Exogenous catechin enhances nitrogen use efficiency in rice by optimizing ammonium nitrogen transport and assimilation

Low nitrogen (N) use efficiency (NUE) constrains sustainable rice production. Although flavonoids have been implicated in improving crop NUE, the direct physiological mechanisms underlying catechin-induced NUE enhancement remain elusive. Here, hydroponic experiments were conducted with high-NUE and low-NUE varieties to analyze the effects of catechin on rice growth, NUE, N content, key enzyme activities involved in N pathways, and the expression of genes related to N transport and assimilation. A concentration screening experiment (0, 5, 10, 20, and 50 mg l-1 catechin) identified 20 mg l-1 as the optimal concentration for maximizing growth performance and NUE improvement across the two rice varieties and three N levels tested. Catechin exhibited a dual regulatory mechanism by simultaneously enhancing ammonium transport capacity and N assimilation efficiency. Specifically, it up-regulated the expression of an ammonium transporter gene (OsAMT1;2) and boosted the activities of key N assimilation enzymes including nitrate reductase, glutamine synthetase, and glutamate synthase. These coordinated responses resulted in a 15.9% elevation in free amino acid content and 28.5% improvement in NUE. Notably, a low-NUE variety demonstrated greater responsiveness to catechin under low-N levels, while a high-NUE variety exhibited a stronger response under medium-N levels. The structural equation modeling revealed that enhanced ammonium transport served as the primary driver of NUE improvement. These findings establish that catechin systemically optimizes N metabolism by synchronizing transport capacity with assimilation efficiency, providing a novel phyto-stimulant strategy particularly effective for improving N-scavenging capacity in low-NUE varieties under N-limited conditions.