Phytohormones and willow gall induction by a gall-inducing sawfly

A variety of insect species induce galls on host plants. Several studies have implicated phytohormones in insect-induced gall formation. However, it has not been determined whether insects can synthesize phytohormones. It has also never been established that phytohormones function in gall tissues. Liquid chromatography and tandem mass spectrometry (LC/MS/MS) were used to analyse concentrations of endogenous cytokinins and the active auxin IAA in the gall-inducing sawfly (Pontania sp.) and its host plant, Salix japonica. Feeding experiments demonstrated the ability of sawfly larvae to synthesize IAA from tryptophan. Gene expression analysis was used to characterize hormonal signalling in galls. Sawfly larvae contain high concentrations of IAA and t-zeatin, and produce IAA from tryptophan. The glands of adult sawflies, the contents of which are injected into leaves upon oviposition and are involved in the initial stages of gall formation, contain an extraordinarily high concentration of t-zeatin riboside. Transcript levels of some auxin- and cytokinin-responsive genes are significantly higher in gall tissue than in leaves. The abnormally high concentration of t-zeatin riboside in the glands strongly suggests that the sawfly can synthesize cytokinins as well as IAA. Gene expression profiles indicate high levels of auxin and cytokinin activities in growing galls.