Phloridzin and apple scab

Phloridzin, sieboldin, trilobatin, phloretin and 3-hydroxyphloretin can all be used as carbon sources by Venturia inaequalis in culture. Resistance to apple scab was not linked with inheritance of sieboldin or trilobatin in seedlings. There is no direct connection between phloridzin or its breakdown products and scab resistance.     

冠盘二胞Marssonina coronaria侵染不同抗性苹果叶片的组织病理学研究

利用光学和电子显微镜,从组织细胞学水平系统研究了冠盘二胞Marssonina coronaria在苹果抗、感病品种叶片上的侵染过程及侵染后寄主细胞的超微结构特征。结果表明：冠盘二胞的侵入和定殖过程可以分为6个阶段：孢子萌发与芽管形成、附着胞形成、侵入细胞角质层、在叶肉细胞内产生吸器、菌丝在叶肉细胞间和细胞内扩展、分生孢子盘形成。随着菌丝扩展,受侵寄主细胞出现细胞壁加厚,细胞壁降解,质壁分离,叶绿体内淀粉粒、嗜饿颗粒积累,叶绿体基粒片层瓦解,线粒体空泡化等现象。在不同抗性的苹果品种上,分生孢子萌发率差别不明     

Sorbitol metabolism and sink-source interconversions in developing apple leaves

In apple (Malus domestica Borkh.) sorbitol is the primary product of photosynthesis, the major translocated form of carbon, and a common fruit constituent and storage compound. Previous work on sorbitol metabolism has revealed a NADPH-dependent aldose 6-phosphate reductase (A6PR) in green tissues, and a NAD-dependent sorbitol dehydrogenase in nongreen tissues. Results here show a decrease in sorbitol dehydrogenase activity and an increase in A6PR activity as leaves developing in the spring undergo the transition from sink to source. Sorbitol dehydrogenase activity reached a minimum as A6PR peaked. These changes were related to increases in leaf carbohydrate levels, especially sorbitol, and to increases in rates of net photosynthesis. Studies conducted in the autumn on senescing leaves also showed changes in enzyme activites, leaf carbohydrate levels, and photosynthesis. At this time, however, sorbitol dehydrogenase increased in specific activity, whereas A6PR activity, leaf carbohydrates, and photosynthetic rates all decreased substantially. Other experiments showed differences in the ability of young and mature leaves to metabolize sorbitol and in the distribution of sorbitol enzymes in leaves at transitional developmental stages. The results suggest that sorbitol metabolism in apple is tightly controlled and may be related to mechanisms regulating partitioning or source and sink activity.     

Long-term plum pox virus infection produces an oxidative stress in a susceptible apricot, Prunus armeniaca, cultivar but not in a resistant cultivar

The effect of plum pox virus (PPV) infection on the response of some antioxidant enzymes was studied in two apricot cultivars, which behaved differently against PPV infection: cultivar Real Fino (susceptible) and cultivar Stark Early Orange (cv. SEO, resistant). In the susceptible cultivar, PPV produced a decrease in Φ _PSII, F '_v/ F '_m and Q _p. PPV infection produced a drop in p -hydroxy mercury benzoic acid (pHMB)-sensitive ascorbate peroxidase, dehydroascorbate reductase and peroxidase in the soluble fraction from susceptible plants, whereas in the resistant apricot cultivar, pHMB-insensitive ascorbate peroxidase, monodehydroascorbate reductase, glutathione reductase and superoxide dismutase increased. However, catalase decreased in the soluble fractions from both infected cultivars. Long-term PPV infection also produced a decrease in the chloroplastic ascorbate–glutathione cycle enzymes only in the susceptible plants. As a consequence of PPV infection, an oxidative stress, indicated by an increase in lipid peroxidation and in protein oxidation, was produced only in the leaves from the susceptible cultivar which was also monitored by the diaminobenzidine peroxidase-coupled H₂O₂ probe. The loss of Φ _PSII, indicative of activated oxygen species production, and the decrease in the levels of antioxidant enzymes in chloroplasts from susceptible plants could be responsible for the chlorosis symptoms observed. The results suggest that the higher antioxidant capacity showed by cv. SEO could be a consequence of a systemic acquired resistance induced by PPV penetration in stem tissue at the graft site and could be related, among other factors, to their resistance to PPV.     

Accumulation of phytoalexins and isoflavone glucosides in a resistant and a susceptible cultivar of Cicer arietinum during infection with Ascochyta rabiei

After infection with spores of a virulent strain of Ascochyta rabiei the chickpea (Cicer arietinum) cultivars ILC 1929 (susceptible) and ILC 3279 (resistant) were compared with regard to pterocarpan phytoalexin and isoflavone accumulation. Quantitative HPLC analyses of total extracts of aerial parts were used to measure the induced formation of the phytoalexins medicarpin and maackiain and the accumulation of the constitutive isoflavones biochanin A and formononetin together with their, 7-0-glucosides and their 7-0-glucoside-6″-0-malonates. The two cultivars showed no significant difference in the level of isoflavones and isoflavone conjugates. On the other hand, the resistant cultivar ILC 3279 rapidly accumulated large amounts of both, phytoalexins (20–26 nmole g^?1 fr.w.) whereas cultivar ILC 1929 only produced very small amounts (5 nmole g^?1 fr.w.) of medicarpin. The data are discussed with regard to isoflavonoid metabolism and the significance of induced and constitutive levels of phytoalexins and isoflavones in resistance of chickpea towards A. rabiei.     

Disease measurement in a study of apple scab epidemics

The growth and yield increases in red raspberry which followed the repeated application of benomyl were not due to the suppression of pests. Benomyl had little effect on numbers of aphids and nematodes which, anyway, were insufficiently abundant to cause damage. Benomyl did, however, decrease the earthworm population. In the following years, when benomyl was no longer applied, significantly more canes died from midge blight in the plots treated most frequently with benomyl than in the untreated. This was probably a consequence of increased infestation by larvae of the raspberry cane midge (Resseliella theobaldi) in the benomyl-treated plots because benomyl increased the extent of cane splitting and hence the number of egg-laying sites for cane midge.     

Phenol metabolism in downy mildew resistant and susceptible genotypes of pearl millet

Biochemical characterisation of pearl millet genotypes was carried at pre- (45 DAS) and post-infection (57 DAS i.e. 7 days after infection) stages. Total phenol content at pre-infection stage did not show inherent resistance or susceptibility. While the total phenol content was found to be higher in susceptible genotypes at post-infection stage, qualitative analysis of phenol through high-performance thin layer chromatography showed absence of ferulic acid in resistant genotypes at pre-infection stage. Peroxidase (POX) activity was higher in susceptible genotypes at both the stages of analysis. Constitutive activity of phenylalanine ammonia lyase was higher in resistant genotype whereas induced activity was recorded higher in susceptible genotypes. Native poly-acrylamide gel electrophoresis isozyme banding pattern of POX showed some inducible band(s) due to disease infection in resistant and susceptible genotypes.     

Effects of Aphis fabae and Uromyces viciae-favae on the growth of a susceptible and an aphid resistant cultivar of Vicia faba

Damaging effects of either black bean aphid (Aphis fabae), broad bean rust (Uromyces viciae-fabae), or the combination of both were investigated on a susceptible (cv. Diana) and an aphid resistant (cv. Bolero) cultivar of Vicia faba. When compared with rust, aphids caused greater reductions of root dry weight, shoot dry weight, leaf area, and mean relative growth rate. The mean unit leaf rate was also reduced whereas the leaf area ratio was not affected. The damage caused per aphid was highest on the susceptible cultivar. Rust induced damage did not differ between the cultivars. Concomitant infestation with both pests only resulted in additive damage. The population development of aphids was delayed on partially resistant plants. High temperature and rust infection reduced the total number of aphids the plants were able to support but not the level of resistance. Thus the specific damaging effect per aphid was increased.     

Erwinia amylovora modifies phenolic profiles of susceptible and resistant apple through its type III secretion system

Fire blight is a disease affecting Maloideae caused by the necrogenic bacterium Erwinia amylovora , which requires the type III protein secretion system (TTSS) for pathogenicity. Profiles of methanol-extractable leaf phenolics of two apple ( Malus  ×  domestica ) genotypes with contrasting susceptibility to this disease were analyzed by HPLC after infection. Some qualitative differences were recorded between the constitutive compositions of the two genotypes but in both of them dihydrochalcones accounted for more than 90% of total phenolics. Principal component analysis separated leaves inoculated with a virulent wild-type strain from those inoculated with a non-pathogenic TTSS-defective mutant or with water. The changes in levels of the various groups of phenolics in response to the virulent bacterium were similar between the two genotypes, with a significant decrease of dihydrochalcones and a significant increase of hydroxycinnamate derivatives. Differences between genotypes were, however, recorded in amplitude and kinetic of variation in these groups. Occurrence of oxidation and polymerization reactions is proposed, based on the browning process of infected tissues, but whether some by-products act in defense as toxic compounds remain to be tested. Among direct antibacterial constitutive compounds present in apple leaves, the dihydrochalcone phloretin only was found at levels close to lethal concentrations in both genotypes. However, E.   amylovora exhibited the ability to stabilize this compound at sublethal levels even in the resistant apple, rejecting the hypothesis of its involvement in the resistance of this genotype.     

In vitro metabolism of pyriproxyfen by microsomes from susceptible and resistant housefly larvae

Levels of cytochrome P450 and b₅ were investigated in microsomal enzymes of houseflies from the gut and fat body of the third instar larvae of a pyriproxyfen-resistant strain (YPPF) and two pyriproxyfen-susceptible strains (YS and SRS). In comparison to the YS and SRS strains, YPPF microsomes had higher levels of total cytochrome P450s in both the gut and fat body. Furthermore, microsomes from the gut and fat body of YPPF larvae were found to have a much greater ability to hydroxylate aniline than YS larvae. In vitro metabolism studies of pyriproxyfen indicated that the metabolic rates were much higher in both the gut and fat body of YPPF larvae than of YS and SRS larvae. The major metabolites of pyriproxyfen in houseflies were identified to be 4′-OH-pyriproxyfen and 5′-OH-pyriproxyfen. Cytochrome P450 inhibitors, piperonyl butoxide (PB) and 2-propynyl 2,3,6-trichlorophenyl ether (PTPE), decreased the metabolic rates significantly in all three strains. This study confirmed that microsomal cytochrome P450 monooxygenases play an important role in the pyriproxyfen resistance of the housefly. Furthermore, it suggests that the fat body must be as important as the gut for the metabolism of pyriproxyfen in resistant housefly larvae. Arch. Insect Biochem. Physiol. 37:215–224, 1998. © 1998 Wiley-Liss, Inc.     

The effect of carbamide and thiocarbamide on the resistance of apple to apple scab

Carbamide and thiocarbamide decreased the resistance of apple to apple scab when infiltrated into apple leaves prior to infection with the disease. In three apple varieties these two substances strikingly stimulated infection with two monosporic isolates of the fungusVenturia inaequalis (Cke.) Wint. It was established that both carbamide and thiocarbamide inhibited polyphenol oxidase activity in apple leavesin vivo, butin vitro only thiocarbamide was inhibitory. It can be concluded that the effect on apple resistance to apple scab is based on an inhibition of polyphenol oxidase.     

Identification of functional apple scab resistance gene promoters

Apple scab (Venturia inaequalis) is one of the most damaging diseases affecting commercial apple production. Some wild Malus species possess resistance against apple scab. One gene, HcrVf2, from a cluster of three genes derived from the wild apple Malus floribunda clone 821, has recently been shown to confer resistance to apple scab when transferred into a scab-susceptible apple variety. For this proof-of-function experiment, the use of the 35S promoter from Cauliflower mosaic virus was reliable and appropriate. However, in order to reduce the amount of non-plant DNA in genetically modified apple to a minimum, with the aim of increasing genetically modified organism acceptability, these genes would ideally be regulated by their own promoters. In this study, sequences from the promoter region of the three members of the HcrVf gene family were compared. Promoter constructs containing progressive 5 deletions were prepared and used for functional analyses. Qualitative assessment confirmed promoter activity in apple. Quantitative promoter comparison was carried out in tobacco (Nicotiana glutinosa) and led to the identification of several promoter regions with different strengths from a basal level to half the strength of the 35S promoter from Cauliflower mosaic virus.