Variants of Coconut cadang-cadang viroid isolated from an African oil palm (Elaies guineensis Jacq.) in Malaysia

Towards Pathogen-Free Coconut Germplasm Exchange

Coconut (Cocos nucifera L.) is an important palm species that serves as the mainstay of several industries and contributes to the livelihoods of millions of smallholder farmers. International exchange of coconut germplasm has been undertaken for several decades to facilitate the conservation of selected varieties within global genebanks and for the distribution to farmers and scientists. In vitro systems are a convenient and an efficient method for the exchange of coconut germplasm. However, it is possible that these tissue culture systems can transfer lethal pathogens causing a threat to the importing countries. In this review, the following topics are discussed: the major disease-causing agents of concern, the various tissues that could be used for coconut germplasm exchange, and the techniques available for the detection and elimination of disease-causing agents from various transmission systems. Additionally, the lack of clear, science-backed guidelines to facilitate the exchange of in vitro coconut materials is raised, along with recommendations for future studies to ensure the safe movement of coconut germplasm without biosecurity risks.

Analysis of Coconut cadang-cadang viroid variants on field samples exhibiting variation in orange spotting symptom expression and severity

BACKGROUND

Sequence variation has been attributed to symptom variations but has not been investigated in Orange Spotting-Coconut cadang-cadang viroid (OS-CCCVd) infected palms. Likewise, the relationship between Coconut cadang-cadang viroid (CCCVd) variants, Orange Spotting (OS) severity and the accumulation of the viroid in the palms have not been elucidated. This paper describes the characterization of CCCVd variants by cloning and sequencing, followed by correlation with symptom expression.

METHODS AND RESULTS

Total nucleic acids were extracted from leaf samples harvested from frond 20 of seven Dura × Pisifera (D × P) African oil palm (Elaeis guineensis Jacq.) aged between 13 and 21 years old collected from local plantations. The nucleic acids were fractionated using 5% non-denaturing polyacrylamide gel electrophoresis (PAGE) before being subjected to detection by reverse transcribed polymerase chain reaction (RT-PCR). The PCR products were cloned into a plasmid vector and the sequence of the clones was analyzed. CCCVd variants were quantified using real-time qPCR assay with CCCVd specific primers. Sixteen randomly selected clones of (OP246) had an arbitrary 100% identity with CCCVdOP246 (GeneBank Accession No: HQ608513). Meanwhile, four clones had >93% similarity with several minor sequence variations forming variants of OP234, OP235, OP251 and OP279.

CONCLUSION

The OS symptoms observed in the field were characterized into three categories based on the size and morphology of the orange spots on the affected fronds. In addition, there was no direct correlation between disease severity and the accumulation of CCCVd variants in oil palm. This finding is the first report describing the sequence variation of the CCCVd RNA and symptom variation in OS oil palm field samples.

EFSA Panel on Plant Health (PLH), Bragard C, Baptista P, Chatzivassiliou E, Gonthier P, Miret JAJ, Justesen AF, MacLeod A, Magnusson CS, Milonas P, Navas‐Cortes JA, Parnell S, Potting R, Stefani E, Thulke H, Van der Werf W, Vicent Civera A, Yuen J, Zappalà L, Carluccio AV, Chiumenti M, Di Serio F, Rubino L, Maiorano A, Pautasso M, Reignault PL. Pest categorisation of coconut cadang-cadang viroid

The EFSA Panel on Plant Health updated its 2017 pest categorisation of coconut cadang cadang viroid (CCCVd) for the EU territory due to new data on its host range. The identity of CCCVd, a member of the genus Cocadviroid (family Pospiviroidae), is established and detection and identification methods are available. It is included as a quarantine pest for the EU in the Commission Implementing Regulation (EU) 2019/2072. CCCVd has been reported from the Philippines and Malaysia. It is not known to be present in the EU. The host range of CCCVd is restricted to Arecaceae species (palms), in particular, coconut palm (Cocos nucifera) to which it causes a lethal disease. Oil palm (Elaeis guineensis) and buri palm (Corypha utan) are other natural hosts of CCCVd. Palm species of several genera, including Phoenix spp. and other species grown and/or cultivated in the EU, have been identified as potential hosts. The viroid is naturally transmitted at low rate by seeds and pollen and possibly by additional not yet identified natural transmission means. It can be transmitted through vegetative propagation applied to some palm species. Plants for planting including seeds of its hosts have been identified as the main entry pathway of CCCVd. Potential hosts of CCCVd are present in the EU, therefore establishment is possible. Should the pest establish in the EU, an impact is expected, with uncertainty on its magnitude. The Panel identified the susceptibility of palm species grown in the EU as a key uncertainty potentially affecting the conclusion of this pest categorisation. Nevertheless, the pest satisfies the criteria that are within the remit of EFSA to assess for this viroid to be regarded as potential Union quarantine pest.

Conserved Motifs and Domains in Members of Pospiviroidae

In 1985, Keese and Symons proposed a hypothesis on the sequence and secondary structure of viroids from the family Pospiviroidae: their secondary structure can be subdivided into five structural and functional domains and “viroids have evolved by rearrangement of domains between different viroids infecting the same cell and subsequent mutations within each domain”; this article is one of the most cited in the field of viroids. Employing the pairwise alignment method used by Keese and Symons and in addition to more recent methods, we tried to reproduce the original results and extent them to further members of Pospiviroidae which were unknown in 1985. Indeed, individual members of Pospiviroidae consist of a patchwork of sequence fragments from the family but the lengths of fragments do not point to consistent points of rearrangement, which is in conflict with the original hypothesis of fixed domain borders.

Jeger M, Bragard C, Caffier D, Candresse T, Dehnen-Schmutz K, Gilioli G, Gregoire JC, Jaques Miret JA, MacLeod A, Navajas Navarro M, Niere B, Parnell S, Potting R, Rafoss T, Rossi V, Urek G, Van Bruggen A, der Werf WV, West J, Chatzivassiliou E, Winter S, Hollo G, Candresse T. Pest categorisation of Cadang-Cadang viroid

The EFSA Panel on Plant Health performed a pest categorisation of Cadang-Cadang viroid for the European Union (EU) territory. Coconut cadang-cadang viroid (CCCVd) is a well-known viroid for which efficient molecular detection assays are available. It is transmitted by vegetative multiplication of infected hosts, by seed and pollen and, possibly, by the action of unknown vector(s). CCCVd is reported from a few countries in Asia and is not known to occur in the EU. It therefore does not meet one of the criteria for being a Union regulated non-quarantine pest. The host range of CCCVd is restricted to Arecaceae species (palms), in particular coconut and it is listed on all known hosts in Annex IIAI of Directive 2000/29/EC. CCCVd is expected to be able to enter in the EU and to be able to establish in the open in the southernmost regions of the EU and elsewhere under protected cultivation. It has the potential to subsequently spread via plants for planting and possibly other mechanisms. CCCVd is able to cause severe symptoms in some Arecaceae species while others seem less affected. The potential impact of CCCVd if introduced in the EU is very difficult to assess. Given that the spread potential is, as for other viroids, likely to be limited, the potential impact is estimated to be limited in extent but this judgement is affected by large uncertainties. Overall, CCCVd meets all the criteria evaluated by EFSA to qualify as Union quarantine pest. The main knowledge gaps concern (1) the relationships between CCCVd-related RNAs and CCCVd, (2) the origin and volume of the trade in palm seeds and plants for planting imported in the EU (3) the efficiency of natural spread under EU conditions and (4) host status and susceptibility of many palm species grown in the EU.

Detection of Coconut cadang-cadang viroid (CCCVd) in oil palm by reverse transcription loop-mediated isothermal amplification (RT-LAMP)

A reverse transcription loop-mediated isothermal amplification (RT-LAMP) detected Coconut cadang-cadang viroid (CCCVd) within 60 min at 60 °C in total nucleic acid extracted from oil palm leaves infected with CCCVd. Positive reactions showed colour change from orange to green in the reaction mix after the addition of fluorescent reagent, and a laddering pattern band on 2% agarose gel electrophoresis. Conventional RT-PCR with LAMP primers produced amplicons with a sequence identical to the 297-nt CCCVd oil palm variant with the primers being specific for CCCVd and not for other viroids such as PSTVd and CEVd. RT-LAMP was found to be rapid and specific for detecting oil palm CCCVd.