The growth defect of lrt1, a maize mutant lacking lateral roots, can be complemented by symbiotic fungi or high phosphate nutrition.

The growth of three maize (Zea mays L.) mutants, each impaired in the formation of one individual element of its root system, was compared under "natural" limiting phosphate conditions (0.1 mM). Mutant plants exhibiting a reduction in root hairs (rth3-1) or a depletion of crown and brace roots (rtcs) grew as well as the corresponding wild-type plants. However, mutant plants lacking lateral roots (lrt1) showed a strong reduction in plant growth. The growth defect of lrt1 was overcome when it was grown in association with an arbuscular mycorrhizal fungus, Glomus mosseae. Establishment of symbiosis was associated with the occurrence of a new type of lateral root. These new lateral roots were stunted and highly branched, giving rise to a bush-like structure. Supply of high phosphate (1 microM) ameliorated the growth of lrt1 plants too, but less efficiently than the symbiosis did. Hence, arbuscular mycorrhizal fungi as well as phosphate functionally complemented the lrt1 mutation.