Mycorrhizal inoculation protected bread wheat against Septoria tritici blotch.
Résumé
Septoria tritici blotch (STB) is one of the most common diseases encountered in bread wheat crops and
can cause yield losses of up to 50%. Its causal agent, the hemibiotrophic fungus Zymoseptoria tritici
(Z. tritici), is developing increasing resistance to conventional fungicide treatments. As a consequence,
STB is difficult to control despite ever more treatments with detrimental impact on environment and
human health. To reduce their use, new control strategies compatible with more sustainable
agriculture have to be developed.
Mycorrhizal symbiosis occurring between plants and arbuscular mycorrhizal fungi (AMF) is known to
improve plant mineral and water nutrition as well as plant resistance against biotic stresses through
Mycorrhiza Induced Resistance (MIR). In this context, the objective of the present work, carried out
within the framework of the Bioscreen project (Smartbiocontrol, Interreg V), was to study the eventual
control of STB through mycorrhizal inoculation with the AMF species Funneliformis mosseae (F.
mosseae), and to get insights in the defense mechanisms involved. For this purpose, 6-week-old
wheats (cv. Alixan) susceptible to STB were inoculated with F. mosseae before infection with Z. tritici
in controlled conditions. At 18 days postinfection (18 dpi), the mycorrhizal root colonization rate was
estimated at 40% and allowed an 80% protection against STB of mycorrhizal wheats compared to
non-mycorrhizal ones. Leaf and root samples were collected during the biotrophic phase of Z. tritici (2
dpi) to monitor the expression of both defense genes induced by the MIR in leaves and genes involved
in nutrient transports relative to mycorrhizal symbiosis in roots. Results show that F. mosseae
inoculated at root level is able to induce genes encoding PR proteins and proteins involved in both
antioxidant systems and the salicylic acid pathway in leaves.
The observed protection was essentially based on a systemic elicitation of defense genes since we did
not observe any priming effect during mycorrhizal wheat infection with Z. tritici. Genes involved in
nutrient exchanges with AMF were induced in mycorrhizal wheat roots with no alteration by STB,
except for SWEET transporters which were induced only in infected wheats. On the whole, these
results lead us to propose F. mosseae as an interesting candidate biocontrol agent for field trials.