Howden Andrew J M, Harrison C Jill, Preston Gail M
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.
Plant J. 2009 Jan;57(2):243-53. doi: 10.1111/j.1365-313X.2008.03682.x. Epub 2008 Oct 7.
Pseudomonas fluorescens SBW25 is a plant growth-promoting bacterium that efficiently colonizes the leaf surfaces and rhizosphere of a range of plants. Previous studies have identified a putative plant-induced nitrilase gene (pinA) in P. fluorescens SBW25 that is expressed in the rhizosphere of sugar beet plants. Nitrilase enzymes have been characterised in plants, bacteria and fungi and are thought to be important in detoxification of nitriles, utilisation of nitrogen and synthesis of plant hormones. We reveal that pinA is a NIT4-type nitrilase that catalyses the hydrolysis of beta-cyano-L-alanine, a nitrile common in the plant environment and an intermediate in the cyanide detoxification pathway in plants. In plants cyanide is converted to beta-cyano-L-alanine, which is subsequently detoxified to aspartic acid and ammonia by NIT4. In P. fluorescens SBW25 pinA is induced in the presence of beta-cyano-L-alanine, and the beta-cyano-L-alanine precursors cyanide and cysteine. pinA allows P. fluorescens SBW25 to use beta-cyano-L-alanine as a nitrogen source and to tolerate toxic concentrations of this nitrile. In addition, pinA is shown to complement a NIT4 mutation in Arabidopsis thaliana, enabling plants to grow in concentrations of beta-cyano-L-alanine that would otherwise prove lethal. Interestingly, over-expression of pinA in wild-type A. thaliana not only resulted in increased growth in high concentrations of beta-cyano-L-alanine, but also resulted in increased root elongation in the absence of exogenous beta-cyano-L-alanine, demonstrating that beta-cyano-L-alanine nitrilase activity can have a significant effect on root physiology and root development.
荧光假单胞菌SBW25是一种促进植物生长的细菌，能有效地定殖在多种植物的叶表面和根际。先前的研究在荧光假单胞菌SBW25中鉴定出一个假定的植物诱导腈水解酶基因（pinA），该基因在甜菜植物的根际表达。腈水解酶已在植物、细菌和真菌中得到表征，被认为在腈的解毒、氮的利用和植物激素的合成中起重要作用。我们发现pinA是一种NIT4型腈水解酶，它催化β-氰基-L-丙氨酸的水解，β-氰基-L-丙氨酸是植物环境中常见的腈，也是植物氰化物解毒途径中的中间体。在植物中，氰化物转化为β-氰基-L-丙氨酸，随后被NIT4解毒为天冬氨酸和氨。在荧光假单胞菌SBW25中，pinA在β-氰基-L-丙氨酸以及β-氰基-L-丙氨酸的前体氰化物和半胱氨酸存在的情况下被诱导。pinA使荧光假单胞菌SBW25能够利用β-氰基-L-丙氨酸作为氮源，并耐受这种腈的有毒浓度。此外，pinA被证明可以弥补拟南芥中的NIT4突变，使植物能够在否则会致命的β-氰基-L-丙氨酸浓度下生长。有趣的是，在野生型拟南芥中过表达pinA不仅导致在高浓度β-氰基-L-丙氨酸下生长增加，而且在没有外源β-氰基-L-丙氨酸的情况下也导致根伸长增加，这表明β-氰基-L-丙氨酸腈水解酶活性可以对根生理学和根发育产生显著影响。
