At least 40 plant diseases are now suspected to be caused by myco
plasmas (11) and no doubt more will be added to this fast-growing list of diseases that were previously assumed to be viral in nature. It should be realized that proof of mycoplasma etiology for such diseases can be obtained only by culturing the pathogen in vitro in order to satisfy Koch's postulates. Such evidence, so far, has been reported for three different diseases (122-124), but the results have not yet been confirmed. It has been assumed, however, that other similar diseases are also caused by mycoplasmas. As yet, specific names have not been as
signed to these plant-pathogenic organisms and therefore, for conveni
ence, the various diseases they cause will be referred to by their common name followed by the term "mycoplasma." Most of these diseases are transmitted by leafhoppers but other insect vectors have also been re
ported. None have been transmitted by mechanical inoculation. Like viruses, mycoplasmas have been shown to multiply in their leafhopper vectors and to undergo an incubation period before they can be trans
mitted to plants. Evidence that these diseases may be caused by myco
plasmas, and not by viruses, came from the work of Doi et al. (10), who demonstrated the presence of such organisms in infected plant tis
sues. Later, mycoplasma cells were also visualized in tissues of leafhopper vectors (125> 126). Symptoms of such diseases have been suppressed in plants and adversely affected in vector tissues by antibiotics of the tetracycline group, such as oxytetracycline (V) and chlortetracycline
(VI), that are known to be effective against diseases caused by myco
plasmas in mammals and avian species.
ο
CONH2 OH
ο
O Hο
OHο
OHOxytetracycline (V)
Chlortetracycline (VI)
Tetracycline treatment has become a standard procedure for indications of mycoplasmal etiology, and susceptibility of more than 20 different
plant diseases to such antibiotics have been reported. The mechanism by which these antibiotics interfere with multiplication of mycoplasmas is not yet known. For details covering various aspects of mycoplasma diseases in plants, readers are referred to a recent review {127).
Ishii et al. (128) were first to study the effects of tetracycline on a plant disease suspected to be caused by a mycoplasma. They observed that symptoms of mulberry dwarf disease were remarkably suppressed by application of tetracycline and chlortetracyline. Kanamycin was found to be ineffective against the disease. It was suggested that the antibiotics inhibited the multiplication of a mycoplasma responsible for the disease. Later, Davis and Whitcomb (129) reported that application of four tetracyclines (chlortetracycline, tetracycline, oxytetracycline, and methacycline) at 50 or 100 ppm to plants infected with aster yellows mycoplasma produced remission of symptoms. Four methods of applying the antibiotics were root immersion, foliar sprays, vacuum infiltration, and hydroponic culture, and all were found to be effective. Remission of symptoms in infected plants treated with chlortetracycline was found to be correlated with inhibition of multiplication of aster yellows myco-plasma, as determined by infectivity bioassays. When healthy plants were first treated with the antibiotics and then inoculated through in-fective leafhoppers, the appearance of symptoms was either delayed or sometimes prevented. Chloramphenicol was found to be slightly effec-tive, but streptomycin, oleandomycin, kanamycin, tylosin, carbomycin, polymyxin, bacitracin, neomycin, sulfanilamide, penicillin, vancomycin, or cycloserine had no discernible effect on development of the disease.
When plants infected with two strains of aster yellows mycoplasma were dipped for 45 seconds in solutions containing 100 ppm of tetra-cycline or chlortetratetra-cycline every third day for 6 weeks, remission of symptoms occurred in some plants (130). Also, fewer leafhopper vectors were able to acquire the mycoplasma when fed on antibiotic-treated plants than when fed on untreated diseased plants. Tylosin tartrate, an antibiotic with a macrolide structure, had little effect on symptom development, although this compound is extremely effective against cer-tain animal mycoplasmas.
Sinha and Peterson (131) studied the effects of oxytetracycline on clover phyllody mycoplasma in aster plants. Immersing the roots of infected plants in solutions containing 100 ppm of the antibiotic for 24 hours (root treatment) resulted in remission of symptoms in most plants. Fewer leafhoppers were able to acquire and transmit the myco-plasma when fed on the antibiotic-treated plants than when fed on in-fected but untreated plants, indicating that the relative concentration
of the pathogen in infected plants was reduced by the antibiotic. Healthy aster plants did not become infected when subjected to root treatment either immediately before or soon after inoculation by infective leaf-hoppers. As the interval between inoculation and antibiotic treatment increased, the number of plants that became infected also increased.
Oxytetracycline was shown, by microbiological assays, to be absorbed from solution by roots of aster plants and translocated to stems, petioles, and leaves. The antibiotic persisted in plants for more than 2 weeks after 1 day of root treatment, but its concentration gradually declined.
Extending the treatment from 1 to 4 days increased the relative concen-tration of the antibiotic in the plants but not its length of persistence.
Susceptibility of healthy plants to mycoplasma infection was dependent on the concentration of the antibiotic in them at the time of inoculation.
Sinha and Peterson {131) also studied the effects of oxytetracycline on the mycoplasma cells found in infected plants. Aster plants showing well-developed symptoms of the disease were root-treated with the anti-biotic for 7 days. Similar plants left in a phosphate buffer served as controls. Examinations of ultrathin sections of infected tissues revealed that, although numerous mycoplasma cells were present in control plants, they occurred very infrequently in the antibiotic-treated plants and, of those present, many were incomplete or broken. Such disrupted myco-plasma cells were not observed in the controls.
The effects of tetracyclines on aster yellows mycoplasma in leafhopper vectors have also been studied in several ways. Addition of the anti-biotics to inocula containing the mycoplasma prevented transmission by the vectors {132) Injection of antibiotics into the bodies of vector insects carrying the mycoplasma reduced their transmitting ability con-siderably. Antibiotics acquired by leafhoppers after feeding through membranes early in the incubation period also delayed or blocked their ability to transmit {130). Similar results have been reported for western X mycoplasma in leafhopper vectors {133). Leafhoppers that were caged on aster plants whose roots were immersed in oxytetracycline solution (100 ppm) accumulated active antibiotic in their bodies, the concentra-tion being dependent on length of feeding period {131). Clover phyllody mycoplasma in leafhoppers was inactivated, as determined subsequently by their transmitting ability, if they were caged on aster plants main-tained in oxytetracycline solution. The degree of inactivation was de-pendent on the length of time the leafhoppers were allowed to ingest the antibiotic through plants. Healthy leafhoppers that ingested the anti-biotic lived much longer than untreated ones. Examination of ultrathin sections of tissues of infective vectors that ingested the antibiotic showed
that mycoplasma cells were devoid of their internal structure (ribosome-like granules and DNA-(ribosome-like fibrils).
A C K N O W L E D G M E NT
I am thankful to Dr. E. A. Peterson for many helpful suggestions in preparing this review. This article is Contribution 721, Chemistry and Biology Research Institute, Research Branch, Canada Agriculture, Ontario, K1A OC6.
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