Session 6: Reproduction II

Chair: E. Meyerowitz, Caltech, Pasedena, USA

email:meyerowitze@starbase1.caltech.edu
The first speaker was Marty Yanofsky (U.C. San Diego), who spoke about AP1, CAL and LFY, and about new data on the AGL MADS box genes. Among the many new data, he pointed out that the initial expression of AP1 is delayed in lfy mutants, indicating that some AP1 activities are downstream of LFY. This is consistent with the p35S-LFY phenotype being attenuated in ap1 mutants, but the p35S-AP1 phenotype not being attenuated in LFY mutants. Yeast 2-hybrid screens to identify CAL and AP1 interacting proteins found AGL9, 20, 22 and 24; the expression of all of them overlaps that of AP1 and CAL. Yanofsky also discussed AGL5, with carpel-specific expression that requires upstream AG activity. p35S-AGL5 was shown to rescue carpel development in ag mutants, resulting in female fertility; in addition, petals are partially converted to stamens in the transgenic plants. AGL5 can thus act as a C function organ identity gene.

I spoke next (E. Meyerowitz, Caltech), and described three different modes of control of the pattern of cell division in developing flowers. One is mediated by the SUP zinc-finger gene, which seems to act in the third whorl to repress cell divisions, and nonautonomously act in the fourth whorl to increase cell division. sup mutants thus have extra stamens, and a reduced fourth whorl. The second is mediated by CLAVATA1, CLAVATA3 and interacting genes such as SHOOT MERISTEMLESS. This is control of the overall number of cell divisions in floral meristems. CLV1 and CLV3 act to repress cell division, their mutants have extra cells and extra floral organs. The molecular cloning of CLV1 shows it to be a leucine-rich-repeat transmembrane receptor kinase. The third mode of cell division control is on the spacing of cell divisions during floral organ formation. the PERIANTHIA gene regulates this; recessive mutants in PAN cause closer spacing of organs, and thus extra floral organs. PAN codes for a basic-leucine zipper protein.

Justin Goodrich (Univ. of Edinburgh) followed, and described the CURLY LEAF gene. clf mutants have curled leaves, carpelloid sepals and staminoid petals; all as a result of ectopic expression of AG. The gene codes for a plant homologue of the Drosophila Enhancer of Zeste protein, a member of the Polycomb group of proteins that act in flies to maintain repression of homeotic selector genes. The plant homologue seems to have a similar function, despite the fly homeotic genes being homeobox proteins, while the plant ones are MADS box proteins.

George Haughn (U. British Columbia) spoke about the cloning of UNUSUAL FLORAL ORGANS, a gene affecting floral meristem identity, floral organ formation, and organ identity. UFO codes for the Arabidopsis homologue of the Antirrhinum FIMBRIATA gene. A yeast 2-hybrid screen with UFO as bait gave a gene called UIP1, which codes for a plant protein related to mammalian p19, involved in cell cycle control. In situ hybridization showed UIP1 to be expressed in dividing cells, a domain that overlaps with that of UFO expression. UFO also shows some similarity to the mammalian p45 protein in a 45 amino acid stretch; p45 interacts with p19. The UFO phenotype may therefore result from loss of cell division in part of the developing flower. A model in which SUP (see above), UFO and LEUNIG all control different aspects of cell division in flowers was presented.

Bob Elliot (Monash Univ., Melbourne) spoke about the AINTEGUMENTA gene. Mutants give reduced floral organ number, reduced integument growth, and abnormal embryo sac development. ANT was cloned and shown to be a member of the AP2 family of DNA binding proteins, with two AP2 domains. The gene is expressed in floral organ primordia, cotyledon primordia, leaf primordia (though cotyledons and leaves are normal in ant mutants), and in developing carpels and ovules. Mosaic analysis shows that ANT in the L1 layer appears to act to normalize development of the L2-derived embryo sac.

David Twell (Univ. Leicester) discussed asymmetric cell divisions in male gametophyte development, by which the pollen mother cell divides unequally to give a large vegetative cell and a small generative cell. Using colchicine to make this division symmetric in cultured microspores prevents inactivation of a generative cell-specific promoter in the vegetative cell, showing that the asymmetry is causal in the functional differences shown by the two cell types. A screen in Arabidopsis for pollen mitosis I cell division mutants was described, with several promising classes of mutations resulting.

Luo Ming (CSIRO, Canberra) described mutations that allow seed development without prior fertilization (FIS, fertilization independent seeds). The mutant screen was to start with self-sterile pistillata homozygotes, and to look for mutant progeny that had some degree of silique elongation. Several different loci (fis1, fis2 and fis3, on chromosomes 1, 2 and 3, respectively) were identified. Unfertilized fis mutant seeds have deformed embryo-like structures and form endosperm. The mutations are embryo lethals even when fertilized, and at least one of them is a gametophyte-effect mutant.