A Family of Four LRR-RLKs Modulate Development and Defense Signaling in Arabidopsis thaliana through Interaction with the Co-receptor BAK1

Abstract

Receptor-like kinases (RLKs) are encoded for by one of the largest gene families in Arabidopsis and represent the predominant form of cell surface receptors in plants. RLKs mediate signal transduction in diverse processes including steroid-mediated growth pathways, pathogen-triggered innate immune responses. Here I present characterization of mutant phenotypes, expression patterns, and genetic interactions for the BAK1 INTERACTING RECEPTOR (BIR) family of Leucine-rich Repeat-RLKs, three members of which have had no previous characterization. Furthermore, I show that cell death, aerial growth, and lateral root development defects in bir1-1 are suppressed by mutations of the LRR-RLK co-receptor BRI1-ASSOCIATED KINASE 1 (BAK1); I identify a novel primary root growth phenotype in bir1-1 mutants, as well as a lateral root development phenotype for bir3 mutants; and primary root growth and aerial defects in bir3.bir4;bak1 triple mutants. Using an allelic series of bak1 mutations I show that bir phenotypes are dependent upon particular functions of BAK1, and propose that the BIR family exhibits a novel function, previously undescribed for LRR-RLKs, as regulators of co-receptor/ligand-binding receptor complex specificity.