The Yield-Associated TaCOL-B5 Locus Activates Its Tightly Linked Target TaDRS1 to Restrict Root Apical Meristem Proliferation and Deep Rooting in Wheat.

Rooting depth underpins subsoil water capture and drought resilience in wheat, yet the regulators coupling root apical meristem (RAM) activity to vertical root growth remain largely unknown. We characterise Deep Root Suppressor 1 (TaDRS1; TraesCS7B01G400700), a transcriptional target of the yield-associated CONSTANS-like locus TaCOL-B5 lying only 32 kb away on chromosome 7B. In the common recessive (Tacol-B5) background of most modern wheat, chromatin immunoprecipitation-qPCR and dual-luciferase assays show that the Tacol-B5 protein occupies a CCAAT-box region of the TaDRS1 promoter and activates its transcription via its conserved CCT domain. TaDRS1 is a nucleus-localised protein that activates transcription when tethered to DNA, but that carries no DNA-binding domain of any recognised class. In 100-cm soil columns, over-expression of either Tacol-B5 or TaDRS1 confined roots to shallow horizons, whereas CRISPR/Cas9 editing of either gene deepened rooting and increased total root length, with no detectable change in the root-branching traits measured. Confocal and 5-ethynyl-2'-deoxyuridine imaging show that TaDRS1 restricts meristem size and S-phase entry rather than mature cell elongation. Editing TaDRS1 sustained higher leaf relative water content and preserved spike number, grain yield and thousand-grain weight under drought, with no penalty under well-watered conditions and no detectable change in spike length or spikelet number per spike. Mechanistically, TaDRS1 is brought to the promoter of CDK-inhibitor TaKRP1B through the promoter-bound C2H2 zinc-finger proteins TaZFP7 and TaZFP22 and potentiates their activation of TaKRP1B. Editing TaZFP7 or TaZFP22 recapitulated the deep-rooting phenotype of TaDRS1 loss of function, and TaKRP1B over-expression in the TaDRS1-edited background partially suppressed it, placing TaKRP1B downstream of TaDRS1. TaDRS1 is thus a candidate for uncoupling deep rooting from the linked yield locus.