</i> silencing, suggesting Mai1 acts upstream of these proteins. Coexpression of Mai1 with M3Kα in leaves enhanced MAPK phosphorylation and accelerated PCD. These findings suggest Mai1 is a molecular link acting between host recognition of pathogens and MAPK signaling.