Antibiotics have been successfully used to control plant diseases for more than fifty years. Recently, oxytetracycline and streptomycin have been approved for the treatment of Huanglongbing, which is threatening the citrus industry in many regions. Because the efficiency of antibiotics in planta is highly affected by their movement and distribution, understanding the mechanism of antibiotics’ uptake and distribution could lead to a better control of plant pathogens. Herein, we investigated the movement of oxytetracycline within citrus plants. Oxytetracycline was applied by root drenching to both girdled and non-girdled citrus seedlings. In addition, oxytetracycline was applied by trunk injection to girdled and non-girdled citrus trees. After the exposure time (24 h), citrus seedlings were dissected and the levels of oxytetracycline in the different tissues were measured using an oxytetracycline ELISA kit. Upon root application (laboratory experiment), oxytetracycline was detected in the inner part of the stem (xylem-associated tissue), cortex (phloem-associated tissue), and leaves above and below the girdled area. Likewise, oxytetracycline was also detected in leaves of trunk-injected field trees (girdled and non-girdled) three days post treatment. Interestingly, cortex girdling did not affect the distribution and translocation of oxytetracycline, indicating that the xylem is the main path for oxytetracycline translocation. Taken together, our results indicate that oxytetracycline translocation mainly occurs via xylem vessels, and that movement into the phloem occurs subsequent to xylem translocation. Our findings also clearly demonstrated that upon trunk injection, only trace levels of oxytetracycline reached the roots, minimizing its therapeutic value there. Thus, our recommendation is to time tree injections to coincide with the flushing periods when the bacteria are moving into new shoots to maximize the efficiency of oxytetracycline.

中文翻译：

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木质部在柑橘树内土霉素移位中的作用。

五十多年来，抗生素已成功用于控制植物疾病。最近，土霉素和链霉素已被批准用于治疗黄龙病，这正在威胁许多地区的柑橘产业。由于植物中抗生素的效率受其移动和分布的高度影响，因此了解抗生素的吸收和分布机理可以更好地控制植物病原体。在这里，我们调查了土霉素在柑橘类植物中的运动。通过根浸将土霉素施用于环剥和未环剥的柑橘幼苗上。另外，土霉素通过树干注射施用于环剥和未环剥的柑橘树上。暴露时间（24小时）后，切开柑橘幼苗，并使用土霉素ELISA试剂盒测量不同组织中土霉素的水平。根部施用（实验室实验）后，在茎的内部（木质部相关组织），皮质（韧皮部相关的组织）以及环绕区域上方和下方的叶片内部检测到土霉素。同样，在处理后三天，在树干注入的田间树木的叶子（环带和非环带）中也检测到土霉素。有趣的是，皮质环剥不影响土霉素的分布和易位，表明木质部是土霉素的主要易位途径。两者合计，我们的结果表明，土霉素主要通过木质部血管发生移位，并且移入韧皮部发生在木质部移位之后。我们的发现还清楚地表明，躯干注射后，只有微量的土霉素可以到达根部，从而使根部的治疗价值最小。因此，我们的建议是在细菌移入新芽的时间冲洗树，以与冲洗期相吻合，以使土霉素的效率最大化。