Summary auto-generated
This study examined three phytoplasma-associated papaya diseases in Australia using molecular analysis. DNA from papaya plants with dieback, yellow crinkle, and mosaic diseases was analyzed using PCR amplification and sequencing of the 16S rRNA gene and 16S–23S rRNA intergenic spacer region. Phylogenetic analysis revealed that papaya yellow crinkle and papaya mosaic phytoplasmas are identical to each other and most closely related to Australian tomato big bud phytoplasma (99.7% identity), clustering within subclade iii. In contrast, papaya dieback phytoplasma is phylogenetically distinct (90.3% identity to yellow crinkle), clustering with New Zealand Phormium yellow leaf and Australian grapevine yellows strains within subclade xii. Based on these phylogenetic relationships and signature sequences, the authors propose that papaya dieback, Phormium yellow leaf, and grapevine yellows be classified as 'Candidatus Phytoplasma australiense,' while papaya yellow crinkle, papaya mosaic, and tomato big bud be assigned to a new taxon, 'Candidatus Phytoplasma australasia.' This classification reflects the geographic origins of these strains in the Asian-Australasian region.
Key findings
- Papaya yellow crinkle and mosaic phytoplasmas are genetically identical and cluster most closely with Australian tomato big bud phytoplasma in subclade iii
- Papaya dieback phytoplasma is phylogenetically distinct from yellow crinkle and mosaic (90.3% 16S rDNA identity) and clusters with New Zealand and Australian grapevine yellows strains in subclade xii
- Two new phytoplasma taxa are proposed based on phylogenetic analysis: 'Candidatus Phytoplasma australiense' and 'Candidatus Phytoplasma australasia'
- Unique 16S rRNA signature sequences distinguish the papaya yellow crinkle/mosaic group from other phytoplasmas and support their classification as a separate taxon
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Abstract
DNA extracted from three papaya (Carica papaya L.) plants, individually affected by dieback, yellow crinkle or mosaic diseases, was subjected to PCR using phytoplasma-specific primers to amplify the 16S rRNA gene plus 16S–23S rRNA intergenic spacer region. Near-complete DNA sequences obtained for the three PCR amplimers were subjected to phylogenetic analyses and direct sequence comparison with other phytoplasma 16S rDNA and 16S-23S spacer region DNA sequences. The papaya yellow crinkle (PpYC) and papaya mosaic (PpM) sequences were identical to each other, but distinctly different from the papaya dieback (PpDB) sequence, showing 90.3% identity in the 16S rDNA and 87.8% identity in the 16S–23S spacer region DNA sequences. A phylogenetic tree based on 16S rDNA sequences was calculated, in which PpYC and PpM are most closely related to the tomato big bud phytoplasma (TBB; 99.7% 16S rDNA sequence identity) from Australia, within subclade iii. This subclade consists of strains only reported occurring in the Southern Asian region and Australia, which indicates an Asian/Australasian origin. PpDB is most closely related to the Phormium yellow leaf phytoplasma from New Zealand (PYL; 99.9% identity) and the Australian grapevine yellows phytoplasma (AGY; 99.7% identity). These three phytoplasma strains form a distinct clade within subclade xii, which also includes the European strains STOL and VK as another distinct clade. The origin of the closely related but geographically separated AGY-like strains and STOL-like strains of subclade xii is unclear. It is proposed that phytoplasma strains PpDB, PYL and AGY be included in the previously described taxon Candidatus Phytoplasma australiense, and that PpYC PpM and TBB be assigned to a new taxon, Candidatus Phytoplasma australasia.