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. <br> 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. <br> 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 <i>Pseudomonas putida</i> 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. <br> 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, <i>Colletotrichum graminicola</i>, and a generalist herbivore, <i>Spodoptera littoralis</i>. 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 <i>S. frugiperda</i>. 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.

Thèse de doctorat : Université de Neuchâtel, 2013