Homeobox Gene Regulation During Lateral Organ Development

Cold Spring Harbor Laboratory, Cold Spg Hbr NY

Investigators

Abstract

0214705 Timmermans Organogenesis in higher plants is primarily post-embryonic in that lateral organs of the shoot, such as leaves and flowers, develop progressively from the shoot apical meristem (SAM). The mechanism that distinguishes organ founder cells from stem cells or their derivatives in the SAM is largely unknown, but the regulation of knotted1-like homeobox (KNOX) genes appears to be a key determinant. The rough sheath2 (rs2), ASYMMETRIC LEAVES1 (AS1), and PHANTASTICA (PHAN) genes from maize, Arabidopsis, and Antirrhinum, respectively, encode highly conserved MYB domain proteins that are required to repress KNOX genes during leaf development. Furthermore, the Arabidopsis KNOX gene SHOOT MERISTEMLESS (STM) negatively regulates AS1. The regulatory hierarchy between the KNOX and PHAN-related genes may thus provide a simple mechanism for distinguishing founder cells from stem cells and their derivatives in the SAM, simply by regulating the expression domain of the PHAN-related genes. The mechanism by which the PHAN proteins regulate KNOX genes is not currently understood. The variegated knox expression pattern in rs2 null-mutant leaves suggests that other factors in the meristematic cells of the SAM are required to control knox gene expression. Several RS2 interacting proteins have previously been identified using the yeast two-hybrid system. Based on their homology with known proteins from yeast and mammals, these interacting proteins can be reconciled with a model in which rs2 is required for the assembly of a transcriptional repressor complex that maintains knox genes silenced during leaf development. In order to dissect the mechanisms that control knox gene expression, the RS2 and AS1 complex as well as the origin of the variegated KNOX phenotype in rs2 mutants will be analyzed. The specific aims are: 1) To analyze the basis underlying the variegated knox expression pattern in rs2 mutants. 2) To characterize the RS2 complex from maize. 3) To begin analysis of the AS1 complex from Arabidopsis. The proposed research will broaden our understanding of the process that distinguishes founder cells from stem cells and their derivatives in the SAM. It will also contribute to our understanding of the role of gene silencing mechanisms in development. The preliminary data indicates that an as of yet unknown silencing mechanism in plants that involves the transcriptional co-repressor HIRA may play a central role.