CoPI: Frank G. Harmon (University of California - Berkeley/USDA-ARS)
Biological clocks orchestrate spatial and temporal control of growth and metabolism, leading to synchronous interactions between circadian rhythms and metabolic homoeostasis. In plants, maintenance of robust circadian rhythms promotes growth and fitness. Hybrids formed between strains or allopolyploids formed between species often show hybrid vigor or heterosis: superior levels of growth, biomass, and fertility in the hybrids compared to the parents. The mechanisms underlying heterosis are poorly understood. A recent study found a direct link between growth vigor and the circadian clock. Alterations in the expression waveform of key circadian clock genes in Arabidopsis hybrids and allotetraploids promote metabolic outputs including chlorophyll biosynthesis and starch metabolism. Upregulation of light signaling, photosynthesis, and starch metabolic genes in hybrids results from epigenetic repression of the circadian clock repressor genes during the day that, in turn, causes upregulation of clock activators and downstream metabolic genes. Although diurnal rhythms in gene expression, photosynthesis, and starch metabolism have been documented in maize, it is unknown whether the circadian-mediated metabolic vigor found in Arabidopsis hybrids and allopolyploids is directly applicable to the improvement of hybrid crops such as maize. Maize is among the best organisms for studying heterosis. Moreover, maize performs C4 photosynthesis, characterized by anatomical and biochemical specialization that increases net carbon assimilation at high temperatures. Part of this specialization in C4 plants may involve clock-regulated metabolic pathways. This project will test how circadian rhythms control metabolism and development in maize, as well as how circadian and other regulatory networks affect growth vigor in maize hybrids. Specific objectives are to (1) determine direct targets of clock genes in biological pathways including metabolism in maize hybrids and their inbred parents; (2) test how circadian clock gene expression and growth vigor in maize hybrids are controlled at levels of chromatin and metabolism; and (3) elucidate molecular bases for circadian regulation of metabolism and growth vigor in maize inbreds and hybrids.
The impact of this research is transformative for many fundamental areas of biology ranging from circadian rhythms, first discovered in heliotrope plants in 1726, to hybrid vigor or heterosis, a widespread phenomenon that has intrigued many scientists, including Charles Darwin. The results from this work impact directly on improvement of hybrid crops, most notably maize and sorghum, which are of great importance to agriculture, food security, biofuels, and human health. Mechanistic insights into circadian regulation of metabolic vigor in plant hybrids are also applicable to the circadian rhythms in metabolism and physiology found in other sexually reproducing organisms like humans. With regard to outreach and training, this project will provide summer student internships for future science teachers through the UTeach program. The University of Texas at Austin (UT Austin) developed the UTeach program, and many other institutions across the country have replicated it. In addition, the project will provide research training opportunities for Master of Science (MSc) students at Alcorn State University and Mississippi Valley State University, two Historically Black Colleges/Universities. Sequence data will be available for public through GEO, MaizeGDB, and Gramene. Requests for seed and information about germplasm generated in this project will be available through MaizeGDB.
