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 x 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 H2O2 content in the leaves but increased it in the roots. The exogenous H2O2 promoted plant growth, increased root Na+ retention and stimulated the gene expressions of NHX1, SOS1 and HKT1. Diphenyleneiodonium (DPI), an inhibitor of H2O2 generation, suppressed the effects of ALA or H2O2 on Na+ retention, gene expressions and salt tolerance. Therefore, we propose that ALA induces H2O2 accumulation in roots, which mediates Na+ transporter gene expression and more Na+ retention in roots, thereby improving plant salt tolerance.