Construction of a multiprobe for the simultaneous detection of viroids infecting citrus trees

Biological and molecular characterization of citrus bent leaf viroid

Background and aim

Citrus bent leaf viroid (CBLVd) is one of the emerging and widely distributed viroids in citrus-growing areas of the world, including Pakistan. Previously, CBLVd has been reported in Pakistan for the first time in 2009. Therefore, characterization of CBLVd is required to monitor the viroid status in the citrus orchards concerning citrus decline.

Methods

Biological and molecular characterization of CBLVd was studied through biological indexing and confirmation through RT-PCR, followed by phylogenetic analysis of selected CBLVd isolates. Among four citrus cultivars viz., Kinnow (Citrus nobilis × Citrus deliciosa), Mosambi (C. sinensis), Futrell's Early (C. reticulata) and Lemon (C. medica) used as indicator plants for two transmission trials viz., graft inoculation and mechanical inoculation. Graft inoculation was more efficient than mechanical inoculation.

Results

Symptoms such as mild mosaic, slight backward leaf bending, and leaf curling were observed after eight months' post-inoculation. Citrus nobilis × Citrus deliciosa, C. reticulata and C. sinensis were more sensitive to CBLVd as compared to C. medica. Inoculated plants were reconfirmed through RT-PCR amplicons of 233 bp. The phylogenetic tree of submitted sequences showed more than 90% relevance of CBLVd in Pakistan compared to the rest of the world.

Conclusions

There was slight genetic variability, but more than 90% relevance was found among the submitted and already reported CBLVd isolate from Pakistan. Scanty literature is available regarding the biological and molecular studies of CBLVd in Pakistan. Therefore, the transmission and molecular characterization of CBLVd in Pakistan were studied for the first time.

Molecular and biological characterization of citrus bent leaf viroid from Malaysia

BACKGROUND

A 328-nucleotide variant of citrus bent leaf viroid (CBLVd) was characterized by citrus varieties in Malaysia. After the first report in Malaysia, the emerging CBLVd was detected in five citrus species, namely Citrofortunella microcarpa, Citrus aurantifolia, Citrus hystrix, Citrus maxima, and Citrus sinensis.

METHODS AND RESULTS

CBLVd was detected in 23 out of 133 symptomatic samples through RT-PCR. Sequence analysis of the RT-PCR amplicons from this study showed 99-100% sequence identity to the reference CBLVd Jp isolate and CBLVd isolates reported in Malaysia. Inoculation of sap, obtained from a CBLVd positive sample, into 6-month old healthy C. microcarpa seedlings showed symptoms of slight leaf bending, reduced leaf size of matured leaves, and mild mosaic between 4 to 6 months after inoculation. Moreover, the observed symptoms of chlorosis, midvein necrosis, leaf rolling, and smalling of leaves in calamondin, C. microcarpa (Bunge) Wijnands, were not reported in earlier studies and opened a new avenue for the study of symptomology. The mechanical transmissibility of CBLVd in the inoculated seedlings was reconfirmed by RT-PCR assay and sequencing.

CONCLUSIONS

Based on the results, the sequence similarity of CBLVd isolates from different areas of Malaysia showed no significant difference among each other and the reference isolate. The CBLVd is mechanically transmissible and could produce variable symptoms in different hosts.

Recent Advances on the Multiplex Molecular Detection of Plant Viruses and Viroids

Plant viruses are still one of the main contributors to economic losses in agriculture. It has been estimated that plant viruses can cause as much as 50 billion euros loss worldwide, per year. This situation may be worsened by recent climate change events and the associated changes in disease epidemiology. Reliable and early detection methods are still one of the main and most effective actions to develop control strategies for plant viral diseases. During the last years, considerable progress has been made to develop tools with high specificity and low detection limits for use in the detection of these plant pathogens. Time and cost reductions have been some of the main objectives pursued during the last few years as these increase their feasibility for routine use. Among other strategies, these objectives can be achieved by the simultaneous detection and (or) identification of several viruses in a single assay. Nucleic acid-based detection techniques are especially suitable for this purpose. Polyvalent detection has allowed the detection of multiple plant viruses at the genus level. Multiplexing RT polymerase chain reaction (PCR) has been optimized for the simultaneous detection of more than 10 plant viruses/viroids. In this short review, we provide an update on the progress made during the last decade on techniques such as multiplex PCR, polyvalent PCR, non-isotopic molecular hybridization techniques, real-time PCR, and array technologies to allow simultaneous detection of multiple plant viruses. Also, the potential and benefits of the powerful new technique of deep sequencing/next-generation sequencing are described.

A universal oligonucleotide microarray with a minimal number of probes for the detection and identification of viroids at the genus level

A major challenge in the agricultural industry is the development of techniques that can screen plant samples for viroid infection. Microarrays are promising in this regard, as their high throughput nature can potentially allow for the detection of a range of viroids in a single test. In this paper we present a microarray that can detect a wide spectrum of all 8 reported viroid genera including 37 known plant viroid species. The array was constructed using an automated probe design protocol which generated a minimal number of probes to detect viroids at the genus level. The designed microarray showed a high specificity and sensitivity when tested with a set of standard virus samples. Finally, the microarray was applied to screen infected field samples, with Hop stunt viroid infection identified as the major disease causing pathogen for an infected citrus sample.