QTL Mapping of Downy Mildew Resistance in an Introgression Line Derived from Interspecific Hybridization Between Cucumber and Cucumis hystrix

Downy mildew (DM), caused by Pseudoperonospora cubensis (Berk. & M.A. Curtis) Rostovzev, is a worldwide major disease of cucumbers (Cucumis sativus L.). By screening 10 introgression lines (ILs) derived from interspecific hybridization between cucumber and the wild Cucumis, C. hystrix, through a whole plant assay, one introgression line (IL52) was identified with high DM‐resistance. IL52 was further used as a resistant parent to make an F₂ population with ‘changchunmici’ (susceptible parent). The F₂ population (300 plants) was investigated for DM‐yellowing, DM‐necrosis and DM‐resistance in the adult stage. A genetic map spanning 642.5 cM with 104 markers was constructed and used for QTL analysis from the population. Three QTL regions were identified on chromosome 5 and chromosome 6. By interval mapping analysis, two QTLs for DM‐resistance were determined on chromosome 5 (DM_5.1 and DM_5.2), which explained 17.9% and 14.2% of the variation, respectively. QTLs for DM‐yellowing were in the same regions as DM‐resistance. For DM‐necrosis, by interval mapping analysis, one QTL was determined on chromosome 5 (Necr_5.1) that explained 18.3% of the variation and one on chromosome 6 (Necr_6.1) that explained 13.9% of the variation. Our results indicated that the identification of molecular markers linked to the QTLs could be further applied for marker‐assisted selection (MAS) of downy mildew resistance in cucumber.     

Purification of an Aminopeptidase Preferentially Releasing N-terminal Alanine from Cucumber Leaves and Its Identification as a Plant Aminopeptidase N

In this study, a highly active foliar aminopeptidase preferentially releasing N-terminal alanine from artificial substrates was purified and characterized from cucumber (Cucumis sativus L. suyo). The enzyme had a molecular mass of 200 kDa consisting of two subunits of 95 kDa. It was a metalloprotease the pH optimum of which was 8 to 9. It cleaved Ala-, Gly-, Met-, Ser-, Leu-, Lys-, and Arg artificial substrates. An internal amino acid sequence was similar to those of aminopeptidase N (clan MA, family M1) of microorganisms, and was very similar to that of a putative aminopeptidase N of Arabidopsis thaliana. From these results, the highly active aminopeptidase in cucumber leaves was identified to be a plant aminopepitdase N.     

Incorporation of Glycine and Serine into Sporulating Cells of Bacillus subtilis

The changes during growth and sporulation in activities of cells of Bacillus subtilis to incorporate various amino acids were investigated with wild-type strain and its asporogenous mutant. In the case of wild type strain the uptake of valine, phenylalanine, and proline was largest during the logarithmic growth period. The uptake of these amino acids decreased rapidly during the early stationary phase. The uptake of valine and cysteine increased again to some extent just prior to the forespore stage. The uptake of glycine and serine, however, was largest at the forespore stage at which the formation of spore coat took place. From these observed phenomena it was assumed that the remarkable incorporation of glycine and serine into the wild type strain during sporulation was closely related to the formation of spore coat.     

Functional compatibility in cucumber mycorrhizas in terms of plant growth performance and foliar nutrient composition

Functional compatibility in cucumber mycorrhizas in terms of plant and fungal growth, and foliar nutrient composition from all possible combinations of six cucumber varieties and three species of arbuscular mycorrhizal (AM) fungi was evaluated. Measurements of foliar nutrient composition included N, P, K, Mg, Ca, Na, Fe, Zn, Mn and Cu. Growth of AM fungi was measured in terms of root colonisation, as examined with microscopy and the AM fungus biomarker fatty acid 16:1ω5 from both phospholipids and neutral lipids. Different responses of plant growth and foliar nutrient profiles were observed for the different AM symbioses examined. The AM fungus Claroideoglomus claroideum caused growth depression in association with four out of six cucumber varieties; Rhizophagus irregularis caused growth promotion in one of six cucumber varieties; whereas Funneliformis mosseae had no effect on the growth performance of any of the cucumber varieties examined. All three AM fungi markedly altered host plant shoot nutrient composition, with the strongest contrast observed between cucumber–R. irregularis symbioses and non‐mycorrhizal cucumber plants, independent of cucumber variety. On the other hand, AM fungal growth in roots differed between the three AM fungi, but was unaffected by host genotype. Strong build‐up of storage lipids was observed for R. irregularis, which was more moderate in the two other AM fungi. In conclusion, strong differential responses of cucumber varieties to inoculation with different AM fungi in terms of growth and shoot nutrient composition revealed high functional diversity in AM symbioses in cucumber plants.     

High and dry: post‐fire tree seedling establishment in subalpine forests decreases with post‐fire drought and large stand‐replacing burn patches

Aim Climate warming and increased wildfire activity are hypothesized to catalyse biogeographical shifts, reducing the resilience of fire‐prone forests world‐wide. Two key mechanisms underpinning hypotheses are: (1) reduced seed availability in large stand‐replacing burn patches, and (2) reduced seedling establishment/survival after post‐fire drought. We tested for regional evidence consistent with these mechanisms in an extensive fire‐prone forest biome by assessing post‐fire tree seedling establishment, a key indicator of forest resilience. Location Subalpine forests, US Rocky Mountains. Methods We analysed post‐fire tree seedling establishment from 184 field plots where stand‐replacing forest fires were followed by varying post‐fire climate conditions. Generalized linear mixed models tested how establishment rates varied with post‐fire drought severity and distance to seed source (among other relevant factors) for tree species with contrasting post‐fire regeneration adaptations. Results Total post‐fire tree seedling establishment (all species combined) declined sharply with greater post‐fire drought severity and with greater distance to seed sources (i.e. the interior of burn patches). Effects varied among key species groups. For conifers that dominate present‐day subalpine forests (Picea engelmannii, Abies lasiocarpa), post‐fire seedling establishment declined sharply with both factors. One exception was serotinous Pinus contorta, which did not vary with either factor. For montane species expected to move upslope under future climate change (Larix occidentalis, Pseudotsuga menziesii, Populus tremuloides) and upper treeline species (Pinus albicaulis), establishment was unrelated to either factor. Greater post‐fire tree seedling establishment on cooler/wetter aspects suggested local topographic refugia during post‐fire droughts. Main conclusions If future drought and wildfire patterns manifest as expected, post‐fire tree seedling establishment of species that currently characterize subalpine forests could be substantially reduced. Compensatory increases from lower montane and upper treeline species may partially offset these reductions, but our data suggest important near‐ to mid‐term shifts in the composition and structure of high‐elevation forests under continued climate warming and increased wildfire activity.     

不同BR施用方式诱导黄瓜幼苗对Ca(NO_3)_2胁迫抗性的研究

为探讨外源油菜素内酯(brassinosteroid,BR)诱导黄瓜幼苗对Ca(NO3)2胁迫抗性的效果,研究了3种外源BR施用方法(0.01mg·L-1 BR浸种、0.1mg·L-1 BR喷叶及其二者结合施用)对Ca(NO3)2胁迫(60mmol·L-1)下黄瓜幼苗生长、生理活动以及光合作用的影响。结果表明:(1)3种外源BR方法处理后,Ca(NO3)2胁迫下的黄瓜幼苗株高、茎粗、展开叶片数、叶面积、干重含水量均显著提高,同时其叶片游离脯氨酸和可溶性糖含量上升,过氧化物酶活性提高,而其丙二醛(MDA)含量趋于无Ca(NO3)2胁迫对照的水平;(2)外源BR处理还提高了Ca(NO3)2胁迫下黄瓜幼苗的净光合速率、蒸腾速率和气孔导度,却抑制了Ca(NO3)2胁迫下胞间CO2浓度的升高。研究认为,适宜浓度的外源BR浸种和喷叶处理均可有效增强黄瓜幼苗渗透调节能力,降低细胞膜质过氧化伤害程度,提高抗氧化酶活性和光合效率,从而表现出对Ca(NO3)2胁迫的抗性,并以操作简便、用量极低的0.01mg·L-1 BR浸种方法效果最佳。     

Activation of transposable elements and insertional polymorphism in Opuntia offspring as assessed by inter-retrotransposon amplified polymorphism markers

Activation of retrotransposon is a pivotal factor in the genesis of genetic polymorphism. Retrotransposon-based molecular markers are excellent tools for detecting genetic diversity and genomic changes associated with their activity. The objective of this study was to use IRAP markers to detect integration events of retrotransposon in Opuntia, and to compare IRAP and ISSR polymorphisms. To achieve these aims, five IRAP and five ISSR markers were analyzed on three varieties and their progenies. All IRAP primers showed an increase in the percentage of polymorphism, number of total bands, and polymorphic bands in the seedlings compared to their mother plants; that is, the offspring showed 13, 24 and 27 more bands than the mother plants from Tobarito, Montesa and Copena varieties, respectively. Conversely, sexual reproduction did not proportionally affect the variation in the number of ISSR bands, and neither did polymorphisms between mother plants and their offspring. Cluster and multivariate analyses based on IRAP and ISSR data revealed a clear separation between varieties, and there was no overlapping between seedlings of different varieties. The activation of retrotransposons in seedlings of opuntia with variable frequencies was evidenced. The presence of insertion and active retrotransposons may help to undertake studies on genetic diversity and evolution of Opuntia.     

CO2加富对盐胁迫下黄瓜幼苗叶片光合特性及活性氧代谢的影响

以‘津优35号’黄瓜(Cucumis sativus L.)水培苗为试材,采用裂区设计,主区因素为CO_2浓度处理,设大气CO_2浓度(Symbol{B@400μmol/mol)和CO_2加富[(800±40)μmol/mol]2个CO_2浓度水平,裂区因素为盐胁迫处理,用NaCl模拟盐胁迫,设对照(0 mmol/L NaCl)、盐胁迫(80 mmol/L NaCl)2个盐分水平,研究了CO_2加富对盐胁迫下黄瓜幼苗生长、光合特性及活性氧代谢的影响。结果表明:盐胁迫显著抑制黄瓜幼苗的生长,并降低了叶绿素含量、ETR、Φ_(PSII)、核酮糖-1,5-二磷酸羧化酶(RuBPCase)活性及净光合速率;盐胁迫增加了丙二醛及活性氧的累积,与此同时也提高了脯氨酸含量及超氧化物歧化酶(SOD)、过氧化氢酶(CAT)活性,但降低了过氧化物酶(POD)活性。CO_2加富显著提高了盐胁迫下黄瓜幼苗的株高、茎粗、叶面积及地上部鲜重,降低了叶绿素a、叶绿素b、类胡萝卜素及叶绿素(a+b)含量,但显著提高了净光合速率和RuBPCase活性,同时降低了气孔导度及蒸腾速率,并且使其具有较高的表观电子传递速率及PSII实际光化学效率;CO_2加富显著提高了盐胁迫下黄瓜幼苗叶片脯氨酸含量及SOD、POD、CAT活性,丙二醛、过氧化氢含量和超氧阴离子产生速率显著降低。综上所述,CO_2加富可通过提高幼苗叶片净光合速率、脯氨酸含量及抗氧化酶活性,降低蒸腾速率、减少丙二醛含量及活性氧的积累,从而缓解盐胁迫对黄瓜植株造成的伤害。