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     

Testing the efficiency of three 15N-labeled nitrogen compounds for indirect labeling of grasshoppers via plants in the field

In field studies of plant–insect herbivore interactions it is often difficult to establish which herbivore has fed on a particular plant. We investigated the suitability of three different ¹⁵N‐labeled nitrogen compounds (ammonium, nitrate, and glycine) for indirect marking of three grasshopper species [Omocestus viridulus (L.), Chorthippus parallelus (Zett.), and Chorthippus biguttulus (L.) (Orthoptera: Acrididae)] through labeling their food plants in the field. In two short‐term experiments grassland plots of 1 m² were separately labeled with either one of the different nitrogen compounds. Grasshoppers were caged on three food‐plant species [Dactylis glomerata L., Holcus lanatus L. (Poaceae), and Trifolium repens L. (Fabaceae)] present in these plots for 72 h. Significantly enriched δ¹⁵N values in grasshoppers were found in all plant/grasshopper combinations. Enrichment in grasshoppers was positively correlated with the enrichment of plants and labeling with nitrate resulted in highest ¹⁵N enrichment. In a long‐term experiment, individuals of C. biguttulus were placed in a cage covering an area of 1 m² for 37 days, with sampling of grasshoppers at regular intervals. δ¹⁵N values of the grasshopper and a common food plant, D. glomerata, increased steadily over time, up to 40‐fold by the end of the experiment. Our results demonstrate that ¹⁵N‐labeling of plants is an appropriate tool for the investigation of insect–plant interactions under natural conditions.     

Reflectance Measurements of Glossy Petals of Ranunculus lingua (Ranunculaceae) and of Non-Glossy Petals of Heliopsis helianthoides (Asteraceae)

Abstract: A spectrophotometric comparison of the reflectance of glossy petals of Ranunculus lingua (Ranunculaceae) and non-glossy petals of Heliopsis helianthoides (Asteraceae) was made in relation to the angle of incidence of the incoming light. Additionally, the reflectance of petals of R. flammula, R. acris and R. repens (Ranunculaceae) was measured for comparative purposes. In the examined Ranunculus species, gloss is present around mirror geometry over the whole spectral range measured, including the UV region where it accentuates the UV peak, and even at the basal UV-absorbing area of the petals. According to a colour hexagon evaluation of the reflectance data, these gloss phenomena diminish the colour contrast of the petal against the background. Nevertheless, due to their brightness, they may serve as an attractant or predictor for pollinators and other visiting insects, possibly acting as a beacon while approaching the flower. The position of the reflectance maximum in the UV around 340 nm is in accordance with the sensitivity maximum of the UV photoreceptor of potential pollinators like Apidae. The mean reflectance values of the four examined Ranunculus species do not show any clear correlation with their anatomical make-up     

Elevated atmospheric CO2 does not affect per se the preference for symbiotic nitrogen as opposed to mineral nitrogen of Trifolium repens L.

The objective of this investigation was to examine the effect of an elevated atmospheric CO₂ partial pressure ( p CO₂) on the N-sink strength and performance of symbiotic N₂ fixation in Trifolium repens L. cv. Milkanova. After initial growth under ambient p CO₂ in a nitrogen-free nutrient solution, T. repens in the exponential growth stage was exposed to ambient and elevated p CO₂ (35 and 60 Pa) and two levels of mineral N (N-free and 7·5 mol m^–3 N) for 36 d in single pots filled with silica sand in growth chambers. Elevated p CO₂ evoked a significant increase in biomass production from day 12 after the start of CO₂ enrichment. For plants supplied with 7·5 mol m^–3 N, the relative contribution of symbiotically fixed N (%N_sym) as opposed to N assimilated from mineral sources (¹⁵N-isotope-dilution method), dropped to 40%. However, in the presence of this high level of mineral N, %N_sym was unaffected by atmospheric p CO₂ over the entire experimental period. In plants fully dependent on N₂ fixation, the increase in N yield reflects a stimulation of symbiotic N₂ fixation that was the result of the formation of more nodules rather than of higher specific N₂ fixation. These results are discussed with regard to physiological processes governing symbiotic N₂ fixation and to the response of symbiotic N₂ fixation to elevated p CO₂ in field-grown T. repens .     

Seasonal differences in foliar content of chlorogenic acid, a phenylpropanoid antioxidant, in Mahonia repens

To assess the possible physiological function of chlorogenic acid (CGA, 3-O-caffeoylquinic acid) in vivo , we characterized the free radical scavenging properties of pure phenylpropanoids and leaf extracts against two free radicals, superoxide and the 2,2'-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical cation. CGA was found to be a highly efficient scavenger of these free radicals, surpassing the activity of all other phenylpropanoids tested, as well as the 'classical' antioxidant ascorbate. Seasonal differences in the leaf content of CGA were examined in field populations of the broadleaf evergreen Mahonia repens growing in different light environments. Leaves of fully sun-exposed plants contained significantly more (74 ± 10 mmol m^–2) CGA in winter than leaves from plants growing under deeply shaded conditions (17 ± 2 mmol m^–2). Sun-acclimated, but not shade-acclimated, leaves also produced high levels of anthocyanins in winter, suggesting a simultaneous increase in carbon flow through the phenylpropanoid and flavonoid pathways in response to high light and seasonal low temperature stress. In summer, high light-acclimated leaves contained ≈ 2-fold less CGA than in winter, whereas CGA levels were similar between seasons in shaded leaves. Consistent with the strong scavenging capacity of CGA measured in vitro , a linear correlation was observed between CGA content and the antioxidant activity of leaf extracts in both scavenging assays. On the basis of these results, we propose that CGA is a powerful hydrogen-donating antioxidant that may play an important role in mitigating the effects of oxidative stress in plants.     

小麦×长穗偃麦草的受精和早期胚胎发育

用石蜡切片法,对小麦（Ｔｒｉｔｉｃｕｍａｅｓｔｉｖｕｍ）和长穗偃麦草（Ｅｌｙｔｒｉｇｉａｅｌｏｎｇａｔａ）杂交的受精和早期胚胎发育进行了观察。结果表明,长穗偃麦草花粉在小麦柱头上萌发良好,花粉管可顺利长入花柱和胚囊。观察的１７０个小麦子房中,１７６５％发生了双受精,产生了胚和胚乳;９４１％发生了单卵受精,只产生胚而无胚乳;４７１％发生了单极核受精,只产生胚乳而无胚;总受精率为３１７７％;成胚率为２７０６％。由于胚乳的缺乏或发育异常及败育,最终难以获得有生活力的种子。为小麦与长穗偃麦草远缘杂交提供了细胞胚胎学证据。     

Antiherbivore defenses alter natural selection on plant reproductive traits

While many studies demonstrate that herbivores alter selection on plant reproductive traits, little is known about whether antiherbivore defenses affect selection on these traits. We hypothesized that antiherbivore defenses could alter selection on reproductive traits by altering trait expression through allocation trade‐offs, or by altering interactions with mutualists and/or antagonists. To test our hypothesis, we used white clover, Trifolium repens, which has a Mendelian polymorphism for the production of hydrogen cyanide—a potent antiherbivore defense. We conducted a common garden experiment with 185 clonal families of T. repens that included cyanogenic and acyanogenic genotypes. We quantified resistance to herbivores, and selection on six floral traits and phenology via male and female fitness. Cyanogenesis reduced herbivory but did not alter the expression of reproductive traits through allocation trade‐offs. However, the presence of cyanogenic defenses altered natural selection on petal morphology and the number of flowers within inflorescences via female fitness. Herbivory influenced selection on flowers and phenology via female fitness independently of cyanogenesis. Our results demonstrate that both herbivory and antiherbivore defenses alter natural selection on plant reproductive traits. We discuss the significance of these results for understanding how antiherbivore defenses interact with herbivores and pollinators to shape floral evolution.     

Effects of temperature and photoperiod on vegetative growth of white clover (Trifolium repens) ecotypes

Effects of photoperiod and temperature on vegetative growth of seedling populations and clones of white clover ( Trifolium repens L.) originating from various latitudes (58°48'–69°54'N) and altitudes (up to 1100 m above sea level) were studied in a phytotron. Dry matter production, stolon elongation, petiole elongation and leaf lamina size were enhanced by long photoperiod. The requirement for long photoperiod increased with decreasing temperature. At 6°C the maximum growth was recorded under 24-h daylength. At 18°C already an increase in photoperiod from 12 to 15 h significantly enhanced growth, and maximum growth was obtained at 21-h photoperiod. The studied populations responded similarly to daylength, and the results did not indicate photoperiodic ecotypes in the material. The southernmost clones and populations generally had the highest dry matter production at all temperature treatments (6–18°C). Variation between clones within one location was, however, significant, and rapidly growing clones were found also in high-latitude locations. Dry matter production was poorly correlated with the morphological characters observed, but in some cases significant correlation with leaf lamina size was found.     

Genetic polymorphism for cyanogenesis and linkage at the linamarase locus in Trifolium nigrescens Viv. subsp. nigrescens

Polymorphisms at two genetic loci conditioning the cyanogenic glucoside linamarin (Ac) and the glucosidase linamarase (Li) are reported for the first time in Trifolium nigrescens Viv. subspecies nigrescens (2n=2x=16). T. nigrescens is one of several possible ancestral species that may have donated a genome to the allotetraploid species white clover (T. repens L., 2n=4x=32). T. nigrescens is a strong candidate because it is the only very close relative that, like white clover, is cyanogenic. Genetic analysis showed that in T. nigrescens, cyanogenesis was inherited as a two-locus genetic system in a similar way to that in white clover. Furthermore, Li, which is linked to the locus Sdh (shikimate dehydrogenase, SDH) at a distance of 6 cM in one genome of white clover, also showed linkage (12 cM) in T. nigrescens. It is concluded that one of the subspecies of T. nigrescens is a likely donor of a genome to white clover. Received: 27 December 2000 / Accepted: 12 April 2001     

长穗偃麦草EeSKOR启动子的克隆及功能分析

为了研究外整流钾通道蛋白(stelar K⁺ outward rectifier channels,SKOR)基因SKOR在长穗偃麦草中的功能,利用热不对称交错PCR(Tail PCR)技术,克隆了长穗偃麦草EeSKOR启动子,并进行启动子顺式作用元件及基因表达分析。结果表明：(1)成功获得长穗偃麦草EeSKOR基因起始密码子上游798 bp启动子序列,命名为pEeSKOR。 (2)EeSKOR启动子除必须具备的核心启动元件外,还含有特异转录因子结合位点、植物激素响应元件、光响应元件、组织特异的启动元件和胁迫响应元件。(3)成功构建植物表达载体pEeSKOR∷GUS,经农杆菌介导的瞬时转化,EeSKOR启动子驱动GUS报告基因可在拟南芥的叶、叶柄和根中表达。(4)实时定量PCR检测显示,在NaCl、PEG、ABA和SA处理下长穗偃麦草EeSKOR基因在根中呈现不同的表达模式,NaCl处理下EeSKOR的表达量呈先下调后上调趋势;PEG处理下EeSKOR的表达量呈上调趋势,且随着时间的延长显著上调;ABA处理下EeSKOR的表达受到抑制且随处理时间延长呈显著下调趋势;SA处理下EeSKOR表现出先上调后下调趋势,且在处理72 h时表达量显著低于正常表达水平。研究认为,EeSKOR基因的表达受NaCl、PEG、ABA和SA的诱导调节。该研究结果为进一步系统研究长穗偃麦草EeSKOR基因功能提供重要理论依据。