The Big Picture

Our research program is focused on understanding one of the most remarkable manifestations of plant evolution - the diversity of floral branching systems called inflorescences. Inflorescence branching is determined by specialized groups of pluripotent stem cells at growing tips called shoot apical meristems (SAM), which undergo reproductive ‘phase transitions’ in response to environmental cues. 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 how reproductive phase transitions drive inflorescence complexity. In particular, one of the most fundamental, yet poorly understood, decisions is whether an inflorescence grows indefinitely (indeterminate growth) or if signals are received to end growth (determinate growth). Determinate plants produce the most elaborate inflorescence architectures, particularly among species representing the ‘sympodial’ growth habit, which defines more than half of all flowering plants, including vines, trees, and several crops. We are taking advantage of developmental, genetic, molecular, and genomic tools in tomato (Solanum lycopersicum) to understand how sympodial growth is controlled to give rise to the tomato multi-flowered zigzag inflorescence, and what mechanisms are responsible for inflorescence variation in the larger nightshade (Solanaceae) family. As the Solanaceae includes well-known crop plants, such at potato, pepper, and petunia, our research addresses questions that are relevant to evolution as well as agriculture.

In a related project, we are exploring why inter-crossing different inbred plants often creates hybrids that produce more flowers, fruits and seeds than their parents. This increased vigor in hybrids, known as heterosis, is widespread in nature and is a driving force behind agricultural productivity and advanced crop breeding efforts. We are testing the hypothesis that heterosis can be caused by subtle changes in gene dosage originating from heterozygous single gene mutations affecting growth and development. In a screen involving dozens of tomato heterozygous mutants, we have identified three heterosis genes, including a reproductive transition gene and an inflorescence branching gene. We are now exploring the role of pleiotropic dosage effects of these genes on plant development as a potential explanation for this type of heterosis.

Sympodial growth in tomato.

Tomato is a powerful model to study mechanisms of sympodial reproductive transitions because it is composed of three distinct shoot systems, each of which undergoes a termination event. The first termination originates from the embryonic meristem, which produces ~8 leaves before ending growth with a multi-flowered inflorescence. The next termination is in the sympodial shoot (SYM), which grows out from the last axillary meristem produced from the primary shoot. All SYMs develop three leaves before producing a final inflorescence, and all upright growth after the primary transition is derived from the indefinite reiteration of SYMs. The third termination occurs in the inflorescence, which produces ~7 flowers in a zigzag arrangement. The inflorescence is also compound, consisting of reiterating sympodial inflorescence meristems (SIM). Finally, each axillary meristem produces primary, sympodial and axillary shoots, and inflorescences. Thus, tomato growth is modular and numerous reproductive transitions throughout life give rise to a plant with hundreds of branches, inflorescences, and flowers.

The Big Picture Continued...