In recent years, the potato crop in Mexico has been notably affected by diseases recognized as potato purple top (PPT) in foliage and potato hair sprouts (PHS) in germinating tubers. In both cases, these syndromes reduce production by affecting viability of the tubers used as seeds. There is evidence indicating that phytoplasmas are associated with these syndromes. This study presents data on the molecular detection, characterization, and ecology of the pathogens related to PPT and PHS. Restriction fragment length polymorphism (RFLP) and sequence analysis indicated that PPT phytoplasma belongs to the 16SrI group and PHS phytoplasma fits in the 16SrII group. In this paper, we report that the two different phytoplasmas have been found coexisting in the same potato plant, which demonstrates the presence of mixed infection in the field. These phytoplasmas were also detected in weeds surrounding potato fields; therefore they should be considered as alternative hosts or natural reservoirs of PPT and PHS phytoplasmas.
The molecular characterization of isolates of citrus tristeza virus (CTV) from eight locations in Mexico was undertaken by analyzing five regions located at the opposite ends of the virus genome. Two regions have been previously used to study CTV variability (coat protein and p23), while the other three correspond to other genomic segments (p349-B, p349-C and p13). Our comparative nucleotide analyses included CTV sequences from different geographical origins already deposited in the GenBank databases. The largest nucleotide differences were located in two fragments located at the 5' end of the genome (p349-B and p349-C). Phylogenetic analyses on those five regions showed that the degree of nucleotide divergence among strains tended to correlate with their pathogenicity. Two main groups were defined: mild, with almost no noticeable effects on the indicator plants and severe, with drastic symptoms. Mild isolates clustered together in every analyzed ORF sharing a genetic distance below 0.022, in contrast with the severe isolates, which showed a more disperse distribution and a genetic distance of 0.276. Analyses of the p349-B and p349-C regions evidenced two lineages within the severe group: severe common subgroup (most of severe isolates) and severe divergent subgroup (T36-like isolates). This study represents the first attempt to analyze the genetic variability of CTV in Mexico by constructing phylogenetic trees based on new genomic regions that use group-specific nucleotide and amino acid sequences. These results may be useful to implement specific assays for strain discrimination. Moreover, it would be an excellent reference for the CTV situation in México to face the recent arrival of brown citrus aphid.
Mexico is considered to be one of the centers of origin of grain amaranth species. Recently, plants with abnormal anatomical features were observed in experimental fields established in Central Mexico. The most noticeable symptoms, which consisted of excessive stem and bud proliferation, mosaics and unusual coloration, suggested that they might be phytoplasma-induced disorders. Thus, different accessions of grain amaranth (Amaranthus hypochondriacus and A. cruentus) plants were subjected to polymerase chain reaction (PCR) analysis specifically designed to detect these pathogens. Two universal phytoplasma-specific primer pairs were tested in a nested PCR assay, with primer pair P1/tint (followed by primer pair R16F2/R16R2). Further DNA sequencing analysis of the resulting amplicons indicated that these phytoplasmas may be related to others already affecting important agricultural crops in Mexico, such as soybean. Data are presented that disclose the etiology of these syndromes by the use of molecular techniques. To the best of our knowledge, this finding constitutes the first report of a phytoplasma-related disease in grain amaranth.
The stability of integration and amplification of an integrational plasmid in Bacillus subtilis was analyzed. A cat-containing plasmid was constructed that could be integrated into the amy locus to facilitate measurement of excision events. Pulse-field gel electrophoresis was used to measure the copy number in strains that were resistant to different levels of chloramphenicol. The stability of the amplified unit in strains containing from 2 to 18 tandem copies of the amplicon in the presence and absence of chloramphenicol and through different generation times was then determined. Our results demonstrate that, for any given strain, the copy number of the amplicon remains stable. Furthermore, this stability is maintained when a clone containing an amplicon of defined size is cultured through as many as 100 generations in the absence of selective pressure.
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