Chair: Elliot Meyerowitz, California Institute for Technology
Detlef Weigel, Salk Institute, La Jolla, California.
USA
Vivian Irish, Yale University, New Haven, Connecticut,
USA
Beth Savidge, University of California San Diego, La
Jolla, California USA
Milo Aukerman, University of Wisconsin, Madison, Wisconsin,
USA
Guillermo Cardon, MPI fur Zuchtungsforschung, Koln,
Germany
Steven Clark, University of Michigan, Ann Arbor, Michigan,
USA
Weigel spoke about the functions of the meristem identity genes LEAFY and UNUSUAL FLORAL ORGANS in establishment of floral meristem identity, and in induction of the B function organ identity genes AETALA3 and PISTILLATA. Ectopic expression of LFY gives ectopic flowers (in the positions where shoots would be), and the flowers are of normal structure. 35S-LFY also causes early flowering in Arabidopsis (as measured by rosette leaf number), but in short days only. Remarkably, expression of LFY from a 35S promoter in Poplar, which normally flowers in five years, allows flowering in six months in noninductive long day conditions. Ectopic expression of UFO does not give ectopic flowers, and gives an abnormal flower phenotype in which sepals become petalloid and carpels, staminoid. This mimics the phenotype of uniform expression of B function genes, showing that UFO is sufficient to activate the B function organ identity genes.
Irish described experiments on the nuclear localization of the B function organ identity gene products, the AP3 and PI proteins. AP3-GUS fusion proteins, when ballistically introduced to onion cells attached to the 35S promoter, show GUS staining in cytoplasm. PI-GUS fusions act similarly. Delivering both genes to the same cells allows nuclear staining. The interpretation given is that an AP3-PI heterodimer localizes to the nucleus, while each protein alone does not. If true, this would represent a new mechanism of nuclear localization in plants and an additional level of control of the B function genes.
Beth Savidge described the expression of the CAULIFLOWER and APETALA1 genes, and the consequences of mutations in them. One consequence is that, in an ap1 cal double mutant, AG is not activated. In either single mutant it is. This indicates a redundancy in the function of AP1 and CAL for the induction of AG. 35S-CAL and 35S-AP1 plants have an apical terminal flower, mimicking the tfl mutant phenotype, and showing that either gene alone is sufficient for converting an inflorescence meristem to a floral meristem.
Milo Aukerman summarized the complex interactions of the late and early flowering genes, and described the molecular cloning of the LUMINIDEPENDENS late flowering gene. It codes for a novel protein, and when fused to GUS, the GUS activity is nuclear. The LD promoter fused to GUS shows that the gene is active in young but not old leaves, and is always on at the shoot apex. Epitope tagging of the protein allowed quantitation, and showed that is on at higher levels in floral apices that in seedling apices.
Guillermo Cardon described work in snapdragon that led to the cloning of two zinc finger proteins that bind upstream of the SQUAMOSA gene, the snapdragon orthologue of APETALA1. Six similar Arabidopsis genes have been cloned, at least three of these bind to the AP1 promoter. One of these, SPL3, is developmentally regulated and comes on before AP1 as assayed by RNA blots. The other two (SPL1 and 2) are constitutively active. 35S-SPL3 plants are early flowering, and also can have bracts under the first flower. It seems plausible then that SPL3 is a genuine upstream regulator of AP1.
Steven Clark described the CLAVATA genes CLV1 and 3, showing that they act to suppress excess cell division in shoot and floral meristems. Genetic interactions between clv1 and clv3 indicate that they act in the same process (e.g. clv1/+ clv3/+ plants show a clv mutant phenotype; clv1/clv1 clv3/clv3 double mutants look no different than either single mutant). clv mutations act to partially suppress SHOOT MERISTEMLESS mutations, indicating that one function of the CLV loci is to act opposite to STM, such that CLV represses meristem cell proliferation (or enhances cell differentiation) and STM promotes cellular proliferation (or acts against differentiation). The molecular cloning of the first of CLV loci, CLV1, was described; it appears to be a transmembrane receptor ser/thr kinase.