Brassinosteroids Modulate Meristem Fate and Differentiation of Unique Inflorescence Morphology in <i>Setaria viridis</i>

Abstract

Inflorescence architecture is a key determinant of yield potential in many crops and is patterned by the organization and developmental fate of axillary meristems. In cereals, flowers and grain are borne from spikelets, which differentiate in the final iteration of axillary meristem branching. In <i>Setaria</i> spp, inflorescence branches terminate in either a spikelet or a sterile bristle, and these structures appear to be paired. In this work, we leverage <i>Setaria viridis</i> to investigate a role for the phytohormones brassinosteroids (BRs) in specifying bristle identity and maintaining spikelet meristem determinacy. We report the molecular identification and characterization of the <i>Bristleless1</i> (<i>Bsl1</i>) locus in <i>S. viridis</i>, which encodes a rate-limiting enzyme in BR biosynthesis. Loss-of-function <i>bsl1</i> mutants fail to initiate a bristle identity program, resulting in homeotic conversion of bristles to spikelets. In addition, spikelet meristem determinacy is altered in the mutants, which produce two florets per spikelet instead of one. Both of these phenotypes provide avenues for enhanced grain production in cereal crops. Our results indicate that the spatiotemporal restriction of BR biosynthesis at boundary domains influences meristem fate decisions during inflorescence development. The <i>bsl1</i> mutants provide insight into the molecular basis underlying morphological variation in inflorescence architecture.