XVI International Botanical Congess

Previous flp mutations have clusters of two, laterally-aligned stomata. A new severe allele, flp-7, has larger elongated clusters. Cluster development was studied by light and electron microscopy. Wild-type (WT) guard mother cells (GMCs) exhibit division site wall thickenings and they divide symmetrically to produce two guard cells (GCs). Differentiating GCs develop a lens-shaped wall thickening that forms the pore. In flp, the first-formed GMC appears normal but produces daughter cells which develop into supernumerary GMCs (sGMCs) instead of GCs. SGMCs have division site thickenings instead of pore thickenings. In weak alleles two sGMCs usually form and divide, and the daughter cells differentiate into GCs. In flp-7, additional generations of sGMCs are produced resulting in larger clusters. Because these loss-of-function mutations allow additional cell cycles of the GMC, the FLP gene product may (1) terminate the GMC program, (2) promote GC specification, or (3) receive signals to switch off the cell cycle after GMC division.