Aphid Herbivory Drives Asymmetry in Carbon for Nutrient Exchange between Plants and an Arbuscular Mycorrhizal Fungus

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小麦防御素基因TaPDF35的克隆与功能分析

从抗纹枯病小麦品种CI12633中克隆出一个小麦防御素基因TaPDF35,并对其表达特性及功能进行了分析。TaPDF35基因包含一个长为249 bp的开放阅读框(ORF,open reading frame),编码82个氨基酸组成的多肽TaPDF35。预测分析表明,TaPDF35能形成由1个α螺旋和3股反向平行的β-折叠片组成的αβ(CSαβ)基序,αβ(CSαβ)基序由8个半胱氨酸形成的4个二硫键固定。TaPDF35的N端有一段27个氨基酸的信号肽。表达分析结果表明,TaPDF35基因在抗纹枯病小麦品种CI12633和山红麦中的表达量显著高于在感纹枯病小麦品种扬麦158和温麦6号中的表达量;该基因在小麦的叶鞘和茎中均有表达,且受纹枯病原菌诱导而上调表达。利用大麦条形花叶病毒(BSMV,barley stripe mosaic virus)诱导的基因沉默技术(VIGS,virus-induced gene silencing)降低抗纹枯病小麦品种CI12633中TaPDF35基因的转录水平,再接种纹枯病原菌进行纹枯病抗性鉴定。结果显示,与接种BSMV:GFP的CI12633对照植株相比,TaPDF35表达水平降低的CI12633植株对纹枯病的抗性显著降低,表明TaPDF35表达是小麦防御纹枯病反应所需的。     

Flavonoid compounds related to seed coat color of wheat

In red wheat, reddish-brown pigments accumulate in testa of mature seeds. Half-cut wheat seeds were immersed in p-dimethylaminocinnamaldehyde (DMACA) reagent that stains flavanol structures blue. Testa of 10–40 days after flowering (DAF) in red wheat (“Norin 61” and “Satonosora”) seeds were stained blue and the reagent color changed to blue with 10–25 DAF seeds. No blue staining was observed in white wheat (“Tamaizumi”) seeds during maturation. “Norin 61” seed coats at 10 DAF contained dihydroquercetin, dihydromyricetin, (+)-catechin, procyanidin B3, and prodelphinidin B3, which were identified by HPLC-diode array detector and LC-MS/MS analyses. These five components began accumulating 7 DAF, reached maxima at 10 or 15 DAF, and then decreased in red wheat seeds, but were not detected in white wheat seeds. These results suggest that flavanol and proanthocyanidins are possible precursors of the reddish-brown pigments of red wheat seeds, and are converted to insoluble compounds as the seeds mature.     

Hypersensitive response of wheat to the Hessian fly

Hessian flyMayetiola destructor (Say) larvae are able to obtain food from their host plant without inflicting mechanical damage to the plant surface, apparently by secreting substances which elicit release of nutrients from plant cells surrounding the feeding site. Cells of fully susceptible plants retain their normal appearances, while in resistant plants extensive areas of cellular collapse occur. These responses indicate that hypersensitivity is the basis of wheat's resistance to the Hessian fly. The fly's feeding mechanism more closely resembles that of a pathogen than of a phytophagous insect; correspondingly, both the genetic relationship and resistance mechanism of the host plant to the parasite are of the sorts commonly associated with bacterial and fungal pathogens.     

The cellular pathway of photosynthate transfer in the developing wheat grain. II. A structural analysis and histochemical studies of the pathway from the crease phloem to the endosperm cavity

In the developing wheat grain, photosynthate is transferred longitudinally along the crease phloem and then laterally into the endosperm cavity through the crease vascular parenchyma, pigment strand and nucellar projection. In order to clarify this cellular pathway of photosynthate unloading, and hence the controlling mechanism of grain filling, the potential for symplastic and apoplastic transfer was examined through structural and histochemical studies on these tissue types. It was found that cells in the crease region from the phloem to the nucellar projection are interconnected by numerous plasmodesmata and have dense cytoplasm with abundant mitochondria. Histochemical studies confirmed that, at the stage of grain development studied, an apoplastic barrier exists in the cell walls of the pigment strand. This barrier is composed of lignin, phenolics and suberin. The potential capacity for symplastic transfer, determined by measuring plasmodesmatal frequencies and computing potential sucrose fluxes through these plasmodesmata, indicated that there is sufficient plasmodesmatal cross-sectional area to support symplastic unloading of photosynthate at the rate required for normal grain growth. The potential capacity for membrane transport of sucrose to the apoplast was assessed by measuring plasma membrane surface areas of the various cell types and computing potential plasma membrane fluxes of sucrose. These fluxes indicated that the combined plasma membrane surface areas of the sieve element–companion cell (se–cc) complexes, vascular parenchyma and pigment strand are not sufficient to allow sucrose transfer to the apoplast at the observed rates. In contrast, the wall ingrowths of the transfer cells in the nucellar projection amplify the membrane surface area up to 22-fold, supporting the observed rates of sucrose transfer into the endosperm cavity. We conclude that photosynthate moves via the symplast from the se–cc complexes to the nucellar projection transfer cells, from where it is transferred across the plasma membrane into the endosperm cavity. The apoplastic barrier in the pigment strand is considered to restrict solute movement to the symplast and block apoplastic solute exchange between maternal and embryonic tissues. The implications of this cellular pathway in relation to the control of photosynthate transfer in the developing grain are discussed.     

Level ofFusarium infection in wheat spikelets related to location and number of inoculated spores

The flowering time is the most susceptible period for primary infection of wheat heads byFusarium spp. During this period spores can be deposited into the opened wheat florets where they may later cause infections. We quantitatively explored the relationship between variables related to the flowering process and the infection level byFusarium graminearum in single spikelets. We imitated open (chasmogamous) and closed (cleistogamous) flowering by injecting well-defined amounts of spores into and between wheat florets. Applying the spores between the florets resulted in weaker disease symptoms and significantly lower amounts ofFusarium mycotoxins. With larger numbers of spores, the disease symptoms became more pronounced and the mycotoxin amounts per spikelet increased significantly. Our results indicate that the probability of primary infection is approximately proportional to the number of spores reaching the open florets during the flowering process. The breeding of wheat lines which flower partially or completely cleistogamously might reduce theFusarium susceptibility in wheat.     

Waterfowl Nesting in Fall-Seeded and Spring-Seeded Cropland in Saskatchewan

Abstract: Waterfowl nesting in annual croplands has remained a little-known aspect of waterfowl nesting ecology because of the inability of many studies to systematically search this habitat through the nesting season. Where searches have been conducted, they are generally restricted to the period prior to seeding, and many nests found are destroyed by the seeding operation. Consequently, fall-seeded crops have been promoted as an alternative cropping practice that could increase nest survival of waterfowl nesting in croplands. During 1996–1999, we conducted 3–4 complete nest searches on 4,274 ha of cropland, including spring-seeded wheat and barley, winter wheat, and fall rye in southern Saskatchewan, Canada. Using suites of predictive models, we tested hypotheses regarding relative nest abundance and nest survival among crop types and tested the influence of several landscape-scale covariates on these metrics. Apparent nest densities were higher in fall-seeded crops (winter wheat: 0.39 nests/ha, fall rye: 0.25 nests/ha) than in spring-seeded crops (0.03 nests/ha), and nest density in spring-seeded croplands increased with percent cropland and percent wetland habitat in the surrounding landscape. Nest survival was higher in winter wheat (38%) than in either fall rye (18%) or spring-seeded crops (12%), and nest survival in spring-seeded crops increased with relative nest initiation date. Nest survival was unaffected by surrounding landscape characteristics but tended to be higher in years of average wetness. Based on our findings, winter wheat and fall rye have the potential to provide productive nesting habitat for ≥7 species of upland nesting ducks and fall-seeded crops are a conservation tool well suited to highly cropped landscapes.