Abstract
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.
Highlights
Citrus greening disease, known as Huanglongbing (HLB), is currently considered to be the most destructive disease of citrus, causing massive production losses in many countries [1]
No oxytetracycline was detected in control tissues, indicating that matrix doesmatrix not interfere with the ELISA
Our current results show that the xylem was the main path of translocation for oxytetracycline Our current results show that the xylem was the main path of translocation for oxytetracycline in citrus plants
Summary
Known as Huanglongbing (HLB), is currently considered to be the most destructive disease of citrus, causing massive production losses in many countries [1]. There is no known cure for HLB, and control of this disease mainly relies on the control of D. citri using a wide range of insecticides. Antibiotics 2020, 9, 691 use of these practices, HLB disease is still considered as the major threat to the citrus industry in several countries. Antibiotics have been marketed and used for controlling a wide range of plant bacterial pathogens for more than 60 years. Oxytetracycline has been approved for the control of the bacterial spot disease in peaches, caused by Xanthomonas arboricola, and the fire blight in peach and nectarine, caused by Erwinia amylovora [4]. Oxytetracycline has been used to control Pseudomonas spp. and Xanthomonas spp. in vegetables, as well as phytoplasmas that cause lethal yellow diseases in coconut palm trees and elm [4]
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