Pre-infection interactions between roots and the mycorrhizal fungus Gigaspora gigantea: Chemotropism of germ-tubes and root growth response

Investigation of the chemotropic response of germinating spores of Gigaspora gigantea to plant roots, using a root organ culture assay, showed that aerial germ-tubes contacted host roots in 87 % of tomato cultures and in 56 % of corn cultures. In the presence of KMnO₄ or KOH this response was essentially eliminated. With non-hosts, germ-tubes were attracted to beet but not to kohlrabi roots. Contact between germ-tubes and roots of host plants led to a significant stimulation of root growth prior to the formation of any infection structures. In tomato root cultures, both root length and number of laterals increased significantly in response to germ-tube contact. Single fibres of glass wool placed on tomato roots failed to elicit a growth response. In corn root cultures, the number of laterals increased but total root length was unaffected. Roots of beet did not respond to germ-tube contact. Emerging lateral roots frequently grew toward germinating spores of G. gigantea, and this response was reduced in the presence of KMnO₄ but not in the presence of KOH. Stimulation of root growth in the pre-infection phase may be a previously unrecognized, non-nutritional benefit of the VAM symbiosis.