Phloem‐derived γ‐aminobutyric acid (GABA) is involved in upregulating nodule N2 fixation efficiency in the model legume Medicago truncatula

Nitrogen fixation in legumes is downregulated through a whole plant N feedback mechanism, for example, when under stress. This mechanism is probably triggered by the impact of shoot‐borne, phloem‐delivered compounds. However, little is known about any whole‐plant mechanism that might upregulate nitrogen fixation, for example, under N deficiency. We induced emerging N‐deficiency through partial excision of nodules from Medicago truncatula plants. Subsequently, the activity and composition of the remaining nodules and shifts in concentration of free amides/amino acids in the phloem were monitored. Furthermore, we mimicked these shifts through artificial feeding of γ‐aminobutyric acid (GABA) into the phloem of undisturbed plants. As a result of increased specific activity of nodules, N₂ fixation per plant recovered almost completely 4–5 d after excision. The concentration of amino acids, sugars and organic acids increased strongly in the upregulated nodules. A concomitant analysis of the phloem revealed a significant increase in GABA concentration. Comparable with the effect of nodule excision, artificial GABA feeding into the phloem resulted in an increased activity and higher concentration of amino acids and organic acids in nodules. It is concluded that GABA might be involved in upregulating nodule activity, possibly because of its constituting part of a putative amino acid cycle between bacteroids and the cytosol.     

<Emphasis Type="Italic">In vitro</Emphasis> plant regeneration of <Emphasis Type="Italic">Alnus acuminata</Emphasis> H.B.K. ssp. <Emphasis Type="Italic">acuminata</Emphasis> and its root nodulation by <Emphasis Type="Italic">Frankia</Emphasis>

A procedure for in vitro plant regeneration of Alnus acuminata from epicotyls with cotyledonary buds was developed using different media formulations with different growth regulators and carbon sources. The development of multiple buds on explants at the initiation step was obtained with MS at 1/2 strength with either 1 or 2M of BAP but not without it. Multiplication gave up to 15 elongating shoots by explant, the best medium being MS supplemented with vitamins from B5 medium, 1M of BAP and 87mM sucrose. Rooting of about 88% occurred in the medium MS with 83 mM sucrose and 1M IBA. Alnus acuminata did not developed well on WPM. Roots of in vitro propagated plants were nodulated by Alnus-infective Frankia. The root nodules show a typical alder root nodule anatomy and differentiation pattern and effectively fixed nitrogen. Rhamnaceae-infective Frankia did not nodulate in vitro cultivated Alnus acuminata suggesting that symbiotic recognition was not altered by in vitro regeneration of the plant.     

植物非共生血红蛋白的研究进展

植物非共生血红蛋白（nsHb）基因在植物界广泛存在。许多生物和非生物胁迫可以诱导nsHb的表达。nsHb在植物的生长发育和逆境胁迫中具有重要功能,其作用机制与NO的代谢密切相关。文章综述了非共生血红蛋白的表达特性、生物学功能及其作用机制等方面的研究进展。     

Legume species identity and soil nitrogen supply determine symbiotic nitrogen-fixation responses to elevated atmospheric [CO2

In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO2] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO2] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. In a field free-air CO2 enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO2] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution 15N methods were used to estimate N fixation. Although N addition did, in general, decrease N fixation, contrary to theoretical predictions, elevated [CO2] did not universally increase N fixation. Rather, the effect of elevated [CO2] on N fixation was positive, neutral or negative, depending on the species and N addition. Our results suggest that legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric [CO2].     

一氧化氮对豆科植物结瘤及固氮的影响机制

豆科植物-根瘤菌共生过程受双方基因复杂且精细的调控, 能够产生特异的根瘤结构并可将大气中的惰性氮气(N₂)转化为可被植物直接利用的氨态氮。结瘤与固氮受多种因素影响, 其中, 一氧化氮(NO)作为一种自由基反应性气体信号分子, 可参与调节植物的许多生长发育过程, 如植物的呼吸、光形态建成、种子萌发、组织和器官发育、衰老以及响应各种生物及非生物胁迫。在豆科植物中, NO不仅影响寄主与菌共生关系的建立, 还参与调控根瘤菌对氮气的固定并提高植株氮素营养利用效率。该文主要从豆科植物及共生菌内NO的产生、降解及其对结瘤、共生固氮的影响和对环境胁迫的响应, 阐述了NO调控豆科植物共生体系中根瘤形成和共生固氮过程的作用机制, 展望了NO信号分子在豆科植物共生固氮体系中的研究前景。     

Medicago-Sinorhizobium symbiotic specificity evolution and the geographic expansion of Medicago

The legume genus Medicago interacts with soil bacteria commonly referred to as rhizobia, in a nitrogen fixing symbiosis. We analysed the diversity of symbiotic association specificity among the two organisms, and its evolution in the plant genus. Nitrogen fixation tests and molecular phylogenetic reconstructions revealed that the genus Medicago includes more symbiotic specificity groups than previously suggested and that plant specificity is highly unstable and has repeatedly switched along the diversification of this genus. A phylogenetic analysis including geographical data shows that bacterial geographical diversity distribution has a strong influence on the geographic distribution of plant species and their ability to colonize new areas. Multiple other modifications of specificity occurred along the diversification of the genus, presumably due to selection for specialization to a single bacterial biovar. Codivergence between plants and bacteria may also have taken place.     

Survival of Bradyrhizobium sp. (Arachis) on fungicide-treated peanut seed in relationship to plant growth and yield

Survival and viability of Bradyrhizobium inoculant on fungicide-treated peanut seed and the resulting effects on nitrogen fixation, plant growth and seed yield were determined. Vitavax and Benomyl had the most and least lethal actions against Bradyrhizobium strains grown on YEM medium containing a fungicide, respectively, while Thiram and Captan effects were intermediate. Survival of Bradyrhizobium USDA 3384 and USDA 3456, as single strain peat inoculants, on peanut (Arachis hypogaea L. var. Florunner) seeds treated with Benomyl or Vitavax at the rate of 3g/kg seed was also examined. Both fungicides inhibited the growth and affected the survival of strain USDA 3384 on peanut seed. Vitavax killed the inoculant in 9 h. In contrast, USDA 3456 resisted both fungicides, and survived for up to 72h. Nodule formation on greenhouse-grown plants inoculated with USDA 3384 was inhibited by all fungicides. Shoot dry weight and plant nitrogen content significantly decreased as compared to controls. Fungicides, except Vitavax, had a slight effect on nodulation and plant growth when USDA 3456 was used as inoculant. The agronomic importance of fungicide-inoculant interaction was examined in field experiments conducted in Egypt in soil free of peanut-nodulating Bradyrhizobium, where seeds were treated with a combination of two fungicides and a single strain peat inoculant of either USDA 3384 or USDA 3456. All fungicides decreased nodulation, nitrogen fixation, plant growth and seed yield, especially with USDA 3384 as inoculant. Fungicides inhibited viability and survival of Bradyrhizobium on peanut seeds which decreased nodule formation leading to reduced peanut seed yield.     

Kinetics of nitric oxide binding to R-state hemoglobin

Despite earlier work indicating otherwise, some recent reports have suggested that nitric oxide (NO) binds to hemoglobin cooperatively. In particular, it has been suggested that, under physiological conditions, NO binds to the high-affinity R-state hemoglobin as much as 100 times faster than to the low-affinity T-state hemoglobin. This rapid NO binding could provide a means of preserving NO bioactivity. However, using a flash-flow photolysis technique, we have determined that the rate of NO binding to normal adult R-state hemoglobin is (2.1 +/- 0.1) x 10(7) (s(-1) M(-1)), which is essentially the same as that reported for T-state NO binding. (c)2002 Elsevier Science (USA).     

Bryophyte-cyanobacteria symbioses and their nitrogen fixation capacity—A review

苔藓-蓝藻共生体(BCS)能固氮, 是养分贫瘠地区森林氮输入的不可忽视的来源。BCS关系与固氮能力研究为科学认识生态系统氮输入与氮循环过程和机理提供了新的视角和有效途径, 具有重要的理论价值。然而, BCS关系、固氮作用与机理的研究迄今未受到足够关注, 报道较少, 认识仍然是零星而片段化的。基于系统查阅的相关文献, 该文综述了BCS的种类组成与共生关系类型、固氮能力及所固定氮的去向及其影响因素和作用机理, 指出了存在的问题及需要深入关注和亟待突破的4个研究方向。     

Root hairs and phosphorus acquisition of wheat and barley cultivars

Several genes that restrict nodulation with specific Bradyrhizobiumstrains are known in Glycine max (soybean), and a similar system of nodulation restriction has recently been discovered in the related North American legume Amphicarpaea bracteata. We analyzed how nodulation-restrictive genotypes of each plant interacted with Bradyrhizobium strains sampled from the other host species. Ten bacterial isolates from A. bracteata that nodulated differentially with genotypes of their homologous host legume showed uniform responses to two soybean isogenic lines that differed at the Rj4 locus controlling nodulation restriction: all isolates formed nodules of normal size and morphology on both isolines. However, little or no nitrogen fixation occurred in any of these symbioses. A. bracteata genotypes that displayed broad vs. restricted symbiotic phenotypes toward naturally-associated bradyrhizobia were also tested with two bacterial isolates from soybean (USDA 76 and USDA 123). Both isolates formed nodules and fixed nitrogen in association with both A. bracteata genotypes. However, symbiotic effectiveness (as measured by acetylene reduction assays) was normal only for the combination of USDA 76 with the restrictive A. bracteata genotype. Overall, these results indicate that plant genes that restrict nodulation by certain naturally-associated bradyrhizobia do not confer comparable specificity when plants interact with bacteria from another related legume species.     

Nitrogen Fixation By Corona Discharge On The Early Precambrian Earth

We report the first experimental study of nitrogen fixation by corona discharge on the anoxic primitive Earth. The energy yields of nitric oxide (NO) and nitrous oxide (N₂O) were experimentally determined over a wide range of CO₂-N₂ mixtures simulating the evolution of the Earth's atmosphere during the Hadean and Archean eras (from 4.5 ba to 2.5 ba). NO, the principal form of fixed nitrogen in lightning and coronal discharge in early Earth, is produced ten times less efficiently in the latter type of electrical discharge with an estimated maximum annual production rate of the order of 10¹⁰ g yr⁻¹. For N₂O the maximum production rate was estimated to be ∼10⁹ g yr⁻¹. These low rates of syntheses indicate that corona discharges as point discharges on the clouds and ground did not play a significant role in the overall pool of reactive nitrogen needed for the emergence and sustainability of life.     

Strains of Mesorhizobium amorphae and Mesorhizobium tianshanense, carrying symbiotic genes of common chickpea endosymbiotic species, constitute a novel biovar (ciceri) capable of nodulating Cicer arietinum

AIMS: To identify several strains of Mesorhizobium amorphae and Mesorhizobium tianshanense nodulating Cicer arietinum in Spain and Portugal, and to study the symbiotic genes carried by these strains. METHODS AND RESULTS: The sequences of 16S-23S intergenic spacer (ITS), 16S rRNA gene and symbiotic genes nodC and nifH were analysed. According to their 16S rRNA gene and ITS sequences, the strains from this study were identified as M. amorphae and M. tianshanense. The type strains of these species were isolated in China from Glycyrrhiza pallidiflora and Amorpha fruticosa nodules, respectively, and are not capable of nodulating chickpea. These strains carry symbiotic genes, phylogenetically divergent from those of the chickpea isolates, whose nodC and nifH genes showed more than 99% similarity with respect to those from Mesorhizobium ciceri and Mesorhizobium mediterraneum, the two common chickpea nodulating species in Spain and Portugal. CONCLUSIONS: The results from this study showed that different symbiotic genes have been acquired by strains from the same species during their coevolution with different legumes in distinct geographical locations. SIGNIFICANCE AND IMPACT OF THE STUDY: A new infrasubspecific division named biovar ciceri is proposed within M. amorphae and M. tianshanense to include the strains able to effectively nodulate Cicer arietinum.     

Seasonal flooding regimes influence survival,nitrogen fixation,and the partitioning of nitrogen and biomass in Alnus incana ssp. rugosa

Alteration of natural flooding regimes can expose lowlands to waterlogged soil conditions during any month of the year. The seasonality of flooding may have profound effects on the carbon and nitrogen budgets of N-fixing alders (Alnus spp.), and in turn, may impact the C and N economy of extensive alder-dominated, wetland ecosystems, including those dominated by speckled alder (Alnus incana ssp. rugosa). To better understand this process, two-year-old, nodulated seedlings of speckled alder were subjected to late spring (May 10 – July 10), summer (July 10 – September 8), and fall (September 8 – November 8) flooding treatments. Alders were root-flooded outdoors in tanks containing an N-free nutrient solution and compared with unflooded alders at the experimental site. Flooding arrested N fixation, photosynthesis, and growth of alders without recovery in all flooding treatments for the remainder of the growing season. Late spring and summer flooding resulted in complete mortality of alders while all seedlings survived flooding in the fall. Fall flooding increased foliar N resorption by 140% over unflooded seedlings. Eighty-seven percent of the total N fixed and 89% of biomass accumulation for the entire growing season occurred in unflooded alders after July 10. In unflooded alders, nitrogen fixation rates per unit mass declined by 63% for nodules, 28% for leaves, and 48% for whole seedlings during the fall, while total N fixed per plant in the fall was similar to that fixed in the summer. The majority of newly fixed N in unflooded alders was allocated to leaves before September 8 and to roots/nodules combined after September 8. In unflooded plants, the greatest proportion of new biomass was partitioned to leaves before July 10, to stems between July 10 and September 8, and equally to stems and roots/nodules after September 8. Fall-flooded alders did not increase root or nodule biomass. Proportional allocation of plant resources were such that the ratio of N fixed to seedling growth of unflooded alders decreased by 19% during summer before rebounding by 6% in fall. Seasonality of flooding alters seedling survival, growth, and resource allocation, and may be a critical determinant of speckled alder recruitment and occurrence in wetlands.