Rhizobium meliloti Genes Encoding Catabolism of Trigonelline Are Induced under Symbiotic Conditions.

Abstract

Rhizobium meliloti trc genes controlling the catabolism of trigonelline, a plant secondary metabolite often abundant in legumes, are closely linked to nif-nod genes on the symbiotic megaplasmid pSym [Boivin, C., Malpica, C., Rosenberg, C., Denarie, J., Goldman, A., Fleury, V., Maille, M., Message, B., and Tepfer, D. (1989). In Molecular Signals in the Microbe-Plant Symbiotic and Pathogenic Systems. (Berlin: Springer-Verlag), pp. 401-407]. To investigate the role of trigonelline catabolism in the Rhizobium-legume interaction, we studied the regulation of trc gene expression in free-living and in endosymbiotic bacteria using Escherichia coli lacZ as a reporter gene. Experiments performed with free-living bacteria indicated that trc genes were organized in at least four transcription units and that the substrate trigonelline was a specific inducer for three of them. Noninducing trigonelline-related compounds such as betaines appeared to antagonize the inducing effect of trigonelline. None of the general or symbiotic regulatory genes ntrA, dctB/D, or nodD seemed to be involved in trigonelline catabolism. trc fusions exhibiting a low basal and a high induced [beta]-galactosidase activity when present on pSym were used to monitor trc gene expression in alfalfa tissue under symbiotic conditions. Results showed that trc genes are induced during all the symbiotic steps, i.e., in the rhizosphere, infection threads, and bacteroids of alfalfa, suggesting that trigonelline is a nutrient source throughout the Rhizobium-legume association.

Visualization of symbiotic tissue in intact root nodules of Vicia tetrasperma using GFP-marked Rhizobium leguminosarum bv. viciae

“…In our previous work (Chovanec and Novák 2005) we used the reporter gene lacZ coding for ȕ-galactosidase to visualize the activity of rhizobial nodulation genes in the early symbiosis with the same host V. tetrasperma. Nevertheless, the removal of indigenous E-galactosidase background by prolonged treatment with glutaraldehyde is inevitable in this case (Boivin et al 1990). Although high sensitivity, robustness and easy detection represent the advantages of enzyme-encoding marker and reporter genes in rhizobial symbiosis (Wilson 1995), the demands for continuous monitoring of nodule development and activities favor the application of nondestructive fluorescence techniques (Xi et al 1999).…”

Changes in pyridine metabolism profile during growth of trigonelline-forming Lotus japonicus cell cultures

“…Trigonelline (10) catabolism may be more important in nodules of leguminous plants. It has been reported that trigonelline (10) is excreted from seeds of Medicago sativa, and induces nodulation (Nod) gene transcription in Rhizobium meliloti by activating the regulatory protein NodD2 (Phillips et al, 1992); R. meliloti genes encoding catabolism of trigonelline (10) are induced under symbiotic conditions (Boivin et al, 1990). Trigonelline (10) may therefore be intimately related to the symbiosis of L. japonicus and leguminous bacteria, Mesorhizobium loti and/or Bradyrhizobium sp.…”

“…Compared with other pyridine alkaloids, such as ricinine (Yang and Waller, 1965), nicotine and anabasin (Häkkinen et al, 2007), trigonelline (10) is a more widely distributed pyridine alkaloid, and is found in most leguminous plants (Matsui et al, 2007;Ashihara, 2008). Trigonelline (10) is produced from nicotinic acid (8) by nicotinate N-methyltransferase (EC 2.1.1.7), using S-adenosyl-L-methionine as a methyl group donor (Joshi and Handler, 1960;Upmeier et al, 1988;Chen and Wood, 2004), and is important in the symbiosis of leguminous plants and bacteria (Boivin et al, 1990;Phillips et al, 1992).…”

Molecular communication in the rhizosphere

“…Whereas many micro-organisms can only utilise rather general plant metabolites, some bacteria have the capacity to catabolize certain plant secondary metabolites providing a selective advantage to colonize the rhizosphere of specific plants (Savka et al 2002). Examples of such nutritional mediators are glycosides and aryl-glycosides (Faure et al 1999(Faure et al , 2001, calystegin (Tepfer et al 1988;Guntli et al 1999), certain flavonoids (Hartig et al 1991), proline (Jiménez-Zurdo et al 1997), 1-aminocyclopropane-1-carboxylic acid (Penrose and Glick 2001), and homoserine and betaines (Boivin et al 1990;Goldmann et al 1991). Another well described effect of often unidentified components of rhizodeposits is the activation of bacterial gene expression culminating in more or less intimate interactions with the producing plant host (Stachel et al 1985;Koch et al 2002;Cooper 2007;Reddy et al 2007;Franks et al 2008;Johnston et al 2008).…”