Genetically, Rhizobium sp. strain NGR234 and R. fredii USDA257 are closely related. Small differences in their nodulation genes result in NGR234 secreting larger amounts of more diverse lipo-oligosaccharidic Nod factors than USDA257. What effects these differences have on nodulation were analyzed by inoculating 452 species of legumes, representing all three subfamilies of the Leguminosae, as well as the nonlegume Parasponia andersonii, with both strains. The two bacteria nodulated P. andersonii, induced ineffective outgrowths on Delonix regia, and nodulated Chamaecrista fasciculata, a member of the only nodulating genus of the Caesalpinieae tested. Both strains nodulated a range of mimosoid legumes, especially the Australian species of Acacia, and the tribe Ingeae. Highest compatibilities were found with the papilionoid tribes Phaseoleae and Desmodieae. On Vigna spp. (Phaseoleae), both bacteria formed more effective symbioses than rhizobia of the "cowpea" (V. unguiculata) miscellany. USDA257 nodulated an exact subset (79 genera) of the NGR234 hosts (112 genera). If only one of the bacteria formed effective, nitrogen-fixing nodules it was usually NGR234. The only exceptions were with Apios americana, Glycine max, and G. soja. Few correlations can be drawn between Nod-factor substituents and the ability to nodulate specific legumes. Relationships between the ability to nodulate and the origin of the host were not apparent. As both P. andersonii and NGR234 originate from Indonesia/Malaysia/Papua New Guinea, and NGR234's preferred hosts (Desmodiinae/Phaseoleae) are largely Asian, we suggest that broad host range originated in Southeast Asia and spread outward.

/pdf/rhizobium-sp-strain-ngr234-and-r-fredii-usda257-share-3pejw75nm4.pdf

Rhizobium sp. strain NGR234 and R. fredii USDA257 share exceptionally broad, nested host ranges.

Colonization of host cells by rhizobium bacteria involves the progressive remodelling of the plant–microbial interface. Following induction of nodulation genes by legume-derived flavonoid signals, rhizobium secretes Nod-factors (lipochitin oligosaccharides) that cause root hair deformations by perturbing the growth of the plant cell wall. The infection thread arises as a tubular ingrowth bounded by plant cell wall. This serves as a conduit for colonizing bacterial cells that grow and divide in its lumen. The transcellular orientation of thread growth is controlled by the cytoskeleton and is coupled to cell cycle reactivation and cell division processes. In response to rhizobium infection, host cells synthesize several new components (early nodulins) that modify the properties of the cell wall and extracellular matrix. Root nodule extensins are a legume-specific family of hydroxyproline-rich glycoproteins targeted into the lumen of the infection thread. They have alternating extensin and arabinogalactan (A...

Plant Cell Wall Remodelling in the Rhizobium–Legume Symbiosis

To elucidate the nature of plant response to infection and transformation by Agrobacterium tumefaciens, we compared the cDNA-amplified fragment length polymorphism (AFLP) pattern of Agrobacterium- and mock-inoculated Ageratum conyzoides plant cell cultures. From 16,000 cDNA fragments analyzed, 251 (1.6%) were differentially regulated (0.5% down-regulated) 48 h after cocultivation with Agrobacterium. From 75 strongly regulated fragments, 56 were already regulated 24 h after cocultivation. Sequence similarities were obtained for 20 of these fragments, and reverse transcription–PCR analysis was carried out with seven to confirm their cDNA-AFLP differential pattern. Their sequence similarities suggest a role for these genes in signal perception, transduction, and plant defense. Reverse transcription–PCR analysis indicated that four genes involved in defense response are regulated in a similar manner by nonpathogenic bacteria, whereas one gene putatively involved in signal transduction appeared to respond more strongly to Agrobacterium. A nodulin-like gene was regulated only by Agrobacterium. These results demonstrate a rapid plant cell response to Agrobacterium infection, which overlaps a general response to bacteria but also has Agrobacterium-specific features.

https://www.pnas.org/content/pnas/98/19/10954.full.pdf

Plant gene expression response to Agrobacterium tumefaciens

The tightly regulated expression patterns of structural cell wall proteins in several plant species indicate that they play a crucial role in determining the extracellular matrix structure for specific cell types. We demonstrate that AtPRP3, a proline-rich cell wall protein in Arabidopsis, is expressed in root-hair-bearing epidermal cells at the root/shoot junction and within the root differentiation zone of light-grown seedlings. Several lines of evidence support a direct relationship between AtPRP3 expression and root hair development. AtPRP3/β-glucuronidase (GUS) expression increased in roots of transgenic seedlings treated with either 1-aminocyclopropane-1-carboxylic acid (ACC) or α-naphthaleneacetic acid (α-NAA), compounds known to promote root hair formation. In the presence of 1-α-(2-aminoethoxyvinyl)glycine (AVG), an inhibitor of ethylene biosynthesis, AtPRP3/GUS expression was strongly reduced, but could be rescued by co-addition of ACC or α-NAA to the growth medium. In addition, AtPRP3/GUS activity was enhanced in ttg and gl2 mutant backgrounds that exhibit ectopic root hairs, but was reduced in rhd6 and 35S- R root-hair-less mutant seedlings. These results indicate that AtPRP3 is regulated by developmental pathways involved in root hair formation, and are consistent with AtPRP39s contributing to cell wall structure in Arabidopsis root hairs.

http://www.plantphysiol.org/content/plantphysiol/122/3/705.full.pdf

Expression of AtPRP3, a proline-rich structural cell wall protein from Arabidopsis, is regulated by cell-type-specific developmental pathways involved in root hair formation.

Rhizobium leguminosarum bv. viciae normally gains access to pea host cells through tubular cell wall ingrowths termed infection threads. Matrix glycoprotein (MGP), a major component of the infection thread lumen, is also secreted from the tips of uninoculated roots and can be released into solution under reducing conditions. Monoclonal antibody MAC265, which recognizes MGP through a carbohydrate epitope, was used for immunoaffinity purification of the glycoprotein from pea roots. Following treatment with chymotrypsin, a peptide fragment was obtained and subjected to N-terminal sequencing. Using PCR primers based on this sequence, cDNA clones were isolated with RNA from inoculated roots and nodules. DNA sequencing of 30 of these clones revealed a family of closely related and repetitive polypeptides with (hydroxy)proline-rich motifs. The cDNA sequences showed over 70% identity with the deduced amino acid sequences of plant extensins, particularly with VfNDS-E from Vicia faba and MtN12 from Medicago truncatula, both of which are strongly upregulated in legume root nodules. Root nodule extensins from pea were of variable length but showed strong sequence conservation of the N-terminus, of the C-terminus, and of a central domain comprising 33 amino acids that were sometimes reiterated. The distribution of tyrosine residues suggested the possible importance of intramolecular and intermolecular cross-linking. There was strong sequence conservation with MtN12 in the 3'-untranslated region, suggesting a possible involvement in posttranscriptional regulation of gene expression.

Identification of a family of extensin-like glycoproteins in the lumen of rhizobium-induced infection threads in pea root nodules.

Three cDNAs (ext3, ext127, and ext26), originally isolated by differential screening from a root-hair cDNA library of Vigna unguiculata, were found to encode extensin-like cell wall proteins. Transcripts homologous to these cDNAs were only detected in root hairs where mRNA levels decreased 1 day after inoculation with rhizobia. This coincided with the onset of root-hair deformation, the first morphological step in the Rhizobium-legume interaction. Decreases in transcript levels following inoculation with wild-type Rhizobium sp. NGR234 were more pronounced than with NGRΔnodABC, a mutant deficient in Nod-factor production. Inoculation with a rhizobial strain carrying a mutation in a gene encoding a transcriptional activator for nod genes (NGRΔnodD1) did not repress mRNA levels, indicating that a second nodulation signal may be present that is nodD dependent. Application of purified NodNGR factors only affected transcript levels of ext3. The genomic locus of the gene homologous to ext26 (Ext26G) was cloned. In the 5′ flanking region, several potential TATA boxes and CAP signals were identified. Part of the promoter region shares homology with the Pisum sativum seed lectin promoter and the Nicotiana tabacum nitrate reductase promoter region. Nonetheless, the function of these homologous regions in gene regulation is unknown. Additional keyword: symbiosis.

/pdf/rhizobia-modulate-root-hair-specific-expression-of-extensin-1sjjmgwhsi.pdf

Rhizobia modulate root-hair-specific expression of extensin genes.

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Rhizobia modulate root-hair-specific expression of extensin genes.