Mycoplasmas, Rickettsiae, and Chlamydiae: Possible Relation to Yellows Diseases and Other Disorders of Plants and Insects

Abstract

In 1967, based on their discovery of mycoplasma- like bodies in si eve ele­ ments of yellows-infected plants and the therapeutic effect of tetracycline antib iot ics, Doi, Ish iie, and coworkers (4 2, 95) pro pose d that the causal agents of yel lows disease might be mycoplasma- or chlam ydia-like organ­ isms. Since that time, considerable rese arch has confirmed the morph ologi­ cal si milari ties between the presumed yellows agents and the Mollicutes (my ­ copla sma) 2 and has greatly stren gthened the hypothesis of mycoplasmal etiology (14, 18, 137, 225). Yet, the intracellular multiplication of the myco­ plasma -li organisms in yello ws-infected hosts contrasts with the usual ex­ trac ellular multiplication of vertebrate mycoplasmas. Moreover , in spite of numerous attem pts, yellows agents have not convincingly been demonstrated to grow in cell-free media. Thes e aspe cts suggest that it may be useful, in st udies on yel lows diseases, to consider certain features of obligate intra­ cellular parasi tism. Thus, although we emphasize the recen t literature on yellows agents and discuss the mycoplasmas in some detail , in this review we also briefly re view othe r pro car yotes, suc h as chlamydiae and rickettsiae, not known to be patho genic in plan ts and therefore generally unfam iliar to plan t pathologists. We know of no fundamen tal reason, however, why such organisms could not reside in plan t tiss ues and even ind uce disease. Indeed, our attention is drawn to this possibilit y by the occurrence of ri cket tsi a- like organisms in phy tophagous insects (13 0), by the possible resemblance of certain Ricket tsi ales to mycoplasma-li ke organisms in some yello ws-in­ fe cted hosts, and by re cent suggestions of rickettsia-like organisms in dodder (69) and in sali va of homo pterous insects ( 128). 1 The following abbreviations will be used: A Y (aster yellows); CS (corn stunt) ; WX (Western X-disease). • The vernacular terms mycoplasma and mycoplasmas properly signify only members of the genus Mycoplasma; however, in this review th ey will be used in a broa der (al though improper ) sense to si gnify any members of the Mollicutes until a suitable ver nacular name for this class of organisms comes into use.