Effect of an IAA overproducer mutant of the fungus hebeloma Cylindrosporum romagnesi on the early stages of ectomycorrhizal infection and carbohydrate content in seedlings of Pinus pinaster (ait.) SOL.

Abstract

Ectomycorrhizal and control seedlings of Pinus pinaster were cultured on a synthetic Melin-Norkrans medium in Petri dishes. Seedlings were inoculated with a mycelial slurry of an indole-3-acetic acid (IAA) overproducer Hebeloma cylindrosporum mutant 331. The wild strain H. cylindrosporum hl was used as a reference. Medium was supplemented or not with glucose. The mycelial slurry appeared to be very effective for mycorrhizal inoculation even on the medium without glucose. In such culture conditions ectomycorrhizal ability of the IAA overproducer mutant 331 was significantly higher than of the comparable wild type. The highest content of soluble sugars was found in stems and roots of plants mycorrhizal with the mutant followed by mycorrhizal plants with the wild type and then by the uninoculated control. Sucrose practically disappeared from roots of mycorrhizal plants. Starch content in roots of mycorrhizal plants with the IAA overproducer mutant was lower as compared with other treatments. Fungal auxin in mycorrhizal symbiosis seems to be responsible for maintaining the source-sink relationship. This is revealed by higher sugar level in the host's photosynthetic tissue (source) and the rise of soluble sugar content in roots (sink) due to enhanced translocation of sugars to the roots and auxin stimulated conversion of carbohydrates (sucrose, starch) of the host.