Indole-3-acetic acid production by Fusarium solani isolated from kiwifruit

Kiwifruit elephantiasis is characterized by hypertrophy of the trunk. Fusarium solani has been frequently isolated from affected plants and its involvement in the disease is being tested with pathogenicity trials underway. F. solani is known for its ability synthesize indole-3-acetic acid (IAA). This auxin is essential for cell growth, affecting both cell division and cellular expansion, and may promote axial elongation, lateral or isodiametric expansions at the tissue level. Monosporic cultures of 14 strains of F. solani were grown on agar-potato-dextrose (PDA) for determining their biological features (colonization ability and pigmentation) and the amount of IAA production. For in vitro tests, 3 cm long kiwifruit sprouts were placed on F. solani colonized PDA and the extent of tissue necrosis was evaluated. Substrate pigmentation induced by the fungus was studied at 25 degree C. Determination of free IAA was done by a capillary gas-chromatography-mass spectrometry-selected ion monitoring (GC-MS-SIM) using a Hewlett Packard 5890-5970 System. Half of the 14 isolates studied showed extensive tissue colonization, whereas most of them pigmented PDA with various levels of yellow/red tinge. IAA production was quite variable for all fungal strains. The high ability to colonize kiwifruit tissues by F. solani strains rarely corresponded to a high IAA production on PDA, except for two of the strains. These results call for further investigations on the relationships between host tissue colonization and IAA production by F. solani strains.