CRABS CLAWandSPATULAGenes Regulate Growth and Pattern Formation during Gynoecium Development inArabidopsis thaliana

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Abstract
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 detail, plotting their time course during gynoecial development, mapping disruptions to xylem lignification, and testing their effects on fertilization. In spt mutants, defects were first seen soon after the gynoecial tube started to elongate. Medial regions where carpels adjoin grew slower than in wild type and were often unfused later at the apex. Development of the septum was severely disrupted, and extracellular matrix‐secreting transmitting tract was not seen in null mutant lines. Even so, some pollination was observed. The amount of stylar and stigmatic tissue was also reduced, and vascular development in medial and stylar regions was disrupted. SPT apparently plays a role in promoting the development of all specialized tissues from carpel margins. In crc mutants, defects were very different. They were seen from the inception of gynoecial development and characterized by additional cells arising across the width of the gynoecium but fewer cells in the longitudinal dimension. In addition, cells were larger on average. All cell types arose in crc mutant gynoecia, but they seemed to differentiate earlier. Mutant gynoecia were always unfused in apical regions, and the medial vasculature was again disrupted. CRC may normally restrain lateral cell division but promote longitudinal division, ensuring the gynoecium adopts an elongated linear form. In crc spt double mutants, nonadditive disruptions were present, with the carpels even shorter, much less fused, and lacking stylar xylem elements. These interactions may be secondary, however, as the expression patterns of the two genes do not overlap. Disruptions in both spt and crc mutants match closely the time and place of expression of the wild‐type genes, indicating that each acts cell autonomously. We have integrated these findings with those of other known regulatory genes to propose a general model of growth and pattern formation in the developing gynoecium. In an initial “neogenic” phase, lateral and medial regions are defined. Meristematic genes (including SPT) are active in the medial zone, maintaining its pluripotent potential, while growth and polarity genes (including CRC) are active 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 subregions, with septum and placentae developing internally and the replum developing externally. At the same time, the lateral regions mature into the tissues of the ovary wall.