Crabs claw and Spatula genes regulate growth and pattern formation during gynoecium development in Arabidopsis thaliana

Development of the gynoecium of Arabidopsis is disrupted in mutants of the regulatory genes SPATULA (SPT; a basic helix-loop-helix family member) and CRABS CLAW (CRC; a YABBY family gene). We have defined the disruptions in d etail, plotting their time course during gynoecial development, mapping dis ruptions to xylem lignification, and testing their effects on fertilization . In spt mutants, defects were first seen soon after the gynoecial tube sta rted to elongate. Medial regions where carpels adjoin grew slower than in w ild type and were often unfused later at the apex. Development of the septu m was severely disrupted, and extracellular matrix-secreting transmitting t ract was not seen in null mutant lines. Even so, some pollination was obser ved. The amount of stylar and stigmatic tissue was also reduced, and vascul ar development in medial and stylar regions was disrupted. SPT apparently p lays a role in promoting the development of all specialized tissues from ca rpel margins. In crc mutants, defects were very different. They were seen f rom the inception of gynoecial development and characterized by additional cells arising across the width of the gynoecium but fewer cells in the long itudinal dimension. In addition, cells were larger on average. All cell typ es arose in crc mutant gynoecia, but they seemed to differentiate earlier. Mutant gynoecia were always unfused in apical regions, and the medial vascu lature was again disrupted. CRC may normally restrain lateral cell division but promote longitudinal division, ensuring the gynoecium adopts an elonga ted linear form. In crc spt double mutants, nonadditive disruptions were pr esent, with the carpels even shorter, much less fused, and lacking stylar x ylem elements. These interactions may be secondary, however, as the express ion patterns of the two genes do not overlap. Disruptions in both spt and c rc mutants match closely the time and place of expression of the wild-type genes, indicating that each acts cell autonomously. We have integrated thes e findings with those of other known regulatory genes to propose a general model of growth and pattern formation in the developing gynoecium. In an in itial "neogenic" phase, lateral and medial regions are defined. Meristemati c genes (including SPT) are active in the medial zone, maintaining its plur ipotent potential, while growth and polarity genes (including CRC) are acti ve in the lateral zones, providing the valve initials with the "competence" to support medial tissue. Later, in a "determination and differentiation" phase, the medial zone is genetically divided into differentiating subregio ns, with septum and placentae developing internally and the replum developi ng externally. At the same time, the lateral regions mature into the tissue s of the ovary wall.