Black spot disease, caused by Alternaria brassicicola in Brassica species, is one of the most devastating diseases all over the world, especially since there is no known fully resistant Brassica cultivar. In this study, the visualization of black spot disease development on Brassica oleracea var. capitata f. alba (white cabbage) leaves and subsequent ultrastructural, molecular and physiological investigations were conducted. Inter- and intracellular hyphae growth within leaf tissues led to the loss of host cell integrity and various levels of organelle disintegration. Severe symptoms of chloroplast damage included the degeneration of chloroplast envelope and grana, and the loss of electron denseness by stroma at the advanced stage of infection. Transcriptional profiling of infected leaves revealed that photosynthesis was the most negatively regulated biological process. However, in infected leaves, chlorophyll and carotenoid content did not decrease until 48 hpi, and several chlorophyll a fluorescence parameters, such as photosystem II quantum yield (F_v/F_m), non-photochemical quenching (NPQ), or plant vitality parameter (Rdf) decreased significantly at 24 and 48 hpi compared to control leaves. Our results indicate that the initial stages of interaction between B. oleracea and A. brassicicola are not uniform within an inoculation site and show a complexity of host responses and fungal attempts to overcome host cell defense mechanisms. The downregulation of photosynthesis at the early stage of this susceptible interaction suggests that it may be a part of a host defense strategy, or, alternatively, that chloroplasts are targets for the unknown virulence factor(s) of A. brassicicola. However, the observed decrease of photosynthetic efficiency at the later stages of infection is a result of the fungus-induced necrotic lesion expansion.

中文翻译：

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甘蓝油菜-Alternaria brasicicola相互作用的复杂性揭示了超微结构，转录和生理水平下光合作用的下调。

黑点病，造成黑斑甘蓝的芸薹属种，是最具破坏性的疾病，全世界之一，特别是因为没有已知的完全的耐药性油菜品种。在这项研究中，油菜黑斑病发展的可视化。实蝇F。阿尔巴（白菜）叶和随后的超微结构，分子和生理学研究。叶片组织内和细胞内菌丝的生长导致宿主细胞完整性的丧失和各种水平的细胞器分解。叶绿体受损的严重症状包括叶绿体包膜和颗粒的变性，以及感染晚期阶段基质的电子密度丧失。感染叶片的转录谱分析表明，光合作用是最负面调节的生物过程。然而，在受感染的叶片中，叶绿素和类胡萝卜素的含量直到48 hpi才降低，并且一些叶绿素a的荧光参数，例如光系统II的量子产率（F _v / F _m），非光化学淬灭（NPQ）或植物活力参数（Rdf）与对照组叶片相比在第24和48 h时显着降低。我们的结果表明，油菜B. oleracea和油菜A. brasicicola之间的相互作用的初始阶段在接种部位内并不均匀，并且显示了宿主反应的复杂性和克服宿主细胞防御机制的真菌尝试。在这种易感相互作用的早期，光合作用的下调表明它可能是宿主防御策略的一部分，或者叶绿体是油菜曲霉未知毒力因子的靶标。。但是，在感染后期观察到的光合作用效率降低是真菌引起的坏死病变扩大的结果。