Resistance to Bremia lactucae in natural populations of Lactuca saligna from some Middle Eastern countries and France

The results of the first detailed screening of a resistance to Bremia lactucae in naturally growing populations of Lactuca saligna are presented here. In total, 146 accessions from 25 populations of L. saligna originating in Israel (N = 136), France (N = 8), Jordan (N = 1) and Turkey (N = 1) were tested at seedling stage for their resistance to 10 highly virulent isolates (races) of B. lactucae from Lactuca sativa (DEG2, Bl:5, Bl:15, Bl:16, Bl:17, Bl:18, Bl:21, Bl:22, Bl:24 and Bl:25). Our study strongly supports the suggestion that L. saligna is indeed generally highly resistant to B. lactucae. However, our results provide evidence that at least at a seedling stage L. saligna may not be a non‐host plant for B. lactucae, as was hypothesised for approximately the last 30 years. Some accessions expressed a differential (i.e. race‐specific) response, which accords with other recently published data for this Lactuca species. Furthermore, some geographical differences in race‐specific resistance were observed, too. Tests performed at an adult‐plant stage, however, did not prove race‐specificity of the respective accessions. To summarise, what is behind the race‐specific character of the responses observed at a seedling stage is still uncertain, as is its comparability with the race‐specific resistance of some other Lactuca species such as L. sativa or L. serriola. The presence of plant stage‐dependent resistance, governed by a combined effect of different quantitative trait loci in young and adult plants of L. saligna, is discussed.     

Characterization of New Highly Virulent German Isolates of Bremia lactucae and Efficiency of Resistance in Wild Lactuca spp. Germplasm

Four German isolates (FS1, SR2, SAW1 and DEG2) of Bremia lactucae originating from lettuce cultivars with R‐factors R18 and Dm6 + R36 were used for detailed characterization of virulence factors (v‐factors) and for the study of the resistance efficiency in wild Lactuca spp. germplasm. The highest complexity of v‐phenotype was recognized in isolate DEG2, which overcomes resistance in cv. Mariska (R18) and line CS‐RL (L. serriola × L. sativa, R18 + ?), until now known as resistant to all known races of B. lactucae in Europe. However, some sparse sporulation also occurred on cv. Titan (Dm6 + R36). The isolates SR2 and SAW1 overcome the resistance based on the gene R36, but are avirulent to R18. The v‐phenotype of SR2 is highly complex with the most important v‐factors being present except for v14 and v18. The isolate FS1 is the first in Germany originating from a cultivar with R18 (cv. Samourai). The search for efficient sources of resistance in 64 accessions of 11 wild Lactuca spp. and primitive forms of L. sativa showed broad variation in accession–isolate interactions. Expression of race‐specific resistance in wild Lactuca spp. (L. serriola, L. viminea, L. virosa) was recorded frequently. Lactuca indica and L. saligna could be considered as the most efficient sources of resistance against isolates FS1, SR2 and SAW1. The isolate DEG2 showed the highest level of virulence. On seedlings of L. saligna, which is considered as a most important source of resistance against B. lactucae, there was frequently recorded limited sporulation, however this response cannot be considered as a susceptible. Except for some L. saligna accessions (CGN 05310 and CGN 05315), the resistance to all studied isolates was only observed in one accession of L. serriola (PI 253467).     

Biodiversity of Lactuca aculeata germplasm assessed by SSR and AFLP markers,and resistance variation to Bremia lactucae

In total, seventy two Lactuca aculeata and three Lactuca serriola samples originating from natural populations of these species in Turkey, Jordan, and Israel were analysed by eight microsatellite and 287 amplified fragment length polymorphism (AFLP) markers. Neighbor–Network and Bayesian clustering were used for visualisation of the differences among the analysed L. aculeata and L. serriola samples, and to confirm hybrid origin (L. aculeata × L. serriola) of three samples (343-8A, 343-8B, 54/07) previously indicated by their morphological traits. Molecular data reflect the geographical origin, i.e., the clustering of samples according to their country of origin. Samples from neighbouring parts of Jordan and Israel expressed similar genetic characteristics, indicating the possibility of migration or artificial introduction of plant material. Forty-one L. aculeata samples were screened for their response to five Bremia lactucae races (Bl: 17, Bl: 18, Bl: 24, Bl: 27, and Bl: 28). Susceptible reactions of L. aculeata prevailed. L. aculeata samples were most frequently susceptible to races Bl: 18, Bl: 24, Bl: 27, Bl: 28; and least susceptible to Bl: 17. No highly efficient source of resistance was detected; however, race-specific reaction patterns were frequently recorded, indicating the possible presence of some race-specific resistance factors/genes in the studied samples of L. aculeata. Conservation and exploitation of this material in lettuce breeding is discussed.     

Recognition of lettuce downy mildew effector BLR38 in Lactuca serriola LS102 requires two unlinked loci

Plant-pathogenic oomycetes secrete effector proteins to suppress host immune responses. Resistance proteins may recognize effectors and activate immunity, which is often associated with a hypersensitive response (HR). Transient expression of effectors in plant germplasm and screening for HR has proven to be a powerful tool in the identification of new resistance genes. In this study, 14 effectors from the lettuce downy mildew Bremia lactucae race Bl:24 were screened for HR induction in over 150 lettuce accessions. Three effectors—BLN06, BLR38 and BLR40—were recognized in specific lettuce lines. The recognition of effector BLR38 in Lactuca serriola LS102 did not co-segregate with resistance against race Bl:24, but was linked to resistance against multiple other B. lactucae races. Two unlinked loci are both required for effector recognition and are located near known major resistance clusters. Gene dosage affects the intensity of the BLR38-triggered HR, but is of minor importance for disease resistance.     

Distribution,Disease Level and Virulence Variation of Bremia lactucae on Lactuca sativa in the Czech Republic in the Period 1999–2011

We report the distribution and disease level of downy mildew on lettuce (Lactuca sativa) and virulence variation in populations of its causal agent (Bremia lactucae) in the Czech Republic during the period 1999–2011. Disease incidence was not high; among a total of 92 different localities surveyed, 43.4% of them were infected by lettuce downy mildew at least once during the whole period. However, among individual years, differences were found in disease incidence that ranged from 4.8% (2009) to 66.7% (2004). A total of 128 isolates of B. lactucae collected from infected leaf samples in 35 different localities during the surveying period were included in the virulence analysis. Virulence was examined on a set of 19 differential genotypes of Lactuca sativa and Lactuca serriola (EU‐A test set). Isolates exhibited quite a broad variation in virulence to individual Lactuca differential genotypes. Eighteen of 19 virulence factors (v‐factors) tested were present in the samples. The most frequently detected factors were v1–4, v5/8, v6, v7, v10–14, v16, v36 and v38; factor v17 was not found. The most pronounced temporal shift was recorded for factors v36 and v38 whose frequency increased during the studied period. V‐factors 15, 17, 18 and 37 were present in low frequencies in a pathogen population, and their corresponding gene (Dm15) or resistance factors (R17, R18 and R37) may have the best potential for resistance breeding in the Czech Republic. Broad diversity of v‐phenotypes (63 different ones) was identified during the study period. The numbers of v‐factors per v‐phenotype (resp. isolate) varied within a range of 5–15. Within the 128 analysed isolates, only 9 v‐phenotypes were recorded repeatedly (three or more times). Possible reasons of recorded virulence variation are discussed.     

Climate change, anthropogenic disturbance and the northward range expansion of Lactuca serriola (Asteraceae)

Aim The distribution range of Lactuca serriola, a species native to the summer‐dry mediterranean climate, has expanded northwards during the last 250 years. This paper assesses the influence of climate on the range expansion of this species and highlights the importance of anthropogenic disturbance to its spread. Location Central and Northern Europe. Methods Data on the geographic distribution of L. serriola were assembled through a literature search as well as through floristic and herbarium surveys. Maps of the spread of L. serriola in Central and Northern Europe were prepared based on herbarium data. The spread was assessed more precisely in Germany, Austria and Great Britain by pooling herbarium and literature data. We modelled the bioclimatic niche of the species using occurrence and climatic data covering the last century to generate projections of suitable habitats under the climatic conditions of five time periods. We tested whether the observed distribution of L. serriola could be explained for each time period, assuming that the climatic niche of the species was conserved across time. Results The species has spread northwards since the beginning of the 19th century. We show that climate warming in Europe increased the number of sites suitable for the species at northern latitudes. Until the late 1970s, the distribution of the species corresponded to the climatically suitable sites available. For the last two decades, however, we could not show any significant relationship between the increase in suitable sites and the distributional range change of L. serriola. However, we highlight potential areas the species could spread to in the future (Great Britain, southern Scandinavia and the Swedish coast). It is predominantly non‐climatic influences of global change that have contributed to its rapid spread. Main conclusions The observation that colonizing species are not filling their climatically suitable range might imply that, potentially, other ruderal species could expand far beyond their current range. Our work highlights the importance of historical floristic and herbarium data for understanding the expansion of a species. Such historical distributional data can provide valuable information for those planning the management of contemporary environmental problems, such as species responses to environmental change.     

First Record of Bremia lactucae Infecting Sonchus oleraceus and Sonchus asper in Brazil and its Infectivity to Lettuce

Bremia lactucae is recorded for the first time causing downy mildew on common sowthistle (Sonchus oleraceus) and spiny sowthistle (Sonchus asper) in Brazil. The disease and etiological agent are described. Pathogencity of sporangia obtained from S. oleraceus was tested on 12 species belonging to the Asteraceae, already recorded in the literature as hosts of B. lactucae, and four commonly cultivated varieties of lettuce. All four cultivars of lettuce, S. oleraceus and S. asper showed symptoms of the disease and sporulation of the pathogen 5 days after inoculation. It has been observed that the disease occurs on Sonchus spp. throughout the year in Viçosa (state of Minas Gerais), being more common on these two hosts than on lettuce. Both weeds are very common invaders of vegetable growing areas in Brazil. This indicates that those two hosts for the fungus may be important inoculum reservoirs for disease occurring in lettuce, highlighting the need for control of these weeds as part of the procedures aimed at controlling this disease. These results are also in agreement with the views that B. lactucae is split into host‐specific infra‐specific taxa. Lettuce and sowthistles are regarded as closely related, belonging to the same subfamily and tribe in the Asteraceae (Subfamily Cichorioideae: Tribe Lactuceae).     

Histochemical Detection and Role of Phenolic Compounds in the Defense Response of Lactuca spp. to Lettuce Downy Mildew (Bremia lactucae)

The occurrence of phenolic compounds (PC) in the defence reaction of Lactuca spp. was detected by histochemical methods. Four staining methods, including three for light and one for fluorescence microscopy, were used for imaging the location of PC in cells of genotypes with different resistance mechanisms after infection by Bremia lactucae , race NL16. The results showed that the major role of phenolic compounds in studied Lactuca spp. is connected with their overexpression and localized accumulation during a hypersensitive response (HR). In incompatible interactions a slight accumulation of phenols near the cell wall of infected cells was detected. The negative reaction to staining with aniline sulphate verified the absence of lignin creation. In both compatible and incompatible interactions structural modifications in the host cells occurred as a callose deposition. Frequently, these deposits were widespread in susceptible genotypes. Intensive and rapid accumulation of autofluorescent phenolics was linked with the onset of HR, the main cytological feature of resistance to lettuce downy mildew in Lactuca spp.     

Temporal Occurrence and Spatial Distribution of Red Tide Events in China's Coastal Waters

Between the years 1933–2001, 460 red tide events were found in China's coastal water. The scope of red tide occurrence has extended over all the coastal provinces of China, and the three major areas with high red tide frequency are the Bohai sea, the sea area near the Shengsi Archipelago and Huaniaoshan Island on the outer side of the Changjiang River estuary and the Hangzhou Bay, and the coastal waters on the east side of the Zhujiang River estuary from Hong Kong to East Guangdong. In the past 20 years, the frequency of red tides has been tending upwards. The years of 1988–1990 and 1998–2001 saw the most serious red tides along China's coastal waters, with the latter period as the peak. The average area of a single red tide, i.e., the scale of red tides, is expanding every year, and in 2001 rose to about 500 km². Every year, China's red tides occur from south to north, with those in the South China Sea occurring from March to May, those in the East China Sea from April to August and those in the Bohai and Huanghai Seas from May to September.     

Distribution,feeding habits and morphology of killer whales Orcinus orca in the Caribbean Sea

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利用MODIS产品中分辨率为1 km的光合有效辐射吸收比例(FPAR)数据,结合植被功能型分类,分析2010—2012年北京植被FPAR的空间分布特征,以及各种植被类型FPAR的多年变化,并进一步探讨了FPAR与叶面积指数(LAI)之间的相关性.结果表明: 研究期间,北京植被的FPAR空间分布均呈现出东北部、西南部高,并向中心城区逐渐递减的分布特征.通过FPAR的叠加分析还发现,各种植被类型FPAR年平均值的波动均较小,针叶树、阔叶树、草地、作物FPAR的年均值波动范围仅分别在0.42～0.44、0.38～0.39、0.32～0.33、0.21～0.22,但各种植被类型FPAR的年内变化范围均较大.各种植被类型的FPAR与LAI也具有较好的线性或对数关系.经过Timesat软件中的Savitzky-Golay平滑滤波后,各种植被类型FPAR的季节性变化特征更加明显.     

三种海水生态养殖池塘细菌数量特征及其与环境因子的相关性

为阐明不同生态养殖池塘浮游细菌数量变化规律及其与环境因子间的相关性,以期为多元立体综合养殖模式中的水质安全与生物疾病防控提供科学依据。利用荧光显微镜细菌计数法(Acridine Orange Direct Counting),研究了“参-虾”“蟹-蛏”和“蜇-蛏”三种海水生态养殖池塘的细菌数量特征,使用典范对应分析(Canonical Correspondence Analysis, CCA)探讨细菌数量与环境因子的相互关系。结果表明,“参-虾”“蟹-蛏”和“蜇-蛏”池塘的细菌平均密度分别为(0.49～3.32)×10⁵ cell/mL、(0.52～2.98)×10⁵ cell/mL和(0.47～2.55)×10⁵ cell/mL,细菌数量有明显的时间差异,夏季密度大于春秋季。三种海水生态养殖池塘的水温、溶解氧和pH变化均不明显,且相差不大,而硝态氮均发生显著变化;其中“蟹-蛏”和“蜇-蛏”池塘的硝态氮、亚硝态氮、活性磷、总氮和总磷均高于“参-虾”池塘,透明度、叶绿素a和化学耗氧量则均低于“参-虾”池塘。典范对应分析(C...