Insect transmitted plant pathogenic mollicutes, Spiroplasma kunkelii and aster yellows witches' broom phytoplasma: From structural genomics to functional genomics

The mollicutes, Spiroplasma kunkelii and aster yellows witches' broom (AY-WB) phytoplasma, are insect-transmitted plant pathogens. These mollicutes invade and replicate in cells of various insect organs and tissues, and inhabit and replicate in plant phloem tissues. They cause severe symptoms to many plant species worldwide, including economically important crops and ornamental plants. Their fastidious nature and lack of genetic tools have hampered the research on these plant pathogenic mollicutes.; I employed various approaches, including genome sequencing, comparative genomics, functional genomics, and conventional molecular techniques, to study the biology and pathogenicity mechanisms of S. kunkelii and AY-WB phytoplasma. The partial genome of S. kunkelii and the complete genome of AY-WB phytoplasma were sequenced. Genome annotation revealed the presence of multiple spiroplasma phage DNA sequences in S. kunkelii and many repetitive elements in both genomes, suggestive of frequent recombination events. The genome sequence data provide genetic basis for the study of the biology and pathogenicity mechanisms of these organisms.; Whereas spiroplasmas and phytoplasmas are distantly related to each other, they share the plant and insect habitats. Therefore, they may share genes involved in insect transmission and plant pathogenicity that are missing from the animal and human pathogenic mycoplasmas. To test this hypothesis, comparative genome analysis among mollicutes was conducted, and resulted in the identification of four genes that are present in the genomes of all plant-pathogenic mollicutes sequenced so far, but missing from the mycoplasmas. Another gene within both genomes might have been derived by horizontal gene transfer between spiroplasmas and phytoplasmas.; The observation of spiroplasma surface appendages prompted the search of genes involved in fimbriae or pili formation. Four traE gene homologs were identified as membrane-bound ATPases in S. kunkelii M2 strain. Two homologs were localized in S. kunkelii chromosome and two in plasmids. The presence of these homologs varied among S. kunkelii strains of different geographical locations. The expression of the genes was detected in culture medium and during infection of insects and plants. Adjacent sequences of traE homologs suggest the involvement of TraE in spiroplasma conjugation and subsequent recombination, and adhesion. (Abstract shortened by UMI.)...