The role of 5-aminolevulinic acid in the response to cold stress in soybean plants

In this study, the possibility of enhancing cold stress tolerance of soybean plants (Glycine max L.) by exogenous application of 5-aminolevulinic acid (ALA) was investigated. ALA was added to the Hoagland solution at various concentrations ranging from 0 to 40 μM for 12 h. After ALA treatment, the plants were subjected to cold stress at 4°C for 48 h. ALA at low concentrations (5-10 μM) provided significant protection against cold stress compared to non-ALA-treated plants, enhancing chlorophyll content (Chl) as well as relative water content (RWC). Increase of thiobarbituric acid reactive species (TBARS) levels was also prevented, whereas exposure to higher ALA concentrations (15-40 μM) brought about a dose dependent increase of these species, reaching a maximum of 117% in plants pre-treated with 40 μM ALA compared to controls. ALA pre-treatment also enhanced catalase (CAT) and heme oxygenase-1 (HO-1) activities. These findings indicate that HO-1 acts not only as the rate limiting enzyme in heme catabolism, but also as an antioxidant enzyme. The highest cold tolerance was obtained with 5 μM ALA pre-treatment. Results show that ALA, which is considered as an endogenous plant growth regulator, could be used effectively to protect soybean plants from the damaging effects of cold stress by enhancing the activity of heme proteins, e.g., catalase (CAT) and by promoting heme catabolism leading to the production of the highly antioxidant biliverdin and carbon monoxide, without any adverse effect on the plant growth.