Branching signal or growth promoting factor?

BUEE, MARC¹, GERALD NAGAHASHI², DAVID D DOUDS², GUILLAUME BECARD¹.

¹Mycologie Végétale, Université Paul Sabatier, 31062 Toulouse FRANCE, ²Eastern Regional Research Center, USDA, 600 E Mermaid Ln, Wyndmoor, PA, 19038 USA.

In the arbuscular mycorrhizal (AM) symbiosis, the exudates from host roots stimulate the growth and branching of mycorrhizal fungi. The bioassay previously described by Nagahashi et al. (last ICOM) consisting of micro-injecting concentrated fractions near growing hyphae, is being exploited for purification of the active molecules. As previously reported, and now confirmed, these compounds are not flavonoids. They appear to be active at very low concentrations. In contrast with results from nonhost roots (sugar beet), semi-purified root exudates from all mycotrophic plant species tested so far were active (carrot, tomato, corn, pea). Interestingly, exudates from carrot cell cultures were not. By using the natural cytoplasmic fluorescence of Gi. gigantea, the morphology and the viability of the branching structures were examined. They share some common features with internal arbuscules. Their functionality as specialized structures to take up nutrients (glucose) is being investigated. The role of the active molecules that trigger their formation needs to be further described at a cellular level. Particularly, the question as to whether or not nuclear division is associated with the formation of the branches needs to be addressed.