The oomycete Phytophthora melonis causes a severe disease in cucumber plants in Asia. In this study, the diameter of cucumber hypocotyl in the resistant variety ‘Shantou qing gua’ was significantly larger than that of the susceptible variety ‘Zhongnong No. 20’. The significantly lower incidence of disease and less invasive hyphae on the epidermis and transverse section of hypocotyls in P plants of the resistant variety than those in susceptible cultivars were also observed. Brassinosteroids are a class of phytohormones that affect plant growth and development and are involved in regulating plant resistance to a variety of biotic and abiotic stresses. 24-Epibrassinolide root drenching significantly enhanced the thickening of cucumber hypocotyl. Thick hypocotyls showed strong resistance to P. melonis, indicating that it significantly reduced the incidence of disease and retarded the hyphae extension for both resistant and susceptible cucumbers. 24-Epibrassinolide pretreatment had no significant effect on the elongation of cucumber hypocotyl. Further histological observation showed that under the condition of infection with P. melonis, exogenous 24-epibrassinolide could induce lignin deposition in external phloem and xylem vessel cell wall of the cucumber hypocotyl vascular bundle. There is also an accumulation of callose in the external phloem sieve plate, which activates the resistance responses in cell walls. It is worth mentioning that in both inoculated and uninoculated conditions, exogenous 24-epibrassinolide enhanced lignin formation in external phloem and xylem vessel cell wall of the vascular bundle. This increased the content of lignin in hypocotyl as well as the number of vascular bundles in the hypocotyl base. The above results show that 24-epibrassinolide constitutively regulates the thickening of cucumber hypocotyl and the development of vascular bundle, hence preventing phytophthora infection and inducing plant resistance to disease.

中文翻译：

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24-表布拉斯内酯防治瓜疫霉猝倒病及黄瓜下胚轴的组织学研究

卵菌 Phytophthora melonis 在亚洲黄瓜植物中引起严重疾病。本研究中，抗病品种‘汕头清瓜’黄瓜下胚轴直径明显大于感病品种‘中农20号’。与易感品种相比，抗性品种的 P 植物的表皮和下胚轴横切面上的病害发生率显着降低，菌丝侵入性更小。油菜素类固醇是一类影响植物生长发育并参与调节植物对多种生物和非生物胁迫的抗性的植物激素。24-Epibrassinolide 根浸液显着增强了黄瓜下胚轴的增厚。厚的下胚轴对 P. melonis 表现出很强的抵抗力，表明它显着降低了疾病的发生率并延缓了抗性和易感黄瓜的菌丝延伸。24-表芸苔素内酯预处理对黄瓜下胚轴的伸长无显着影响。进一步的组织学观察表明，在黄瓜下胚轴维管束感染条件下，外源性24-表布拉斯内酯可诱导木质素沉积在黄瓜下胚轴维管束的外部韧皮部和木质部血管细胞壁中。外部韧皮部筛板中也有胼胝质的积累，激活细胞壁的抵抗反应。值得一提的是，在接种和未接种条件下，外源性 24-表布拉斯内酯增强了维管束的外部韧皮部和木质部血管细胞壁中木质素的形成。这增加了下胚轴中木质素的含量以及下胚轴基部中维管束的数量。以上结果表明，24-表布拉斯内酯组成性调节黄瓜下胚轴的增厚和维管束的发育，从而防止疫病菌侵染并诱导植物抗病性。