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Le mildiou causé par Phytophthora infestans est une des maladies les plus dévastatrices de la pomme de terre et son contrôle requiert de nombreuses applications de fongicides souvent chimiques et toxiques. Pour se substituer à ces molécules, l’utilisation d’agents de lutte biologiques connait un engouement croissant ces dernières années. Dans notre étude, quatre filtrats (sur 149) de levures et champignons filamenteux sélectionnés d’après leur effet sur la croissance de P. infestans in vitro, ont été combinés avec le CMA Rhizophagus irregularis MUCL 41833 en billes d’alginate pour tester tout d’abord leur compatibilité, à la fois en conditions in vitro et in vivo. Ensuite, l’efficacité de la combinaison CMA avec Trichoderma virens MUCL 18139 a été évaluée sur le biocontrôle du mildiou chez la variété sensible Annabelle en conditions de serres. Pour cela, le CMA a été introduit dans le substrat au moment de la plantation et le filtrat (1) appliqué sur les parties aériennes en spray un jour avant ou un ou cinq jours après l’inoculation de P. infestans, (2) encapsulé dans des billes d’alginates avec le CMA au moment de la plantation, (3) soit les deux. Les résultats ont montré qu’après 15 jours de développement du mildiou sur les feuilles de pomme de terre, l’inoculation des racines avec le CMA seul ou encapsulé avec le filtrat de T. virens diminuait significativement l’aire sous la courbe de progression de la maladie de 69 et 77% respectivement (résultats similaires à ceux obtenus avec le fongicide Mancozèbe), en comparaison avec les plantes non-traitées. A l’inverse, l’application foliaire du filtrat, quelque soit la colonisation des racines par le CMA, n’a pas réduit les symptômes de la maladie. Nos résultats démontrent le potentiel offert par la combinaison de filtrats d’agents de bio-contrôle et de CMA pour contrôler le mildiou de la pomme de terre.

Late blight caused by Phytophthora infestans is the most devastating disease of potato crops worldwide. Control practices mostly rely on synthetic fungicides or copper-based products. In recent years, biological control agents have generated significant enthusiasm as eco-friendly sustainable alternatives. Here, the filtrates of 149 filamentous fungi and yeasts were tested in vitro against P. infestans MUCL 54981. The most effective filtrates were then combined in alginate beads with the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis MUCL 41833 and tested in vitro for their compatibility, and finally in the greenhouse for their efficacy against P. infestans. The filtrates of Penicillium aurantiogriseum MUCL 47586, Penicillium griseofulvum MUCL 29201, Trichoderma virens MUCL 18139, and Verticillium psalliotae MUCL 18310 totally suppressed conidial germination and growth of P. infestans in vitro. In whole potato plants in vitro, only the filtrate of T. virens significantly suppressed P. infestans plant infestation. With the exception of P. griseofulvum, none of the selected filtrates affected the germination, root colonization, and spore production of R. irregularis following their combined entrapment in alginate beads. According to these results, the filtrate of T. virens was selected for the greenhouse experiment with the potato variety Annabelle. The filtrate was used as leaf spray and/or as root treatment in combination with R. irregularis entrapped in alginate beads. Root treatments with AMF alone as well as with the AMF entrapped with the filtrate of T. virens significantly reduced the area under disease progress curve compared to the non-treated plants. LC-MS analyses of the filtrate showed the production of koninginin D, gliotoxin, and koningic acid, three antifungal compounds, which could have impacted P. infestans. Conversely, foliar application of the filtrate, whether the plant was colonized by the AMF or not, did not reduce symptoms of the dis

Most plants are connected belowground via common mycorrhizal networks (CMNs). In their presence, the transmission of warning signals from diseased to uninfected plants has been reported. However, current studies have all been conducted in pots making it difficult to discriminate direct from indirect contribution of hyphae to the transmission of the signals. Here, we conducted an in vitro study with potato plantlets connected by a CMN of the arbuscular mycorrhizal fungus Rhizophagus irregularis. The plantlets were grown in physically separated compartments and their connection ensured only by the CMN. The donor potato plantlets were infected by Phytophthora infestans and defense genes analyzed 24, 48 and 120 h post-infection (hpi) in the uninfected receiver potato plantlets. Twenty-four hpi by the pathogen, PAL, PR-1b, ERF3, and LOX genes were significantly upregulated, whereas no significant transcript variation was noticed 48 and 120 hpi. The exact nature of the warning signals remains unknown but was not associated to microorganisms other than the AMF or to diffusion mechanisms through the growth medium or induced by volatile compounds. The defense response appeared to be transitory and associated with the jasmonic acid or ethylene pathway. These findings demonstrate the direct involvement of hyphae in the transmission of warning signals from diseased to uninfected potato plantlets and their indubitable role in providing a route for activating defense responses in uninfected plants.

In organic potato production in Europe, only copper-based fungicides allow to directly control Phytophthora infestans (the causal agent of late blight). Due to environmental concerns caused by the repeated and excessive use of Cu before the nineties, the EU legislation has promoted alternatives approaches such as the use of biocontrol agents (e.g. arbuscular mycorrhizal fungi – AMF). Here, two field trials were conducted over two climatic-contrasting growing seasons. Trial 1 was characterized by a dry and hot cultural season with low pressure of P. infestans, while trial 2 was conducted under high humidity and relatively low temperatures with high pressure of the pathogen. In both trials, sprouted potato tubers were inoculated with AMF in the greenhouse before transplanting to the field. A Real-Time quantitative PCR assay was designed to target the inoculant strain Rhizophagus irregularis MUCL 41833 as well as the native Rhizophagus irregularis strains. In both trials, the inoculated AMF was detected in the roots at harvest, demonstrating the capacity of the inoculated strain to incorporate the microbiome of the potato plants. In the first trial, disease severity in AMF pre-colonized potato plants was markedly decreased and the onset of late blight symptoms was delayed by 10 days. In contrast, in the second trial no differences were noticed between AMF pre-colonized and control plants. In both trials, no mycorrhizal effect was noticed on tuber yield. As a conclusion, disease severity of P. infestans, measured by symptoms development on leaves, was decreased in AMF pre-colonized plants under conditions of low pressure of late blight and over a short period of time, while under conditions more adequate to the pathogen, no reduction in symptoms was noticed.

Long-term maintenance of arbuscular mycorrhizal fungi (AMF) by in vitro or in vivo subcultivation is often expensive and time-consuming and could present the risk of contaminations and possibly morphological, physiological, and genetic variations over time. Recently, in vitro produced AMF isolates belonging to the genus Rhizophagus were successfully cryopreserved at -130 °C following encapsulation-drying. Here, this method was tested on 12 single species cultures belonging to six different genera (i.e., Rhizophagus, Glomus, Claroideoglomus, Septoglomus, Paraglomus, and Gigaspora) produced in vitro or in vivo. Their viability was estimated, after 1 month of cryopreservation at -130 °C, by the percentage of potentially infective beads (i.e., the percentage of beads that contained at least one germinated propagule) for the in vitro produced species and the percentage of infective beads (i.e., the percentage of beads that contained at least one propagule able to colonize a new host plant in pot culture) for the in vivo produced species. With the exception of Gigaspora sp. MUCL 52331 and Septoglomus constrictus PER 7.2, no significant differences were observed in the viability of the single species cultures before and after cryopreservation. These results, thus, demonstrated the suitability of the cryopreservation method by encapsulation-drying for AMF species belonging to different genera and produced in vitro or in vivo. This method opens the door to the long-term preservation at ultra-low temperature of a large number of AMF species and for the preservation of species that are still recalcitrant to in vitro cultivation. © 2013 Springer-Verlag Berlin Heidelberg.

Due to their low substrate specificity, fungal laccases have a great potential in industrial applications, including the bioremediation of colored wastewaters from textile industry. However, the presence of halides in these effluents (up to 1M NaCl) which inhibit laccases is a drawback for bioremediation processes. In order to develop an efficient enzymatic remediation process for textile dye effluent, the possibility to reduce this halide inhibition is conditioned by a better understanding of the phenomenon. The present study gives a detailed account of the kinetics of chloride inhibition of both ABTS (a model substrate) and ABu62 (an anthraquinonic acid dye) oxidations catalyzed by Trametes versicolor laccase (LacIIIb). Chloride inhibition can be described by a mixed model for ABTS and a non-competitive model for ABu62 and both inhibitions are linear suggesting a single inhibitory site for chloride. Experiments were also conducted in presence of both substrates. An apparent activation of laccase was observed in the presence of ABu62 leading to an enhancement of the oxidation rate of ABTS. The extent of activation increased in the presence of chloride anions. Finally, for the first time to our knowledge, we evidenced that inhibition of ABTS oxidation by chloride can be reduced in the presence of ABu62.