Microaerobiosis is not involved in the induction of pea nodulin-gene expression

Oxygen regulation of uricase and sucrose synthase synthesis in soybean callus tissue is exerted at the mRNA level

The effect of lowering oxygen concentration on the expression of nodulin genes in soybean callus tissue devoid of the microsymbiont has been examined. Poly(A)+ RNA was isolated from tissue cultivated in 4% oxygen and in normal atmosphere. Quantitative mRNA hybridization experiments using nodule-specific uricase (Nodulin-35) and sucrose synthase (Nodulin-100) cDNA probes confirmed that the synthesis of the uricase and sucrose synthase is controlled by oxygen at the mRNA level. The steady-state levels of uricase and sucrose synthase mRNA increased significantly (5-6- and 4-fold respectively) when the callus tissue was incubated at reduced oxygen concentration. Concomitant with the increase in mRNA level a 6-fold increase in specific activity of sucrose synthase was observed. Two messengers representing poly-ubiquitin precursors also responded to lowering the oxygen concentration. The increase was about 5-fold at 4% oxygen. No expression at atmospheric oxygen or in response to low oxygen was observed when using cDNA probes for other nodulin genes such as leghemoglobin c3, nodulin-22 and nodulin-44.

Effect of oxygen pressure on synthesis and export of nitrogenous solutes by nodules of cowpea

Nodules of cowpea plants (Vigna unguiculata (L.) Walp. cv. Vita 3 :Bradyrhizobium CB756) cultured for periods of 23 d with their root systems maintained in atmospheres containing a range of partial pressures of O2 (pO2; 1-80%, v/v, in N2) formed and exported ureides (allantoin and allantoic acid) as the major products of fixation at all pO2 tested. In sub-ambient pO2 (1 and 2.5%) nodules contained specific activities of uricase (urate: O2 oxidoreductase; EC 1.7.3.3) and allantoinase (allantoin hydrolyase; EC 3.5.2.5) as much as sevenfold higher than in those from air. On a cell basis, uninfected cells in nodules from 1% O2 contained around five times the level of uricase. Except for NAD: glutamate synthase (EC 1.4.1.14), which was reduced in sub-ambient O2, the activities of other enzymes of ureide synthesis were relatively unaffected by pO2. Short-term effects of pO2 on assimilation of fixed nitrogen were measured in nodules of air-grown plants exposed to subambient pO2 (1, 2.5 or 5%, v/v in N2) and(15)N2. Despite a fall in total(15)N2 fixation, ureide synthesis and export was maintained at a high level except in 1% O2 where formation was halved. The data indicate that in addition to the structural and diffusional adaptations of cowpea nodules which allow the balance between O2 supply and demand to be maintained over a wide range of pO2, nodules also show evidence of biochemical adaptations which maintain and enhance normal pathways for the assimilation of fixed nitrogen.

The occurrence of leghemoglobin protein in the uninfected interstitial cells of soybean root nodules

The distribution of leghemoglobin (Lb) in resin-embedded root nodules of soybean (Glycine max (L.) Merr.) was investigated using immunogold labeling. Using anti-Lb immunoglobulin G and protein A-gold, Lb or its apoprotein was detected both in cells infected by Bradyrhizobium japonicum and in uninfected interstitial cells. Leghemoglobin was present in the cytoplasm, exclusive of the organelles, and in the nuclei of both cell types. In a comparison of the density of labeling in adjacent pairs of infected and uninfected cells, Lb was found to be about four times more concentrated in infected cells. This is the first report of Lb in uninfected cells of any legume nodule; it raises the possibility that this important nodule-specific protein may participate in mediating oxygen flow to host plant organelles throughout the infected region of the nodule.

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.