Ethylene induces stomata differentiation in Arabidopsis.

A common feature to multicellular organisms is the acquisition of different cell fates d~ring development. In this process, from an apparently homogeneous cell layer or cell mass, an array of different cell types is built into a differentiated tissue. The presence of root-hair cells and hairless cells in the root epidermis, and of trichomes, guard cells and pavement cells in the leaf epidermal layer of Arabidopsis is a clear examples that show the wide diversity of cell fates that can be generated during the development of the specialized epidermai tissue. It is not a new obseNation that the distribution of these specialised celi types in the epidermis is not at random, but that they follow instead an ordered arrangement which is remarkably conseNed. During the last years, a tremendous progress has been made in the genetic control of the developmental pathways that lead to trichome and root hair patterning and differentiation in Arabidopsis. In contrast, very little is known about the mechanisms that regulate stomata differentiation and spatial distribution in the leaf blade. Stomatal compiexes in many Brassicaceae species (the family which includes Arabidopsis) are anisocytic. This term applies to complexes of epidermai cells which include the two guard cells (the stoma proper), surrounded by three subsidiary cells. Both the guard cells and the subsidiary cells that integrate these complexes derive from a single protodermal cell (Paliwal, 1967; Pant and Kidwai, 1967). Equivalent information for the model plant Arabidopsis has been lacking. The presence of similar complexes in Arabidopsis leaves, together with the expression analysis of severai genes which are markers for cell division and cell identity, strongly suggests that a cell lineage may be associated with the development of stomatal complexes (Serna and Fenoll, in preparation). This fact imposes that stomata should always be separated by at least two non-guard cells, and could explain why stomatal complexes are regularly spaced in the epidermis. Recently, two mutants exhibiting clustered stomata in the cotyledons have been described with the self-explanatory names of too many mbuths (ftm) and four lips (fls) by Yang and Sack (1995). These mutants have opened the possibility for genetic dissection of stomata spacing. in a previous work, we described the induction by growth conditions of phenocopies of these mutants in wild-type Arabidopsis plants, and traced the ontogeny of stomatai clusters with molecular markers (Serna and Fenoll, 1996). Because these stomatai ciusters were induced in piants grown in different types of sterile cultures whereever gas exchange was limited, we suggested that endogenously produced ethylene could accumulate to high levels under these conditions and could be related to cluster formation.