Source: UNIV OF CALIFORNIA-SAN DIEGO submitted to
THE ABCD MODEL OF FLOWER ORGAN IDENTITY
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
TERMINATED
Funding Source
Reporting Frequency
Annual
Accession No.
0186858
Grant No.
2001-35304-09987
Project No.
CALR-2000-01554
Proposal No.
2002-04557
Multistate No.
(N/A)
Program Code
53.0
Project Start Date
Nov 15, 2000
Project End Date
Nov 14, 2004
Grant Year
2002
Project Director
Yanofsky, M. F.
Recipient Organization
UNIV OF CALIFORNIA-SAN DIEGO
9500 GILMAN DRIVE
LA JOLLA,CA 92093
Performing Department
BIOLOGY
Non Technical Summary
Easily the most recognizable accomplishment in plant developmental biology is the proposal of the landmark ABC model of flower organ identity, where the individual and combined activities of the ABC genes specify the identity of flower organs. We have characterized the closely-related SEP1, SEP2 and SEP3 MADS-box genes from Arabidopsis which share overlapping expression patterns early in flower development. Our data indicate that sep1 sep2 sep3 triple mutants display the striking phenotype in which all flower organs appear sepal-like. Thus, these three genes are functionally redundant and are required for petal, stamen and carpel development. The triple mutant appears indistinguishable from the previously described bc double mutants, suggesting that these genes are required for the activities of the previously described B and C genes. Based on these and other observations, we propose that the SEP1/2/3 genes encode a D-function that is active in the three inner whorls where it specifies organ identity by interacting with B- and C-functions. In the current proposal, we plan to directly test our revised ABCD model of flower organ identity by establishing the role of the SEP1/2/3 gene set in flower development.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
20624201050100%
Knowledge Area
206 - Basic Plant Biology;

Subject Of Investigation
2420 - Noncrop plant research;

Field Of Science
1050 - Developmental biology;
Goals / Objectives
To incorporate the roles of the SEPALLATA1-3 MADS-box genes into the widely accepted ABC model of flower organ identity. Identify genetic interactions that are responsible for the normal development of petals, stamens and carpels in flowers. To determine the genes sufficient to convert vegetative leaves into each of the flower organs. To provide a revised model that fully incorporates the roles of all known genes in specifying the identity of flower organs.
Project Methods
Single, double, and triple mutant combinations of the sepallata mutants will be combined with mutations in the previously described ABC organ identity genes by crossing. RNA tissue in situ hybridizations and reporter gene fusions will be analyzed in each of the mutant combinations produced. Genes that act downstream of the ABC organ identity genes will be analyzed by in situ hybridizations in the various mutant backgrounds. Transgenic plants that ectopically express each of the SEPALLATA and ABC genes will be produced by Agrobacterium-mediated transformation of intact plants and the various transgenes will be combined by crossing. PCR-based Reverse genetic screens will be used to obtain loss-of-function mutant alleles.

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.