Regulation of stem cell patterning and activity in Citrus

Yale University, New Haven CT

Investigators

Abstract

Non-technical paragraph: Plants grow by the action of stem cell populations, which give rise to branches, leaves and flowers in a highly regulated pattern around the axis. This pattern is called phyllotaxy. While most plant species show spiral phyllotaxy, citrus species present an unusual variation on phyllotaxy; instead of single branch primordia arising in a spiral pattern, many citrus species instead produce two adjacent primordia with differing developmental potentials. One primordium develops into a typical branch, while the other primordium differentiates as a thorn. Previous work has shown that the branch and the thorn primordia regulate stem cell activity in different ways. The goals of this project are to understand how this unusual phyllotactic pattern is formed in citrus, and to examine the related issue of how differences in stem cell activity are regulated in branch versus thorn primordia. The arrangement of leaves, branches, and flowers is a key component of crop yield. As such, carrying out this work in citrus will support the bioeconomy through knowledge of the mechanisms controlling growth and yield of this economically important crop. In addition, this work has the potential to provide new tools for the engineering of this fruit commodity, as well as providing information on mechanistic aspects of plant growth and development more broadly. This project will also contribute to the career development of undergraduate, graduate student and postdoctoral researchers, as well as provide a means for outreach to the local New Haven area community and underserved high school students. Technical paragraph: The plant hormone auxin is transported in a directional manner and auxin activity at the sites of primordia initiation is thought to define both the pattern of phyllotaxy and organ outgrowth. Understanding how auxin signaling interacts with other signaling components to define two different developmental outcomes, the thorn versus the branch primordium, will provide insight into stem cell patterning processes and how they translate into organismal forms. The proposed work includes assessing auxin distribution and activity in Citrus, as well as multiple experimental approaches to perturb auxin action in Citrus. These experiments will be complemented by genome wide evaluation of the thorn versus branch transcriptome, as well as an assessment of the genome-wide binding sites of THORN IDENTITY1 (TI1), encoding a CYC/BRC1 class TCP transcription factor, that functions to specify thorn identity through binding to the promoter of the stem cell regulator WUS and downregulating its expression. Collectively, the proposed experiments will help to define how auxin signaling influences the decision to produce a thorn versus a branch primordium, and how such signals interface with the components of the gene regulatory network regulating thorn versus branch identity. Defining the mechanisms regulating the interplay between auxin signaling and stem cell activity should help to reveal new regulatory targets that could be genetically manipulated to alter plant architecture and influence crop productivity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.