The response of Alnus glutinosa to Frankia alni ACN14a is driven by several sequential physiological events from calcium spiking and root hair deformation to the development of the nodule. Early stages of actinorhizal symbiosis were monitored at the transcriptional level to observe plant host responses to Frankia. Forty-two genes were significantly upregulated in inoculated compared to non-inoculated roots. Most of these genes encode proteins involved in biological processes induced during microbial infection such as oxidative stress or response to stimuli but a large part of them are not differentially modulated or downregulated later in the process of nodulation. In contrast, several of them remained upregulated in mature nodules, and this included the gene most upregulated, which encodes a non-specific lipid transfer protein (nsLTP). Classified as an antimicrobial peptide, this nsLTP, was immunolocalized on the deformed root hair surfaces that are points of contact for Frankia during infection. Later in nodules, it binds to the surface of Frankia’s vesicles, which are the specialized cells for nitrogen fixation. This nsLTP, named AgLTP24, was biologically produced in a heterologous host and purified for assay on F. alni ACN14a to identify physiological effects. Thus, the activation of the plant immunity response occurs upon first contact, while the recognition of Frankia switches off part of the defense system during nodulation. AgLTP24 constitutes a part of the defense system that is maintained all along the symbiosis with potential functions such as the formation of infection threads or nodule primordia to the control of Frankia proliferation.
SummaryThe response of Alnus glutinosa to Frankia alni is complex with several sequential physiological modifications that include calcium spiking, root hair deformation, penetration, induction of primordium, formation and growth of nodule. A transcriptomic study of seedlings in hydroponics after early contact (2.5 days) with Frankia alni, either with a culture supernatant or with living cells separated from the roots by a dialysis membrane, permitted to identify plant genes which expression level was modified upon early contact with Frankia.Forty-two genes were significantly up-regulated in both experiments, most of them encoding biological processes such as oxidative stress or response to stimuli. Among them, the most upregulated gene was a non-specific lipid transfer protein encoding gene with a fold change of 141. This nsLTP was found to increase Frankia nitrogen fixation at sub-lethal concentration. Interestingly, it was immunolocalized to a region of the deformed root hair at an early infection stage and later in nodules, it was localized around bacterial vesicles suggesting a role in early and late stages of symbiosis.
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