The in vivo Three-dimensional Form of a Plant Mycoplasma-like Organism by the Analysis of Serial Ultrathin Sections

Abstract

Mycoplasma-like organisms (MLO) associated with lethal yellowing disease of coconut palms exhibited a range of different morphologies even within the same sieve element. The morphologies were revealed by graphic reconstruction from ribbons of serial ultrathin sections containing 45 sections. Five different morphotypes were recognized amongst the 120 organisms reconstructed. MLO could be saccate, erythrocyte-like, cylindrical, monili-form or filiform. The different morphotypes showed differences in linear dimensions apart from their particular morphological characteristics. Saccate MLO were generally the smallest, the maximum length being 1 μm, whilst the filiform organisms could be more than 16 μm long. The morphologies of the MLO were analogous to those now recognized as usual for Mycoplasma and Acholeplasma species in vitro during the later stages of development. Comparison of the individual profile shapes used in the reconstructions with those of MLO from other diseased plants suggest that many previous interpretations of MLO morphology may have been too simple. With the insight provided by the understanding of the relationship between profile shape and three-dimensional form, the ultrastructure of individual profiles is shown to be an important indicator of form.