The procambium/cambium is the stem cell population that controls the cell division and differentiation of plant vascular tissues, conduits of water and nutrients. The extent of vascular cell division and differentiation from the procambium/cambium determines diverse growth forms of vascular plants, such as trees, shrubs, and herbs. Considering these various growth forms are essential for plants to grow and reproduce in dynamic environmental conditions, a thorough understanding of genetic regulation operating in the stem cell population for vascular tissue formation is very important. Arabidopsis root is an ideal model system for studying gene regulatory networks in the procambium/cambium because of its feasibility of tracking cell types and extensive molecular information and tools. Newly developed genome-wide cell type specific expression data and identification of an important vascular stem cell regulatory pathway regulated by two transcription factors, SHORT ROOT (SHR) and PHABULOSA (PHB), provide an excellent starting point for investigating the stem cell regulatory networks in root vascular tissue development. In this project, the downstream regulatory pathways and procambium functions led by SHR and PHB will be identified by employing systematic approaches that combine molecular genetics, genomics, and systems biology. Investigation of the fundamental regulation for vascular tissue production, the main source of future renewable energy, will have a broad impact on other fields of science and industry, such as agriculture and bioenergy. The proposed research will integrate training post-doc, graduate students, and undergraduate students, together forming a team. The research topics will educate the next generation of scientists who can address interdisciplinary scientific problems with the most up-to-date knowledge and skills. Results of the project, including the collection of the gene expression data, mutant lines, and cell type specific GFP lines, will be disseminated via scientific publications and a website (www.bti.cornell.edu/JiYoungLee.php).
Root is a plant organ essential for the plant growth and survival in diverse terrestrial environments. Roots absorb water and nutrients from soil and redistribute them to the other body parts through vascular tissues. They also provide physical support to plants by anchoring to the soil. To achieve these, roots grow deep into the underground and branch out laterally. The stem cell population at the tip of roots serves as a reservior that generates new cells as building blocks for growth. A comprehensive understanding of the mechanisms underlying root growth and vascular tissue development could lead to developing crop plants with a more efficient usage of water and fertilizers.Under this NSF support, we made three key discoveries that reveal the mechanisms underlying root growth and vascular tissue development. First, we identified the regulatory mechanism of small microRNAs that serve as an intercellular signal for development of a water conducting vascular tissue, xylem (Figure). These microRNAs modulate target transcription factors by binding to their messenger RNAs and leading their degradation. Target transcription factors under microRNA regulation in turn promote xylem development in a dosage-dependent manner. Second, we found that the aforementioned microRNAs and their target transcription factors also regulate root growth. They do this by affecting the action of a plant hormone, cytokinin. Lastly, we found a novel regulatory program that regulates the formation of a sugar conducting vascular tissue, phloem. This involves a transcription factor that travels from one cell to the other. In summary, we uncovered the identities of important cell-to-cell communications that direct root growth and vascular tissue development.Under this NSF support for three years, the PI trained one postdoc, one graduate student, five undergraduate students and one high school student. The PI introduced root biology and its impact on agriculture and environment to high school teachers in the curriculum workshop hosted by the Boyce Thompson Institute.
