Control of ripening in transgenic tomatoes

Major progress has been made over the last few years in the identification and regulation of tomato ripening genes. At least 25 genes showing elevated expression during ripening have been cloned and several, including polygalacturonase, which modifies fruit textures, have been shown to be ripening-specific. In addition, genes have been cloned for ACC synthase and ACC oxidase, which control the synthesis of ethylene, which plays a critical role in ripening. Inhibition of expression of polygalacturonase, pectinesterase, ACC synthase, ACC oxidase and phytoene synthase has been achieved in transgenic plants, using antisense technology. The expression of several genes has also been inhibited by sense gene suppression. New traits caused by these transgenes are stably inherited. Antisense tomatoes with reduced polygalacturonase have improved textural qualities which are being exploited commercially for the fresh and processed markets. Overexpression of phytoene synthase has been shown to restore carotenoid production in the yellow flesh mutant and can be used to enhance colour in other cultivars. Antisense fruit in which ACC synthase or ACC oxidase are inhibited show slower ripening and reduced over-ripening. ACC oxidase antisense genes have also been shown to delay leaf senescence. It is to be expected that further genes determining other quality traits will be identified and manipulated soon.     

Counteracting virulence mechanisms of grain legume pathogens

Summary Grain legumes, in common with all other plants, are subject to biotic constraints of which pathogens form an important group. They are variable in type, number, space and time and, most insidiously, in genetic constitution. Consequently, resistance in the plant to a given pathogen may be quickly nullified by genetic alteration of the pathogen, particularly where this is conferred by a single resistance gene. The products of such resistance genes usually recognise, directly or indirectly, a component of the pathogen, which is encoded by a corresponding avirulence gene. Thus resistance and avirulence genes are specific and complementary and the arrangement is referred to as a gene-for-gene relationship. It follows that alteration of the avirulence gene of the pathogen to give a product that is no longer recognised by the product of the resistance gene of the plant gives rise to a susceptible reaction. A possible solution to this problem is to pyramid several resistance genes, a procedure now facilitated by the techniques of genetic modification. In other interactions genes that reduce susceptibility rather than confer complete resistance have been found and in some cases the loci (quantitative trait loci) responsible have been mapped to specific regions of particular chromosomes. The mechanisms by which these genes limit the virulence of the pathogen are generally unknown. However, by gaining an understanding of the fundamental properties of a pathogen that are necessary for pathogenicity or virulence it may be possible to counteract them. Candidates for such properties are toxins, enzymes and mechanisms that interfere with constitutive or active defence of the plant. Reciprocally, understanding the properties of the plant that confer susceptibility may allow selection of germplasm that lacks such properties. Among the candidates here are germination stimulants of pathogen propagules and signals that promote the formation of infection structures.     

Exploitation of bacterial pectinolytic strains for improvement of hemp water retting

Summary Retting is the major limitation to an efficient production of textile hemp fibres. Traditional retting has been carried out by autochthonous bacterial community. Aerobic and anaerobic pectinolytic strains were isolated from hemp or flax sources and characterised. Anaerobic pectinolytic strains had a wide range of acid polygalacturonase (PG) activity, whereas aerobic isolates did not produce any acid PG activity, but only an alkalophylic one, suggesting they could play a minor role in the retting process, except in the early stages. Analysis of 16S rDNA sequences assigned anaerobic strains to the Clostridium genus and aerobic isolates to the Bacillus and Paenibacillus genus. C. felsineum and C. acetobutylicum were confirmed as the main anaerobic agents. Nevertheless, a high proportion of anaerobic and aerobic pectinolytic strains was assigned to C. saccharobutylicum and B. pumilus, respectively, both species never being described as involved in water retting. Anaerobic and aerobic strains with high PG activity were selected and characterized. PG activity is well correlated with the strain retting efficiency and improvement of the process was obtained by inoculating the retting water with spores of selected aerobic and anaerobic bacteria. An advisable feature of retting strains is the absence of cellulosolytic activity. An aerobic strain with no cellulosolytic activity was identified. In contrast, all the anaerobic isolates showed cellulosolytic activity. Mutagenesis was ineffective for selection of Cel-Pec⁺ mutants. Localization of the C. felsineum L1/6 PG activity was investigated.     

Effect of Trichoderma viride on activities of polygalacturonase of Rhizoctonia

The pectin is a backbone of the plant cell wall, its network structure will systemicly resolve when the plant cell splits up and forms. The pectinase produced by Rhizoctonia mainly acts on the pectin of cell wall, and causes the maceration of tissue and the death of protoplast. Polygalacturonase (PG) can decompose the galacturonic acid of disease tissue. The research defined the PG activities of extracellular metabolite of the different virulence Rhizoctonia isolates, and testifid the e…     

A synopsis of symbiotic nitrogen fixation and other apparently similar host‐microorganism interactions

Physical, physiological and biochemical processes occurring during colonization, infection, and nodule development and maintenance phases of an effective legume‐rhizobia symtiotic nitrogen fixation system are discussed. Limited knowledge of host reactions to invasion by rhizobia which result in failure to establish a symbiotic nitrogen fixation system are related to more thoroughly researched reactions of resistant host plants to invastion by pathogenic organisms. The most common resistant host responses are an increase in the production of phenolic compounds and phenol oxidizing enzymes. Many of the pheolic compounds or their quinones produced by enzymatic oxidation inhibit the action cell wall degrading enzymes and phytohormones, are antibiotic toward pathogenic organisms, and are phytotoxic to host plant cells. It is postulated that similar host responses result with rhizobia invasion and that the magnitude of these responses determine specificity of legume‐rhizobia symbiosis.     

5种瓜类植物多聚半乳糖醛酸酶抑制蛋白的诱导表达及活性测定

多聚半乳糖醛酸酶抑制蛋白(PGIP)是一种富含亮氨酸重复单位的糖蛋白,能非竞争性地抑制真菌多聚半乳糖醛酸酶(PG)的活性,从而提高植物的抗病性.以黄瓜、苦瓜、冬瓜、节瓜和甜瓜幼苗为材料,研究PGIP的诱导表达特性及在不同组织中的含量差异.通过比较经孢子诱导、SA诱导和黑暗诱导的黄瓜幼苗中的PGIP的相对含量,发现经病原菌孢子悬浮液处理48 h时PGIP的表达量最高.对经孢子诱导48 h的黄瓜幼苗的根、茎、叶中的PGIP含量进行比较,发现茎中PGIP的相对含量最高.测定这5种瓜类的PGIP对6种尖孢镰刀菌粗PG的抑制作用,发现苦瓜PGIP的相对活性较高,并证实了不同的PGIP能够特异性地识别不同的PG,PGIP对不同PG存在着选择性抑制.     

柚多聚半乳糖醛酸酶基因家族的鉴定和分析

草莓、桃、苹果等果实在生长后期和贮藏过程中逐渐软化,而柑橘类特别是柚在成熟后期和贮藏过程中出现粒化,严重影响其经济价值.本研究分析和鉴定了柚基因组中多聚半乳糖醛酸酶基因(CgPG)家族成员以及PG酶活的变化,旨在揭示柚果实成熟过程中CgPG表达与汁胞粒化形成的关系.本研究对柚基因组库中CgPG基因家族成员的数量、基因定...     

Modulation of host pH during the wheat–Fusarium culmorum interaction and its influence on the production and activity of pectolytic enzymes

The effect on ambient pH of Fusarium culmorum during its growth on mineral medium and in apoplastic fluids from infected wheat seedlings, and the effect on the production and activity of the enzymes pectin lyase (PNL) and polygalacturonase (PG), were investigated. Fungal development on a weakly buffered mineral medium in the pH range 5·0–8·0, with pectin as the sole carbon source, led to pronounced alkalinization, reaching values above 8·0. The increase in ambient pH was accompanied by enhancement of total PNL activity. Pectin lyase secretion was detected at pH 5·0 as a single isoenzyme. An additional isoenzyme was apparent during the increase in medium pH. Polygalacturonase was detected as a single isoenzyme only during early growth on medium buffered at pH 5·0. At this stage, the initial medium pH of 5·0, corresponding to the optimum pH for PG activity, appeared to be the most suitable for the activation of early production of this enzyme. During growth in acidified yeast extract medium the fungus secreted ammonia and increased medium pH. Similarly, in apoplastic fluids from inoculated seedlings the concomitant ammonia accumulation and rise in pH were recorded. This trend was accompanied by an increase in PNL, which could therefore function at close to its optimal pH. The results suggest that during infection of wheat seedlings by F. culmorum , pH modulation can lead to PNL production and activity, thus promoting colonization of host tissue.     

果胶酶高产菌微紫青霉SW09的鉴定及发酵特征

从云南烟田采取土样多份,以果胶为惟一碳源,通过初筛和摇瓶复筛,筛选到l株果胶酶高产真菌SW09。通过菌落形态观察、生理生化指标试验和l8S r DNA保守序列分析,经初步鉴定为微紫青霉,将其命名为Penicillium janthinellum SW09,并以发酵罐深层液体培养确定其发酵生长周期为72 h,产聚半乳糖醛酸酶活力最大达到3 071.99 U/m L,为烟梗果胶质的生物降解奠定基础。