Root Exudates Promote Biosynthetic and Metabolic Pathways in the Nitrogen-Fixing Actinobacteria Frankia
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Actinorhizal plants form a symbiotic association with nitrogen-fixing actinobacteria Frankia, leading to the development of root nodules. The symbiotic interaction begins with an exchange of signals between the host plant and the bacteria. A molecule present in root exudates from Casuarina glauca plants induced molecular and physiological changes in Frankia, including the ability to establish root nodules significantly earlier than untreated cells. The presence of an extracellular signaling molecule(s) produced by the exudates-treated Frankia was identified using a bioassay with transgenic C. glauca plants and specific genetic markers in the nodulation pathway. The global gene expression via RNA Seq was determined in response to host- specific (C. glauca) exudates compared to the control (BD medium) and nonhost ( Hippophae rhamnoides) exudates over several days (1, 2, 3, and 5). Differentially expressed genes were further analyzed using antiSMASH and BlastKOALA, where an increase in biosynthetic and metabolic pathways was observed after exposure to root exudates. Several genes were significantly upregulated (p-value < 0.05, fold change> 2), belonging to the biosynthetic gene clusters for lanthipeptide production and polyketide synthases across the various days for both root exudate treatments. An increase in pathways involved in various metabolic pathways, such as carbon and amino acid metabolism, was also observed. These findings demonstrate that treatment of Frankia casuarinae with host plant root exudates increases the expression of pathways involved in metabolism and the biosynthesis of secondary metabolite, furthering the understanding of the early stages of actinorhizal symbiosis.