From grasses to succulents - development and function of distinct stomatal subsidiary cells.

Stomata are breathing pores on leaves that balance photosynthetic carbon dioxide uptake and water vapor loss. Stomatal morphology and complexity are rather diverse when considering stomatal subsidiary cells (SCs). Subsidiary cells are adjacent to the central guard cells (GCs) and are morphologically distinct from other epidermal cells. Yet, how various SCs develop and whether and how they support stomatal gas exchange physiology outside of the grass family is largely unknown. Here, we discuss the development, ontogeny, and putative function of paracytic vs anisocytic SCs, which can be found in grasses and Crassulaceae succulents, respectively. First, we highlight recent advances in understanding how grasses form stomatal SCs. We then summarize novel insights into stomatal development in SC-less Arabidopsis to speculate on how this stomatal program might be rewired to enable anisocytic SC formation. Finally, we discuss the functional relevance of paracytic SCs in grasses and the putative roles of anisocytic SCs in succulents.