Abstract
Nitric oxide (NO) is a gaseous molecule that participates in numerous plant signalling pathways. It is involved in plant responses to pathogens and development processes such as seed germination, flowering and stomatal closure.Using a permeable NO-specific fluorescent probe and a bacterial reporter strain expressing the lacZ gene under the control of a NO-responsive promoter, we detected NO production in the first steps, during infection threads growth, of the Medicago truncatula–Sinorhizobium meliloti symbiotic interaction. Nitric oxide was also detected, by confocal microscopy, in nodule primordia.Depletion of NO caused by cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl-3-oxide), an NO scavenger, resulted in a significant delay in nodule appearance. The overexpression of a bacterial hmp gene, encoding a flavohaemoglobin able to scavenge NO, under the control of a nodule-specific promoter (pENOD20) in transgenic roots, led to the same phenotype. The NO scavenging resulting from these approaches provoked the downregulation of plant genes involved in nodule development, such as MtCRE1 and MtCCS52A. Furthermore, an Hmp-overexpressing S. meliloti mutant strain was found to be less competitive than the wild type in the nodulation process.Taken together, these results indicate that NO is required for an optimal establishment of the M. truncatula–S. meliloti symbiotic interaction.
Highlights
Nitric oxide (NO) is a gaseous signalling molecule that has been established as a major signal in mammals (Ignarro, 2000)
Nitric oxide plays an important role in the plant hypersensitive response (HR), a localized programmed cell death (PCD) that confines the pathogen to the site of attempted infection, and in non PCD defence (Mur et al, 2006)
The results presented here clearly show that NO is present in the first steps of the M. truncatula–S. meliloti symbiosis, that is, in the root hair infection pockets as well as in the nodule primordium, clearly pointing to a role of this reactive nitrogen species in the early exchange of signals between the two partners
Summary
Nitric oxide (NO) is a gaseous signalling molecule that has been established as a major signal in mammals (Ignarro, 2000). The participation of NO in a large number of plant signalling pathways is well established (Grun et al, 2006) and there is increasing evidence of its role in plant growth and development (del Rio et al, 2004; Delledonne, 2005). Nitric oxide signalling in the induction of cell death, defence genes and interaction with reactive oxygen species (ROS) during plant defence against pathogen attack is well documented (Neill et al, 2003; Delledonne, 2005; Mur et al, 2006; Besson-Bard et al, 2008). Nitric oxide plays an important role in the plant hypersensitive response (HR), a localized programmed cell death (PCD) that confines the pathogen to the site of attempted infection, and in non PCD defence (Mur et al, 2006).
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