Structure and role in symbiosis of the exoB gene of Rhizobium leguminosarum bv trifolii

The Rhizobium leguminosarum bv trifolii exoB gene has been isolated by heterologous complementation of an exoB mutant of R. meliloti. We have cloned a chromosomal DNA fragment from the R. leguminosarum bv trifolii genome that contains an open reading frame of 981 bp showing 80% identity at the amino acid level to the UDP-glucose 4-epimerase of R. meliloti. This enzyme produces UDP-galactose, the donor of galactosyl residues for the lipid-linked oligosaccharide repeat units of various heteropolysaccharides of rhizobia. An R. leguminosarum bv trifoliiexoB disruption mutant differed from the wild type in the structure of both the acidic exopolysaccharide and the lipopolysaccharide. The acidic exopolysaccharide made by our wild-type strain is similar to the Type 2 exopolysaccharide made by other R. leguminosarum bv trifolii wild types. The exopolysaccharide made by the exoB mutant lacked the galactose residue and the substitutions attached to it. The exoB mutant induced the development of abnormal root nodules and was almost completely unable to invade plant cells. Our results stress the importance of exoB in the Rhizobium-plant interaction. Received: 31 May 1996 / Accepted: 18 December 1996     

Phosphorus availability and elevated CO2 affect biological nitrogen fixation and nutrient fluxes in a clover-dominated sward

The response of biological nitrogen fixation (BNF) to elevated CO(2) was examined in white clover (Trifolium repens)-dominated swards under both high and low phosphorus availability. Mixed swards of clover and buffalo grass (Stenotaphrum secundatum) were grown for 15 months in 0.2 m2 sand-filled mesocosms under two CO2 treatments (ambient and twice ambient) and three nutrient treatments [no N, and either low or high P (5 or 134 kg P ha(-1)); the third nutrient treatment was supplied with high P and N (240 kg N ha(-1))]. Under ambient CO2, high P increased BNF from 410 to 900 kg ha(-1). Elevated CO2 further increased BNF to 1180 kg ha(-1) with high P, but there was no effect of CO2 on BNF with low P. Allocation of N belowground increased by approx. 50% under elevated CO2 irrespective of supplied P. The results suggest that where soil P availability is low, elevated CO2 will not increase BNF, and pasture quality could decrease because of a reduction in aboveground N.     

Population genetic diversity in the polyploid complex of wheatgrasses using isoenzyme and RAPD data

Thirty five bands (alleles) from six enzyme systems and fifty seven random amplified polymorphic DNA (RAPD) fragments were selected to analyse the genetic diversity of 33 polyploid wheatgrasses (Triticeae) populations of species Thinopyrum junceiforme and Elytrigia pycnantha, and two hybrids, one pentaploid and one novel 9-ploid. Dice’s similarity coefficient, the UPGMA-derived phenograms from RAPD, and allozymes markers showed that the clustering of wheatgrass populations was based on ploidy level. These markers had similar levels of diversity between populations, with high genetic similarity within the same ploidy-level and within population’s individuals. The tetraploid Th. junceiforme populations are closely related, with a large similarity distances varied from 0.8 to 1. Based on the isozyme and RAPD analyses, diploid taxa are related to polyploids with similarity coefficients 0.4.     

Rhizobium trifolii mutants deficient in exopolysaccharide production

Spontaneous mutants of Rhizobium trifolii 24AR5 which did not produce exopoly-saccharide were isolated. The non-mucoid mutants formed small white and ineffective nodules on both red and white clover. These nodules contained infection threads, but only a small number of bacteria were released into nodule cells, and bacteroids were rarely observed. The non-mucoid phenotype was not complemented by the symbiotic plasmid (pJB5JI) of Rhizobium leguminosarum.     

马蹄金基因组DNA的提取方法比较

对马蹄金基因组DNA的提取方法———快速提取法、小量提取法和大量提取法进行了对比分析,结果表明,利用快速提取法可在20 min内快速可靠地从马蹄金(Dichondra repensForst.)组织中提取DNA,与小量提取法和大量提取法获得的DNA用于PCR检测结果一致,可为转基因植物的快速检测提供方法。     

长穗偃麦草DREB类基因EeAP2.2 的克隆与序列分析

由于一些基因的特殊碱基序列限制,使得应用一种技术获得基因的全长cDNA序列比较困难。本研究结合RACE和Genome Walking技术从十倍体长穗偃麦草（Elytrigia elongata,2n=70）中克隆了AP2家族的一个全长cDNA序列,命名为EeAP2.2。序列分析表明,该基因具有一个837bp的开放阅读框,编码279个氨基酸残基,含有一个保守的AP2结构域,是AP2大家族的一个新成员。该基因编码的氨基酸序列与GenBank已有的普通小麦AP2家族两个同源基因编码蛋白TaDREB1和TaDREBW50（登录号分别为：AAL01124.1和AAY44605.1）具有98%的氨基酸序列一致性, 与大麦AP2蛋白HvDREB1-a（登录号AAY25517.1）,高羊茅AP2蛋白FaDREB2A （登录号CAG30547.1） 及水稻OsDREB2.2（登录号AY064403）的氨基酸序列一致性分别为 93%、86%、69%。说明该基因与小麦AP2家族基因的同源性最高。本研究除获得了长穗偃麦草一个重要抗逆转录因子基因EeAP2.2的全长cDNA序列外,也提供了一种快速、有效克隆功能基因的方法。