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Mycorrhiza-induced resistance against Thielaviopsis basicola in the ornemental crop Petunia hybrida

Abstract : Petunia hybrida is an ornamental crop of high economic interest but diverse root pathogens can cause high ¬losses, especially in soilless greenhouse production systems, and their control by conventional methods implies an excessive use of pesticides. A more sustainable horticulture requires alternative methods to counter these chemical inputs. The introduction of arbuscular mycorrhiza (AM), known to reduce a number of root diseases in other plant species, into the production itinerary could form an integral part of an appropriate strategy. However, mycorrhizal effects against soil-borne pathogens are not always predictable and mechanisms behind the protective effects of mycorrhiza are largely unknown. In this context, mycorrhiza-induced resistance (MIR) was studied in P. hybrida in an inert soilless substrate, and the underlying mechanisms were investigated.After testing different soil-borne pathogenic fungi causing disease in petunia nursery production, Thielaviopsis basicola was selected as a model pathosystem. Three AM fungal species were evaluated for their ability to protect petunia against T. basicola; only Glomus mosseae BEG 12 turned out to reduce disease symptoms and pathogen spread in roots. Split root experiments showed that this protective effect was systemic and could be induced in non-mycorrhizal parts of mycorrhizal root systems, in agreement with previous studies in other plant pathosystems. The AM fungus, moreover, reduced the amount of phosphate fertiliser input fivefold, and provides tolerance against high salt concentrations in the horticultural substrate.In order to gain insight into molecular mechanisms involved in the MIR to T. basicola in petunia roots, hypotheses were tested by analysing the expression patterns of plant genes which are involved in various pathways of known plant defence responses. Nine genes related to the jasmonic acid pathway of induced systemic resistance (ISR) by plant growth promoting bacteria and three genes activated by salicylic acid, a key molecule in systemic acquired resistance (SAR), were selected. Expression profiles of these genes indicated that local MIR to T. basicola in petunia roots does not primarily involve either pathway, whilst systemic MIR in this pathosystem could include elements of both SAR and ISR. The activation of seven AM-related genes was unaffected by T. basicola infection of mycorrhizal petunia roots showing that the pathogen does not affect symbiotic functionality. Results suggest that the part of the symbiotic cell programme covering AM-regulated plant defence genes may constitutively contribute to the expression of local MIR; the role of such genes in this phenomenon merits further attention and analyses
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Soukayna Hayek. Mycorrhiza-induced resistance against Thielaviopsis basicola in the ornemental crop Petunia hybrida. Agricultural sciences. Université de Bourgogne; Humboldt-Universität (Berlin), 2012. English. ⟨NNT : 2012DIJOS018⟩. ⟨tel-00808074⟩

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