芸薹根肿菌活细胞PMAxx-qPCR快速定量检测方法的建立与应用

【目的】由芸薹根肿菌（Plasmodiophora brassicae）侵染引起的十字花科根肿病是一种世界性土传病害,病原菌长期存在于土壤中,对十字花科作物造成严重威胁。改良叠氮溴化丙锭（propidium monoazide xx,PMAxx）可选择性地穿透受损的死细胞膜,并抑制死细胞DNA的实时荧光定量PCR（qPCR）扩增。本文将PMAxx与qPCR技术相结合,建立一种快速检测芸薹根肿菌活菌的方法,为根肿病的早期诊断及制定科学的防控措施提供依据。【方法】配置浓度分别为0、5、10、20、40、60 µmol·L^-1的叠氮溴化丙锭PMA和改良叠氮溴化丙锭PMAxx,比较两种核酸染料对芸薹根肿菌死细胞DNA扩增的抑制效果,确定最佳核酸染料及工作浓度;设置光照时间分别为0、2、5、10、15和20 min,进行最佳光照时间的优化,建立芸薹根肿菌活细胞PMAxx-qPCR快速检测体系。设置芸薹根肿菌活孢子百分比为0、0.01%、0.1%、1%、10%、25%、50%、75%和100%的混合体系,验证PMAxx-qPCR体系的准确性,并应用于田间土壤样本中芸薹根肿菌活孢子的定量检测。【结果】PMAxx对芸薹根肿菌死细胞DNA的扩增抑制效果更好,当芸薹根肿菌浓度为1×10⁸个孢子/mL,PMAxx预处理的最适终浓度为4 µmol·L^-1,最佳光照时间为10 min时,可有效地抑制死孢子DNA的扩增,仅以有活力孢子DNA为靶标选择性地扩增。利用PMAxx-qPCR技术检测已知不同活孢子比例的菌悬液样品,各样品实测孢子存活率和理论存活率之间呈正相关（R²=0.992）。对田间采集的25份土壤样本,采用PMAxx-qPCR方法检测到11份样本中携带芸薹根肿菌,活细胞DNA浓度为32.35—6.97×10³ fg·g^-1。【结论】建立了基于PMAxx-qPCR的芸薹根肿菌活细胞定量检测技术,该技术具有快速、准确、灵敏的特点,解决了qPCR不能仅对活体病原菌进行准确鉴别和定量分析的问题,为制定有效的根肿病防控策略提供了依据。

Establishment and Application of Rapid Quantitative Detection of Viable Plasmodiophora brassicae by PMAxx-qPCR Method

【Objective】Plasmodiophora brassicae is an obligate endoparasite that causes clubroot disease, which is the most devastating soil-borne disease in brassica crops. The propidium monoazide xx (PMAxx) could selectively bind to the chromosomal DNA of dead spores and therefore block DNA amplification by real-time fluorescent quantitative PCR (qPCR). In the present study, a strategy involving a PMAxx pre-treatment followed by the qPCR (PMAxx-qPCR) assay was developed for quantifying viable spores of P. brassicae, so as to provide a basis for early detection and prevention measurement of cloobroot disease. 【Method】 PMA and PMAxx with concentrations of 0, 5, 10, 20, 40 and 60 µmol·L^-1 were prepared, respectively, and were used to pre-treat P. brassicae prior to DNA extraction, followed by qPCR. The inhibitory effects of PMA and PMAxx on DNA amplification of P. brassicae dead spores were compared, and the optimal nucleic acid dye and concentration to distinguish between live and dead spores were determined. The illumination time was set as 0, 2, 5, 10, 15 and 20 min, respectively, and the optimal exposure time was optimized to establish a PMAxx-qPCR assay for selectively detection of viable spores of P. brassicae. The mixed suspensions with different ratios of dead and viable spores (0, 0.01%, 0.1%, 1%, 10%, 25%, 50%, 75% and 100% viable spores) were prepared to determine the suitability of PMAxx-qPCR assay for distinguishing viable and dead spores. The assay was also applied to quantitative detection of viable spores of P. brassicae in 25 field soil samples. 【Result】 PMAxx showed a better discrimination effect than PMA on the viable and dead spores of P. brassicae. When the concentration of P. brassicae was 1×10⁸ spores/mL, the optimal PMAxx concentration and light exposure time were 4 μmol·L^-1 and 10 min, respectively. The amplification of dead spores could be inhibited effectively, and only the DNA of living spores was targeted for selective amplification. For pre-defined ratio of viable spores, there was a good linear relationship between the lg of the P. brassicae DNA concentration assessed by PMAxx-qPCR and the theoretical viability (R²=0.992). For soil samples, viable P. brassicae was quantified in 11 of 25 samples, with infestation levels of approximately 32.35-6.97×10³ fg·g^-1. 【Conclusion】 The established method could quantitatively detect the viable spores of P. brassicae, with advantages of rapid, efficiency and sensitivity, which could be useful for avoiding the inability of qPCR method to distinguish between viable and nonviable spores. Application of the assay may potentially improve P. brassicae control and disease management.