Abstract
Molecular signaling networks in the actinorhizal rhizosphere select host-compatible Frankia strains, trigger the infection process and eventually the genesis of nitrogen-fixing nodules. The molecular triggers involved remain difficult to ascertain. Root exudates (RE) are highly dynamic substrates that play key roles in establishing the rhizosphere microbiome. RE are known to induce the secretion by rhizobia of Nod factors, polysaccharides, and other proteins in the case of legume symbiosis. Next-generation proteomic approach was here used to decipher the key bacterial signals matching the first-step recognition of host plant stimuli upon treatment of Frankia coriariae strain BMG5.1 with RE derived from compatible (Coriaria myrtifolia), incompatible (Alnus glutinosa), and non-actinorhizal (Cucumis melo) host plants. The Frankia proteome dynamics were mainly driven by host compatibility. Both metabolism and signal transduction were the dominant activities for BMG5.1 under the different RE conditions tested. A second set of proteins that were solely induced by C. myrtifolia RE and were mainly linked to cell wall remodeling, signal transduction and host signal processing activities. These proteins may footprint early steps in receptive recognition of host stimuli before subsequent events of symbiotic recruitment.
Highlights
The cohabitation of plant roots and soil microbes has been shaped by a long and complex coevolutionary process (Morgan et al, 2005), leading to specialized and durable interactions ranging from cheating to altruism (Mendes et al, 2011)
After 15 days’ growth, BD+N medium was replaced by BD medium without nitrogen source (BD-N) and Root exudates (RE) were collected after 3–4 weeks of plant growth, filter sterilized through a 0.22 μm polycarbonate membrane and freshly added to BMG5.1 cultures
An average of 21% of each protein sequence was covered by certified tryptic peptides, and a high redundancy in terms of spectral counts was obtained with an average of 166 MS/MS spectra recorded per protein
Summary
The cohabitation of plant roots and soil microbes has been shaped by a long and complex coevolutionary process (Morgan et al, 2005), leading to specialized and durable interactions ranging from cheating to altruism (Mendes et al, 2011). The multiple plant-microbe interactions taking place in the rhizosphere are mediated through a dynamic network, via molecular signals secreted as a response to attack in the case of disease, or as a developmental root structure such as in rhizobial and arbuscular mycorrhizal symbiosis (Oldroyd, 2013). In addition to their nutritional value for almost all rhizosphere microbes, root exudates (RE) contain chemoattractants, repellents and/or defensins that play key roles in priming and sustaining molecular dialogs with beneficial and pathogenic microbes (Bais et al, 2006). Frankia are nitrogen-fixing soil actinobacteria that are best known for their symbiotic lifestyle with a wide range of dicotyledonous host plants collectively designated as actinorhizal plants (Gtari et al, 2013).
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