The aberrant cell walls of boron-deficient bean root nodules have no covalently bound hydroxyproline-/proline-rich proteins.

B-deficient bean (Phaseolus vulgaris L.) nodules examined by light microscopy showed dramatic anatomical changes, mainly in the parenchyma region. Western analysis of total nodule extracts examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that one 116-kD polypeptide was recognized by antibodies raised against hydroxyproline-rich glycoproteins (HRGPs) from the soybean (Glycine max) seed coat. A protein with a comparable molecular mass of 116 kD was purified from the cell walls of soybean root nodules. The amino acid composition of this protein is similar to the early nodulin (ENOD2) gene. Immunoprecipitation of the soybean ENOD2 in vitro translation product showed that the soybean seed coat anti-HRGP antibodies recognized this early nodulin. Furthermore, we used these antibodies to localize the ENOD2 homolog in bean nodules. Immunocytochemistry revealed that in B-deficient nodules ENOD2 was absent in the walls of the nodule parenchyma. The absence of ENOD2 in B-deficient nodules was corroborated by performing hydroxyproline assays. Northern analysis showed that ENOD2 mRNA is present in B-deficient nodules; therefore, the accumulation of ENOD2 is not affected by B deficiency, but its assembly into the cell wall is. B-deficient nodules fix much less N2 than control nodules, probably because the nodule parenchyma is no longer an effective O2 barrier.     

cDNA cloning and developmental expression of pea nodulin genes

A cDNA library prepared from pea nodule poly(A)⁺ RNA was screened by differential hybridization with cDNA probes synthesized from root and nodule RNA respectively. From the cDNA clones that hybridized exclusively with the nodule probe five clones, designated pPsNod 6, 10, 11, 13 and 14 and each containing unique sequences, were further characterized together with one leghemoglobin and one root-specific cDNA clone. In vitro translation of RNA selected by the pPsNod clones showed that the corresponding genes encode nodulins with molecular weights ranging from 5 800 to 19 000. During pea root nodule development expression of the five PsNod genes starts more or less concomitantly with the onset of nitrogen fixing activity in the nodules and the time course of appearance and accumulation of the nodulin mRNAs is similar to that of leghemoglobin mRNA. In ineffective pea root nodules expression of the PsNod genes is induced but the final accumulation levels of the mRNAs are markedly reduced to various degrees. The expression of another nodulin gene, designated ENOD2, was followed using a heterologous soybean cDNA clone as probe. In pea root nodules the ENOD2 gene is expressed at least five days before the PsNod and leghemoglobin genes, and in contrast to the PsNod mRNAs the concentration of the ENOD2 mRNA is the same in wild type and fix ^- nodules. The results described suggest that in root nodules several regulatory mechanisms exist which determine the final nodulin mRNA amounts accumulating in the root nodule.     

Minimal enhancer elements of the leghemoglobinlba andlbc3 gene promoters fromGlycine max L. have different properties

The characteristics of the soybean leghemoglobinlba gene promoter were analyzed and important promoter elements from thelba andlbc3 promoters were compared using transgenicLotus corniculatus plants. A 5 deletion analysis of thelba promoter delimited twocis-acting elements controlling expression: a distal positive element (–1254, –884) required for expression and a proximal element (–285, –60) essential for full-level activity. In contrast to the corresponding region of thelbc3 promoter, thelba proximal element is unable to control expression from the heterologous CaMV 35S enhancer. The upstream positive element of thelba gene contains a position- and orientation-independent enhancer between positions (–1091, –788). The sequence of this enhancer region is conserved in thelbc3 gene upstream (–1333, –1132) of the previously assigned strong positive element (SPE; –1090, –947). The present analysis revealed some of the properties of this extendedlbc3 SPE element. The extended element (–1364, –947) functions in both orientations from 5 locations whereas the SPE2 subcomponent (–1364, –1154) containing the conserved sequence is only active in the correct orientation. Removal of the SPE2 by internal deletion demonstrates that the SPE2 subcomponent is indispensable for the activity of thelbc3 upstream positive element. These results indicate that the upstream positive elements of thelba andlbc3 genes possess different properties although their conserved minimal enhancer sequence has similar function. This may reflect the differential expression of the twolb genes ofGlycine max L.     

The expression of a chimeric Phaseolus vulgaris nodulin 30-GUS gene is restricted to the rhizobially infected cells in transgenic Lotus corniculatus nodules

In Phaseolus vulgaris there is a nodulin family, Npv30, of ca. 30 kDa, as detected in an in vitro translation assay [2]. We isolated a gene (npv30-1) for one of the members of this family. The nucleotide sequence of the promoter of npv30-1 contains nodule-specific motifs common to other late nodulin genes. The promoter was fused to the GUS reporter gene; this chimeric fusion was introduced into Lotus corniculatus via Agrobacterium rhizogenes transformation. GUS activity was only detected in the infected cells of the nodules of transgenic plants. By contrast, the expression of a 35S-GUS construct was restricted to the uninfected cells and the vascular tissue.     

VsENBP1 regulates the expression of the early nodulin PsENOD12B

A DNA-binding protein, VsENBP1, previously isolated from Vicia sativa was shown to bind in a sequence-specific manner to the early nodulin ENOD12 gene promoter from Pisum sativum. Here, the functional importance of the VsENBP1 binding sites on the PsENOD12B promoter has been studied in vivo. A promoter-gusA fusion in which a mutation was introduced at the putative target sequence, AATAA, was inactive in nodules of transgenic Vicia hirsuta roots. Gel retardation assays showed that VsENBP1 does not bind to the mutated promoter segment, suggesting that VsENBP1 activates the PsENOD12B expression in nodules through its interaction with its target sequence. In the presence of the 35S enhancer, an ENOD12 promoter-GUS construct gave expression in root vascular tissue in addition to the root nodules. Overexpression of Vsenbp1 in transgenic V. hirsuta roots reduced the leaky expression in root vascular tissue in contrast to nodules in which a small increase in GUS expression was observed. The results indicate that VsENBP1 acts as a repressor of ENOD12 expression in root tissue.     

Expression of LjENOD40 genes in response to symbiotic and non-symbiotic signals: LjENOD40-1 and LjENOD40-2 are differentially regulated in Lotus japonicus

Nitrogen fixation in nodules provides leguminous plants with an ability to grow in nitrogen-starved soil. Infection of the host plants by microsymbionts triggers various physiological and morphological changes during nodule formation. In Lotus japonicus, expression of early nodulin (ENOD) genes is triggered by perception of bacterial signal molecules, nodulation factors (Nod factors). We examined the expression patterns of ENOD40 genes during the nodule formation process. Two ENOD40 genes of L. japonicus were specifically expressed in the nodule formation process, but they showed different expression patterns upon infection. Each ENOD40 gene demonstrates an individual specificity and regulation with regard to rhizobial infection.     

Expression of the early nodulin, ENOD40, in soybean roots in response to various lipo-chitin signal molecules

The lipo-chitin (LCO) nodulation signal (nod signal) purified from Bradyrhizobium japonicum induced nodule primordia on soybean (i.e. Glycine soja) roots. These primordia were characterized by a bifurcated vascular connection, cortical cell division, and the accumulation of mRNA of the early nodulin gene, ENOD40. A chemically synthesized LCO identical in structure to the Nod signal purified from B. japonicum cultures showed the same activity when inoculated on to soybean roots. Surprisingly, synthetic LCO or chitin pentamer, inactive in inducing root hair curling (HAD) or cortical cell division (NOI) in G. soja, induced the transient accumulation of ENOD40 mRNA. In roots inoculated with such LCO, ENOD40 mRNA was abundant at 40 h after inoculation but decreased to the background levels 6 days after inoculation. In contrast, nod signals active in inducing HAD and NOI induced high levels of ENOD40 accumulation at 40 h and 6 days after inoculation. In situ hybridization analysis showed that ENOD40 mRNA accumulated in the pericycle of the vascular bundle at 24 h after root inoculation with nod signal. At 6 days post-inoculation with nod signal, ENOD40 expression was seen in dividing subepidermal cortical cells. These results provide morphological and molecular evidence that nodule induction in soybean in response to purified or synthetic nod signal is similar, if not identical, to nodule formation induced by bacterial inoculation. Surprisingly, ENOD40 mRNA accumulation occurs in response to non-specific chitin signals. This suggests that, in the case of ENOD40, nodulation specificity is not determined at the level of initial gene expression.     

Cells expressing ENOD2 show differential spatial organization during the development of alfalfa root nodules

We have used in situ hybridization to examine the spatial organization of cells expressing the early nodulin gene (ENOD2) during the development of alfalfa root nodules. ENOD2 gene expression was found in the nodule parenchyma, uninfected cells surrounding the symbiotic region of both effective and ineffective nodules. However, in empty nodules, ENOD2 gene expression was found in a mass of parenchyma cells at the base of the nodule. Similar results were also observed in 11-day-old nodules that contained infected cells but that had not yet begun to express leghemoglobin. Although early events of nodulation result in the induction of ENOD2 expression in cells at the nodule base, the pattern of cells expressing ENOD2 during nodule growth appears to be correlated with the development of other peripheral tissues.     

Regulation of nodulin gene expression

The expression of plant genes specifically induced during rhizobial infection and the early stages of nodule ontogeny (early nodulin genes) and those induced in the mature, nitrogen-fixing nodule (late nodulin genes) is differentially regulated and tissue/cell specific. We have been interested in the signal transduction pathway responsible for symbiotic, temporal and spatial control of expression of an early (Enod2) and a late (Leghemoglobin;lb) nodulin gene from the stem-nodulated legumeSesbania rostrata, and in identifying thecis-acting elements andtrans-acting factors involved in this process (De Bruijn and Schell, 1992). By introducing chimericS. rostrata lb promoter-gus reporter gene fusions into transgenicLotus corniculatus plants, we have been able to show that thelb promoter directs an infected-cell-specific expression pattern inLotus nodules. We have been able to delimit thecis-acting element responsible for nodule-infected-cell-expression to a 78 pb region of thelb promoter (NICE Element) and have analyzed this element in detail by site-specific mutagenesis. We have studied the interaction of the NICE element, and further upstreamcis-acting elements, withtrans-acting factors of both plant- and rhizobial origin. We have obtained evidence for the involvement of rhizobial proteins in infected-cell-specific plant gene expression (Welters et al., 1993). We have purified one of the bacterial binding proteins from theS. rostrata symbiontAzorhizobium caulinodans (AcBBP1), and cloned and mutated the corresponding gene, in order to examine its symbiotic phenotype. We have also found that theS. rostrata Enod2 gene is rapidly induced by physiologically significant concentrations of cytokinins, suggesting the role of cytokinin as a potential secondary signal involved in nodulation (Dehio and De Bruijn, 1992). We are examining whether the observed cytokinin induction, as well as the nodule-specific expression pattern, are modulated by theSrEnod2 promoter.     

Symbiotic Nitrogen Fixation at the Late Stage of Nodule Formation in Lotus japonicus and Other Legume Plants

Lotus japonicus nodules are reviewed, and current research data on Nod-35 (uricase) gene, including that of Lotus japonicus as an example of a late nodulin gene are presented. Received 5 September 2000/ Accepted in revised form 2 October 2000