Search

The shoot meristem (SM) in higher plants is responsible for all above-ground organ formation. The SM maintains a population of stem cells and directs progenitor cells towards differentiation and organ formation. I have used Arabidopsis thaliana as a model system to study SM function. Previous work had shown that the CLV loci promote cell differentation at the shoot meristem, and that clv mutants accumulate stem cells. Mutations in CORONA (CNA) were identified in genetic screens for modifiers of the Clv- phenotype. cna clv double mutant plants, when compared to clv single mutants, exhibit dynamic and dramatic changes in SM structure as they proceed through development. These changes include an often massive enlargement of the shoot meristem. This clv cna enlargement differs from that observed in clv single mutants in that most of the additional cells are not stem cells. This is based on three lines of evidence. (1) The SMs of clv cna plants fully differentiate into carpelloid structures at approximately 4 weeks. (2) clv cna shoots lose the ability to initate new organs early in development. (3) clv cna shoots show variable but early loss of expression of key meristem regulators WUS and CLV3. The CNA gene was isolated by positional cloning and encodes a member of the plant-specific homeodomain - leucine zipper III class of transcription factors. The lesion found in the dominant-negative cna-1 is a missense mutation in a conserved domain of unknown function. The null cna-2 allele contains a T-DNA insertion in the THIRD exon. CNA is expressed in developing vascular tissue, as well as diffusely through developing SMs and flower meristems. The primary defect in clv cna plants remains to be determined. Perhaps clv cna plants are unable to maintain a population of stem cells resulting in premature organ termination and eventual differentiation. Alternatively, premature organ termination might lead to the subsequent breakdown of meristem identity. Whichever mechanism best describes the function of CNA, it is a novel component of meristem development representing a new pathway likely regulating the balance between stem cell identity and differentiation.