Speaker: Erik Vollbrecht, professor of genetics, development and cell biology at Iowa State University
Title: “Interspecies transfer of RAMOSA1 orthologs and cis regulation of grass inflorescence architecture”
Abstract — A key objective in biology is to understand the genetic basis of morphological diversity between and within species. In grasses including cereal crops such as maize, rice and sorghum and a plethora of wild grasses, grains are borne in reproductive shoots that are called inflorescences. There is tremendous intra- and interspecific variation among inflorescence morphology in the grasses. These similarities and differences provide an effective model for studying genetic mechanisms that underly evolutionary change in plant architecture including variation in inflorescence morphology impacting yield in cereal crops and weedy grass species. Despite such agronomical and ecological significance, the genes that underlie diverse inflorescence forms in the grasses have not been fully elucidated, and tests of functional conservation of syntenic orthologous genes are limited.
Using genetic approaches in maize, our lab has identified plant developmental mechanisms that regulate the architecture of both vegetative shoots and inflorescence shoots. For example, the ramosa1 (RA1) transcription factor gene is central to a genetic network regulating floral shoot (inflorescence) branching. To better understand unique and conserved aspects of that network we carried out a cross-species investigation of ramosa1 function. We performed genetic tests for functional conservation of syntenic orthologous RA1 genes from maize, sorghum and a wild grass called setaria. In sorghum and setaria inflorescences, RA1 expression marked meristem boundary domains in concordance with RA1 transcript accumulation in maize. To directly test function, RA1 transgene constructs from maize (Zm), sorghum (Sb) and setaria (Sv) were transformed into maize and introgressed into a common inbred genetic background containing the strong ra1-R mutant, so that the only functional RA1 came from the transgene. The ZmRA1, SbRA1 and SvRA1 transgenes showed differential complementation and effects on the regulation of branching in maize tassels and ears. Through examining two different SbRA1 constructs and a maize-setaria promoter swap construct we identified cis-regulatory differences that impact ear and tassel morphology, and correlations with open chromatin signatures and conserved upstream sequences including known transcription factor binding motifs, as potential drivers of inflorescence diversity in the grasses.