Our research program is focused on understanding one of the most remarkable manifestations of plant evolution – the diversity of floral branching systems known as inflorescences, which are the foundation for flower production and crop yield. Inflorescences originate from stem cells at the growing tips of shoots called apical meristems (AM). Inflorescences can be simple, producing only a single flower, or they can be highly complex, producing dozens of branches and hundreds of flowers. Beyond genetic frameworks from only a few model systems, little is known about the genes and networks that control inflorescence complexity and flower production. We are studying plants that show ‘sympodial growth’, which defines more than half of all flowering plants, including vines, trees, and many crops. We are taking advantage of developmental, genetic, molecular, and genomic tools in tomato (Solanum lycopersicum) to understand how sympodial growth gives rise to many multi-flowered inflorescences throughout life, and what mechanisms are responsible for inflorescence variation in the larger nightshade (Solanaceae) family. As the Solanaceae includes many important crops, such at pepper and potato, our research addresses questions that are relevant to evolution as well as agriculture.
Tomato is an excellent system to study sympodial growth, because meristems are large and easily accessed for morphological and molecular analyses. Tomato vegetative and inflorescence shoot architecture depends on continuous cycling between meristem termination and renewal. Following germination, the primary shoot meristem (PSM) from the embryo produces 8-13 leaves before terminating in the first flower of the primary inflorescence. A sympodial vegetative meristem (SYM) forms in the axil of the last PSM leaf and develops three leaves before terminating in the first flower of the next inflorescence. This process continues indefinitely from the last leaf of each SYM to produce a compound shoot with multiple inflorescences. Each inflorescence develops from a sympodial inflorescence meristem (SIM) that forms just below the PSM and each SYM as they terminate in flowers. This SIM gives rise to one new SIM before quickly maturing into a flower, and several iterations of this process results in a multi-flowered inflorescence. Canonical axillary meristems from lower leaves recapitulate the PSM. Thus, tomato plants can develop hundreds of multi-flowered inflorescences, which in turn, drive fruit production and yield.
Some of the most extensive variation in inflorescence production and architecture is found in the Solanaceae family. There are species whose inflorescences are composed of a single flower (e.g. gooseberry: Physalis peruviana), whereas others are highly branched and produce dozens of flowers (e.g. Chilean potato tree: S. crispum). In between are species with 2-3 flowers on each inflorescence (e.g. passion berry plant: S. cleistogamum), tomato-like inflorescences (e.g. American black nightshade: S. americanum), and inflorescences that make a few branches (e.g. the wild tomato S. peruvianum). By taking advantage of a unique germplasm resource comprised of tomato mutants and Solanaceae species representing a continuum of variation in vegetative and inflorescence shoot architecture, we are testing the hypothesis that differences in the heterochronic regulation of meristem maturation, including both the identity and timing of expression of key regulatory factors, explains the evolution and agricultural selection of variation in inflorescence production, branching, and flower production in sympodial plants