Impact of culture conditions on root growth and mycorrhization of Hevea brasiliensis in vitro

(eng) Hevea brasiliensis is cultivated worldwide for the production of natural rubber. In vitro studies on the mycorrhization of this tree are essential to increase the knowledge about the biology of the symbiosis, and to validate the utilization of arbuscular mycorrhizal fungi (AMF) in rubber nurseries. The objective of this thesis was to study the effect of the culture conditions on the association between the AMF Rhizophagus irregularis and H. brasiliensis plantlets growing in an autotrophic in vitro culture system. The combination of high atmospheric CO2 with addition of MES and activated charcoal to the culture medium improved root growth and colonization. The pruning of the taproot did not significantly impact these parameters. None of the treatments accelerated the formation of new roots or their colonization, suggesting that other factors (e.g. plant genetics and physiology) may be involved. We hypothesized that the delay in root colonization may be related to root exudates produced by H. brasiliensis. To test this hypothesis, we placed Medicago truncatula seedlings in an actively growing extraradical mycelium network of R. irregularis, in close vicinity of a H. brasiliensis plantlet. We observed a decrease in the root colonization of M. truncatula, and a negative effect on the metabolic activity of the AMF, suggesting that roots exudates containing antifungal molecules were produced. The experimental system adapted during this thesis offers large opportunities for molecular and physiological studies that could contribute to elucidate the factors (e.g. plant secondary metabolism) that are regulating the first steps of the symbiosis.