ZmmiR1432-ZmCML21-ZmPMA2 Module Affects Maize Low Phosphate Tolerance via Regulating Organic Acid Secretion

Phosphorus is indispensable in agricultural production. The growing global demand for food necessitates the development of crops with enhanced phosphorus utilisation efficiency. However, the molecular mechanisms coordinating phosphorus utilisation efficiency in plants remain incompletely characterised. MicroRNAs, pivotal regulators of plant developmental and physiological processes, have emerged as key targets for deciphering the regulatory networks underlying low phosphate (Pi) tolerance. Herein, we delineate the regulatory role of ZmmiR1432 in maize and elucidate its mechanistic basis in conferring low Pi tolerance. Suppression of ZmmiR1432 markedly improved tolerance to Pi deficiency via enhanced organic acid exudation, whereas its overexpression had the opposite effect. It is also indicated that ZmmiR1432 regulates low Pi tolerance through direct modulation of its target gene, ZmCML21, a calmodulin-like protein coding gene that also plays a key role in organic acid secretion and Pi-deficiency response. Metabolomic and transcriptomic analyses revealed that overexpression of ZmCML21 severely affected organic acid secretion and altered the expression of genes involved in the citrate cycle (TCA cycle). Furthermore, it is demonstrated that ZmCML21 directly interacts with plasma membrane H⁺-ATPase (ZmPMA2). Overexpression of ZmPMA2 phenocopied the ZmmiR1432 knockdown plants and ZmCML21 overexpression plants. Collectively, our findings uncover a ZmmiR1432-ZmCML21 regulatory module that governs low Pi tolerance by modulating ZmPMA2 activity, thereby influencing organic acid secretion and ultimately determining Pi use efficiency. These results provide mechanistic insights and actionable genetic targets for improving Pi use efficiency in maize through molecular breeding and genetic engineering.