Cloning and characterization of MxHA7, a plasma membrane H+ - ATPase gene related to high tolerance of Malus xiaojinensis to iron deficiency

Malus xiaojinensis, an iron-efficient apple rootstock, was used to study the molecular mechanisms of iron uptake. Increased H⁺ extrusion under iron-deficient conditions has been related with H⁺-ATPases. Thus, a 2,901-bp plasma membrane H⁺-ATPase gene, MxHA7, encoding 966 amino acids was isolated. Quantitative realtime PCR showed that MxHA7 was specifically induced in the roots of M. xiaojinensis during iron-deficient conditions, not in M. baccata. A functional complementation assay indicated that the high tolerance of MxHA7-transgenic aha7 Arabidopsis thaliana (HA7) plants to iron deficiency was significantly enhanced. Under iron-deficient conditions, Fe²⁺ contents in the roots and chlorophyll concentrations in the leaves of HA7 plants were increased up to about 2 to 3 times compared to Col-0, aha7 and empty vector (EV) (aha7 transformed with an empty vector) plants. The zinc and manganese contents in the roots of HA7 plants were also higher significantly than in aha7 and EV plants under iron-deficient conditions. Meantime, the HA7 plants have less increasing for iron uptake-related genes than those Col-0, aha7 and EV other plants after iron deficiency, which means MxHA7 gene apparently contributed to help Arabidopsis tolerance to iron deficiency.