Enhancing crop legume N2 fixation through selection and breeding

Legume N₂ fixation is variable, but nonetheless is a valuable process in world agriculture. There is great potential to increase the contribution by the crop legumes to the world's supply of soil.N. This will be achieved by (i) increasing the area of legumes sown by farmers; (ii) improved management of the crops in order that the major determinants of productivity, e.g. land area, water availability, are converted to harvested product with maximum efficiency; and (iii) genetic modification of the commonly-grown species to ensure high dependence of the legume crop on N₂ fixation at all levels of productivity. Currently-used methods for measuring N₂ fixation and for assessing heritability and repeatability of N₂ fixation in breeding and selection programs are reviewed. Results from research programs to define genetic variation in N₂ fixation and to enhance N₂ fixation through selection and breeding are presented with particular emphasis on common bean (Phaseolus vulgaris) and soybean (Glycine max).     

Diversity in symbiotic specificity of cowpea rhizobia indigenous to Zimbabwean soils

Tropical cowpea rhizobia are often presumed to be generally promiscuous but poor N fixers. This study was conducted to evaluate symbiotic interactions of 59 indigenous rhizobia isolates (49 of them from cowpea (Vigna unguiculata)), with up to 13 other (mostly tropical) legume species. Host ranges averaged 2.4 and 2.3 legume species each for fast- and slow-growing isolates respectively compared to 4.3 for slow-growing reference cowpea strains. An average of 22% and 19% of fast- and slow-growing cowpea isolates respectively were effective on each of 12 legume species tested. We conclude that the indigenous cowpea rhizobia studied have relatively narrow host ranges. The ready nodulation of different legumes in tropical soils appears due to the diversity of indigenous symbiotic genotypes, each consisting of subgroups compatible with a limited number of legume species.     

西北部分地区苦马豆根瘤菌的表型多样性研究

将采集自甘肃、新疆、宁夏等地区的苦马豆根瘤,经分离、纯化获得48株未知菌株,并选取7株参比菌株,进行唯一碳、氮源利用、对抗生素及染料抗性、耐盐性、初始pH生长、生长温度范围及酶活性等共113项生理生化测定;采用数值分类方法对未知根瘤菌进行表型多样性分析。结果表明：供试菌株在碳氮源利用、抗生素敏感性、抗逆性等方面存在着差异。该地区苦马豆根瘤菌具有较强的耐盐、耐碱能力,所有菌株均能在初始pH9~12的YMA培养基上生长,35%菌株可耐受6%的NaCl。从数值分类树状图可见,未知供试菌株在56%的相似水平上聚在一起,在72%的相似水平上分为5个表观群。群Ⅰ有39株菌,在74.6%相似水平聚合,中心菌株为CCNWGS0215;群Ⅱ有5株菌,在76%的相似水平聚合,中心菌株为CCNWGS0228;群Ⅳ有2株菌,在81.5%的相似水平聚合,群Ⅲ和群Ⅴ分别只有1株菌,它们与模式株分离,可能为潜在的新种。     

Invasive legume management strategies differentially impact mutualist abundance and benefit to native and invasive hosts

Determining the best management practices for plant invasions is a critical, but often elusive goal. Invasive removals frequently involve complex and poorly understood biotic interactions. For example, invasive species can leave potent legacies that influence the success of native species restoration efforts, and positive plant‐microbial feedbacks may promote continued reinvasion by an exotic species following restoration. Removal methods can vary in their effects on plant–soil feedbacks, with consequences for restoration of native species. We determined the effects of invasion by a leguminous shrub (French broom; Genista monspessulana) on the density and community composition of, and benefit conferred by, its microbial mutualists in its invading range. Densities of soil‐dwelling rhizobia were much higher in areas invaded by G. monspessulana relative to uninvaded areas, and this increased density of rhizobia fed back to increase seedling growth of both the invader and native legumes. We further compared how three techniques for removing G. monspessulana affected the densities of rhizobia relative to areas where G. monspessulana was still present. Removal by hand‐pulling reduced soil rhizobial densities, and reduced growth of one native legume, while having no effect on the growth of the invader. Overall, our results show that the consequences of restoration techniques, both above‐ and belowground, could be critical for the successful removal of an invasive legume and restoration of native species.     

Novel associations between rhizobial populations and legume species within the genera Lathyrus and O xytropis grown in the temperate region of China

Fifty rhizobial isolates of Lathyrus and Oxytropis collected from northern regions of China were studied in their genotypic characterization based upon analyses of ARDRA, 16S-23S IGS PCR-RFLP, TP-RAPD, MLEE, sequences of 16S rDNA gene and housekeeping genes of atpD, recA and glnII. The results demonstrated that most of the Lathyrus rhizobia belonged to Rhizobium and most of the Oxytropis rhizobia belonged to Sinorhizobium. A novel group of Rhizobium sp. I and S. meliloti were identified as the main microsymbionts respectively associated with Lathyrus and Oxytropis species in the collection area, which were new associations between rhizobia and the mentioned hosts. This study also provides new evidence for biogeography of rhizobia. Supported by the National Program for Basic S&T Platform Construction (Grant No. 2005DKA21201-1), the National Natural Science Foundation of China (Grant No. 30670001), and the National Basic Research Program of China (Grant No. 2006CB100206)     

Continuous ATP Regeneration Using Immobilized Yeast Cells

Arthrobacter simplex was screened as an α-keto-δ-guanidinovalerate (ketoarginine) assimilating organism. A characteristic feature was its growth on ketoarginine as a carbon source; it began to grow after an extremely long lag. Its growth was stimulated by addition of 0.02% yeast extract to the medium.

The results indicated the transamination of arginine-α-ketoglutarate (α-KGA) and the hydrolyzing reaction of ketoarginine into α-keto-δ-aminovalerate and urea. Two intermediates, ketoarginine and α-keto-δ-aminovalerate, were isolated and identified by various procedures. Coupling of the two reactions was demonstrated in cell-free extracts of arginine-grown cells; ketoarginine formed from arginine by transamination with α-KGA was hydrolyzed directly to α-keto-δ-aminovalerate and urea. The metabolic routes of arginine in microorganisms were discussed.