土霉素处理对柑橘黄龙病的防治效果及PP2基因表达的影响

【目的】评价土霉素(oxytetracycline,OTC)药剂对不同发病程度柑橘黄龙病（Huanglongbing,HLB）的防治效果,并检测OTC处理后韧皮部关键基因——韧皮部蛋白2（PP2）的表达量变化,为HLB的有效防控提供科学依据,也为OTC作用机理研究提供参考。【方法】应用可有效抑制、杀死病原菌的OTC,树干定量注射0.1 g/树于不同发病程度HLB（初感染、轻度发病和重度发病3组）的4年生Valencia夏橙,注射后7、30、60、90 d定期采集Valencia叶片样品监测HLB致病菌Cadidatus Liberibacter asiaticus(Las)含量、淀粉含量及PP2基因表达量变化。树干注射OTC后90 d采集不同处理Valencia的嫩叶、成熟叶片及茎,进行淀粉染色光学显微观察（LM）,直观反映植株累积淀粉的变化。树干注射OTC后80 d调查柑橘嫩叶抽发的情况,综合一系列指标评价OTC对HLB的防治效果。【结果】注射OTC 0.1 g/树对初感染HLB的4年生Valencia植株治疗作用最明显,7 d后检测即为阴性,并可保持90 d,其成熟叶片的淀粉含量明显减少,但茎内仍有淀粉粒富集;轻度发病夏橙注射90 d后叶片内的Las含量从（1.68×10 ⁶±858884）cells/g叶片减至（7.21×10 ⁴±30981）cells/g叶片,下降幅度极为明显,其成熟叶片内的淀粉含量在90 d内一直呈下降趋势;重度发病植株注射后叶片内的Las含量从（4.10×10 ⁸±3.04×10 ⁸）cells/g叶片减至（2.80×10 ⁷±2.70×10 ⁷）cells/g叶片,但从数量而言仍然有较多分布,除了新长出的秋梢,其成熟叶及茎内的淀粉粒含量仍旧很大,说明0.1 g/株OTC不足以治愈4年生重度发病Valencia夏橙。对PP2基因表达分析结果显示,注射OTC后30 d,Valencia夏橙体内的PP2基因表达量大幅度下降,随后90 d内表达量稳定,与注射后30 d的表达量较为一致。 【结论】OTC可以用于HLB的防控,对初感染（Las含量为<9.00×10 ⁵cells/g叶片）及轻度发病（Las含量为9.00×10 ⁵—9.00×10 ⁷ cells/g叶片）的柑橘植株治疗作用较好,不适用于重度发病植株（Las含量为>9.00×10 ⁷ cells/g叶片）的治疗。注射OTC后,PP2基因表达量下降明显,说明OTC可有效减少韧皮部病菌等的胁迫压力。

Effects of Oxytetracycline Treatment on the Control of Sweet Orange Huanglongbing and the Expression of PP2 Gene

【Objective】The objective of this study is to evaluate the control efficacy of oxytetracycline (OTC) on sweet orange Huanglongbing (HLB), detect the expression of phloem protein 2 (PP2) gene, and to provide a scientific basis for the effective prevention and control of HLB as well as a reference for OTC mechanism research. 【Method】OTC, which can effectively inhibit and kill pathogenic bacteria, was injected with 0.1 g/tree of 4-year-old Valencia sweet orange with different severity of HLB (newly infected, mild infected and severely infected plants), and then the contents of Cadidatus Liberibacter asiaticus (Las), starch and PP2 gene of leaves were evaluated at 7, 30, 60, 90 days post injection, respectively. Furthermore, 90 days after injection, the young and mature tissues of the plants were observed by using starch staining light microscopy (LM). The extraction of tender leaves was investigated 80 days after OTC injection. 【Result】Trunk injection of OTC 0.1 g/tree had the most obvious therapeutic effect on newly infected Valencia sweet orange. Seven days after injection, Las could not be detected and it maintained for 90 days. The starch content in mature leaves decreased obviously, but starch grains were still enriched in stems. The Las content of mild infected Valencia leaves decreased from (1.68×10 ⁶±858884) cells/g leaf to (7.21×10 ⁴±30981) cells/g leaf, the descending range was obvious, and the starch content in mature leaves showed a decreasing trend within 90 days. The Las content of severely infected Valencia leaves decreased from (4.10×10 ⁸±3.04×10 ⁸) cells/g leaf to (2.80×10 ⁷±2.70×10 ⁷) cells/g leaf, but the starch content in mature tissues of severely infected plants was still high, except the newly autumn shoots. The results indicated that 0.1 g/tree OTC was not enough to cure 4-year-old severely infected Valencia. The expression of PP2 gene in Valencia significantly decreased 30 days after OTC injection, and remained stable in the subsequent 90 days, which was consistent with the expression level at 30 days after injection.【Conclusion】OTC can be used in the prevention and control of HLB, and has better therapeutic effect on newly infected Valencia (Las content is less than 9.00×10 ⁵ cells/g leaf) and mild infected Valencia (Las content is 9.00×10 ⁵-9.00×10 ⁷ cells/g leaf). It was not suitable for the treatment of severely infected plants (Las content is more than 9.00×10 ⁷ cells/g leaf). After injection of OTC, the expression of PP2 gene decreased obviously, suggesting that OTC can effectively reduce the stress of phloem pathogens.