Regulation of stomatal development by receptor-like kinases and receptor-like proteins in Arabidopsis and grasses

Stomata are core channels for plant gas exchange and water transpiration, and precise regulation of their development directly impacts photosynthetic efficiency, water use, and stress resistance. Plant receptor kinases, particularly leucine-rich repeat receptor-like kinases (LRR-RLKs), function as key signal sensors: they perceive endogenous and exogenous signals, trigger downstream cascades, and finely regulate stomatal initiation, differentiation, and patterning. Deciphering these mechanisms is therefore critical for improving crop stomatal traits, stress tolerance, and yield. Among receptors regulating stomatal development, LRR-RLKs and leucine-rich repeat receptor-like proteins (LRR-RLPs) are the best studied. Too Many Mouths (TMM), the first identified stomatal receptor (LRR-RLP), forms a multiprotein complex with ERECTA family (ERf) and SERK family LRR-RLKs. This complex recognizes Epidermal Patterning Factors (EPFs)/EPF-like factors (EPFLs) peptides., activates the YODA (YDA)-MAPK cascade, and inhibits the key stomatal lineage transcription factor SPEECHLESS (SPCH), thereby precisely regulating stomatal patterning and differentiation in <i>Arabidopsis</i>. Beyond this core complex, other LRR-RLKs (e.g., HSL1, CLV1, MUS) also regulate <i>Arabidopsis</i> stomatal development or morphogenesis. HSL1 recruits SERK co-receptors and perceives CLE9/10 ligands, but the ligands for CLV1 and MUS in stomatal development remain unknown. Notably, maize PAN1 and PAN2 are essential for asymmetric division of subsidiary mother cells (SMCs) and subsidiary cell (SC) formation, with their cognate ligands also uncharacterized. This review summarizes key advances in stomatal receptor (especially LRR-RLK) mediated stomatal development in <i>Arabidopsis</i> and grasses and highlights core issues such as ligand recognition.