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Background  Terpenoids constitute the largest class of secondary metabolites made by plants and display vast chemical diversity among and within species. Terpene synthases (TPSs) are the pivotal enzymes for terpenoid biosynthesis that create the basic carbon skeletons of this class. Functional divergence of paralogous and orthologous  TPS  genes is a major mechanism for the diversification of terpenoid biosynthesis. However, little is known about the evolutionary forces that have shaped the evolution of plant  TPS  genes leading to terpenoid diversity.    Results  The orthologs of  Oryza Terpene Synthase 1  ( OryzaTPS1 ), a rice terpene synthase gene involved in indirect defense against insects in  Oryza sativa , were cloned from six additional  Oryza  species.  In vitro  biochemical analysis showed that the enzymes encoded by these  OryzaTPS1  genes functioned either as ( E )-β-caryophyllene synthases (ECS), or ( E )-β-caryophyllene & germacrene A synthases (EGS), or germacrene D & germacrene A synthases (DAS). Because the orthologs of OryzaTPS1 in maize and sorghum function as ECS, the ECS activity was inferred to be ancestral. Molecular evolutionary detected the signature of positive Darwinian selection in five codon substitutions in the evolution from ECS to DAS. Homology-based structure modeling and the biochemical analysis of laboratory-generated protein variants validated the contribution of the five positively selected sites to functional divergence of OryzaTPS1. The changes in the  in vitro  product spectra of OryzaTPS1 proteins also correlated closely to the changes in  in vivo  blends of volatile terpenes released from insect-damaged rice plants.    Conclusions  In this study, we found that positive Darwinian selection is a driving force for the functional divergence of OryzaTPS1. This finding suggests that the diverged sesquiterpene blend produced by the  Oryza  species containing  DAS  may be adaptive, likely in the attraction of the natural enemies of insect herbivores.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-014-0239-x) contains supplementary material, which is available to authorized users.

Positive Darwinian selection is a driving force for the diversification of terpenoid biosynthesis in the genus Oryza