Role of Flavonol Synthesized by Nucleus FLS1 in Arabidopsis Resistance to Pb Stress

Lead (Pb) is an important pollutant of worldwide concern with respect to extensive pollution sources and highly toxic effect. Flavonol can improve plant resistance to abiotic stress and is also responsible for the alleviating effect under Pb stress. The relationship between Pb stress and flavonol and the knowledge about the mechanisms of flavonol function are very limited. Pb affected the energy metabolism process and, thus, inhibited plant growth and development. Flavonol accumulation controlled by FLS1 (flavonol synthase) could alleviate the toxic effect. Importantly, nes (mutant of NES that allows FLS1 to enter the nucleus expression) showed better growth status and lighter oxidative damage than NES (N-terminal nucleus exclusion signal peptide prevents FLS1 from entering the nucleus expression), which indicated that nucleus flavonol synthesized by nucleus FLS1 plays a key role in plant resistance to Pb stress. Although FLS1 signals were detected in the cell membrane, cytoplasm, and nucleus, membrane flavonol, cytoplasm flavonol, and nucleus flavonol were not exercising their function in the corresponding position. The expression of nucleus FLS1 intervened in the total content and composition of flavonol. The results also revealed that nucleus flavonol could regulate the ascorbate metabolism for alleviating the damage on the chloroplast, thus maintaining the photophosphorylation pathway. Our findings provided new insights for the molecular basis of Pb tolerance and response mechanism of the plant.