The miR156h–TaSPL4– <i>TaPIN18</i> Module Regulates Plant Architecture and Grain Size by Modulating Auxin Transport in Wheat

Understanding the regulatory mechanisms underlying wheat plant architecture is essential for yield improvement and molecular breeding. The miR156-SPL module plays a pivotal role in controlling plant architecture and agronomic traits, however, its regulatory functions in wheat remain incompletely understood. In this study, we first report the miR156h-TaSPL4-TaPIN18 module regulates plant architecture and grain size by modulating auxin transport in wheat. Overexpression of miR156h resulted in reducing the size of leaf, spikelet and grain, accompanied by increased tillering and a more compact plant architecture. And miR156h directly cleaves and suppresses the expression of TaSPL4. CRISPR/Cas9-mediated knockout of TaSPL4 leads to increased tiller number, compact growth, and significant reductions in grain width and thousand-grain weight. In contrast, overexpression of TaSPL4 reduced tillering, increased tiller angle, and significantly enhanced grain length, grain width, and thousand-grain weight, highlighting its critical role in regulating plant architecture and grain size. Furthermore, we identified TaPIN18 as a novel downstream gene regulated by TaSPL4 and demonstrated that TaSPL4 positively regulates TaPIN18 expression and mediates auxin transport and distribution. Taken together, our findings reveal a previously uncharacterised miR156h-TaSPL4-TaPIN18 regulatory module that modulates wheat plant architecture and grain size, and provides potential molecular targets for yield improvement.