COLLABORATIVE RESEARCH: Evolution of genetic networks in grass abscission zones

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Agriculture depends on plants that hold on to their seeds until harvest, yet wild plants need to drop their seeds, in a process known as shattering or abscission. Therefore, domestication in cereal crops such as wheat, rice, and corn was the result of human selection for spontaneous mutant plants that did not shatter. In a world facing rapid population growth and changing climates, it is crucial to understand the mechanism of shattering to be able to domesticate and improve new wild or less domesticated cereal crops, thereby diversifying food choices and improving agriculture in marginal environments. This project will examine the shattering process, which is controlled by a specialized set of cells known as an abscission zone (AZ). Cell development of the AZ will be compared in different cereal crops and their wild relatives, including pearl millet and sorghum, two economically valuable and drought tolerant cereals. The project aims to identify genes that are required for a functional AZ, and to understand how modification of these genes causes morphological differences in the AZ in different species. High school students, undergraduates and secondary school teachers will be involved in conducting the research. The research experience provided to secondary school teachers will be used to develop new lesson plans, and further impact next generation education in science. Gene regulatory networks are continually modified over evolutionary time, but the speed of modification and the nature of the changes are generally not known. This project addresses both the tempo and mode of change in the network controlling the abscission zone (AZ) in plants, a specialized cell layer that permits plants to drop seeds, fruits, petals or leaves. The AZ of three distantly related grass species (weedy rice, Brachypodium, and Setaria) is controlled by distinct sets of genes, suggesting rewiring of the underlying genetic network. This project will investigate the AZ networks among additional species in grass subfamily Panicoideae, including wild and cultivated pearl millet (Cenchrus americanus, closely related to Setaria) and sorghum (Sorghum bicolor), a more distant relative. Histology (using light and transmission electron microscopy) and gene expression patterns (RNA-seq) will be characterized at several stages of development. To determine whether the extensive rewiring comes from changes in cis-regulatory modules, the upstream and downstream regulators of two conserved transcription factors, YAB2/SH1 and MYB26, will be characterized in the genetically tractable species Setaria and Brachypodium, with a combination of mutant analysis, RNA-seq, DAP-seq and bioinformatics. To determine if the disparate early regulatory networks converge on a conserved late stage set of genetic interactions, transcriptomic, enzymatic, and cell wall changes will be evaluated right before and after abscission. Together, these data will determine how and how fast the gene network regulating AZ formation is remodeled over time. High school students and teachers will be involved in many aspects of plant phenotyping.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.