Auxin induction is a trigger for root gall formation caused by root-knot nematodes in white clover and is associated with the activation of the flavonoid pathway

We studied the expression of the auxin responsive promoter (GH3) fused to the gusA reporter gene in white clover (Trifolium repens cv. Haifa) during the initiation of root galls by root-knot nematodes (Meloidogyne javanica) to investigate whether nematode infection affects auxin distribution in developing galls. In search for a plant signal that would mediate changes in auxin location we studied the induction of the flavonoid pathway because flavonoids can act as auxin transport regulators. Three chalcone synthase (CHS1, CHS2 and CHS3) promoter:gusA fusions were examined in transgenic plants and flavonoids were detected using fluorescence microscopy. Within 24 h post inoculation CHS:gusA expression occurred around the invading nematode. At 48 h post inoculation CHS:gusA expression and flavonoids were detected throughout the infection site, followed by high GH3:gusA expression in the gall 48-72 h post inoculation. Initially (48-72 h post inoculation) high GH3:gusA expression in giant cell precursors was followed by low expression in the enlarging giant cells (96-120 h post inoculation), suggesting that auxin is needed as a trigger for giant cell initiation but not for later enlargement. We suggest that nematodes control auxin distribution in the root and that flavonoids could be responsible for controlling auxin accumulation.