Higher plants initiate new organs (e.g., leaves) throughout their life. Thus they face two problems. Plants must maintain a pool of undifferentiated cells from which to draw upon for organ initiation, and must balance the proliferation of these undifferentiated cells and their incorporation into organ primordia. In other words, given a constant number of undifferentiated cells, the cell division rate of the undifferentiated cells must equal the rate at which these cells are directed toward organ formation. These processes occur at a structure termed a meristem. In the plant embryo, two meristems are established: the root meristem, which will give rise to the root system, and the shoot meristem, which will give rise to the above-ground portion of the plant. At the growing tip of the shoot meristem, cells in the very center are maintained in an undifferentiated state, while cells surrounding this central region are directed toward organ formation. The goal of the proposed research is to provide a better understanding of the genes and processes directing organ formation at the shoot meristem. While the proposed project is basic science, any applied projects to alter the number, spatial patterning or identity of plant organs will require an understanding of the shoot meristem. The CLAVATA loci (CLV1, CLV2, CLV3) function in the same pathway to promote organ formation at the shoot and flower meristem based on loss-of-function phenotypes. The CLV1 and CLV2 genes have been cloned and appear to code for receptors involved in signal transduction. During the previous funding period the CLV1 gene was isolated and its expression pattern, which is tightly restricted to the shoot and flower meristem, was determined. CLV1 becomes ectopically expressed in mutants that develop ectopic or enlarged meristems. Constitutive expression of CLV1 by the 35S promoter was not possible, apparently due to the toxicity of CLV1 during early stages of development. CLV1 could be expressed, however, under control of the flower-specific AP1 promoter, and this rescues the clv1 mutant phenotype, but does not affect wild-type development. It was also determined that the primary defect in clv1 and clv3 mutants is in organ formation, not cell division. Analysis of clv2 mutants revealed that CLV2 regulates both meristem and organ development. CLV2 functions in the same pathway as CLV1/CLV3 to regulate meristem development, but in a separate pathway to regulate organ development. The requirement for CLV2 is very sensitive to physiological conditions. Mutations in a number of novel regulators of meristem development were isolated. The goal in the current funding period is to generate inducible ectopic activation of CLV1. This will allow questions of CLV1 function to be addressed, and will allow the characterization of potential targets of CLV1 signal transduction. The process of suppression of clv2 flower phenotypes under short day photoperiod conditions will be investigated. The investigations will determine whether this is a response to day length or growth rates, and what genes are involved. CLV2 will be expressed under various tissue-specific and inducible promoters to address questions of CLV2 function. Finally, a mutation in a novel regulator of meristem development, corona (cor), will be studied. The cor mutant was identified as a strong enhancer of the clv1 organ formation defect. cor clv1-1 double mutants completely lose the ability to initiate new organ primordia shortly after the transition to flowering. Preliminary genetic evidence suggest that COR promotes organ formation in a separate pathway from the CLV loci.
