The role of nitrilases in auxin biosynthesis, plant development and stress responses

NITRILASEs (NITs) are enzymes that have been identified across kingdoms. NITs are industrially important hydrolases, which are widely used in the production of valuable chemicals and medicines. In plants, NITs are phylogenetically divided into two groups: NIT1 and NIT4. The NIT1 (NIT1-3) subfamily that detoxifies nitriles is specific to the Brassicaceae and catalyzes the conversion of indole-3-acetonitrile, derived from indole glucosinolates or indole-3-acetaldoxime, into indole-3-acetic acid, the principal auxin, which provides an evolutionary advantage as it is a growth hormone. The NIT1 subfamily has been implicated in the catabolism of indole acetamide, although this has yet to be confirmed in planta. NIT4 appears to function in cyanide detoxification and exhibits strong specificity toward β-cyanoalanine. Additionally, it is hypothesized that NIT4, as well as enzymes of the NIT1 subfamily, might be involved in phenylacetic acid formation from phenylacetonitrile/benzyl cyanide. Crop plants, such as Zea mays and Oryza sativa, have been used to study NITs sporadically, consequently, our understanding of the role of NITs is primarily derived from studies of the model plant Arabidopsis thaliana, including single or sparse multiple mutants, reporter lines, or overexpressing lines. This review mainly focuses on the NIT1 subfamily, which plays a role in root and flower development. However, NITs expression and activity have been demonstrated mainly under plant stress conditions, including both biotic and abiotic stresses, such as saline, drought, sulfate deficiency, and thermomorphogenesis, during which NIT-dependent auxin biosynthesis is activated. In addition, the role of NITs has been confirmed in morphogenetic processes in in vitro cultures, highlighting their role in stress-induced developmental reprogramming.