Silicon alleviates aluminum toxicity in apple seedlings by enhancing the lignin biosynthesis pathway through MdCOMT9

Aluminum (Al) toxicity profoundly restricts the growth of apples and results in major reductions in their yield. Among the elements present in the Earth's crust, silicon (Si) reduces the impact of heavy metals on the growth of plants by reducing the uptake of these metals and thereby minimizes oxidative damage. The aim of this study was to clarify the role of Si in relieving Al stress on apple seedlings. Our findings demonstrated that the application of exogenous Si lowered the concentration of Al in apple plants, alleviated Al-induced stress, and promoted seedling growth. The transcriptomic and metabolomic analyses indicated that Si alleviated the toxicity of Al primarily through phenylpropane biosynthesis, cyanuric acid metabolism, and flavonoid biosynthesis, with phenylpropane biosynthesis being the crucial one. Within this pathway, the expression of the key genes responsible for lignin synthesis-such as CYP98A, COMT, HCT, and 4CL-were upregulated and thereby enhanced the synthesis of lignin in cell walls along with the improved binding of Al to the cell wall. To further study the functions of the key genes involved in lignin synthesis, we combined transcriptomic data and gene family analysis and selected MdCOMT9 for overexpression. Under Al stress, MdCOMT9-overexpressing plants demonstrated a 95.8 % increase in the lignin content when compared with the content in wild-type plants. The length, surface area, tip volume, and tip number of the root increased by 40.5 %, 40.4 %, 39.6 %, and 34.3 %, respectively. Moreover, the activities of root, superoxide dismutase, peroxidase, and catalase enhanced. In conclusion, this study revealed the alleviating effect of exogenous Si on Al stress at the molecular level, highlighted the key role of MdCOMT9 in the Al stress response, provided a theoretical basis for further investigation into the molecular mechanisms underlying apple tolerance to Al, and offered a reference for breeding superior Al-resistant varieties.