Involvement of nitrilases in auxin biosynthesis and their role in plant development and stress response

Abstract NITRILASEs (NITs) are enzymes that have been identified across kingdoms. Nitrilases are industrially important hydrolases widely used in the production of valuable chemicals and medicines. In plants, nitrilases are phylogenetically divided into two groups: NIT1 and NIT4. The NIT1 (NIT1–3) subfamily detoxifying nitriles is specific to the Brassicaceae and catalyze the conversion of indole-3-acetonitrile (IAN), derived from indole glucosinolates (IGs) or indole-3-acetaldoxime (IAOx), into indole-3-acetic acid (IAA), the principal auxin, which provides an evolutionary advantage since it's a growth hormone. The NIT1 subfamily has been implicated in the catabolism of indole acetamide (IAM), 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 (PAA) formation from phenylacetonitrile/benzyl cyanide (PAN/BnCN). Crop plants, such as Zea mays and Oryza sativa, have been used to study NITs sporadically, consequently, our understanding of the role of nitrilases 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 primarily been demonstrated under plant stress conditions, biotic and abiotic stress, such as saline, drought, sulfate deficiency, 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.