Both induction and morphogenesis of cyst nematode feeding cells are mediated by auxin

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