Figure 7.

The involvement and crosstalk of plant growth substances against Al sensitivity in plants. Growth hormones like Aux, GA, ABA, CK, JA, and ET induce several cellular processes in roots of plants exposed to Al hyper-accumulation. Both external and internal detoxification by exclusion, sequestering, and chelation reactions are important under hormonal influence. Reduction of H⁺ by inhibition of H⁺/ATPase activity is important to downregulate the acidification of apoplasts of roots to reduce Al ions (Al³⁺). ET induces auxin accumulation as well as auxin transportation by pin formed 2 (PIN2)-mediated polar transport that results in root growth inhibition. Acidification of cell wall by organic acids (malate, citrate, oxalate) is important for ligand formation with metal. ABA and ET have synergistic actions with negative regulation, where RBOH and short-term response, respectively, are important. ABA can also induce ROS biosynthesis, which directly influences Al tolerance genes. Other substances like JA and CK are distantly related to Al tolerance by overexpression of coronatine insensitive 1 and myeloblastosis viral oncogene homologue 2 (COI1, MYC2) and direct inhibition of root growth.