Suppression of nodulation gene expression in bacteroids of Rhizobium leguminosarum biovar viciae

Abstract

The expression of nod genes of Rhizobium leguminosarum bv. viciae in nodules of Pisum sativum was investigated at both the translational and transcriptional levels. By using immunoblots, it was found that the levels of NodA, NodI, NodE, and NodO proteins were reduced at least 14-fold in bacteroids compared with cultured cells, whereas NodD protein was reduced only 3-fold. Northern (RNA) blot hybridization, RNase protection assays, and in situ RNA hybridization together showed that, except for the nodD transcript, none of the other nod gene transcripts were present in bacteroids. The amount of nodD transcript in bacteroids was reduced only two-to threefold compared with that in cultured cells. Identical results were found with a Rhizobium strain harboring multicopies of nodD and with a strain containing a NodD protein (NodD604) which is activated independently of flavonoids. Furthermore, it was found that mature pea nodules contain inhibitors of induced nod gene transcription but that NodD604 was insensitive to these compounds. In situ RNA hybridization on sections from P. sativum and Vicia hirsuta nodules showed that transcription of inducible nod genes is switched off before the bacteria differentiate into bacteroids. This is unlikely to be due to limiting amounts of NodD, the absence of inducing compounds, or the presence of anti-inducers. The observed switch off of transcription during the development of symbiosis is a general phenomenon and is apparently caused by a yet unknown negative regulation mechanism.Bacteria of the genus Rhizobium are able to establish a symbiosis with leguminous plants, resulting in formation of root nodules in which the bacteria, in an altered form designated as bacteroids, reduce atmospheric nitrogen to ammonia. Successful nodulation is a host-specific process in the sense that Pisum and Vicia species are host plants for Rhizobium leguminosarum biovar (bv.) viciae, alfalfa is a host for R. meliloti, and Trifolium sp. is a host for R. leguminosarum bv. trifolii.Bacterial nod (for nodulation) genes localized on a Sym (for symbiosis) plasmid code for proteins involved in early steps in nodulation. The nodD gene is the only constitutively transcribed nod gene in free-living cells. In R. leguminosarum bv. viciae and R. leguminosarum bv. trifolii, nodD is present as a single copy whereas in R. meliloti four allelic forms, designated nodDi, nodD2, nodD3, and syrM, have been identified. The NodD protein binds specifically to nod boxes (18,19,22,28), conserved DNA sequences in the upstream untranslated region of other nod genes (11,40,46,49), and induces transcription of the other nod genes, provided that NodD protein is activated by an inducer of plant origin. These inducers have been identified as flavones and flavanones (17,34,37,65), while isoflavones and coumarins act as anti-inducers for these species (13,17 early steps of nodulation, as reflected by the Nod-phenotype of nodABC mutants and the strongly reduced nodulation of nodFEL mutants. The products of these genes function in root...

Developmental downregulation of rhizobial genes as a function of symbiosome differentiation in symbiotic root nodules of Pisum sativum

“…1a,f ). This result was consistent with earlier reports on nodA gene expression (Schlaman et al, 1991), but was unexpected in the case of the dctA gene. In alfalfa nodules, expression of Sinorhizobium meliloti dctA was not observed in infection threads (Boesten et al, 1998).…”

Developmental downregulation of rhizobial genes as a function of symbiosome differentiation in symbiotic root nodules of Pisum sativum

“…1a,f ). This result was consistent with earlier reports on nodA gene expression (Schlaman et al, 1991), but was unexpected in the case of the dctA gene. In alfalfa nodules, expression of Sinorhizobium meliloti dctA was not observed in infection threads (Boesten et al, 1998).…”

Transcriptome analysis of Sinorhizobium melilotiduring symbiosis

“…Conversely, nod genes were expressed in 8 dai nodules as well as in 1021-TE7 and bacA -J6 nodules, whereas expression levels were lower in 18 dai wild-type bacteroids from both M. sativa and M. truncatula . This result is consistent with the observation that the nodABC genes from R. leguminosarum are transcribed in infection threads and that transcription is switched off at the onset of bacteroid differentiation [19] and with the detection of Nod factors in the infection zone of alfalfa nodules, which contains the infection threads [20]. …”

Unravelling the molecular basis for symbiotic signal transduction in legumes

“…These data strongly suggest that the proteins required for Nod factor perception function well after recognition and infection have taken place in root hairs. They are also consistent with previous studies reporting the synthesis and secretion of Nod factor by bacteria up to the point of bacterial release from infection threads (Schlaman et al ., 1991; Timmers et al ., 1998). Two recent reports extended these gene expression data and investigated the role of Nod factor signalling in later developmental stages.…”

Developmental downregulation of rhizobial genes as a function of symbiosome differentiation in symbiotic root nodules of Pisum sativum

“…1a,f ). This result was consistent with earlier reports on nodA gene expression (Schlaman et al, 1991), but was unexpected in the case of the dctA gene. In alfalfa nodules, expression of Sinorhizobium meliloti dctA was not observed in infection threads (Boesten et al, 1998).…”

Transcriptome analysis of Sinorhizobium melilotiduring symbiosis

“…Conversely, nod genes were expressed in 8 dai nodules as well as in 1021-TE7 and bacA -J6 nodules, whereas expression levels were lower in 18 dai wild-type bacteroids from both M. sativa and M. truncatula . This result is consistent with the observation that the nodABC genes from R. leguminosarum are transcribed in infection threads and that transcription is switched off at the onset of bacteroid differentiation [19] and with the detection of Nod factors in the infection zone of alfalfa nodules, which contains the infection threads [20]. …”

Unravelling the molecular basis for symbiotic signal transduction in legumes

“…These data strongly suggest that the proteins required for Nod factor perception function well after recognition and infection have taken place in root hairs. They are also consistent with previous studies reporting the synthesis and secretion of Nod factor by bacteria up to the point of bacterial release from infection threads (Schlaman et al ., 1991; Timmers et al ., 1998). Two recent reports extended these gene expression data and investigated the role of Nod factor signalling in later developmental stages.…”

Developmental downregulation of rhizobial genes as a function of symbiosome differentiation in symbiotic root nodules of Pisum sativum

“…1a,f ). This result was consistent with earlier reports on nodA gene expression (Schlaman et al, 1991), but was unexpected in the case of the dctA gene. In alfalfa nodules, expression of Sinorhizobium meliloti dctA was not observed in infection threads (Boesten et al, 1998).…”

Transcriptome analysis of Sinorhizobium melilotiduring symbiosis

“…Conversely, nod genes were expressed in 8 dai nodules as well as in 1021-TE7 and bacA -J6 nodules, whereas expression levels were lower in 18 dai wild-type bacteroids from both M. sativa and M. truncatula . This result is consistent with the observation that the nodABC genes from R. leguminosarum are transcribed in infection threads and that transcription is switched off at the onset of bacteroid differentiation [19] and with the detection of Nod factors in the infection zone of alfalfa nodules, which contains the infection threads [20]. …”

Unravelling the molecular basis for symbiotic signal transduction in legumes

“…These data strongly suggest that the proteins required for Nod factor perception function well after recognition and infection have taken place in root hairs. They are also consistent with previous studies reporting the synthesis and secretion of Nod factor by bacteria up to the point of bacterial release from infection threads (Schlaman et al ., 1991; Timmers et al ., 1998). Two recent reports extended these gene expression data and investigated the role of Nod factor signalling in later developmental stages.…”