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One of the fundamental questions in developmental biology is how a cell is specified to differentiate as a specialized cell type. Traditionally, plant cell types were defined based on their function, location, morphology, and lineage. Currently, in the age of single-cell biology, researchers typically attempt to assign plant cells to cell types by clustering them based on their transcriptomes. However, because cells are dynamic entities that progress through the cell cycle and respond to signals, the transcriptome also reflects the state of the cell at a particular moment in time, raising questions about how to define a cell type. We suggest that these complexities and dynamics of cell states are of interest and further consider the roles signaling, stochasticity, cell cycle, and mechanical forces play in plant cell fate specification. Once established, cell identity must also be maintained. With the wealth of single-cell data coming out, the field is poised to elucidate both the complexity and dynamics of cell states. 

Abstract Arabidopsis leaf epidermal cells have a wide range of sizes and ploidies, but how large cells are spatially patterned alongside smaller cells remains unclear. Here, we demonstrate that the same genetic pathway that creates giant cells in sepals is also responsible for their formation in the leaf epidermis. In both sepals and leaves, giant cells are scattered among smaller cells; therefore, we asked whether the spatial arrangement of giant cells is random. By comparing sepal and leaf epidermises with computationally generated randomized tissues we show that giant cells are clustered more than is expected by chance. Our cell-autonomous and stochastic computational model recapitulates the observed giant cell clustering, indicating that clustering emerges as a result of the cell division pattern. Overall, cell size patterning is developmentally regulated by common mechanisms in leaves and sepals rather than a simple byproduct of cell growth. TeaserThe spatial pattern of giant cells becomes non-random as the surrounding cells divide.