Blue light promotes and dark inhibits aluminum resistance in Arabidopsis

Aluminum (Al³⁺) ions in acid soil are a major threat to crop production. Aluminum (Al) toxicity inhibits primary root elongation; however, whether light regulates this process is not known. We found that blue light, in a cryptochrome (CRY)-dependent manner, alleviates Al-mediated root growth inhibition by lowering internal Al accumulation in the roots. Blue light enhances HY5 and STOP1 accumulation in Al-treated roots. HY5 induces ALMT1 and MATE expression by directly binding to their promoters and enhances carboxylate exudation to chelate Al outside and prevent its entry into the roots. In darkness, the Al-mediated root growth inhibition is enhanced. COP1 physically interacts with STOP1 and degrades it to inhibit Al resistance in the dark. The accumulation of HY5 in the roots of Al-exposed plants is also downregulated in a COP1-dependent manner in the dark. hy5 and cop1 seedlings show hypersensitive and hyposensitive responses to Al, respectively, validating the role of these light-signaling regulators in modulating Al resistance. Taken together, the results of this study suggest that blue light promotes Al resistance by enhancing HY5 and STOP1 accumulation, whereas these proteins are degraded in the dark in a COP1-dependent manner to inhibit Al resistance. The light- and dark-mediated modulation of carboxylate exudation might offer a unique way of balancing Al resistance and growth.