Summary auto-generated
This study establishes that the phytoplasma causing rice yellow dwarf disease in Asia represents a novel, distinct species-level taxon. Researchers analyzed 16S rDNA sequences from rice yellow dwarf phytoplasma isolates collected in Japan and Thailand, comparing them with related phytoplasmas infecting other gramineous plants. Two RYD isolates were nearly identical (99.2% similarity) but showed significantly lower similarity (96.3–97.9%) to other phytoplasmas in the RYD group, including those causing sugarcane white leaf and bermuda grass white leaf diseases. The RYD phytoplasma possesses unique signature sequences in its 16S rDNA not found in any other phytoplasmas. Supporting its status as a discrete taxon are its unique characteristics: exclusive transmission by Nephotettix leafhopper species, restriction to rice as its natural host, geographic limitation to Asia, and Southern blot evidence showing DNA sequences not shared with closely related phytoplasmas. Based on these molecular, biological, and ecological criteria, the researchers propose designating this pathogen as 'Candidatus Phytoplasma oryzae', a novel species within the RYD 16S-group of phytoplasmas.
Key findings
- RYD phytoplasma isolates from Japan and Thailand show 99.2% 16S rDNA sequence identity but differ by 96.3–97.9% from other RYD group phytoplasmas, meeting species-level distinction criteria.
- RYD phytoplasma possesses unique signature sequences in its 16S rDNA (positions 604–610, 999–1002, and others) not found in any other phytoplasma species.
- The pathogen meets all three criteria for species designation: transmitted exclusively by Nephotettix spp. leafhoppers, naturally infects only rice (Oryza sativa), and shows molecular diversity distinct from related phytoplasmas.
- Geographic isolation in Asia combined with vector and host specificity likely contributed to the evolutionary divergence of this phytoplasma into a separate taxon.
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Abstract
In addition to rice yellow dwarf (RYD) phytoplasma, several phytoplasmas infect gramineous plants, including rice orange leaf, bermuda grass white leaf, brachiaria grass white leaf and sugarcane white leaf phytoplasmas. To investigate whether the RYD phytoplasma is a discrete, species-level taxon, several isolates of the aforementioned phytoplasmas were analysed using PCR-amplified 16S rDNA sequences. Two RYD isolates, RYD-JT and RYD-Th, were almost identical (99·2 %), but were distinct (similarities of 96·397·9 %) from other phytoplasma isolates of the RYD 16S-group. The notion that the RYD phytoplasma constitutes a unique taxon is also supported by its unique insect vector (Nephotettix sp.), its unique host plant in nature (rice) and its limited geographical distribution (Asia). In Southern blot analysis, chromosomal and extrachromosomal DNA probes of the RYD phytoplasma reportedly did not hybridize with those of closely related phytoplasmas. These properties of the RYD phytoplasma clearly indicate that it represents a novel taxon, Candidatus Phytoplasma oryzae.