Interactions and physiological effects of diazotrophic bacteria and arbuscular mycorrhizal fungi in cassava plants

Abstract

For better understanding of interactions between arbuscular mycorrhizal fungi and diazotrophic bacteria in cassava plants (Manihot esculenta Crantz), several experiments were performed to check the effects of plant and bacteria exsudates on arbuscular mycorrhizal fungi and diazotrophic bacteria indoleacetic acid (IAA) production. Diazotrophic bacteria stimulated mycorrhizal root colonization of Glomus clarum after 30 days. The beneficial effects could be related to growth promoting compounds able to stimulate plant susceptibility to mycorrhizal infection, spore germination or the growth of mycelium, being able to increase the contact between fungal hypha and plant roots and, consequently, to increase mycorrhizal colonization. Addition of cassava exsudates stimulated growth of diazotrophic bacteria in vitro, showing that they could contain molecular signals or growth stimulators, once the effects did not seem to be related to a nutritional factor. It was also observed that diazotrophic bacteria could produce IAA in vitro. Azospirillum lipoferum isolated from cassava produced up to 130 µM of IAA after 48 h of incubation, while Klebsiella sp. accumulated about 60 µM and Bacterium E approximately 20 µM of IAA.