Green leaf volatiles (GLVs) are commonly emitted by green plants, and their production is drastically enhanced when they are under biotic stress. To clarify the ecological function of naturally emitted GLVs, we studied the response of
Arabidopsis
, whose GLV biosynthesis had been modified, when subjected to herbivory or a pathogenic infection. There was a significant increase in GLV production after herbivory by cabbage white butterfly larvae and pathogen (gray mold) infection in hydroperoxide lyase (HPL) sense
Arabidopsis
compared with WT controls. The
HPL
sense modification resulted in the plant being more attractive to the parasitic wasp
Cotesia glomerata
, leading to higher mortality of the herbivores. The
HPL
sense modification also resulted in greater inhibition of growth of the fungus. By contrast,
HPL
antisense
Arabidopsis
produced fewer GLVs, attracted fewer parasitoids, and was more susceptible to the pathogens than the WT control. These data show that (
i
) one of the ecological functions of GLV biosynthesis related to resistance against both herbivores and pathogens, and (
ii
) the genetic modification of GLV biosynthesis could be a unique approach for improving plant resistance against such biotic stresses.
