Exogenous 5-aminolevulinic acid improves strawberry tolerance to osmotic stress and its possible mechanisms

Cultivated strawberry, one of the major fruit crops worldwide, is an evergreen plant with shallow root system, and thus sensitive to environmental changes, including drought stress. To investigate the effect of 5-aminolevulinic acid (ALA), a new environment-friendly plant growth regulator, on strawberry drought tolerance and its possible mechanisms, we treated strawberry (Fragaria × annanasa Duch. cv. ‘Benihoppe’) with 15% polyethylene glycol 6000 to simulate osmotic stress with or without 10 mg l⁻¹ ALA. We found that ALA significantly alleviated PEG-inhibited plant growth and improved water absorption and xylem sap flux, indicating ALA mitigates the adverse effect of osmotic stress on strawberry plants. Gas exchange and chlorophyll fluorescence analysis showed that ALA mitigated PEG-induced decreases of P_n, G_s, T_r, P_n/C_i, photosystem I and II reaction center activities, electron transport activity, and photosynthetic performance indexes. Equally important, ALA promoted PEG-increased antioxidant enzyme activities and repressed PEG-increased malondialdehyde and superoxide anion in both leaves and roots. Specially, ALA repressed H₂O₂ increase in leaves, but stimulated it in roots. Furthermore, ALA repressed abscisic acid (ABA) biosynthesis and signaling gene expressions in leaves, but promoted those in roots. In addition, ALA blocked PEG-downregulated expressions of plasmalemma and tonoplast aquaporin genes PIP and TIP in both leaves and roots. Taken together, ALA effectively enhances strawberry drought tolerance and the mechanism is related to the improvement of water absorption and conductivity. The tissue-specific responses of ABA biosynthesis, ABA signaling, and H₂O₂ accumulation to ALA in leaves and roots play key roles in ALA-improved strawberry tolerance to osmotic stress.