Abstract
<title>Abstract</title> <p>The transcription factor TEOSINTE BRANCHED1 (TB1) and its orthologs were pivotal to cereal domestication. TB1 suppresses tillering but also influence inflorescence architecture and other yield-related traits, raising the question whether these functions can be molecularly separated. Here we show that the barley TB1 ortholog VULGARE ROW-TYPE SIX 5 (VRS5), which regulates both tiller bud outgrowth and lateral spikelet development, generates two transcript isoforms, VRS5α and VRS5β. Targeted mutagenesis of VRS5α, while retaining the downstream VRS5β transcript, increased tillering but maintained two-rowed spike architecture, genetically uncoupling vegetative branching from row-type regulation. Mechanistically, VRS5α and VRS5β showed promoter- and COMPOSITUM 1 (HvCOM1)-dependent differences in DNA binding: both isoforms bound the promoter of VRS1, a core regulator of row-type, in absence or presence of HvCOM1; in contrast, VRS5α, but not VRS5β, binds to the promoter of HvGT1, a regulator associated with tiller repression, in the presence of HvCOM1. These findings show that alternative transcript isoforms of VRS5 can be used to uncouple pleiotropic functions.</p>