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

Authors: Gallagher, Joseph - Joe; MAN, JARRETT - University Of Massachusetts, Amherst; CHIARAMIDA, ADRIANA - University Of Massachusetts, Amherst; ROZZA, ISABELLA - University Of Massachusetts, Amherst; PATTERSON, ERIN - University Of Massachusetts, Amherst; POWELL, MORGAN - University Of Massachusetts, Amherst; SCHRAGER-LAVELLE, AMANDA - University Of Massachusetts, Amherst; MULTANI, DILBAG - Corteva Agriscience; MEELEY, ROBERT - Corteva Agriscience; BARTLETT, MADELAINE - University Of Massachusetts, Amherst

Submitted to: bioRxiv
Publication Type: Pre-print Publication
Publication Acceptance Date: 3/15/2023
Publication Date: 3/15/2023
Citation: Gallagher, J.P., Man, J., Chiaramida, A., Rozza, I., Patterson, E., Powell, M., Schrager-Lavelle, A., Multani, D.S., Meeley, R., Bartlett, M.E. 2023. Duplicate transcription factors GT1 and VRS1 regulate branching and fertile flower number in maize and Brachypodium distachyon. bioRxiv. https://doi.org/10.1101/2023.03.15.532786.
DOI: https://doi.org/10.1101/2023.03.15.532786

Interpretive Summary: Plant yield can be impacted by the fertility of reproductive structures. In grasses, this fertility is regulated in part by genes that regulate growth repression. Genes GRASSY TILLERS1 (GT1) and SIX-ROWED SPIKE1 (VRS1) both regulate growth repression and have been identified as playing a role in regulating floral development in maize and barley, respectively. In this study, the roles of GT1-like and VRS1-like genes in maize and brachypodium were examined. CRISPR-Cas9 gene editing was used to develop mutants in these genes. Maize double mutants in gt1 and vrs1-like1 (vrl1) developed more and longer tillers, more ears, and fertile pistils in typically sterile lower ear flowers. Brachypodium double mutants in gt1 and vrl1 developed more branches, more spikelets, and more flowers. Repression of floral tissues caused by transgenic expression of GT1 in the model plant Arabidopsis thaliana confirmed that these genes can regulate growth repression across long divergence times. These genes display a highly conserved role in regulating grass reproduction via growth repression.

Technical Abstract: 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.