Hydrogen peroxide as a mediator of 5‐aminolevulinic acid‐induced Na + retention in roots for improving salt tolerance of strawberries

To explore the mechanisms of 5‐aminolevulinic acid (ALA)‐improved plant salt tolerance, strawberries ( Fragaria × ananassa Duch. cv. ‘Benihoppe’) were treated with 10 mg l −1 ALA under 100 mmol l −1 NaCl stress. We found that the amount of Na + increased in the roots but decreased in the leaves. Laser scanning confocal microscopy (LSCM) observations showed that ALA‐induced roots had more Na + accumulation than NaCl alone. Measurement of the xylem sap revealed that ALA repressed Na + concentrations to a large extent. The electron microprobe X‐ray assay also confirmed ALA‐induced Na + retention in roots. qRT‐PCR showed that ALA upregulated the gene expressions of SOS1 (encoding a plasma membrane Na + /H + antiporter), NHX1 (encoding a vacuolar Na + /H + antiporter) and HKT1 (encoding a protein of high‐affinity K + uptake), which are associated with Na + exclusion in the roots, Na + sequestration in vacuoles and Na + unloading from the xylem vessels to the parenchyma cells, respectively. Furthermore, we found that ALA treatment reduced the H 2 O 2 content in the leaves but increased it in the roots. The exogenous H 2 O 2 promoted plant growth, increased root Na + retention and stimulated the gene expressions of NHX1 , SOS1 and HKT1 . Diphenyleneiodonium (DPI), an inhibitor of H 2 O 2 generation, suppressed the effects of ALA or H 2 O 2 on Na + retention, gene expressions and salt tolerance. Therefore, we propose that ALA induces H 2 O 2 accumulation in roots, which mediates Na + transporter gene expression and more Na + retention in roots, thereby improving plant salt tolerance.