Arbuscular mycorrhizal responsiveness of in vitro tomato root lines is not related to growth and nutrient uptake rates

Variability in growth and nutritional dynamics of in vitro tomato hairy root lines and their relationship with responsiveness to mycorrhizal colonization were studied. Four tomato cultivars were transformed with three Agrobacterium rhizogenes strains to obtain several hairy root lines, which were compared for growth and receptivity to Glomus intraradices. Four transformed hairy root lines were further characterized and compared with excised roots of the nontransformed tomato cultivar 'Cobra' and with Ri-T-DNA carrot hairy roots. Lines were compared during 4 months on minimal medium in terms of growth, nutrient uptake, and mycorrhizal colonization. In a subexperiment, the cultures were grown on a modified minimal medium to assess the contribution of initial inorganic phosphate concentration in mycorrhizal susceptibility of the three initially nonreceptive lines. On minimal medium, growth and nutrient uptake rates were highly correlated, but both were unrelated to mycorrhizal receptiveness. All the lines successfully established the symbiosis when the initial phosphate concentration was significantly reduced. No association was found between the origin of lines from the different tomato cultivar-bacterial strain combinations and the absence of symbiosis establishment on minimal medium. Decrease of inorganic phosphate concentration at the beginning of the culture was a key factor involved in precolonization steps of mycorrhizal symbiosis.