5-Aminolevulinic acid-induced salt tolerance in strawberry (cv. ‘Benihoppe’): Possible role of nitric oxide on interception of salt ions in roots

•Confirm that ALA induces Na+ interception in the roots of strawberry under salt stress.•Find that ALA induces Cl− interception in the roots of strawberry under salt stress.•Find that NO is necessary for ALA inducing ion homeostasis of strawberry under salt stress.•Find that the signaling routes of ALA possess tissue specificity between roots and leaves. 5-Aminolevunic acid (ALA) significantly improves salt tolerance of plants. In the previous work, we proposed that ALA induced H2O2 accumulation in roots of strawberry, which was involved in up-regulating Na+ transporter gene expressions to intercept Na+ in roots with less upward transport. In this study, we further hypothesize that nitric oxide (NO) is involved in ALA signaling cascade. Therefore, we applied sodium nitroprusside (SNP, NO donor), Na2WO4 (NO biosynthetic inhibitor), and 2, 4-carboxyphenyl-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, NO scavenger) to the culture solution when strawberry (Fragaria × ananassa Duch. cv. ‘Benihoppe’) was stressed by 100 mM NaCl with or without exogenous ALA. The results revealed that salinity greatly impaired plant growth while 10 mg L−1 ALA or 10 µM SNP ameliorated the inhibition. When 5 µM Na2WO4 or cPTIO was co-treated, the ALA-improved salt tolerance was almost eliminated. This suggests that ALA-improved salt tolerance is dependent on NO presence. We found that salt stress caused NO, H2O2, Na+ and Cl− increases in the whole plants, while ALA induced additional increases in roots but significantly inhibited in leaves. These tissue-specific responses to ALA are important for plant salt tolerance. Our results suggest that the regulation of ALA to roots is very critical, which is mediated through NO and then H2O2 signaling to up-regulate gene expressions related with Na+ and Cl− transport, selectively retaining the harmful ions in roots with less upward transport. The hypothesis can reasonably explain how ALA-treated plants cope with harmful ions under salinity.