Progress 11/15/00 to 11/14/04
Outputs The ABC model of flower development explains how three classes of floral genes specify the identity of the four whorls of the flower. Our initial study related to this grant focused on three closely related MADS-box genes, termed SEP1, SEP2 and SEP3. We found that simultaneous inactivation by mutation of all three genes in the sep1,2,3 triple mutant led to the replacement of petals, stamens and carpels by sepals. This triple mutant allowed us to conclude that these three SEP proteins are required for petal, stamen and carpel identity. We also found that these SEP genes were required to prevent the indeterminate growth of the flower meristem. Protein-protein interaction data by our group as well as others indicate that the SEP1,2,3 proteins are required for the activity of the B and C gene products. A model emerged in which high order complexes of MADS-domain proteins direct flower organ identity. The earlier studies by the Meyerowitz lab established that the ABC genes
are necessary for flower organ identity. One obvious question that stemmed from these observations was whether or not these genes were sufficient to convert normal vegetative leaves into flower organs. For example, the A and B genes together, according to the ABC model, are necessary for the formation of petals. Would ectopic expression of the A and B genes be sufficient to convert normal leaves into petals? The answer turned out to be no, indicating that one ore more factors (genes) must be missing to achieve this transformation. With our recent discovery of the SEP1,2,3 genes as important players in flower organ identity, we reasoned that these may represent the missing factors. Indeed, when we ectopically expressed the A and B genes, together with SEP, we found that all leaves were converted into petals. These results extend the previous findings by showing that the combined action of the A, B and SEP genes is sufficient to convert leaves into petals. More than 200 years ago Goethe
hypothesized that flower organs represent modified leaves. Support for this hypothesis was provided by the observation that ABC triple mutant produce flower organs that are all leaf-like and by the observation that ectopic expression of various combinations of the ABC genes, together with SEP, is sufficient to convert leaves into flower organs. We became interested in a gene that was closely related to the SEP1,2,3 genes and shared an overlapping expression pattern with the other SEP genes. We named this gene SEP4, and although sep4 single mutants appear similar to the wild type, sep1,2,3,4 quadruple mutant flowers produced flowers with leaf-like organs. Our recent studies extended our previous observations and indicate that the four SEP proteins have largely redundant activities required for the identity of all four organ types, sepals, petals, stamens and carpels. We also extended these studies by showing that the SEP proteins contribute to the earlier step of flower meristem
identity as well. Our USDA support also contributed to a number of additional studies, including the finding that AGL24 plays a key role in determining the time at which Arabidopsis plants flower.
Impacts Our studies have identified major regulators of flower development and I expect our findings will be incorporated into college level textbooks of plant development. Our studies will also catalyze related studies in diverse plant species.
Publications
- Pelaz, S., Gustafson-Brown, Kohalmi, S.E., Crosby, W.L., and Yanofsky, M.F. (2001) APETALA1 and SEPALLATA3 interact to promote flower development. The Plant J. 26: 385-394.
- Ng, M. and Yanofsky, M.F. (2001). Function and evolution of the plant MADS-box gene family. Nature Reviews Genetics2: 186-195.
- Pelaz, S., Tapia-Lopez, R., Alvarez-Buylla, E.R., and Yanofsky, M.F. (2001). Conversion of leaves into petals in Arabidopsis. Current Biology 11:182-184.
- Michaels, S.D., Ditta, G., Gustafson-Brown, C., Pelaz, S., and Yanofsky, M.F. and Amasino, R. (2003). AGL24 acts as a promoter of flowering in Arabidopsis and is positively regulated by vernalization. The Plant J. 33:867-874.
- Ditta, G., Pinyopich, A., Robles, P., Pelaz, S., and Yanofsky, M.F. (2004). The SEP4 gene of Arabidopsis thaliana functions in floral organ and meristem identity. Current Biology, 14:1935-1940.
- Yanofsky, Martin F., Pelaz, S., and Ditta, G. Combinations of genes for producing seed plants exhibiting modulated reproductive development. (2004). United States Patent No.: US 6,828,478,B2.
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