Plant cells communicate to coordinate their cycle of growth, flowering and fruiting, and activities in roots, shoots, and leaves in response to the changing environmental conditions. Plant signaling is distinct from animal signaling. Plants primarily utilize enzyme-linked receptors, whereas the largest class of cell-surface receptors in animals are G-protein coupled receptors (GPCRs). Unlike animals, receptor tyrosine kinases are rare in plants. Instead, plants have a diverse class of transmembrane kinase receptors called receptor-like kinases (RLKs). The most abundant types of these receptors, with about 175 identified in Arabidopsis, are the leucine-rich repeat (LRR) receptor kinases which have serine/threonine kinase cytoplasmic domain. They consist of repeating units of ~20-24 amino acids rich in leucine and involved in protein-protein interactions. The LRR-RLKs are involved in various plant processes, including pathogen resistance, flagellin sensing, meristem proliferation, abscission, etc.

Plant hormone receptors

Brassinosteroids (BRs) are plant steroids analogous to animal steroid hormones in structure and are essential for normal plant growth and development. The BR receptor, BRI1, is a cell-surface LRR-RLK with 20 leucine-rich repeats. The binding of BR to BRI1 leads to recruitment of the co-receptor kinase BRI1-associated receptor kinase 1 (BAK1) and dissociation of the BRI1 inhibitory protein (BKI1). The signaling pathway is activated by a series of transphosphorylation between the kinase domains of BRI1 and BAK1. BRI1 directly phosphorylates and activates its downstream kinase and phosphatase partners to regulate the phosphorylation and degradation of specific transcription regulatory proteins.