Balance between nitrogen use efficiency and cadmium tolerance in Brassica napus and Arabidopsis thaliana.

The transmembrane transport of NO ₃ ^- and Cd ²⁺ into plant cell vacuoles relies on the energy from their tonoplast proton pumps, V-ATPase and V-PPase. If the activity of these pumps is reduced, it results in less NO ₃ ^- and Cd ²⁺ being transported into the vacuoles, which contributes to better nitrogen use efficiency (NUE) and lower Cd ²⁺ tolerance in plants. The physiological mechanisms that regulate the balance between NUE and Cd ²⁺ tolerance remain unknown. In our study, two Brassica napus genotypes with differential NUEs, xiangyou 15 and 814, and Atclca-2 mutant and AtCAX4 over-expression line (AtCAX4-OE) of Arabidopsis thaliana, were used to investigate Cd ²⁺ stress responses. We found that the Brassica napus genotype, with higher NUE, was more sensitive to Cd ²⁺ stress. The AtCAX4-OE mutant, with higher Cd ²⁺ vacuolar sequestration capacity (VSC), limited NO ₃ ^- sequestration into root vacuoles and promoted NUE. Atclca-2 mutants, with decreased NO ₃ ^- VSC, enhanced Cd ²⁺ sequestration into root vacuoles and conferred greater Cd ²⁺ tolerance than the WT. This may be due to the competition between Cd ²⁺ andNO ₃ ^- in the vacuoles for the energy provided by V-ATPase and V-PPase. Regulating the balance between Cd ²⁺ and NO ₃ ^- vacuolar accumulation by inhibiting the activity of CLCa transporter and increasing the activity of CAX4 transporter will simultaneously enhance both the NUE and Cd ²⁺ tolerance of Brassica napus, essential for improving its Cd ²⁺ phytoremediation potential.
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