Overexpression of AspAT alleviates the inhibitory effects of ammonium on root development in Populus tomentosa

Ammonium toxicity, resulting from prolonged use of ammonium as the sole nitrogen source, can lead to physiological and morphological disorders, ultimately stunting plant growth. Enhancing ammonium assimilation efficiency has been extensively explored as a strategy to mitigate ammonium toxicity. However, the role of AspAT, a key enzyme in nitrogen assimilation, remains underexplored. This study investigates the function of AspAT in alleviating ammonium toxicity and uncovers the underlying physiological mechanisms. The results show that two Populus AspAT genes, AspAT13 and AspAT15, exhibit the highest expression levels in roots and are induced by exogenous ammonium. Overexpression of AspAT13 and AspAT15 in transgenic plants results in increased root biomass. In these plants, the activities of key nitrogen assimilation enzymes (GS and GOGAT) are significantly enhanced, along with increases in soluble protein, soluble sugar, and free amino acid contents. Additionally, the activities of antioxidant enzymes, such as SOD and CAT, are elevated, and ammonium content in the roots is significantly reduced. Moreover, the levels of hormones, including IAA, ACC, IBA, and BR, are significantly increased in the roots of transgenic plants. Our findings suggest that AspAT13 and AspAT15 play essential roles in mitigating ammonium toxicity, a process closely linked to enhanced nitrogen assimilation, antioxidant systems, and the regulation of auxin and brassinosteroid (BR) signaling.