Mycorrhyza-induced differential response to a yellow disease in tomato

The protective effects induced by arbuscular mycorrhizal (AM) fungi against a phytoplasma of the Stolbour group have been investigated in tomato by morphometry and flow cytometry. Symptoms induced by the phytoplasma were less severe when the plants also harboured AM fungi. Morphological parameters such as shoot and root fresh weight, shoot height, internode lenght, leaf number and adventitious root diameter were closer to those of healthy plants when arbuscular mycorrhyza were present. Reduced nuclear senescence was observed in AM plants infected with phytoplasmas; the parcentages of nuclear populations with different ploidy levels were intermediate between AM and phytoplasma-infected plants. The mechanisms underlying these interactions are discussed and a direct action of the AM fungus is hypothesized.