Loss of Pleiotropic Regulatory Functions in <i>Tannin1</i>, the Sorghum Ortholog of Arabidopsis Master Regulator <i>TTG1</i>

Transcriptional master regulators are often targeted to improve plant traits, but antagonistic pleiotropic effects of these regulators can hamper this approach. The Myb-bHLH-WDR (MBW) complex is a broadly conserved transcriptional regulator affecting pigmentation, biotic stress resistance, and abiotic stress tolerance. We investigated the function of sorghum grain pigmentation regulator <i>Tannin1</i>, the ortholog of Arabidopsis pleiotropic WD40 regulator <i>TTG1</i>, to test for conserved pleiotropic regulatory effects and to better understand the evolution of the MBW complex in Poaceae. We characterized genome-wide differential expression of leaf tissue using RNA sequencing in near-isogenic lines (NILs) that contrasted wildtype <i>Tan1</i> and loss-of-function <i>tan1-b</i> alleles, under optimal temperature and chilling stress. Notably, Gene Ontology analyses revealed no pathways with differential expression between <i>Tan1</i> and <i>tan1-b</i> NILs, suggesting that, in contrast to Arabidopsis <i>TTG1</i>, <i>Tannin1</i> has no pleiotropic regulatory role in leaves. Further, NILs had no visible difference in anthocyanin pigmentation, and no genes with known or expected function in flavonoid synthesis were differentially expressed. Genome-wide, only 18 total genes were differentially expressed between NILs, with six of these genes located inside the NIL introgression region, an observation most parsimoniously explained by <i>cis</i>-regulatory effects unrelated to <i>Tannin1</i> regulation. Comparing our findings with known function of <i>TTG1</i> orthologs in maize, rice, and Arabidopsis, we conclude that pleiotropic regulatory function in leaf tissue was likely lost in panicoid grass evolution before the sorghum-maize split. These findings inform future molecular breeding of MBW regulated traits and highlight the benefit of subfunctionalization to relieve pleiotropic constraints.