Hydrogen Sulfide Alleviates Aluminum Toxicity via Decreasing Apoplast and Symplast Al Contents in Rice.

Hydrogen sulfide (HS) plays a vital role in Al stress resistance in plants, but the underlying mechanism is unclear. In the present study, pretreatment with 2 μM of the HS donor NaHS significantly alleviated the inhibition of root elongation caused by Al toxicity in rice roots, which was accompanied by a decrease in Al contents in root tips under 50 μM Al treatment. NaHS pretreatment decreased the negative charge in cell walls by reducing the activity of pectin methylesterase and decreasing the pectin and hemicellulose contents in rice roots. This treatment also masked Al-binding sites in the cell wall by upregulating the expression of and in roots and reduced Al binding in the cell wall by stimulating the expression of the citrate acid exudation gene and increasing the secretion of citrate acid. In addition, NaHS pretreatment decreased the symplasmic Al content by downregulating the expression of , and increasing the translocation of cytoplasmic Al to the vacuole via upregulating the expression of . The increment of antioxidant enzyme [superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and peroxidase (POD)] activity with NaHS pretreatment significantly decreased the MDA and HO content in rice roots, thereby reducing the damage of Al toxicity on membrane integrity in rice. HS exhibits crosstalk with nitric oxide (NO) in response to Al toxicity, and through reducing NO content in root tips to alleviate Al toxicity. Together, this study establishes that HS alleviates Al toxicity by decreasing the Al content in the apoplast and symplast of rice roots.