Constitutive OsCIN1 Expression Reprograms Source–Sink Dynamics and Compromises Agronomic Traits in Rice

Cell wall invertases (CINs) establish sucrose gradients between source and sink tissues, essential for the allocation of photoassimilates. Rice possesses nine <i>CIN</i> genes, among which <i>OsCIN1</i> and <i>OsCIN2</i> have been reported as key regulators of sink strength. To test whether increasing CIN activity enhances grain yield, we generated <i>OsCIN1</i> overexpression lines in rice driven by the CaMV 35S promoter. Subcellular localization analysis of <i>OsCIN1</i>-GFP confirmed its apoplastic localization. <i>OsCIN1</i> promoter::GUS analyses verified expression in vascular tissues and revealed predominant signals in the ovular vascular and lateral stylar vascular traces during seed development. Although CIN activity was markedly elevated throughout the plant, the resulting phenotypes were unexpected. Sugar profiling of flag leaves at the flowering stage showed almost complete sucrose depletion in the overexpression (OX) lines, accompanied by increased hexose and starch accumulation. Under field conditions, <i>OsCIN1</i> OX plants exhibited ~50% fewer tillers and a lower 1000-grain weight relative to wild type (WT), resulting in reduced productivity. Ectopic expression of <i>OsCIN1</i> disrupted the sucrose concentration gradient, weakened carbon partitioning to sink tissues, and impaired key agronomic traits. Collectively, sugar flux is governed by the spatiotemporal patterning of CINs, highlighting that precise spatial and temporal control of CIN activity is required to increase yield.