Huanglongbing (HLB, also commonly known as citrus greening disease) is caused by the bacterium Candidatus Liberibacter asiaticus (CLas) in the US. It is a serious plant disease causing tremendous economic damages and posing an existential threat to the citrus industry worldwide. CLas proliferates inside the phloem, clogs the transportation system, stunts plant growth and renders trees barren. Currently there is no cure or effective treatments for this disease. In this study, qPCR and scanning electron microscopy (SEM) were used to examine orange plants with or without HLB symptoms. We found that symptomatic HLB plants showed high levels of CLas DNA and its prophage in leaves and had heavily clogged phloem tissues. In contrast, the asymptomatic, healthy looking plants had detectable, low levels of CLas and prophage DNA by qPCR but the SEM imaging revealed phloem clear of clogging deposits. Transcription of several CLas phage genes was detected in both healthy and diseased plants, however, higher transcription levels were observed in the healthy plants than in the diseased plants. As part of our efforts to explore the possibility of using a plant-based antibacterial concoction named Agent G to mitigate HLB, NMR and GC/MS were used to assess its organic chemical composition and migration through the phloem tissue. It was found that the major compounds of Agent G were allyl polysulfides and cinnamaldehyde. The phloemic migrate rate in planta was estimated between 1.5 and 3.25 cm/h. Our studies suggest that a combination of qPCR test and SEM imaging may prove to be more informative for gauging growth status of citrus plants and for early HLB detection. We also showcase a feasible approach of treating HLB by delivering antibacterial reagents directly into the phloem tissue where most CLas reside.

中文翻译：

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感染黄龙病（柑橘绿化病）的柑橘植物的分子和细胞分析

黄龙病（HLB，俗称柑橘绿化病）是由美国念珠菌（Candidatus Liberibacter asiaticus，CLas）引起的。它是一种严重的植物病害，会造成巨大的经济损失，并对全球柑橘产业构成生存威胁。CLas 在韧皮部内增殖，堵塞运输系统，阻碍植物生长并使树木变得贫瘠。目前，这种疾病没有治愈方法或有效的治疗方法。在这项研究中，qPCR 和扫描电子显微镜 (SEM) 用于检查有或没有 HLB 症状的橙色植物。我们发现有症状的 HLB 植物在叶子中显示出高水平的 CLas DNA 及其原噬菌体，并且韧皮部组织严重堵塞。相比之下，无症状的、看起来健康的植物具有可检测的、通过 qPCR 检测到低水平的 CLas 和原噬菌体 DNA，但 SEM 成像显示韧皮部没有堵塞沉积物。在健康和患病植物中均检测到几种 CLas 噬菌体基因的转录，然而，在健康植物中观察到的转录水平高于患病植物。作为我们探索使用名为 Agent G 的基于植物的抗菌混合物减轻 HLB 的可能性的努力的一部分，使用 NMR 和 GC/MS 来评估其有机化学成分和通过韧皮部组织的迁移。发现G剂的主要化合物是烯丙基多硫化物和肉桂醛。植物中韧皮迁移率估计在 1.5 到 3.25 厘米/小时之间。我们的研究表明，qPCR 测试和 SEM 成像的组合可能被证明为衡量柑橘植物的生长状态和早期 HLB 检测提供更多信息。我们还展示了一种通过将抗菌试剂直接输送到大多数 CLas 所在的韧皮部组织来治疗 HLB 的可行方法。