Pseudomonas fluorescens and Glomus mosseae Trigger DMI3-Dependent Activation of Genes Related to a Signal Transduction Pathway in Roots of Medicago truncatula  

Abstract

Plant genes induced during early root colonization of Medicago truncatula Gaertn. J5 by a growth-promoting strain of Pseudomonas fluorescens (C7R12) have been identified by suppressive subtractive hybridization. Ten M. truncatula genes, coding proteins associated with a putative signal transduction pathway, showed an early and transient activation during initial interactions between M. truncatula and P. fluorescens, up to 8 d after root inoculation. Gene expression was not significantly enhanced, except for one gene, in P. fluorescens-inoculated roots of a Myc Nod2 genotype (TRV25) of M. truncatula mutated for the DMI3 (syn. MtSYM13) gene. This gene codes a Ca 21 and calmodulin-dependent protein kinase, indicating a possible role of calcium in the cellular interactions between M. truncatula and P. fluorescens. When expression of the 10 plant genes was compared in early stages of root colonization by mycorrhizal and rhizobial microsymbionts, Glomus mosseae activated all 10 genes, whereas Sinorhizobium meliloti only activated one and inhibited four others. None of the genes responded to inoculation by either microsymbiont in roots of the TRV25 mutant. The similar response of the M. truncatula genes to P. fluorescens and G. mosseae points to common molecular pathways in the perception of the microbial signals by plant roots.Root exudates play an active role in the regulation of symbiotic and pathogenic interactions with microbes (Hirsch et al., 2003). Rhizosphere microorganisms in turn can have a decisive influence on plant health. The most commonly studied beneficial interactions are the arbuscular mycorrhizal (AM) symbiosis, between the majority of land plant families and fungi in the phylum Glomeromycota (Smith and Read, 1997;Schü ssler et al., 2001), and the nodule symbiosis restricted to legumes and bacteria of the family Rhizobiaceae (Dénarié et al., 1992;Franssen et al., 1992;Perret et al., 2000). Several other genera of soil bacteria, including Pseudomonas and Bacillus species, can stimulate root proliferation or have antagonistic effects on pathogens in the rhizosphere (Kloepper and Schroth, 1978;Ellis et al., 2000;Whipps, 2001). Contrary to the AM and nodule symbioses, few investigations have focused on the molecular bases of plant responses to rhizobacteria such as Pseudomonas, and studies have mainly been concerned with elucidating the basis of induced systemic resistance by Pseudomonas species in Arabidopsis (Arabidopsis thaliana; Cartieaux et al., Verhagen et al., 2004;Wang et al., 2005).The lack of information about plant gene expression during root colonization by Pseudomonas fluorescens prompted us to exploit suppressive subtractive hybridization (SSH; Diatchenko et al., 1996) to identify plant gene responses during early-stage colonization of Medicago truncatula Gaertn. roots by P. fluorescens strain C7R12. P. fluorescens strain C7R12 is a spontaneous rifampicin-resistant mutant of a wild-type strain C7 (Eparvier et al., 1991) previously isolated from the rhizosphere of flax cultivated in a soil ...

Plant responses to plant growth-promoting rhizobacteria

“…In roots of the legume species Medicago trunculata, Sanchez et al (2005) found 58 genes to be upregulated in response to colonization by the growth-promoting strain Pseudomonas fluorescens C7R12, a number in line with that found by Verhagen et al (2004) in Arabidopsis roots colonized by strain WCS417. Out of 10 of the C7R12-induced genes, 9 were not upregulated in the M. trunculata dmi3 mutant, which is impaired in the signal transduction pathway of the Nod factor from Sinorhizobium meliloti as well as in mycorrhization by the fungus Glomus mosseae.…”

Jasmonate signaling in plant interactions with resistance-inducing beneficial microbes

“…identified an increase in JA-regulated PR-, salt stress-relatedand putative receptor like-proteins, especially in less-compatible interactions (Miché et al, 2006). In Medicago truncatula the initial local transcriptional responses to the mycorrhizal fungus Glomus mosseae showed significant overlap to those initiated by the PGPR P. fluorescens C7R12 (Sanchez et al, 2005). Moreover, both beneficials were unable to elicit these shared transcriptional responses in the symbiosis-defective mutant dmi3 (does not make infections 3), suggesting that the signaling pathways that are triggered by these different beneficials converge (Sanchez et al, 2005).…”

Plant immune responses triggered by beneficial microbes

“…In rice, 40% of the mycorrhiza-responsive genes were also responsive to infection by fungal pathogens, indicating that the responses to beneficial and pathogenic fungi partly overlap [45]. Interestingly, some of the initial responses of Medicago truncatula to the mycorrhizal fungus Glomus mosseae and to the beneficial rhizobacterium P. fluorescens C7R12 have been shown to be overlapping as well [46]. Both beneficial microbes failed to elicit these shared responses in the symbiosisdefective mutant dmi3, which is affected in the calciumdependent and calmodulin-dependent protein kinase DMI3 [46].…”

“…Interestingly, some of the initial responses of Medicago truncatula to the mycorrhizal fungus Glomus mosseae and to the beneficial rhizobacterium P. fluorescens C7R12 have been shown to be overlapping as well [46]. Both beneficial microbes failed to elicit these shared responses in the symbiosisdefective mutant dmi3, which is affected in the calciumdependent and calmodulin-dependent protein kinase DMI3 [46]. Again, this suggests that the signaling pathways triggered by very different beneficial microbes converge.…”