Researchers have identified the gene that controls the distribution of stomatal cells on leaves. These cells, named after the Greek word stoma, meaning mouth, are key components for the healthy growth of all plants. Stomatal cells function as pores on the underside of leaves and control the entry of gases into the leaf for photosynthesis. Normally, stem cells in the Arabidopsis epidermis will divide asymmetrically into a smaller daughter cell, which eventually form stomata, and a larger daughter cell. Signalling between cells tells the smaller cell to move away from existing stomata, insuring a fairly even distribution of stoma across the leaf surface. The new research shows that mutation of the Too Many Mouths (TMM) gene in Arabidopsis causes stomatal cells to increase in number and clump together, rather than maintain their characteristic spacing. ‘Genes like TMM exist in crop plants such as rice,’ Fred Sack, professor at the University of Ohio, said. ‘If TMM has the same function in crop plants that it does in Arabidopsis, then it could be a key regulator.’ (Nadeau, J.A. and Sack, F.D. [2002] Science 296, 1697–1700) PL