Enhanced accumulation of gibberellins rendered rice seedlings sensitive to ammonium toxicity

Ammonium (NH4+) phytotoxicity is a worldwide phenomenon, but the primary toxic mechanisms are still controversial. In the present study, we investigated the physiological function of gibberellins (GAs) in the response of rice plants to NH4+ toxicity and polyamine accumulation using GA biosynthesis-related rice mutants. Exposure to NH4+ significantly decreased GA(4) production in shoots of wild-type (WT) plants. Both exogenous GA application to the WT and increases in endogenous GA levels in eui1 mutants rendered them more sensitive to NH4+ toxicity. In contrast, growth of sd1 GA-deficient mutants was more tolerant to NH4+ toxicity than that of their WT counterparts. The role of polyamines in GA-mediated NH4+ toxicity was evaluated using WT rice plants and their GA-related mutants. The eui1 mutants with GA overproduction displayed a higher endogenous putrescine (Put) accumulation than WT plants, leading to an enhanced Put/[spermidine (Spd)+spermine (Spm)] ratio in their shoots. In contrast, mutation of the SD1 gene encoding a defective enzyme in GA biosynthesis resulted in a significant increase in Spd and Spm production, and reduction in the Put/(Spd+Spm) ratio when exposed to a high NH4+ medium. Exogenous application of Put exacerbated symptoms associated with NH4+ toxicity in rice shoots, while the symptoms were alleviated by an inhibitor of Put biosynthesis. These findings highlight the involvement of GAs in NH4+ toxicity, and that GA-induced Put accumulation is responsible for the increased sensitivity to NH4+ toxicity in rice plants.