Caractérisation moléculaire d'un extrait d'algues vertes, stimulateur des défenses des plantes contre les agents pathogènes

La capacité des plantes à se défendre contre les parasites dépend de la perception d'éliciteurs qui induisent les défenses des plantes. Afin de les utiliser comme méthode alternative pour contrôler les maladies, il est nécessite d'identifier des sources abondantes et de caractériser leur activité. Au cours de ce travail, nous avons identifié un nouveau polysaccharide extrait d'algue verte, Ulva spp., et caractérisé son activité biologique. Les molécules présentes dans l'extrait ont été séparées par leur poids moléculaire (PM). Seule la fraction de haut PM, qui contient essentiellement un polysaccharide sulfaté appelé ulvane, possède une activité. Les effets des ulvanes sur le transcriptome de M. truncatula ont été comparés à ceux du méthyl jasmonate (MeJA) et de l'acibenzolar-S-methyl. La signature transcriptomique obtenue en réponse aux ulvanes présente de nombreuses similitudes à celle obtenue en réponse au MeJA. Ces deux traitements induisent des réponses régulées par la voie du JA telle que l'activation des inhibiteurs de protéases. L'expression de gènes dépendants cette voie est aussi induite chez A. thaliana en réponse aux ulvanes. Enfin, les ulvanes entraînent une inhibition de la croissance in-vitro, qui est partiellement affectée chez le mutant PEPR2. L'efficacité protectrice de l'extrait brut a été testée contre divers parasites. Ainsi, il protège contre les oomycetes et les champignons responsables, respectivement, d'oïdium et de mildiou. Nos résultats montrent que les ulvanes induisent les défenses des plantes via la voie du JA et soulignent l'intérêt des outils d'analyse transcriptomique pour identifier de nouveaux éliciteurs capables de protéger les plantes.The ability of plants to defend themselves against pathogens depends on the perception of elicitors, which induce plant defence.They might be considered as alternative tools for disease control in agronomic crops. Their industrial use needs to identify abundant sources and characterize their activity. Here we report on the identification of a new polysaccharide purified from the green algae Ulva spp. and the characterization of it biological activity in plants. Molecules contained in the extract were sized-fractionated. Analyses of the fractions revealed that biological activity was present only in the fraction of high molecular weight which contained most exclusively a sulfated polysaccharide named ulvan. Response to ulvan were compared to those induced upon methyl jasmonate (MeJA) and acibenzolar-S-methyl on the legume M. truncatula using microarrays. Interestingly, ulvan gene expression signature showed significant similarity to MeJA and typical responses controlled by the JA pathway, such as induction of protease inhibitor activity. Expression of JA responsive genes, like PDF1.2, was also induced in A. thaliana after ulvan treatment. Ulvan provoked an inhibition of in-vitro growth, which was partially impaired in the mutant PEPR2. The efficient protection induced by the crude extract was tested against pathogens on various crops. So, it protect well against oomycetes and fungus that respectively cause downy and powdery mildews. Our results show that ulvan induces plant defences through the jasmonate signalling pathway and highlight the use of functional genomics to develop new bioactive compounds for plant protection

Ulvan, a Sulfated Polysaccharide from Green Algae, Activates Plant Immunity through the Jasmonic Acid Signaling Pathway

The industrial use of elicitors as alternative tools for disease control needs the identification of abundant sources of them. We report on an elicitor obtained from the green algae Ulva spp. A fraction containing most exclusively the sulfated polysaccharide known as ulvan-induced expression of a GUS gene placed under the control of a lipoxygenase gene promoter. Gene expression profiling was performed upon ulvan treatments on Medicago truncatula and compared to phytohormone effects. Ulvan induced a gene expression signature similar to that observed upon methyl jasmonate treatment (MeJA). Involvement of jasmonic acid (JA) in ulvan response was confirmed by detecting induction of protease inhibitory activity and by hormonal profiling of JA, salicylic acid (SA) and abscisic acid (ABA). Ulvan activity on the hormonal pathway was further consolidated by using Arabidopsis hormonal mutants. Altogether, our results demonstrate that green algae are a potential reservoir of ulvan elicitor which acts through the JA pathway

An Ulva armoricana extract protects plants against three powdery mildew pathogens

International audienceThe protective activity of a crude extract prepared from the green macroalga, Ulva armoricana, previously shown to induce plant defence responses, was evaluated on three plant species, common bean, grapevine and cucumber, cultivated in the greenhouse and inoculated with three powdery mildew pathogens Erysiphe polygoni, E. necator and Sphareotheca fuliginea respectively. Chemical analyses showed that the extract was enriched in ulvans, which are green algae polysaccharides essentially composed of uronic acid and sulphated rhamnose. Weekly applications were performed by spraying of the green algal extract at various dilutions on bean, grapevine and cucumber leaves. A significant effect (50% protection) was observed using a dilution corresponding to about 3 gl(-1) dry matter and up to 90% reduction of symptom severity was obtained for the highest concentration (1/9 dilution, 6 gl(-1) dry matter) for the three plant species. To study the natural variability of the protective activity, five extracts prepared from algae batches harvested at different year periods were evaluated. Although polysaccharide composition varied among batches, all extracts elicit a reporter gene regulated by a defence-gene promoter in a transgenic tobacco line, and protect cucumber plants against powdery mildew infection. Together, these data demonstrate that U. armoricana is a reproducible source of active compounds which can be used to efficiently protect crop plants against powdery mildew diseases