Detection of avocado sunblotch viroid (ASBV) by dot-spot self-hybridization with a [(32)P]-labelled ASBV-RNA

Pest categorisation of the avocado sunblotch viroid

The EFSA Panel on Plant Health conducted a pest categorisation of the avocado sunblotch viroid (ASBVd) for the EU. The identity of ASBVd, a member of the genus Avsunviroid (family Avsunviroidae) is clearly defined and detection and identification methods are available. The pathogen is not included in the EU Commission Implementing Regulation 2019/2072. ASBVd has been reported in Australia, Ghana, Guatemala, Israel, Mexico, Peru, South Africa, USA (California, Florida) and Venezuela. In the EU, it has been reported in Greece (Crete Island) and Spain. The pathogen could establish in the EU wherever avocado (Persea americana) is grown. The only known natural host of ASBVd is avocado to which it causes the severe 'avocado sunblotch' disease, characterised by white, yellow, red or necrotic depressed areas or scars on the fruit surface, bleached veins and petioles of the leaf, and rectangular cracking patterns in the bark of the old branches. Fruit yield and quality are severely diminished. ASBVd infects under experimental conditions a few more species in the family Lauraceae. The viroid is naturally transmitted at an extremely high rate by seeds (up to 100% in asymptomatically infected trees), but with a low efficiency by pollen (only to the produced seeds), and possibly through root grafts. Plants for planting, including seeds, and fresh avocado fruits were identified as the most relevant pathways for further entry of ASBVd into the EU. Avocado crops are cultivated in southern EU countries. Should the pest further enter and establish in the EU, impact on the production of avocado is expected. Phytosanitary measures are available to prevent entry and spread of the viroid in the EU. ASBVd fulfils the criteria that are within the remit of EFSA to assess for it to be regarded as a potential Union quarantine pest.

Diversity of citrus tristeza virus strains indicated by hybridization with cloned cDNA sequences

Cloned cDNA sequences 500-2000 base pairs long, derived from a severe citrus tristeza virus (CTV) isolate, were used to study sequence homology with RNA of nine other serologically indistinguishable CTV isolates which differed in their biological properties. Six of the nine CTV isolates hybridized positively with the tested cDNA clones, while three others hybridized differentially with these cDNA clones. The potential use of cloned viral sequences for typing of virus strains is discussed.

Avsunviroidae family: viroids containing hammerhead ribozymes

This chapter focuses on the second viroid family, whose members are also referred to as hammerhead viroids, taking into account their most outstanding feature. If the word “small” is the first to come to mind when considering viroids, perhaps the second word is “hammerhead,” because this class of ribozymes, which because of its structural simplicity has an enormous biotechnological potential, is described in avocado sunblotch viroid (ASBVd) as well as in a viroid-like satellite RNA. The most outstanding feature of the Avsunviroidae members is their potential to adopt hammerhead structures in both polarity strands and to self-cleave in vitro accordingly. Viroids differ from viruses not only in their genome size but also in other fundamental aspects, prominent among which is the lack of messenger activity of both viroid RNAs and their complementary strands.

Enhanced resolution of circular and linear molecular forms of viroid and viroid-like RNA by electrophoresis in a discontinuous-pH system

A discontinuous-pH polyacrylamide gel electrophoresis system is described. An increase in the pH differential between the gel and the running buffer enhances the separation of low molecular weight circular and linear RNA molecules. Highly purified preparations of the circular form of viroids can be obtained with this procedure. Since all the linear RNAs of similar molecular weight migrate with the front, a relatively clean background can be obtained even when crude extracts are used. This facilitates an improved separation and identification of similarly sized viroid-like RNAs. The conditions of electrophoresis in low salt and 8 M urea also permit the effective transfer of RNA molecules directly to nylon-based membranes without any additional denaturation treatment.

Dot-blot procedure with [32P]DNA probes for the sensitive detection of avocado sunblotch and other viroids in plants

Avocado sunblotch viroid (ASBV) has been detected down to a level of about 20 pg per gram fresh weight of leaves by the use of a dot-blot hybridization procedure and partially purified nucleic acid extracts. Three [32P]DNA probes were compared, two prepared from full-length ASBV clones in the single-strand M13mp93 vector and the other by primer extension on purified ASBV. All three probes gave the same sensitivity of detection of ASBV. The methods developed have also been used successfully for the routine detection of potato spindle tuber viroid, citrus exocortis viroid, and coconut cadang cadang viroid.

Detection of avocado sunblotch viroid by hybridization with synthetic oligonucleotide probes

Two short (20 and 17 nucleotides) DNA hybridization probes, complementary to avocado sunblotch viroid (ASBV) RNA nucleotides 68-87 and 88-104 respectively (Symons, R.H., Nucleic Acid Res. 9, 6527, 1981) were synthesized. The sensitivity and specificity of these radioactively labelled probes for hybridization with RNA of several ASBV isolates are demonstrated.