A conserved mechanism for nitrile metabolism in bacteria and plants

Pseudomonas fluorescens SBW25 is a plant growth-promoting bacterium that efficiently colonises 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 β-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 β-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 β-cyano-l-alanine, and the β-cyano-l-alanine precursors cyanide and cysteine. pinA allows P. fluorescens SBW25 to use β-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 β-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 β-cyano-l-alanine, but also resulted in increased root elongation in the absence of exogenous β-cyano-l-alanine, demonstrating that β-cyano-l-alanine nitrilase activity can have a significant effect on root physiology and root development.