Host recognition in the Rhizobium-soybean symbiosis: detection of a protein factor in soybean root exudate which is involved in the nodulation process

The mechanism of host-symbiont recognition in the soybean-Rhizobium symbiosis was investigated utilizing mutants of R. japonicum defective in nodulation. Soybeans were grown in clear plastic growth pouches allowing the identification of the area on the root most susceptible to Rhizobium nodulation; the area between the root tip (RT) and smallest emergent root hair (SERH). The location of nodules in relation to this developing zone is an indication of the rate of nodule initiation. Nodules were scored as to the distance from the RT mark made at the time of inoculation. Seventy-eight per cent of the plants nodulate above the RT mark when inoculated with the wild type R. japonicum strain 3I1b110 with the average distance of the uppermost nodule being approximately 2 millimeters above the RT mark. These data indicate that the wild type strain initiates nodulation rapidly within the RT-SERH zone following inoculation. However, inoculation with the slow-to-nodulate mutant strain HS111 resulted in 100% of the plants nodulating only below the RT mark with the average distance of the uppermost nodule being approximately 56 millimeters below the RT mark. Thus, mutant strain HS111 is defective in the ability to rapidly initiate infection leading to nodulation within the RT-SERH zone. The location of the nodules suggest that stain HS111 must `adapt' to the root environment before nodulation can occur. To test this, strain HS111 was incubated in soybean root exudate prior to inoculation. In this case, 68% of the plants nodulated above the RT mark with the average distance of the uppermost nodule being approximately 1 millimeter below the RT mark. Experiments indicated that the change in nodule initiation by strain HS111 brought about by incubation in soybean root exudate was due to a phenotypic, rather than a genotypic change. The half-time of root exudate incubation for strain HS111 necessary for optimal nodulation enhancement was less than 6 hours. Heat sensitivity and trypsin sensitivity of the nodulation enhancement factor(s) in soybean root exudate indicate a protein was involved in the reversal of the delay in nodulation by mutant strain HS111.     

Pollen frequencies reflect vegetation patterns in a great basin (U.S.A.) mountain range

Five indices are used to quantify the relationship between vegetation and pollen in a mountain range in the arid Great Basin. Computations are based on vegetation coverage and pollen percentages from 63 stands. Association is a measure of whether the presence of the pollen type in a surface sample is an indication of the presence of the parent plant in the local vegetation. Over-representation and under-representation measure tendencies for pollen to occur where the parent plants are absent and vice versa. The correlation coefficient measures the relationship between plant and pollen in stands where both are present. Twenty-nine trees, shrubs, and herbs accurately reflect local vegetation conditions. A percentage diagram shows elevational trends in abundant pollen types. Regional pollen types are used to compute the accumulation rate of pollen in the surface samples. A diagram of pollen accumulation rates shows trends similar to those shown in the percentage diagram. The moss polsters used in this study may collect pollen over a fifteen-year interval.     

Xylem Fluids of White Lupin (Lupinus albus) Cultivars Susceptible and Resistant to Fusarium oxysporum f. sp. Lupini Race 3

Xylem fluids from white lupin cv. Neuland, susceptible to wilt causes by Fusarium oxysporum f. sp. lupini , promoted germination of conidia and chlamydospores of he pathogen, and the production of micro-conidia, to a greater extent than did xylem fluids from the resistant cv. Primorskij No sugars, organic acids or phenolic compounds were detected in the xylem fluids, but a number of amino acids were found, and the concentration of some differed in xylem fluids from the two cultivars; on the whole, concentrations were greater in the susceptible cv. Neuland. Synthetic amino acids were supplied to the fungus in vitro and both stimulatory and inhibitory effects (according to the compound and its concentration) were noted upon the germination of conidia and chlamydospores and on mycelial growth and the production of conidia. The role of amino acids and other compounds in the susceptibility of white lupin to wilt is discussed.     

Studies on the generation of B lymphocytes in fetal liver and bone marrow.

With the use of immunofluorescence techniques, cells containing cytoplasmic IgM (cIgM+), but lacking detectable surface IgM (sIgM+), have been identified in mouse fetal liver and adult bone marrow as a distinct cell population to sIgM+ B lymphocytes. We have shown that there is a considerable difference in the rate of entry of cIgM+ and sIgM+ cells into DNA synthesis in these locations. Moreover, within the cIgM+ population, the largest cells are the main group entering DNA synthesis. Our results are compatible with the notion that a pool of rapidly proliferating, large cIgM+ cells is present in fetal liver and adult bone marrow and that these cells give rise to populations of smaller cIgM+ cells, which move out of cell cycle, and convert to sIgM+ B lymphocytes. However, we recognize that this interpretation is speculative. Finally, we have shown that fetal bone marrow is a site of generation of sIgM+ B lymphocytes, but the question as to whether these cells are derived from Ig- precursors within marrow itself remains open.     

Effect of heparin on the extent of inhibition of thrombin by heparin cofactor.

A heparin preparation obtained by gel chromatography is compared to unfractionated heparin with respect to the effects of heparin on the reaction between thrombin and heparin cofactor. Whereas both preparations enhance the rate of inhibition of thrombin by heparin cofactor, the extent of inhibition is decreased by the unfractionated, but not by the fractionated heparin. The decreased extent of inhibition is accounted for by residua of unreacted and undegraded heparin cofactor and thrombin, as demonstrated by gel electrophoresis in dodecyl sulfate. However both heparin preparations enhance the rate of degradation by thrombin of the thrombin-heparin cofactor complex.     

A fluorometric method for monitoring the enzymic hydrolysis of terminal galactose from GM1-ganglioside.

A fluorometric method for monitoring the enzymic hydrolysis of the terminal galactose from GM₁-ganglioside has been developed. The released galactose is oxidized with galactose dehydrogenase and NAD and the fluorescence of the product NADH measured. This method can detect as little as 0.1 nmol of galactose. β-Galactosidase from the gastropod Turbo cornutus was employed for the hydrolysis reaction. The rate of GM₁-ganglioside hydrolysis is linearly proportional to incubation time for 30 min under the assay conditions employed. In addition to galactose, the other product of hydrolysis, GM₂-ganglioside, is identified by thin-layer chromatography. This procedure provides a convenient and specific method for measuring the release of galactose from GM₁-ganglioside.     

Nitrogen and ammonia assimilation in the cyanobacteria: regulation of glutamine synthetase.

Glutamine synthetase, the first enzyme of the ammonia assimilatory pathway, has been purified from Anabaena sp. CA by use of established procedures and by affinity chromatography as a final step. No adenylylation system controlling glutamine synthetase activity was found. The enzyme shows a marked specificity for Mg²⁺ in the biosynthetic assay and Mn²⁺ in the transferase assay. Under physiological conditions, Co²⁺ produces a large stimulatory effect on the Mg²⁺-dependent biosynthetic activity. The enzyme is inhibited by the feedback modifiers l-alanine, glycine, l-serine, l-aspartate, and 5′-AMP. Inhibition by l-serine and l-aspartate is linear, noncompetitive with respect to l-glutamate with apparent K_i values of 3 and 13 mm, respectively. Cumulative inhibition is seen with mixtures of l-serine, l-aspartate, and 5′-AMP. The results indicate that, in vivo, divalent cation availability and the presence of feedback inhibitors may play the dominant role in regulating glutamine synthetase activity and hence ammonia assimilation in nitrogen-fixing cyanobacteria.     

Expression of a subgenomic retroviral messenger RNA

When mRNA for avian retroviral envelope glycoprotein (env) was injected into cells transformed by env-deficient Bryan Rous sarcoma virus, the env deficiency of the injected cells was complemented to allow the release of transforming virus for up to 40 hr. When virus spread within the injected culture was allowed to occur, a second phase of transforming virus production by the injected culture began approximately 2 days following injection, continued for many days and often increased to titers well above those seen soon after injection. The requirement for virus spread, along with the genetic properties of virus released long after injection, supported the hypothesis that the second phase of virus production resulted when injected env mRNA was packaged into virus released by injected cells. When this virus infected other cells within the culture the env mRNA was reverse-transcribed to form a subgenomic, proviral-like molecule able to direct the synthesis of env mRNA. Accordingly, it was shown that neither DNA nor full genomic viral RNA contaminating injected mRNA preparations could account for the results. Evidence that an mRNA can be reverse-transcribed into an active, proviral-like molecule may be of importance in the relationship between retroviruses and their hosts.