Abstract
Nitrilases consist of a group of enzymes that catalyze the hydrolysis of organic cyanides. They are found ubiquitously distributed in the plant kingdom. Plant nitrilases are mainly involved in the detoxification of ß-cyanoalanine, a side-product of ethylene biosynthesis. In the model plant Arabidopsis thaliana a second group of Brassicaceae-specific nitrilases (NIT1-3) has been found. This so-called NIT1-subfamily has been associated with the conversion of indole-3-acetonitrile (IAN) into the major plant growth hormone, indole-3-acetic acid (IAA). However, apart of reported functions in defense responses to pathogens and in responses to sulfur depletion, conclusive insight into the general physiological function of the NIT-subfamily nitrilases remains elusive. In this report, we test both the contribution of the indole-3-acetaldoxime (IAOx) pathway to general auxin biosynthesis and the influence of altered nitrilase expression on plant development. Apart of a comprehensive transcriptomics approach to explore the role of the IAOx route in auxin formation, we took a genetic approach to disclose the function of NITRILASE 1 (NIT1) of A. thaliana. We show that NIT1 over-expression (NIT1ox) results in seedlings with shorter primary roots, and an increased number of lateral roots. In addition, NIT1ox plants exhibit drastic changes of both free IAA and IAN levels, which are suggested to be the reason for the observed phenotype. On the other hand, NIT2RNAi knockdown lines, capable of suppressing the expression of all members of the NIT1-subfamily, were generated and characterized to substantiate the above-mentioned findings. Our results demonstrate for the first time that Arabidopsis NIT1 has profound effects on root morphogenesis in early seedling development.
Highlights
Auxins are major plant growth regulators that control virtually all aspects of plant growth and development, including embryo development, cell expansion growth, cambial activity, and formation of lateral and adventitious roots
Most interesting was the finding that many plant hormone-related processes, such as the responses to salicylic acid (SA), ethylene (ET), abscisic acid (ABA), jasmonic acid (JA), and GA seem to be altered in cyp79b2/cyp79b3, which suggests a central role in integrating and processing plant hormone crosstalk for the IAOx pathway
We show that the reason for this is most likely not the compensation of the loss an auxin biosynthetic pathway by other metabolic routes that lead to indole3-acetic acid (IAA), but rather that the IAOx pathway does not contribute to general auxin biosynthesis under normal growth conditions
Summary
Auxins are major plant growth regulators that control virtually all aspects of plant growth and development, including embryo development, cell expansion growth, cambial activity, and formation of lateral and adventitious roots. In particular after the identification of nitrilases in the model plant Arabidopsis thaliana they were suggested to be key enzymes of IAA biosynthesis in that they catalyze the conversion of indole-3-acetonitrile (IAN) to IAA (Bartling et al, 1992, 1994; Bartel and Fink, 1994). This initial assumption did not stand the proof of time, since we know today that other enzymes, such as the members of the YUCCA-family, are the main players in auxin biosynthesis (Stepanova et al, 2011; Won et al, 2011)
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