Physiological, anatomical, and transcriptomic analyses reveal the potential mechanism of resistance of Akebia trifoliata to acid rain stress and mitigation effects of curcumin

Acid rain is a global ecological issue severely threatening crop growth. Curcumin (CUR), a natural antioxidant, can enhance the tolerance of plants to abiotic stresses via physiological and molecular modes. As both medicine and food, Akebia trifoliata exhibits high economic value. The resistance mechanism of A. trifoliata to acid rain and mitigation effects of CUR remain unclear. Therefore, in this study, we investigated the plant growth, physiological characteristics of leaves, anatomical structure, and gene expression of A. trifoliata under acid rain stress before and after treatment with exogenous CUR. The results indicated that under acid rain stress, the contents of chlorophyll a, chlorophyll b, total chlorophyll, and starch and thicknesses of upper and lower epidermis of leaves decreased by 58.16%, 77.88%, 64.77%, 63.85%, 58.93%, and 35.57%, respectively. Moreover, MDA, soluble sugar, soluble protein, and proline contents and production rate of oxygen free radicals increased by 82.55%, 43.20%, 44.55%, 64.40% and 345.77%, respectively. This suggested that acid rain stress affected the growth and development of A. trifoliata . A. trifoliata resisted acid rain stress by increasing SOD and CAT activities; thickness of leaf, palisade tissue, and spongy tissue; and ratio of palisade/spongy tissue. However, exogenous CUR could effectively facilitate plant growth, maintain integrity of anatomical structure of leaf, and relieve the damages to A. trifoliata caused by acid rain stress, and 50 µmol/L (CUR50) was the most optimal concentration. Transcriptomic analysis revealed that CUR0 vs Control, CUR50 vs Control, and CUR50 vs CUR0 had 2978, 1760, and 323 DEGs, respectively. KEGG pathway enrichment analysis revealed that these DEGs were involved in eight pathways, among which protein processing in endoplasmic reticulum, plant hormone signal transduction, phenylpropanoid biosynthesis, and starch and sucrose metabolism were the key metabolic pathways via which CUR mitigated the effects of acid rain stress. This study revealed the potential mechanism of response of A. trifoliata to acid rain stress and mitigation effects of exogenous CUR via physiological, anatomical, and transcriptomic analyses, thereby providing theoretical references for phytoremediation in the acid rain zone.