Teosinte branched and the evolution of development

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Doebley 9513573 teosinte branchedl (tbl ) is a recessive mutant of maize that conditions the plant to have long lateral branches tipped by tassels at the upper nodes and many tillers at the basal nodes. As its name implies, tbl mutant plants show a striking similarity to teosinte, the wild progenitor of maize. Recently, my laboratory determined that tbl was involved in the evolution of maize plant architecture (short lateral branches tipped by ears) from teosinte plant architecture (long lateral branches tipped by tassels). Our model for tbl is that it acts as a repressor of axillary branch development such that the maize allele (Tbl +Maize) engenders strong repression (short branches) and the teosinte allele (tbl +teosinte) weak repression (normally long branches). This model further suggests that tbl +teosinte is environmentally regulated so that the extent of branch elongation in teosinte is a function of the degree of shading from surrounding vegetation. The evolution of maize would have involved a change in tbl expression from environmentally regulated expression to constitutive expression. Recently, my laboratory has molecularly cloned tbl using the Mutator transposable element system. The proposed research will utilize this tbl clone to test the proposed model for tbl and to elucidate the nature of the molecular changes in tbl that were responsible for the evolution of maize from teosinte. First, the molecular structure of tbl including nucleic acid sequence, intron-exon borders, and homology to other known genes will be determined. The sequences of 10 maize and 1() teosinte copies of tbl will also be determined. Phylogenetic and visual comparison of this sample of maize and teosinte alleles will allow the identification of sequence motifs that are found only in maize and not in teosinte. These motifs will be candidates for the molecular event(s) responsible for the functional difference between Tbl +Maize and tbl +teosinte. Second, the pattern of expression of Tbl +Maize and tbl +teosinte will be analyzed to determine if and how the expression of these alleles differs. Under the proposed model, the expression of tbl +teosinte should be influenced by the degree of shading, while Tbl +Maize should be strongly expressed under all environments. Third, the phenotypic effects of the different tbl alleles will be determined. Tests for epistasis between tbl and two other loci thought to be involved in the evolution of plant architecture in maize will be made. The evolution of development has been identified as a focus for developmental biology over the next decade. The proposed research will make significant contributions to the understanding of the evolution of development by assessing how changes in gene structure and expression alter the normal course of plant development leading to the evolution of new adult forms. The proposed research will also make contributions to plant developmental biology and biotechnology. This research will provide for the cloning and structural characterization of an important gene involved in maize development. The cloning and characterization of tbl will provide new tools for the analysis of meristem fate and apical dominance, processes of long-standing interest in plant development. Finally, since increased apical dominance is a common feature of plant domestication including all cereals, many tree species and herbs such as sunflower, the possibility exists that engineering of tbl will enable new advances in crop or tree improvement. 3