Abstract
Vacuolar processing enzymes (VPEs) play an important role during regular growth and development and defence responses. Despite substantial attempts to understand the molecular basis of plant-cyst nematode interaction, the mechanism of VPEs functioning during this interaction remains unknown. The second-stage Heterodera filipjevi juvenile penetrates host roots and induces the formation of a permanent feeding site called a syncytium. To investigate whether infection with H. filipjevi alters plant host VPEs, the studies were performed in Hordeum vulgare roots and leaves on the day of inoculation and at 7, 14 and 21 days post-inoculation (dpi). Implementing molecular, biochemical and microscopic methods we identified reasons for modulation of barley VPE activity during interaction with H. filipjevi. Heterodera filipjevi parasitism caused a general decrease of VPE activity in infected roots, but live imaging of VPEs showed that their activity is up-regulated in syncytia at 7 and 14 dpi and down-regulated at 21 dpi. These findings were accompanied by tissue-specific VPE gene expression patterns. Expression of the barley cystatin HvCPI-4 gene was stimulated in leaves but diminished in roots upon infestation. External application of cyclotides that can be produced naturally by VPEs elicits in pre-parasitic juveniles vesiculation of their body, enhanced formation of granules, induction of exploratory behaviour (stylet thrusts and head movements), production of reactive oxygen species (ROS) and final death by methuosis. Taken together, down-regulation of VPE activity through nematode effectors promotes the nematode invasion rates and leads to avoidance of the induction of the plant proteolytic response and death of the invading juveniles.