Various lines of evidence show that local changes in the auxin concentration are involved in the initiation and directional expansion of syncytia induced by cyst nematodes. Analysis of nematode infections on auxin-insensitive tomato and Arabidopsis mutants revealed various phenotypes ranging from complete inhibition of syncytium development to a decrease in hypertrophy and lateral root formation at the infection site. Specific activation of an auxin-responsive promoter confirmed the role of auxin and pointed at a local accumulation of auxin in developing syncytia Disturbance of auxin gradients by inhibiting polar auxin transport with N-(1-naphthyl)phtalamic acid (NPA) resulted in abnormal feeding cells, which were characterized by extreme galling, massive disordered cell divisions in the cortex, and absence of radial expansion of the syncytium initial toward the vascular bundle. The role of auxin gradients in guiding feeding cell morphogenesis and the cross-talk between auxin and ethylene resulting in a local activation of cell wall degrading enzymes are discussed.