Silicon- and Boron-Induced Physio-Biochemical Alteration and Organic Acid Regulation Mitigates Aluminum Phytotoxicity in Date Palm Seedlings

The current study aimed to understand the synergistic impacts of silicon (Si; 1.0 mM) and boron (B; 10 µM) application on modulating physio-molecular responses of date palm to mitigate aluminum (Al(3+); 2.0 mM) toxicity. Results revealed that compared to sole Si and B treatments, a combined application significantly improved plant growth, biomass, and photosynthetic pigments during Al toxicity. Interestingly, Si and B resulted in significantly higher exudation of organic acid (malic acids, citric acids, and acetic acid) in the plant’s rhizosphere. This is also correlated with the reduced accumulation and translocation of Al in roots (60%) and shoots (56%) in Si and B treatments during Al toxicity compared to in sole Al(3+) treatment. The activation of organic acids by combined Si + B application has significantly regulated the ALMT1, ALMT2 and plasma membrane ATPase; PMMA1 and PMMA3 in roots and shoots. Further, the Si-related transporter Lsi2 gene was upregulated by Si + B application under Al toxicity. This was also validated by the higher uptake and translocation of Si in plants. Al-induced oxidative stress was significantly counteracted by exhibiting lower malondialdehyde and superoxide production in Si + B treatments. Experiencing less oxidative stress was evident from upregulation of CAT and Cyt-Cu/Zn SOD expression; hence, enzymatic activities such as polyphenol oxidase, catalase, peroxidase, and ascorbate peroxidase were significantly activated. In the case of endogenous phytohormones, Si + B application demonstrated the downregulation of the abscisic acid (ABA; NCED1 and NCED6) and salicylic acid (SA; PYL4, PYR1) biosynthesis-related genes. Consequently, we also noticed a lower accumulation of ABA and rising SA levels under Al-stress. The current findings illustrate that the synergistic Si + B application could be an effective strategy for date palm growth and productivity against Al stress and could be further extended in field trails in Al-contaminated fields.