Role of root plasma membrane H⁺-ATPase in enhancing Cucumis sativus adaptation to microcystins

Microcystins (MCs) are the most widespread and hazardous cyanotoxins posing a huge threat to agro-ecosystem by irrigation. Some adaptive metabolisms can be initiated at the cellular and molecular levels of plant to survive environmental change. To find ways to improve plant tolerance to MCs after recognizing adaptive mechanism in plant, we studied effects of MCs on root morphology, mineral element contents, root activity, H⁺-ATPase activity, and its gene expression level in cucumber during exposure and recovery (without MCs) periods. After being exposed to MCs (1, 10, 100 and 1000 μg L^-1) for 7 days, we found 1 μg L^-1 MCs did not affect growth and mineral elements in cucumber. MCs at 10 μg ·L^-1 increased root activity and H⁺-ATPase activity partly from upregulation of genes (CsHA2, CsHA3, CsHA8, and CsHA9) expression, to promote nutrient uptake. Then, the increase in NO₃^-, Fe, Zn, and Mn contents could contribute to maintaining root growth and morphology. Higher concentration MCs (100 or 1000 µg L^-1) inhibited root activity and H⁺-ATPase activity by downregulating expression of genes (CsHA2, CsHA3, CsHA4, CsHA8, CsHA9, and CsHA10), decreased contents of nutrient elements except Ca largely, and caused root growing worse. After a recovery, the absorption activity and H⁺-ATPase activity in cucumber treated with10 μg L^-1 MCs were closed to the control whereas all parameters in cucumber treated 1000 μg L^-1 MCs were even worse. All results indicate that the increase in H⁺-ATPase activity can enhance cucumber tolerance to MC stress by regulating nutrient uptake, especially when the MCs occur at low concentrations.