Colorado State University

Although plants are photoautotrophic organisms, they are composed of many heterotrophic tissue systems, such as roots, flowers, seeds, and developing leaves, that depend on carbon and nitrogen import for growth and development. In general, sucrose and amino acids are transported to the heterotrophic cells from mature leaves. This process is known as assimilate partitioning and it is a fundamental activity that allows plants to function as multicellular organisms. My laboratory provided the first biochemical and molecular descriptions of several plant sugar and amino acid transport systems that are key contributors to resource allocation within cells and between organs. We initially described these transporters using an in vitro biochemical assay that allowed us to define the transport properties and bioenergetics of these important carriers. To identify the genes encoding the plant's sugar and amino transporters, we used functional complementation of yeast transport mutants with plant cDNA expression libraries (PNAS 90:7441-7445). The yeast system is very useful because it allows us measure key transport properties and protein structure/function relationships when plant transport proteins expressed in yeast cells (PNAS 95:9025-9030). The unifying theme of our research today is understanding how plants regulate resource allocation between "source and sink" tissues. To tackle this complex question, we are using genetic and biochemical strategies to identify the signal transduction pathways that regulate assimilate partitioning. We discovered a unique sucrose-mediated signal transduction pathway that regulates the expression and protein abundance of the sucrose transporter that is responsible for phloem loading (PNAS 95:4784-4788; PNAS 99:10876-10880).  We also discovered that multiple nitrogen-metabolites regulate amino acid transporter gene expression, and have now focused on understanding nitrate as a signal molecule that regulates unique patterns of plant gene expression.  That research led to the discovery of a novel protein that appears to regulate cell wall structure (Plant Phys 152: 904-917, 2010). In a complementary biofuels research project, we are using genetic and genomic tools in rice as a model plant to identify genes that control primary productivity (biomass per m²).  As part of that work, we discoved a unique transcription factor, that when exressed out of its normal context, increases yield by 3-fold! We're currently focused on understanding the underlying molecular mechanism.