CsABCG11.2 mediates theanine uptake to alleviate cadmium toxicity in tea plants (Camellia sinensis)

Abstract

Theanine (Thea) is a unique metabolite in tea plants, but its physiological functions remain elusive. A low soil pH increases cadmium (Cd) availability, affecting the quality of tea plant products. In this study, we found that Thea reversed the Cd-induced reduction in free amino acid (FAA) and caffeine (CAF) in the young tea leaves, as well as the down-regulation in the expression of nitrate transporters CsNRT1.2 and CsNRT2.5, and genes responsible for the nitrogen (N) assimilation. We demonstrated that Thea could alleviate Cd-induced oxidative stresses and enhance photosynthesis. Moreover, an ATP-binding cassette (ABC) transporter, CsABCG11.2, could uptake distinct Cd substrates and the five major amino acids in tea plants. Heterologous expression of CsABCG11.2 in yeast indicated a competitive absorption between Cd and Thea in a concentration-dependent pattern. CsABCG11.2-overexpressing Arabidopsis plants exhibited increased sensitivity to Cd due to enhanced Cd concentration, accumulation in the shoots, and reduction in the primary root length. Exogenous application of Thea at environmentally regular levels attenuated the adverse effects of Cd-induced growth inhibition and chlorosis in CsABCG11.2-overexpressing Arabidopsis plants. Knockdown of CsABCG11.2 tea plants significantly lowered Cd levels in young shoots. Our results suggest that Thea plays beneficial roles in alleviating Cd stress directly or indirectly by modulating CsABCG11.2-mediated Cd uptake and translocation within plants.