Diverse functions of FOUR LIPS: Stomatal development and beyond

Stomata, composed of paired guard cells, serve as essential pores for plant-atmosphere gas exchange and undergo tightly regulated development. FOUR LIPS (FLP), the first characterized regulator of stomatal development, has been studied for three decades. It encodes an atypical R2R3-MYB transcription factor that restricts guard mother cell division by repressing several core cell cycle genes. This factor exhibits partial functional redundancy with bHLH proteins FAMA and MUTE during stomatal fate commitment and differentiation. Its stomatal regulatory function is conserved across plant species. Notably, broad FLP expression across tissues enables functions beyond stomata, including roles in root gravitropism, lateral root initiation, female gametophyte development, and leaf angle determination. Furthermore, FLP is induced by environmental cues such as drought, salt, and cold stress to mediate adaptive responses. Here, we summarize the current understanding of FLP, focusing on conserved stomatal mechanisms and highlighting its functional versatility across plant systems.