Auxin homeostasis in maize (Zea mays) is regulated via 1-O-indole-3-acetyl-myo-inositol synthesis at early stages of seedling development and under abiotic stress

MAIN CONCLUSION: Indole-3-acetyl-myo-inositol biosynthesis is regulated during maize seedling development and in response to drought and cold stress. The main purpose of this pathway is maintenance of auxin homeostasis. ABSTRACT: Indole-3-acetic acid (IAA) conjugation to myo-inositol is a part of a mechanism controlling free auxin level in maize. In this work, we investigated changes in the indole-3-acetyl-myo-inositol (IAInos) biosynthesis pathway in 3-d- and 6-d-old maize seedlings and germinating seeds as well as in seedlings subjected to drought and cold stress to evaluate a role of this pathway in maize development and stress response. In germinating seeds, activity of the enzymes involved in IAInos biosynthesis remains unchanged between 3-d- and 6-d-old material but increases in coleoptiles and radicles of the seedlings. Under cold stress, in germinating seeds and in coleoptiles, activity of the enzymes decreases and increases, respectively; however, it does not entail changes in auxin level. In drought-exposed germinating maize seeds, totally diminished activities of IAInos synthesis pathway enzymes resulted in almost twofold increase of free IAA content. Similar increase of auxin level was observed in radicles of drought-subjected seedlings together with lack of catalytic activity of the first enzyme of the pathway. Exogenous IAInos has no effect on the level of non-enzymatic antioxidant, ascorbate. It has also either no effect on the protein carbonylation and lipid peroxidation, or it affects it in a similar way as exogenously applied IAA and myo-inositol, which are products of IAInos hydrolysis. Thus, IAInos biosynthesis pathway acts in maize development and stress responses by regulation of free IAA concentration, as IAInos itself does not appear to have a distinct role in these processes.