Molecular Diagnosis of Phytoplasmas

Abstract

Phytoplasmas are wall-less prokaryotes associated with diseases in numerous plant species worldwide. In nature they are transmitted by phloem-sucking insects. Yellowing, decline, witches’ broom, leaf curl, floral virescence and phyllody are the most conspicuous symptoms associated with phytoplasmas, although infections are sometimes asymptomatic. Since phytoplasmas cannot be cultured in vitro, molecular techniques are needed for their diagnosis and characterization. The titer of phytoplasma cells in the phloem of infected plants may vary according to the season and the plant species, and it is often very low in woody hosts. Different DNA extraction procedures have therefore been tried out to obtain phytoplasma DNA at a concentration and purity high enough for effective diagnosis. DNA/DNA hybridization methods were reported in the nineties to be appropriate for the detection of phytoplasmas, but at present PCR is considered the most suitable. Universal and group-specific primers have been designed on the rRNA operon of the phytoplasma genome and on plasmid sequences. RFLP analysis of the obtained amplicons has classified these pathogens into major 16Sr RNA groups. Group-specific primers have also been designed on other genomic sequences. PCR is a very sensitive technique, but due to the low titre of phytoplasmas a further increase in sensitivity may be required for accurate diagnosis. This is routinely obtained with a second round of PCR (nested PCR). The drawback of nested PCR is that there is a greater chance of obtaining false positives due to contamination. Many authors have therefore developed protocols based on hybridization (PCR/dot blot) or serological approaches (PCR/ELISA) to increase the sensitivity and specificity of the direct PCR, reducing the risks due to nested PCR. Real time PCR protocols may also improve the sensitivity and specificity of the direct PCR assay.