Effects of calcium deficiency on symbiotic nitrogen fixation

The mechanism of the effect of mild calcium deficiency on nitrogen metabolism of the symbiotic plant was studied from the distribution of calcium and of nitrogen and carbohydrate fractions in plant organs.

Nitrogen concentrations of all plant organs decreased with calcium deficiency. Addition of either a nitrogen or a calcium salt increased nitrogen concentrations. For roots as well as whole plants the effects of one salt were largely replaced by the other. These effects establish that calcium deficiency decreased the supply of fixed nitrogen from nodules to other organs. As weight of nodules was independent of calcium it follows that nodular efficiency was impaired.

Since nitrogen concentrations of nodules decreased rather than increased with calcium deficiency it is suggested that calcium deficiency interferes with the rate of reduction of nitrogen in the nodule rather than with the export of reduced nitrogen. Distribution of α-amino, amide, and ammonium nitrogen are consistent with this suggestion.

Nitrogen fixation was not limited by translocation of carbohydrates to nodules as calcium deficiency had little effect on the concentration of soluble carbohydrates and actually increased the concentration of starch in nodules.

Calcium deficiency depressed the calcium content of nodules so that nitrogen fixation may have been impaired by inadequate calcium for nodular structure or metabolism.

     

Genomics insights into symbiotic nitrogen fixation

Following an interaction with rhizobial soil bacteria, legume plants are able to form a novel organ, termed the root nodule. This organ houses the rhizobial microsymbionts, which perform the biological nitrogen fixation process resulting in the incorporation of ammonia into plant organic molecules. Recent advances in genomics have opened exciting new perspectives in this field by providing the complete gene inventory of two rhizobial microsymbionts. The complete genome sequences of Mesorhizobium loti, the symbiont of several Lotus species, and Sinorhizobium meliloti, the symbiont of alfalfa, were determined and annotated in detail. For legume macrosymbionts, expressed sequence tag projects and expression analyses using DNA arrays in conjunction with proteomics approaches have identified numerous genes involved in root nodule formation and nitrogen fixation. The isolation of legume genes by tagging or positional cloning recently allowed the identification of genes that control the very early steps of root nodule organogenesis.     

Bacteria-plant interactions in symbiotic nitrogen fixation

In the inter- and intracellular N₂-fixing symbioses between plants and micro-symbionts, the development of an endophytic form of the micro-symbiont is essential. This development includes a series of steps consisting of plant-bacteria interactions. Considerable progress in the elucidation of these steps has been made by applications of the methods of molecular genetics. Several genes with a role during infection and nodulation have been indicated in Rhizobium and Bradyrhizobium like the common nod genes A, B, C, I and J, and the host-specific genes nod E, F and H. The nod D gene is the only constitutive gene, and its product is essential for activity of all other nod genes, provided some flavonoids from the root exudate are present as well. Mutants in these genes show phenotypic effects, in which the products of the genes must be involved. Far more difficult is the biochemical and physiological study of these products and their direct effects. The difficulties involved in such biochemical-physiological studies is illustrated by a short discussion of the controversies around the possible role of plant lectins. While in Rhizobium the nod genes are present on a large sym-plasmid, other essential genes must be present on the bacterial chromosome and on other plasmids. Induction of plant genes is evident from the formation of nodule-specific proteins, the nodulins. Though many different plant and bacterial genes are involved in the series of steps in the development of an effective root nodule, there are indications that regulation is affected by a smaller number of essential regulatory genes. This is illustrated by the effect of the regulatory nod D gene during infection and nodulation, and of ntrA and nifA genes for the formation and activation of the nitrogen-fixing systems. Moreover, every step, once initiated, may lead to cascade effects on subsequent reactions. Finally, some further consequences of the endophytic way of life are discussed, which affect either the metabolic and transport activities of the endophytes or their viability. This is illustrated by the possible role of membrane integrity as evident during the isolation of Frankia from its endophytic form.     

Methods for enhancing symbiotic nitrogen fixation

Biological nitrogen fixation of leguminous crops is becoming increasingly important in attempts to develop sustainable agricultural production. However, these crops are quite variable in their effectiveness in fixing nitrogen. By the use of the ¹⁵N isotope dilution method some species have been found to fix large proportions of their nitrogen, while others like common bean have been considered rather inefficient. Methods for increasing N₂ fixation are therefore of great importance in any legume work. Attempts to enhance nitrogen fixation of grain legumes has been mainly the domain of microbiologists who have selected rhizobial strains with superior effectiveness or competitive ability. Few projects have focused on the plant symbiont with the objective of improving N₂ fixation as done in the FAO/IAEA Co-ordinated Research Programme which is being reported in this volume. The objective of the present paper is to discuss some possibilities available for scientists interested in enhancing symbiotic nitrogen fixation in grain legumes. Examples will be presented on work performed using agronomic methods, as well as work on the plant and microbial symbionts. There are several methods available to scientists working on enhancement of N₂ fixation. No one approach is better than the others; rather work on the legume/Rhizobium symbiosis combining experience from various disciplines in inter-disciplinary research programmes should be pursued.     

干旱胁迫对毛竹幼苗生理特性的影响

以1年生毛竹为试验材料,采用盆栽方式,设置对照(CK)、轻度(LS)、中度(MS)和重度干旱胁迫(HS)4个不同处理,研究不同水分条件下毛竹幼苗生理响应。结果表明:随着水分胁迫程度的加强,毛竹幼苗净光合速率(Pn)、蒸腾速率(Tr)显著下降(P<0.01),降幅分别达54.7%和49.7%,水分利用效率(WUE)在轻度胁迫条件下明显提高,但在中度和重度胁迫下又有一定程度下降;叶绿素a和叶绿素b含量显著下降(P<0.05和P<0.01),降幅分别为42.1%和28.2%,水分胁迫对叶绿素a/b(Chl-a/Chl-b)影响不显著;SOD活性、CAT活性和MDA含量显著上升(P<0.01、P<0.05和P<0.01);各生理指标间存在着相关性,经主成分分析,可将9个单项耐旱生理指标综合成为2个相互独立的综合指标,为评价毛竹苗期抗旱性提供参考。     

Legume haemoglobins: symbiotic nitrogen fixation needs bloody nodules

How do plants create an environment in which symbiotic bacteria can reduce enough N2 to provide the plant with sufficient ammonium for growth? Gene silencing has now been used to show that legume haemoglobins are crucial.     

Enhanced expression of Rhizobium etli cbb 3 oxidase improves drought tolerance of common bean symbiotic nitrogen fixation

To investigate the involvement of Rhizobium etli cbb (3) oxidase in the response of Phaseolus vulgaris to drought, common bean plants were inoculated with the R. etli strain, CFNX713, overexpressing this oxidase in bacteroids (cbb (3) (+)) and subjected to drought conditions. The negative effect of drought on plant and nodule dryweight, nitrogen content, and nodule functionality was more pronounced in plants inoculated with the wild-type (WT) strain than in those inoculated with the cbb (3) (+) strain. Regardless of the plant treatment, bacteroids produced by the cbb (3) (+) strain showed higher respiratory capacity than those produced by the WT strain. Inoculation of plants with the cbb (3) (+) strain alleviated the negative effect of a moderate drought on the respiratory capacity of bacteroids and the energy charge of the nodules. Expression of the FixP and FixO components of the cbb (3) oxidase was higher in bacteroids of the cbb (3) (+) strain than in those of the WT strain under all experimental conditions. The decline in sucrose synthase activity and the decrease in dicarboxylic acids provoked by moderate drought stress were more pronounced in nodules from plants inoculated with the WT strain than in those inoculated with the cbb (3) (+) strain. Taken together, these results suggest that inoculation of plants with a R. etli strain having enhanced expression of cbb (3) oxidase in bacteroids reduces the sensitivity of P. vulgaris-R. etli symbiosis to drought and can modulate carbon metabolism in nodules.     

Enhancement of symbiotic nitrogen fixation by vitamin-secreting fluorescent Pseudomonas

Fluorescent Pseudomonas sp. strain 267 promotes growth of nodulated clover plants under gnotobiotic conditions. In the growth conditions (60 M FeCl₃), the production of siderophores of the pseudobactin-pyoverdin group was repressed. Plant growth enhancement results from secretion of B vitamins by Pseudomonas sp. strain 267. This was proven by stimulation of clover growth by naturally auxotrophic strains of Rhizobium leguminosarum bv. trifolii and marker strains E. coli thi^- and R. meliloti pan^- in the presence of the supernatant of Pseudomonas sp. strain 267. The addition of vitamins to the plant medium increased symbiotic nitrogen fixation by the clover plants.