MUTE drives asymmetric divisions to form stomatal subsidiary cells in Crassulaceae succulents.

Abstract

Among the evolutionary innovations of many succulents is a photosynthetic lifestyle, where stomatal gas exchange is decoupled from light-dependent carbon fixation. Many Crassulaceae leaf succulents form a stomatal morphotype consisting of kidney-shaped guard cells surrounded by three anisocytic subsidiary cells (SCs), whose function and development remained unknown. Here, we established <i>Kalanchoë laxiflora</i> as a developmental model. Potassium staining suggested SCs to shuttle osmolytes and support turgor-driven stomatal movements. Gene editing, reporter lines, protein overexpression, and RNA sequencing implicated the stomatal transcription factor <i>MUTE</i> in facilitating the additional rounds of asymmetric divisions required to form SCs in succulents. This is opposite to the role of <i>MUTE</i> in <i>Arabidopsis thaliana</i>, where it stops rather than induces asymmetric divisions but reminiscent of <i>MUTE</i>'s subsidiary cell-related function in grasses. Our work firmly establishes <i>K. laxiflora</i> as a model for succulent development and deciphers an intricate genetic mechanism that generates innovative stomatal morphology in Crassulaceae succulents.