Structural basis of carbohydrate recognition by a Man(alpha1-2)Man-specific lectin from Bowringia milbraedii

The crystal structure of the seed lectin from the tropical legume Bowringia milbraedii was determined in complex with the disaccharide ligand Man(alpha1-2)Man. In solution, the protein exhibits a dynamic dimer-tetramer equilibrium, consistent with the concanavalin A-type tetramer observed in the crystal. Contacts between the tetramers are mediated almost exclusively through the carbohydrate ligand, resulting in a crystal lattice virtually identical to that of the concanavalin-A:Man(alpha1-2)Man complex, even though both proteins have less than 50% sequence identity. The disaccharide binds exclusively in a "downstream" binding mode, with the non-reducing mannose occupying the monosaccharide-binding site. The reducing mannose is bound in a predominantly polar subsite involving Tyr131, Gln218, and Tyr219.     

Unique and conserved features of floral evocation in legumes

Legumes, with their unique ability to fix atmospheric nitrogen, play a vital role in ensuring future food security and mitigating the effects of climate change because they use less fossil energy and produce less greenhouse gases compared with N-fertilized systems. Grain legumes are second only to cereal crops as a source of human and animal food, and they contribute approximately one third of the protein consumed by the human population. The productivity of seed crops, such as grain legumes, is dependent on flowering. Despite the genetic variation and importance of flowering in legume production, studies of the molecular pathways that control flowering in legumes are limited.Recent advances in genomics have revealed that legume flowering pathways are divergent from those of such model species as Arabidopsis thaliana. Here, we discuss the current understanding of flowering time regulation in legumes and highlight the unique and conserved features of floral evocation in legumes.     

Presence of unique repeated insertion sequences in nodulation genes of Rhizobium ‘hedysari’

Cloning and sequencing of DNA from a symbiotic large plasmid in Rhizobium hedysari strain IS 123 required for its nodulation of the mediterranean legume crop Hedysarum coronarium (sulla) and complementation studies of nod^- mutant derivatives led to the characterization of a 30-kb region containing common and host-specific nod genes. This DNA region also contained at least six copies of a novel insertion sequence-like structure, some of which appeared to have suffered deletions. This 0.8 kb novel element carries two 17-bp flanking inverted repeats and an open reading frame showing homology with a transposase from Staphylococcus aureus. Hybridization studies revealed that several strains of Rhizobium hedysari carry this element in various copy number. The six copies in strain IS 123 appear clustered specifically within the pSym nod region.The significance of this IS element in rhizobia and its possible use as a probe for taxonomic and phylogenetic studies of Rhizobiaceae is addressed.     

有益微生物缓解花生连作障碍机理研究进展

花生是我国重要的油料作物,种植过程中会遇到连作障碍问题.有益微生物可以有效缓解花生连作障碍,其机理包括有益微生物分解化感物质、改变花生根际微生物区系、增加花生抗病能力、促进花生结瘤固氮、促进花生养分吸收五个方面.阐明该机理,对花生连作障碍缓解有指导意义,对其他作物连作障碍的缓解也有参考价值.     

Effects of continuous nitrogen application and nitrogen preconditioning on nodulation and growth ofCeanothus griseus varhorizontalis

Rooted cuttings ofCeanothus griseus varhorizontalis were irrigated with 0, 10, 20, 50, 75 or 100ppm nitrogen as NH₄NO₃ for eight weeks prior to inoculation with infectiveFrankia. After inoculation, half of the plants for each treatment nitrogen level continued to be irrigated with the preconditioning nitrogen level and half were given no more supplemental nitrogen. For plants continuously receiving nitrogen, nodule initiation (nodule number) was inversely correlated with increasing supplemental nitrogen levels, and suppressed above 50 ppm N. Leaf nitrogen above 2% in continuous-N plants correlated with greatly reduced or suppressed nodulation. Plants maintained after inoculation without supplemental nitrogen showed influence of the prior nitrogen treatment on nodulation. Preconditioning at 50 ppm and above greatly reduced the number of nodules formed. The evidence suggests that stored internal nitrogen can regulate nodulation.Plant biomass accumulated maximally when nodulation was suppressed, at 75 and 100 ppm supplemental N applied continuously. Internode elongation during the nodulation period occurred only on nodulated plants, or in the presence of supplemental N (10 ppm and above).     

根瘤菌结瘤基因研究的新进展

简要综述了目前根瘤菌结癌基因研究的3个热点方向,即结瘤因子、nodlD基因的调控和结瘤基因系统发育分析的新进展。结瘤因子的骨架核心是结瘤基因中的共同性基因nodABC表达的产物,宿主专一性基因则进行骨架结构的修饰,所形成的特异性结瘤因子是根瘤苗宿主范围的主要决定因素。结瘤调控基因nodD的作用方式与其存在的拷贝数目和产物NodD蛋白活性有关,同时NodD的敏感性还影响到根瘤菌的宿主范围。结瘤基因的系统发育揭示出根瘤菌宿主范围与共同性结瘤基因间比其它基荫的相关性更高。结瘤基因与豆科宿主之间存在一定的共进化关系。     

刺槐核糖体蛋白同源基因RpRPL22在共生结瘤过程中功能研究

目的: 核糖体蛋白(RPs)属于多功能蛋白,能够参与调控细胞生长和响应胁迫条件。RpRPL22是一个从豆科植物刺槐中分离得到的结瘤相关基因,通过序列比对发现其与核糖体大亚基蛋白RPL22高度同源。对其如何通过调控根瘤菌侵染而在共生结瘤过程中发挥重要作用进行了较为深入的探索。方法: 利用实时荧光定量PCR技术(qRT-PCR)分析RpRPL22在接菌后不同时间及不同植物组织的表达变化。利用cDNA末端快速扩增技术(RACE)获得目的基因cDNA全长。通过GFP报告基因进行RpRPL22亚细胞定位分析。通过Gateway BP重组技术构建RNA干扰(RNAi)重组载体,借助电转化法将重组载体转至农杆菌K599,利用农杆菌介导植物根部,接菌后观察和测量植株表型。首先从宏观水平统计观察目的基因是否对结瘤过程有影响,其次从分子水平揭示目的基因在共生结瘤过程的重要功能。结果: 不同接菌时期、不同植物组织目的基因qRT-PCR相对表达量结果显示,几乎在所有取样的接菌时间,目的基因RpRPL22在接菌根中的相对表达量都低于未接菌对照根,只有接菌后第25天除外。在成熟的根瘤中,接菌后第25天该基因的表达量也最高。洋葱表皮和毛状根亚细胞定位结果均显示在椰菜花叶病毒(CaMV)的35S启动子控制下,RpRPL22融合绿色荧光蛋白GFP的荧光信号在细胞核和细胞质有明显的表达。RNAi转化植株的表型统计观察结果,比如植株鲜重、植株的有效结瘤数目较对照组均有明显的降低;同时RNAi转化植株在根瘤菌侵染过程形成的侵染线数目和根瘤原基数目较对照均显著降低。根瘤切片实验用于观察根瘤显微超微结构,结果显示RNAi植株根瘤中固氮区的受菌侵染细胞数目与对照相比明显减少。电镜观察根瘤单个受菌侵染细胞中类菌体形态显示,RNAi根瘤中类菌体侵染细胞胞体多呈不规则形状,皱缩变形严重,环类菌体周间隙空间增大,多共生体融合,表现出细胞凋亡的迹象。对照根瘤中的受菌侵染细胞胞体多呈圆形椭圆形,胞质饱满丰富且分布均匀,细胞发育正常,表明RNAi植株根瘤发育过程明显受阻。结论: 核糖体蛋白(RP)能够参与调控豆科植物共生结瘤过程,相关同源基因RpRPL22可能在起始根瘤菌侵染植物和阻止类菌体降解过程中起重要作用。     

Nitrogen economy of alpine plants on the north Tibetan Plateau: Nitrogen conservation by resorption rather than open sources through biological symbiotic fixation

Nitrogen (N) is one of the most important factors limiting plant productivity, and N fixation by legume species is an important source of N input into ecosystems. Meanwhile, N resorption from senescent plant tissues conserves nutrients taken up in the current season, which may alleviate ecosystem N limitation. N fixation was assessed by the ¹⁵N dilution technique in four types of alpine grasslands along the precipitation and soil nutrient gradients. The N resorption efficiency (NRE) was also measured in these alpine grasslands. The aboveground biomass in the alpine meadow was 4–6 times higher than in the alpine meadow steppe, alpine steppe, and alpine desert steppe. However, the proportion of legume species to community biomass in the alpine steppe and the alpine desert steppe was significantly higher than the proportion in the alpine meadow. N fixation by the legume plants in the alpine meadow was 0.236 g N/m², which was significantly higher than N fixation in other alpine grasslands (0.041 to 0.089 g N/m²). The NRE in the alpine meadows was lower than in the other three alpine grasslands. Both the aboveground biomass and N fixation of the legume plants showed decreasing trends with the decline of precipitation and soil N gradients from east to west, while the NRE of alpine plants showed increasing trends along the gradients, which indicates that alpine plants enhance the NRE to adapt to the increasing droughts and nutrient‐poor environments. The opposite trends of N fixation and NRE along the precipitation and soil nutrient gradients indicate that alpine plants adapt to precipitation and soil nutrient limitation by promoting NRE (conservative nutrient use by alpine plants) rather than biological N fixation (open sources by legume plants) on the north Tibetan Plateau.     

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.