On the cover: Multicellular organisms must control the cell number during development. The plant epidermis consists mostly of stomatal guard cells and pavement cells. During epidermal development, a fraction of protodermal cells are selected to be the meristemoid mother cells (MMCs), which initiate cell lineages producing all stomatal cells and a majority of non-stomatal pavement cells. In this issue, Hara et al. (pp. 1019–1031) report that the epidermal cell density is autoregulated by a secretory peptide, EPIDERMAL PATTERNING FACTOR 2 (EPF2), which is produced by the MMC and early descendants. The epidermal cell density was decreased in EPF2-overexpressors and was increased in epf2 loss-of-function mutant plants. The action point of EPF2 was delimited to a process of MMC formation. EPF2 requires TOO MANY MOUTHS (TMM) receptor-like protein, ERECTA (ER), ERECTA LIKE 1 (ERL1) and ERECTA LIKE 2 (ERL2) receptor kinases and YODA (YDA) mitogen-activated protein kinase (MAPK) kinase kinase, for its function. Thus the epidermal cell number is autoregulated by the EPF2-mediated feedback loop, and EPF2 is likely to be perceived by the putative receptor complex constituting TMM, ER, ERL1 and ERL2, and then the signal is transmitted through a MAPK cascade inhibiting the formation of the MMC. The cover picture shows that the EPF2 promoter–GFP signal (green) is present in the MMC and its early descendants.
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