Duplicate transcription factors GT1 and VRS1 regulate branching and fertile flower number in maize and Brachypodium distachyon

Crop engineering and de novo domestication using genome editing are new frontiers in agriculture. However, outside of well-studied crops and model systems, prioritizing engineering targets remains challenging. Evolution can serve as our guide, revealing high-priority genes with deeply conserved roles. Indeed, GRASSY TILLERS1 (GT1), SIX-ROWED SPIKE1 (VRS1), and their homologs have repeatedly been targets of selection in domestication and evolution. This repeated selection may be because these genes have an ancient, conserved role in regulating growth repression. To test this, we determined the roles of GT1 and VRS1 homologs in maize (Zea mays) and the distantly related grass brachypodium (Brachypodium distachyon) using CRISPR-Cas9 gene editing and mutant analysis. GT1 and VRS1 have roles in floral development in maize and barley, respectively. Grass flowers are borne in branching structures called spikelets. In maize spikelets, carpels are suppressed in half of all initiated ear flowers. These spikelets can only produce single grains. We show that gt1; vrs1-like1 (vrl1) mutants have derepressed carpels in ear flowers. Importantly, these plants can produce two grains per spikelet. In brachypodium, bdgt1; bdvrl1 mutants have more branches, spikelets, and flowers than wildtype plants, indicating conserved roles for GT1 and VRS1 homologs in growth suppression. Indeed, maize GT1 can suppress growth in Arabidopsis thaliana, separated from the grasses by ca. 160 million years of evolution. Thus, GT1 and VRS1 maintain their potency as growth regulators across vast timescales and in distinct developmental contexts. Modulating the activity of these and other conserved genes may be critical in crop engineering.