Exogenous allantoin improves waterlogging tolerance of summer maize by regulating the endogenous allantoin and nitrogen metabolism

Waterlogging has emerged as one of the main disasters limiting maize production in the Huang-Huai-Hai region. The physiological mechanisms by which allantoin exogenously regulated waterlogging tolerance of maize seedlings remained unclear. Based on the concentration screening test, the optimal allantoin concentration was found to be 5 mM. Then, the endogenous allantoin metabolism, nitrogen metabolism, and transcriptomic profiles of three plant organs were explored. Results showed that 5 mM allantoin significantly boosted peroxidase and superoxide dismutase activities, enhanced reactive oxygen species scavenging, while increasing soluble protein and sugar accumulation across leaves, seminal roots, and adventitious roots under waterlogging stress. Compared with the control treatment, waterlogging (WL) significantly suppressed nitrate metabolism by decreasing the activities of nitrate reductase (NR), glutamate synthase, and glutamine synthetase, while downregulating NR gene expression in all organs and reducing total nitrogen content on the 6th day of waterlogging. The waterlogging plus 5 mM allantoin treatment (WL + ALN5) restored NR activity, increasing it by 46.71 %, 47.91 %, and 34.61 % in the leaves, seminal roots, and adventitious roots, respectively. Furthermore, WL + ALN5 treatment reduced the activities of xanthine dehydrogenase and uric acid oxidase in seminal and adventitious roots, while it stimulated allantoinase activity and enhanced endogenous allantoin and allantoic acid levels. The gene expression of allantoinase was downregulated in leaves but upregulated in seminal roots. This finding provides a theoretical basis for the cultivation of maize with stable waterlogging resistance and yield in the Huang-Huai-Hai region.