| Continuous organogenesis in plants is possible because of the specification of stem cells at the meristems. Genetic studies using Arabidopsis thaliana have isolated a multitude of mutants showing defects in meristem development. Identifying the mutated genes and elucidating how their products interact will be the key to understand how meristems function. Mutations at the CLAVATA (CLV) 1, CLV2 and CLV3 loci lead to the accumulation of stem cells in the shoot and flower meristems. CLV1 and CLV3 genes are specifically expressed in these meristems and encode a receptor-like kinase (RLK) and a putative ligand, respectively, indicating that these genes are involved in signal transduction events regulating stem cells. The CLV2 gene regulates both meristem and organ development. Genetic studies suggested that CLV1, CLV2 and CLV3 genes function in the same pathway for meristem development. I have isolated the CLV2 gene and found it encodes a receptor-like protein (RLP), with a presumed extracellular domain composed of leucine-rich repeats (LRRs), but with a very short predicted cytoplasmic tail. In addition, the chromosomal region in which CLV2 is located contains an extremely high rate of polymorphism. CLV2 is the first RLP shown to regulate development in plants. CLV2 mRNA is expressed in many tissues including the shoot apex and flowers. These results are consistent with a model in which CLV2 forms a signaling complex with CLV1 and CLV3. Analysis of predicted sequences show that plant RLPs and RLKs have several structural features for potential protein-protein interactions. I have shown that clv1, clv2, and clv3 flower defects are suppressed under short day (SD) growth condition, and that this is a true photoperiodic response regulated by genes in the long day (LD)-dependent flowering promotion pathway in Arabidopsis. Inactivation of the LD pathway is sufficient to suppress clv2 under LD and activation of the LD pathway restores the clv phenotypes under SD. These results reveal a novel role of photoperiod in flower meristem development in Arabidopsis. Analysis of the dominant negative alleles of clv1 under SD suggest that the SD suppression may be mediated by a CLV1-like protein specific to the flower meristem. |
