Crop Diversification Centre North, Alberta Agriculture and Rural Development, Edmonton, AB, Canada, T5Y 6H3.
Mol Plant Pathol. 2012 Feb;13(2):105-13. doi: 10.1111/j.1364-3703.2011.00729.x. Epub 2011 Jun 1.
Plasmodiophora brassicae causes clubroot disease in cruciferous plants, and is an emerging threat to Canadian canola (Brassica napus) production. This review focuses on recent studies into the pathogenic diversity of P. brassicae populations, mechanisms of pathogenesis and resistance, and the development of diagnostic tests for pathogen detection and quantification.
Plasmodiophora brassicae is a soil-borne, obligate parasite within the class Phytomyxea (plasmodiophorids) of the protist supergroup Rhizaria.
Clubroot development is characterized by the formation of club-shaped galls on the roots of affected plants. Above-ground symptoms include wilting, stunting, yellowing and premature senescence. DISEASE CYCLE: Plasmodiophora brassicae first infects the root hairs, producing motile zoospores that invade the cortical tissue. Secondary plasmodia form within the root cortex and, by triggering the expression of genes involved in the production of auxins, cytokinins and other plant growth regulators, divert a substantial proportion of plant resources into hypertrophic growth of the root tissues, resulting in the formation of galls. The secondary plasmodia are cleaved into millions of resting spores and the root galls quickly disintegrate, releasing long-lived resting spores into the soil. A serine protease, PRO1, has been shown to trigger resting spore germination. PHYSIOLOGICAL SPECIALIZATION: Physiological specialization occurs in populations of P. brassicae, and various host differential sets, consisting of different collections of Brassica genotypes, are used to distinguish among pathotypes of the parasite. DETECTION AND QUANTIFICATION: As P. brassicae cannot be cultured, bioassays with bait plants were traditionally used to detect the pathogen in the soil. More recent innovations for the detection and quantification of P. brassicae include the use of antibodies, quantitative polymerase chain reaction (qPCR) and qPCR in conjunction with signature fatty acid analysis, all of which are more sensitive than bioassays. RESISTANCE IN CANOLA: Clubroot-resistant canola hybrids, recently introduced into the Canadian market, represent an important new tool for clubroot management in this crop. Genetic resistance must be carefully managed, however, as it has been quickly overcome in other regions. At least three resistance genes and one or two quantitative trait loci are involved in conferring resistance to P. brassicae. Root hair infection still occurs in resistant cultivars, but secondary plasmodia often remain immature and unable to produce resting spores. Fewer cell wall breakages occur in resistant hosts, and spread of the plasmodium through cortical tissue is restricted. More information on the genetics of clubroot resistance in canola is needed to ensure more effective resistance stewardship.
http://www.canolacouncil.org/clubroot/resources.aspx, http://tu-dresden.de/die_tu_dresden/fakultaeten/fakultaet_mathematik_und_naturwissenschaften/fachrichtung_biologie/botanik/pflanzenphysiologie/clubroot, http://www.ohio.edu/people/braselto/plasmos/
根肿菌引起十字花科植物的根肿病，是对加拿大油菜（甘蓝型油菜）生产的一个新出现的威胁。本综述重点介绍了根肿菌种群的致病多样性、发病机制和抗性、以及用于病原体检测和定量的诊断测试的最新研究。
根肿菌是一种土壤传播的、专性寄生的原生动物门（根肿菌类）中的植物原生动物超群根足虫的一员。
根肿病的发展特征是受感染植物的根部形成棒状的根瘤。地上部分的症状包括萎蔫、生长迟缓、叶片发黄和过早衰老。
根肿菌首先感染根毛，产生游动的游动孢子，游动孢子侵入皮层组织。次级原生质体在根皮层内形成，通过触发参与生长素、细胞分裂素和其他植物生长调节剂产生的基因表达，将植物资源的很大一部分转移到根组织的过度生长上，导致根瘤的形成。次级原生质体被切割成数百万个休眠孢子，根瘤迅速解体，将长寿命的休眠孢子释放到土壤中。一种丝氨酸蛋白酶 PRO1 已被证明能触发休眠孢子的萌发。
根肿菌种群中存在生理特化现象，不同的寄主差异集由不同的甘蓝型基因型组成，用于区分寄生虫的不同致病型。
由于根肿菌不能被培养，传统上使用诱饵植物的生物测定法来检测土壤中的病原体。最近用于检测和定量根肿菌的创新方法包括使用抗体、定量聚合酶链反应（qPCR）和 qPCR 结合特征脂肪酸分析，所有这些方法都比生物测定法更敏感。
最近引入加拿大市场的抗根肿病油菜杂交种是管理这种作物根肿病的一个重要新工具。然而，必须谨慎管理遗传抗性，因为它在其他地区已经很快被克服。至少有三个抗性基因和一个或两个数量性状位点参与赋予对根肿菌的抗性。在抗性品种中，根毛感染仍然会发生，但次级原生质体往往仍然不成熟，无法产生休眠孢子。在抗性宿主中，细胞壁的破裂较少，原生质体通过皮层组织的传播受到限制。需要更多关于油菜根肿病抗性的遗传学信息，以确保更有效的抗性管理。
