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Molecular virology and bacteriology marriage - a case of rape phyllody disease

Studies in plant pathology often require interdisciplinary research in molecular aspects of unrelated microbes. Oilseed rape (Brassica napus ssp. oleifera (DC.) Metzg.) is important industrial plant and rape phyllody disease, previously ascribed to specific class of bacteria called phytoplasmas, can have devastating effects on its yield. In this case study, plants showing typical rape phyllody symptoms(green and deformed flowers, small and deformed siliques, seed reduction) along with atypical stem necrosis, leaf chlorotic spots and necrotic changes were investigated for the presence of phytoplasmas and viruses. Expectedly, a phytoplasma was identified by PCR-RFLP and sequencing of 16S rRNA gene. Multi-gene sequence characterization of phytoplasma house-keeping tufB, secY, groEL and ribosomal protein genes and the highly variable and specific amp gene was performed. Phylogenetic analyses of all phytoplasma genes confirmed the affiliation of this strain to the ‘Candidatus Phytoplasma asteris’ (aster yellows, AY) species with the closest relatedness to the 16SrI-B subgroup strains.As opposed to the analyzed house-keeping genes, the amp gene encoding an immunodominant membrane protein, showed a significant variability suggesting possible influence on insect vector specificity and transmissibility. Biological and serological tests revealed the co-infection with Turnip mosaic virus (TuMV). The full genome TuMV sequence obtained for the Croatian isolate CRO184A and its phylogenetic analysis classified it into the world-B phylogenetic lineage, whilst no evidence for inter-lineage recombination was obtained. This way, the first detailed molecular characterization of ‘Ca. P. asteris’ associated with the rape phyllody disease was obtained. Also, the first finding of a ‘Ca. P. asteris’ and TuMV co-infection in oilseed rape, or of any phytoplasma virus simultaneously infecting a brassicaceous host, was presented (Šeruga Musić et al. 2014: J. Appl. Microbiol. 117: 774-785.). In view of the worldwide distribution and pathogenicity of the AY phytoplasma and TuMV as individual plant pathogens, their co-occurrence should be considered in future rape phyllody cases and may have consequences in the disease management, especially if this phytoplasma-virus complex role is demonstrated in the rape phyllody pathophysiology.

Oilseed rape; Turnip mosaic virus; phytoplasmas; molecular analysis

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