Dissecting a core mechanism controlling spikelet meristem fate and inflorescence architecture in panicoid cereals

Inflorescences are flower-bearing structures that ultimately produce fruits and grains. In cereal crops such as economically important corn, wheat and rice, inflorescences have complex branching patterns. The number, length and position of branches collectively compose architecture, an agronomic trait that impacts grain yield and harvesting ability. Inflorescence architecture in cereals is controlled in part by the timing in which a developing branch terminates in a grain-bearing unit called a spikelet. While spikelet development is generally conserved across cereals, little is known about the mechanisms that control it at the molecular level. This project aims to dissect the molecular underpinnings of spikelet development and identify control points for precision breeding or engineering high-yielding cereal crops. The proposed work leverages the model cereal Setaria viridis, which is closely related to corn but is small and rapid cycling, making it ideal for lab-based hypothesis testing. It also has a unique inflorescence architecture where spikelets can be converted to sterile branches, and vice versa, through applications of growth hormones. This genetic system enables rapid discovery that can be directly translated to corn and other cereal crops, ultimately impacting crop productivity and food security. The project also provides cross-disciplinary training in advanced molecular and microscopy techniques and computational biology for undergraduates and high school students. A curriculum based on this project will be deployed in high schools and community colleges, from rural to city schools, providing research experiences that link molecular lab skills to concepts in food security and impacting future workforce development. Inflorescence architecture is an important agronomic trait, directly related to grain yield and harvesting ability. Architecture is determined by position and fate of differentiating stem cell populations called meristems. In grasses, different meristem types and variations in their determinacies give rise to diverse, complex branching patterns of the inflorescence. This proposal is aimed at understanding the molecular mechanisms controlling cell fate decisions during inflorescence development and how those decisions determine architecture and grain bearing potential. The work takes advantage of the model grass Setaria viridis because of its unique inflorescence architecture and emerging breadth of genetics and genomics tools. In Setaria spp., inflorescence branches terminate in either a grain-producing spikelet or sterile bristle, and these structures are paired. The loss-of-function bristleless1 mutant is defective in brassinosteroid biosynthesis and fails to initiate a bristle identity program, resulting in homeotic conversion of bristles to spikelets. In contrast, mutations in the spikeletless (spkl) gene convert spikelets to bristles. Spkl encodes a panicoid grass-specific transcription factor and functional ortholog of maize ramosa1 (ra1), a domestication gene that impacts harvestability by conferring meristem determinacy in ears. Despite its importance in evolution and development, the molecular mechanisms surrounding RA1 function in maize and other panicoid cereals is largely unknown. This proposal aims to i) leverage the unique spikelet vs bristle program in Setaria to dissect molecular mechanisms controlling this core genetic module involving RA1 and growth hormones in shaping meristem fate and inflorescence architecture, and ii) extend these findings through comparative approaches to closely related cereal crops maize and sorghum. 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.