Mycorrhizal colonization with Glomus intraradices and development stage of transformed tomato roots significantly modify the chemotactic response of zoospores of the pathogen Phytophthora nicotianae

The chemotaxic response of zoospores of the plant pathogen, Phytophthora nicotianae, towards exudates from mycorrhizal and non-mycorrhizal transformed tomato roots was studied. A bi-compartmental in vitro system was used to grow Ri T-DNA-transformed tomato roots colonized or non-colonized with the arbuscular mycorrhizal fungus, Glomus intraradices, and to collect root and mycorrhizal exudates. The root and mycorrhizal growth dynamics were first characterized in order to determine two times of exudate sampling. Exudates collected from 16-wk-old mycorrhizal roots were significantly more attractive for P. nicotianae zoospores than exudates from non-inoculated roots. On the contrary, concentrated exudates harvested from 24-wk-old mycorrhizal roots were repulsive to zoospores compared to exudates from non-colonized roots and the water control. In exudates of G. intraradices-inoculated roots, HPLC–MS analyses revealed significantly higher concentrations of proline and isocitrate after 24 wk of growth, while after 16 wk of growth, proline concentration did not differ between exudate types, and the isocitrate concentration was lower in mycorrhizal root exudates. Mycorrhizal inoculation had no effect on the amounts of other amino acids and organic acids and on the sugars quantified within exudates. Our results suggest that modification in exudate composition of mature roots by mycorrhizal colonization may provoke the repulsion of P. nicotianae, and that their capacity to infect host roots may in this manner be reduced.