Localized alteration in lateral root development in roots colonized by an arbuscular mycorrhizal fungus

Abstract

The morphological responses of root systems to localized colonization by endophytes is not well understood. We examined the responses of lateral roots to the arbuscular mycorrhizal (AM) fungus Gigaspora margarita Becker & Hall inoculated locally into the soil. Peanut (Arachis hypogaea L.) and pigeon pea (Cajanus cajan (L.) Millsp.) were examined. Root boxes filled with nutrient-poor soil in were inoculated in one half with the fungus and in the other half with a sterilized inoculum. Responses were apparent after 30 days but not after 20 days. Overall, lateral root development was more advanced in inoculated soil. This was clearly observed for 2nd- and 3rd-order lateral roots, but less clear for 1st-order lateral roots in both species, although percentage of colonized root length was higher in 1st-order lateral roots. Whilst in peanut the responses were clearly evident at the level of lateral roots initiated on more proximal parts of the tap root axis, they occurred on more distal parts in pigeon pea. We conclude that plants under nutrient-poor conditions give priority to mycorrhizal roots when partitioning assimilation products within the root system. Thus, AM formation may induce local morphological alteration of root systems.