Insect-damaged rice plants emit a complex mixture of volatiles that are highly attractive to parasitic wasps. Indole is one constituent of insect-induced rice volatiles, and is produced in plants by the enzyme indole-3-glycerol phosphate lyase (IGL). The alpha-subunit of tryptophan synthase (TSA) is the IGL that catalyses the conversion of indole-3-glycerol phosphate to indole in the alpha-reaction of tryptophan synthesis; however, TSA is only active in the complex with the beta-subunit of tryptophan synthase and is not capable of producing free indole. In maize a TSA homolog, ZmIgl, is the structural gene responsible for volatile indole biosynthesis. Bioinformatic analysis based on the ZmIgl-sequence indicated that the rice genome contains five homologous genes. Three homologs Os03g58260, Os03g58300 and Os07g08430, have detectable transcript levels in seedling tissue and were expressed in both insect-damaged and control rice plants. Only Os03g58300, however, was up-regulated by insect feeding. Recombinant proteins of the three rice genes were tested for IGL activity. Os03g58300 had a low K(m) for indole-3-glycerol phosphate and a high k(cat), and hence can efficiently produce indole. Os07g08430 exhibited biochemical properties resembling characterized TSAs. In contrast, Os03g58260 was inactive as a monomer. Analysis of Os03g58300 expression and indole emission provides further support that Os03g58300 is the bona fide rice IGL for biosynthesis of indole, in analogy to maize, this gene is termed OsIgl. Phylogenetic analysis showed that the rice genes are localized in two distinct clades together with the maize genes ZmIgl and ZmBx1 (Os03g58300) and ZmTSA (Os03g58260 and Os07g08430). The genes in the two clades have distinct enzyme activities and gene structures in terms of intron/exon organization. These results suggest that OsIgl evolved after the split of monocot and dicot lineages and before the diversification of the Poaceae.
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