Sublethal fluazaindolizine doses inhibit development of the cyst nematode Heterodera schachtii during sedentary parasitism.

BACKGROUND Fluazaindolizine is a new compound for the control of plant-parasitic nematodes with an unknown and novel mode of action. This compound is very effective against important plant-parasitic nematodes. However, investigations elucidating the impact of sublethal fluazaindolizine doses on early nematode virulence and plant - nematode interaction parameters are lacking. RESULTS The effect of direct exposure of Heterodera schachtii juveniles to 50 ppm fluazaindolizine was negligible. Infection assays revealed a 57% reduction in adult females at 1.25 ppm and a 46% reduction in offspring at 40 ppm when juveniles were soaked in the compound for 48 hours and subsequently inoculated onto Arabidopsis thaliana. Pre-incubation of A. thaliana roots with fluazaindolizine was not effective against H. schachtii. Conversely, supplementing the plant growth medium with fluazaindolizine led to a significant reduction of adults (-35 %), females (-75 %) and female size at 1.25 ppm and nearly completely inhibited nematode parasitism at 5 ppm. The impact of fluazaindolizine on A. thaliana was dependent on plant age, compound concentration and duration of contact. Very low sublethal fluazaindolizine concentrations, 5 or 10 ppm, did not interfere with nematode mobility, host finding, penetration, and induction of the feeding site, but specifically inhibited sedentary nematode development inside the root in a concentration-dependent manner. CONCLUSION Fluazaindolizine does not have direct toxicity against infective juveniles, but has a clear effect on nematodes during sedentary development. The formation of females and the development of offspring are strongly reduced. It will be tempting to identify the underlying mechanism in the future.