Axis-dependent Regulation of Lateral Organ Development in Plants

Summary Leaves and floral organs are known as lateral organs formed from meristem at the top of shoots. Distinct from other plant organs, such as stems or roots, the lateral organs are flat with two faces, the adaxial side and the abaxial side. The structural principle of the lateral organs suggests that their development is dependent on three crossing axes, the apical-basal axis, the adaxial-abaxial axis and the lateral axis, although the molecular nature of the axes is not known. Recent studies of Arabidopsis mutant show a couple of examples that putative axes control the expression pattern of genes working in the spatial specification of lateral organs. One of the genes, FILAMENTOUS FLOWER (FIL), a member of the YABBY/FIL gene family encoding a protein with a zinc finger and HMG-related domains, is involved in the specification of the abaxial side of lateral organs. FIL gene expression was restricted at the abaxial side of the lateral organ primordia, suggesting that FIL gene expression is under the control of the putative adaxial-abaxial axis in the lateral organ primordia. Another gene, PRESSED FLOWER (PRS), is a member of the horneobox gene family. Loss of function mutant of PRS lacks two sepals at lateral positions. Two sepals at the adaxial and the abaxial positions are present, but the marginal cell files of the remaining sepals are missing. PRS expression is restricted at the lateral regions of flower primordia and of lateral organs. The expression patterns of PRS strongly suggest that it is controlled by the putative lateral axis formed in the primordia. During development of floral meristems, the axis-dependent expression of FIL and PRS are transiently reduced at stage 2, a stage just before the floral organ primordia appear. Their expression recovers after stage 3, but the expression pattern is different before and after the tentative reduction, suggesting that the center of axes formed in the primordia shifted from the inflorescence meristem to the floral meristem. This shift of the axis center suggests the timing when the floral meristem acquires independence from the inflorescence meristem.