Indole-3-acetic acid production by Fusarium solani and its levels in plant tissues of actinidia deliciosa

Since 2001 an unusual disease, named elephantiasis, was observed in the kiwifruit cultivar ‘Hayward’ in orchards of Northern Italy. The symptom most typical of elephantiasis is trunk hypertrophy. Fusarium solani has been frequently isolated from affected plants and its involvement in the disease is being tested with pathogenicity trials that are underway. F. solani is known for its ability to synthesize indole-3-acetic acid (IAA). This auxin is essential for cell growth, affecting both cell division and cellular expansion, and it may promote at tissue level axial elongation, lateral or isodiametric expansions. F. solani strains were grown in vitro on agar-potato-dextrose (PDA) to screen their biological and growth promoting traits (colonization ability and IAA production). Furthermore, to investigate whether the fungus affects plant hormone metabolism during the course of plant-fungus interaction, we used an in vitro system that allowed us to determine the IAA levels in plant tissues by using kiwifruit shoot cultures that where inoculated with fungal filtrate of different strains. All the tested F. solani strains were shown to produce IAA but at different concentrations. When the shoots were inoculated with high- and low-IAA producer strains, the concentration of this hormone in the plant tissue followed a trend similar to that produced by the fungus. The excess production of this growth hormone could be an important aspect to explain the hypetrophy trunk symptom found in diseased plants, thus the effect of F. solani on the IAA metabolism in affected plant tissues is discussed.