Pathogenicity of Plasmodiophora brassicae populations from small, spheroid, resistant-type clubroot galls on roots of clubroot-resistant cultivars of Chinese cabbage (Brassica rapa L. subsp. pekinensis)

A resistant type of small, spheroid clubroot galls (SSGs) containing resting spores formed on the root surface of clubroot-resistant (CR) cultivars of Chinese cabbage (Brassica rapa L. subsp. pekinensis) inoculated with an avirulent population of Plasmodiophora  brassicae. Populations isolated from the SSGs severely affected a common (susceptible) cultivar but did not have the typical pathogenicity on CR cultivars, indicating similarity in pathogenicity between SSG and original spore populaions. Populations pathogenic on CR cultivars were not detected from SSGs. Therefore, the ability of the avirulent population among the SSGs to form resting spores may need to be considered to achieve clubroot control in common cultivars of crucifers.     

氟唑活化酯诱导大白菜抗根肿病效果与机理初步研究

为明确新型植物诱导抗病剂氟唑活化酯对大白菜根肿病的诱导抗病效果及抗病机理,研究了氟唑活化酯的诱导浓度、在白菜根部的运输过程以及氟唑活化酯诱导白菜后相关防御基因的表达情况。结果显示以氟唑活化酯25 mg ? L-1对大白菜进行叶面喷雾诱导时根肿病的病情指数最低。通过台盼蓝（Trypan blue）染色观察到氟唑活化酯诱导大白菜叶片后,根部对根肿菌也产生了一定的抗性。利用Real-time PCR技术检测大白菜防御相关基因的表达,发现氟唑活化酯诱导大白菜2 h后JA途径中的COI1和LOX2基因以及SA途径中的PR-1基因都有显著的过量表达,其中JA信号途径相关基因表达变化更明显。结果说明氟唑活化酯可以诱导大白菜产生系统获得抗性,两种信号传导途径都参与了抗病过程,但以JA途径为主,SA途径为辅。     

基于根肿菌早期侵染量的白菜抗性分析

为探究不同抗性白菜品种对根肿菌侵染量的影响,通过水培法观测4个白菜品种接种根肿菌后其在寄主根内的发展动态,实时荧光定量PCR检测技术对同期根内含菌量进行分析,并结合温室盆栽试验获得的各品种发病数据进行相关性分析。结果表明,早熟长江5号根肿病的发病率和病情指数分别为82.5%和36.2,华良早五号为66.7%和37.5,CR-春美为68.9%和36.0,可归为感病品种,而CR-英雄为52.5%和16.5,可归为抗病品种;水培法接种观测和PCR定量结果显示,根毛侵染率、皮层侵染数以及根内含菌量均表现为CR-英雄最低,显著低于其它3个品种。根毛侵染率、皮层侵染数、根内含菌量、发病率四者间呈显著相关,其R2最大值变幅为0.84~0.98,且根毛侵染率与根内含菌量、发病率与根内含菌量的最大相关系数均出现在第6天,说明根肿菌早期侵染量直接影响白菜根肿病的发生程度。表明根内早期侵染是根肿菌致病的关键环节,通过水培法观测并结合实时荧光定量PCR检测可以评价品种的抗性,可作为根肿病抗性评价的一种方法。     

Dissemination of Plasmodiophora brassicae in livestock manure detected by qPCR

The present study was performed to investigate whether Plasmodiophora brassicae can be disseminated by livestock manure. A quantitative PCR (qPCR) assay was developed and used to detect and quantify P. brassicae in manure samples from naturally and artificially infested chickens and pigs. In naturally infested manure, quantifiable levels of infestation were observed in 7 out of the 28 samples, ranging from 10³ to 10⁷ resting spores per g of manure. The vast majority of the resting spores (76–91%) were viable, as determined by a dual fluorescence viability assay. Clubroot symptoms developed on plants inoculated with P. brassicae resting spores isolated from all seven qPCR‐positive samples. Artificially infested manure samples were produced by feeding chickens and pigs on P. brassicae‐contaminated feed. The levels of infestation were 10³ resting spores per g of manure for both chicken and pig manure sampled 24 and 48 h after feeding, respectively. Spore viability was >80% for both samples, and the disease severity indices were both >40 as indicated by bioassay. This showed that resting spores can survive the digestive tracts of chickens and pigs, and retain strong pathogenicity. The findings indicate that manure dissemination is possible for P. brassicae. Farmers should avoid feeding livestock on P. brassicae‐contaminated feed or applying infested manure as fertilizer on land intended for crucifer crops.     

Electron microscopic studies on the root hairs and cortex of a susceptible and a resistant variety of Brassica campestris infected with Plasmodiophora brassicae

The cortex of the roots of a susceptible and a resistant variety ofBrassica campestris var.rapa infected with sterile resting spores ofPlasmodiophora brassicae from senescent callus was studied at a stage prior to disease symptom development. Electron micrographs show the presence of amoeboid structures within the cortical cells of the susceptible variety 10 days after inoculation. Cell wall perforations, hypertrophied host cell nuclei, nucleoli and broken tonoplasts were frequently found in the susceptible variety. It has been concluded that amoeboid structures of the parasite penetrate the cell wall and disrupt the cortical cells.Electron micrographs of the resistant variety show the presence of zoosporangia with secondary zoospores in the root hairs nine days after inoculation. Two to four days later a large number of dead host cells can be observed in the outer cortical layer of the resistant variety, whereas no apparent changes are found in the inner cortex. The results suggest the occurrence of a hypersensitive host reaction which terminates further growth ofPlasmodiophora brassicae.Samenvatting Kiemplanten van een vatbare en een resistente variëteit vanBrassica campestris var.rapa werden geïnoculeerd met een suspensie van rustsporen vanPlasmodiophora brassicae. De rustsporen waren afkomstig uit verouderd ziek callusweefsel. Elektronenmicroscopisch onderzoek werd verricht aan de cortex van de hoofdwortel van vatbare planten in de periode die vooraf gaat aan de ontwikkeling van ziektesymptomen (10–13 dagen na inoculatie). Tien dagen na inoculatie werden amoebe-achtige structuren in de cortex gevonden (Fig. 1–4). Herhaaldelijk werden gaten in de celwand van de cortex gevonden (Fig. 5). De waardplantkern en kernlichaampje vertoonden een hypertrofische reactie terwijl de tonoplast vaak kapot was (Fig. 3). Deze resultaten vormen een aanwijzing dat de parasiet in de vorm van een amoebe door de celwand dringt en de celinhoud van de cortex verstoort. Dertien dagen na inoculatie werden de eerste secondaire plasmodiën in het binnenste gedeelte van de cortex of in de mergstraalcellen van de hoofdwortel gevonden (Fig. 6). Vanaf dat moment traden de typische knolvoetsymptomen op.Uit elektronen-microscopisch onderzoek van de resistente variëteit bleek dat negen dagen na inoculatie zoösporangien en secondaire zoösporen aanwezig waren in de wortelharen van de zijwortels (Fig. 7–9). Twee tot vier dagen later werden een groot aantal dode waardplantcellen in de buitenste cortexcellen van de hoofdwortel gevonden, terwijl het binnenste gedeelte van de cortex geen veranderingen vertoonde (Fig. 10 en 11). Deze resultaten wijzen er op dat in de resistente variëteit een overgevoeligheidsreactie in de cortex opgetreden is waardoor verdere groei vanPlasmodiophora brassicae verhinderd werd.