Résultat de la recherche
Root inoculation with Pseudomonas putida KT2440 induces transcriptional and metabolic changes and systemic resistance in maize plants
Direct and indirect effect of the rhizobacteria 'Pseudomonas putida' KT2440 on maize plants
To protect themselves against biotic and abiotic stresses, plants have developed a broad range of defense mechanisms that are constitutively present or that can be induced in response to a stress. Such induced defense can be the result of a resistance elicitation by non-pathogenic organisms that are present in soil and is then called induced systemic resistance (ISR). ISR confers plant resistance against a large variety of attackers such as pathogens and herbivores. In monocotyledonous plants this phenomenon has been less studied than in dicots such as Arabidopsis. Hence, the aim of this thesis was to enhance our knowledge on ISR in monocots and more specifically in maize plants.
To facilitate plant root manipulation for our experiments, we established a soil-free system for growing maize plants. Based on an existing system for root observation, we adapted a system that is convenient for working with beneficial as well as pathogenic microbes.
ISR establishment necessitates the local recognition of the beneficial microbe by the plant. Thus, we studied the reaction of maize after inoculation with a well-known maize colonizer rhizobacterium Pseudomonas putida KT2440. The presence of these bacteria activated plant immunity early in the interaction. We hypothesized that KT2440 manipulates root defense to be able to colonize roots. We observed that KT2440 had a beneficial effect on plant growth, showing their capacity to be a plant growth promoting rhizobacteria.
After analyzing the local plant response to KT2440 inoculation we tested the efficiency of KT2440 to induce a systemic defense against various types of attacks. We demonstrated that ISR triggered by KT2440 was efficient against an hemibiotrophic fungus, Colletotrichum graminicola, and a generalist herbivore, Spodoptera littoralis. However, the efficacy of ISR induced by KT2440 was dependent of the host-plant specialization of the leaf herbivores as ISR triggered by KT2440 did not affect the specialist S. frugiperda. Our transcript and metabolite analyses revealed the involvement of phenolic compounds as well as ethylene-dependent signaling in maize ISR. However, mechanisms involved in ISR induced by KT2440 in maize remain to be further investigated.
Signal signature of aboveground-induced resistance upon belowground herbivory in maize
Plants activate local and systemic defence mechanisms upon exposure to stress. This innate immune response is partially regulated by plant hormones, and involves the accumulation of defensive metabolites. Although local defence reactions to herbivores are well studied, less is known about the impact of root herbivory on shoot defence. Here, we examined the effects of belowground infestation by the western corn rootworm Diabrotica virgifera virgifera on aboveground resistance in maize. Belowground herbivory by D. v. virgifera induced aboveground resistance against the generalist herbivore Spodoptera littoralis, and the necrotrophic pathogen Setosphaeria turcica. Furthermore, D. v. virgifera increased shoot levels of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), and primed the induction of chlorogenic acid upon subsequent infestation by S. littoralis. To gain insight into the signalling network behind this below- and aboveground defence interaction, we compiled a set of 32 defence-related genes, which can be used as transcriptional marker systems to detect activities of different hormone-response pathways. Belowground attack by D. v. virgifera triggered an ABA-inducible transcription pattern in the shoot. The quantification of defence hormones showed a local increase in the production of oxylipins after root and shoot infestation by D. v. virgifera and S. littoralis, respectively. On the other hand, ABA accumulated locally and systemically upon belowground attack by D. v. virgifera. Furthermore, D. v. virgifera reduced the aboveground water content, whereas the removal of similar quantities of root biomass had no effect. Our study shows that root herbivory by D. v. virgifera specifically alters the aboveground defence status of a maize, and suggests that ABA plays a role in the signalling network mediating this interaction.
A soil-free root observation system for the study of root-microorganism interactions in maize
Induced resistance in maize is based on organ-specific defence responses
Synergies and trade-offs between insect and pathogen resistance in maize leaves and roots
Determining links between plant defence strategies is important to understand plant evolution and to optimize crop breeding strategies. Although several examples of synergies and trade-offs between defence traits are known for plants that are under attack by multiple organisms, few studies have attempted to measure correlations of defensive strategies using specific single attackers. Such links are hard to detect in natural populations because they are inherently confounded by the evolutionary history of different ecotypes. We therefore used a range of 20 maize inbred lines with considerable differences in resistance traits to determine if correlations exist between leaf and root resistance against pathogens and insects. Aboveground resistance against insects was positively correlated with the plant's capacity to produce volatiles in response to insect attack. Resistance to herbivores and resistance to a pathogen, on the other hand, were negatively correlated. Our results also give first insights into the intraspecific variability of root volatiles release in maize and its positive correlation with leaf volatile production. We show that the breeding history of the different genotypes (dent versus flint) has influenced several defensive parameters. Taken together, our study demonstrates the importance of genetically determined synergies and trade-offs for plant resistance against insects and pathogens.