Effect of a Field-Source Mixture of Citrus Viroids on the Performance of 'Nules' Clementine and 'Navelina' Sweet Orange Trees Grafted on Carrizo Citrange

Identification and primary distribution of Citrus viroid V in citrus in Punjab, Pakistan

BACKGROUND

Citrus plants are prone to infection by different viroids which deteriorate their vigor and production. Citrus viroid V (CVd-V) is among the six citrus viroids, belongs to genus Apscaviroid (family Pospiviroidae) which induces symptoms of mild necrotic lesions on branches and cracks on trunk portion.

METHODS AND RESULTS

A survey was conducted to evaluate the prevalence of CVd-V in core and non-core citrus cultivated areas of Punjab, Pakistan. A total of 154 samples from different citrus cultivars were tested for CVd-V infection by RT-PCR. The results revealed 66.66% disease incidence of CVd-V. Citrus cultivars Palestinia Sweet lime, Roy Ruby, Olinda Valencia, Kaghzi lime, and Dancy were identified as new citrus hosts of CVd-V for the first time from Pakistan. The viroid infection was confirmed by biological indexing on indicator host Etrog citron. The reported primers used for the detection of CVd-V did not amplify, rather showed non-specific amplification, which led to the designing of new primers. Whereas, new back-to-back designed primers (CVd-V AF1/CVd-V AR1) detected CVd-V successfully and obtained an expected amplified product of CVd-V with 294 bp. Sequencing analysis confirmed the new host of CVd-V showing 98-100% nucleotide sequence homology with those reported previously from other countries while 100% sequence homology to the isolates reported from Pakistan. Based on phylogenetic analysis using all CVd-V sequences in GenBank, two main CVd-V groups (I and II) were identified, and newly identified isolates during this study fall in the group I.

CONCLUSION

The study revealed that there are some changes in the nucleotide sequences of CVd-V which made difficult for their detection using reported primers. All isolates of Pakistan showed high sequence homology with other isolates of CVd-V from Iran and USA whereas; the isolates from China, Japan, Tunisia, and Africa are distantly related. It is evident that CVd-V is spreading in all citrus cultivars in Pakistan.

Viroids as Companions of a Professional Career

Since the early 1970s when “virus-like” agents were considered as the cause of two diseases (potato spindle tuber and citrus exocortis), their study and further characterization have been linked to the development and use of molecular biology tools. Sucrose density gradient centrifugation and polyacrylamide gel electrophoresis (PAGE) played a critical role in the pioneering studies of PSTVd and citrus exocortis viroid (CEVd). This was later modified by using other PAGEs (sequential PAGE, return PAGE, two-dimensional PAGE), and/or different staining methods (ethidium bromide, silver nitrate, etc.). Since then, disease-causing agents suspected to be viroids were usually subjected to a number of tests to define their: (i) Molecular nature (RNA or DNA; single stranded or double stranded; circular or linear RNA); (ii) molecular weight; (iii) secondary and tertiary structure. Further biological assays are also essential to establish the relationship of a viroid with plant disease and to fulfill Koch’s postulates.

One-step multiplex RT-qPCR detects three citrus viroids from different genera in a wide range of hosts

A one-step multiplex reverse transcription real-time quantitative polymerase chain reaction (RT-qPCR) based on species-specific minor groove binding (MGB) probes, was developed for the simultaneous detection, identification, and quantification of three citrus viroids belonging to different genera. Citrus exocortis viroid (Pospiviroid), Hop stunt viroid (Hostuviroid), and Citrus bark cracking viroid (Cocadviroid) cause a variety of maladies in agriculturally significant crops. Therefore, reliable assays for their detection are essential tools for various government and industry organizations implementing disease management programs. Singleplex qPCR primers and MGB probes were designed individually for the detection of the three targeted viroids, and subsequently combined in a one-step multiplex RT-qPCR reaction. A wide host range of woody plants, including citrus, grapevines, apricots, plums and herbaceous plants such as tomato, cucumber, eggplant and chrysanthemum different world regions were used to validate the assay. Single, double and triple viroid infections were identified in the tested samples. The developed multiplex RT-qPCR assay was compared with a previously reported SYBR Green I RT-qPCR for the universal detection of citrus viroids. Both assays accurately identified all citrus viroid infected samples. The multiplex assay complemented the SYBR Green I universal detection assay by differentiating among citrus viroid species in the positive samples. The developed multiplex RT-qPCR assay has the potential to simultaneously detect each targeted viroid and could be used in high throughput screenings for citrus viroids in field surveys, germplasm banks, nurseries and other viroid disease management programs.

Citrus Viroids: Symptom Expression and Performance of Washington Navel Sweet Orange Trees Grafted on Carrizo Citrange

Citrus are natural hosts of several viroid species. Citrus exocortis viroid (CEVd) and Hop stunt viroid (HSVd) are the causal agents of two well-known diseases of citrus, exocortis and cachexia. Other viroids have been found to induce specific symptoms and different degrees of stunting in trees grafted on trifoliate orange and trifoliate orange hybrids. A field assay was initiated in 1989 to establish the effect of CEVd, HSVd, Citrus bent leaf viroid (CBLVd), Citrus dwarfing viroid (CDVd), and Citrus bark cracking viroid (CBCVd) on Washington navel sweet orange trees grafted on Carrizo citrange rootstock. Here we report the effect of viroid infection on symptom expression, tree size, fruit production and quality evaluated from 2004 to 2007. Vegetative growth was affected by viroid infection with height and canopy volume being reduced. No bark scaling symptoms were observed in CEVd-infected trees albeit they presented lesions and blisters in the roots. Bark cracking symptoms were consistently observed in CBCVd-infected trees that were smaller with enhanced productivity and fruit size. No major effects were found as a result of infection with CBLVd, HSVd, or CDVd. The quality of the fruits was not affected by viroid infection, except for the low diameter of the fruits harvested from HSVd-infected trees. An interesting effect was identified in terms of tree productivity increase (yield/canopy volume) as a result of infection with CEVd, CDVd, and especially CBCVd.

Microarray analysis of Etrog citron (Citrus medica L.) reveals changes in chloroplast, cell wall, peroxidase and symporter activities in response to viroid infection

Viroids are small (246-401 nucleotides), single-stranded, circular RNA molecules that infect several crop plants and can cause diseases of economic importance. Citrus are the hosts in which the largest number of viroids have been identified. Citrus exocortis viroid (CEVd), the causal agent of citrus exocortis disease, induces considerable losses in citrus crops. Changes in the gene expression profile during the early (pre-symptomatic) and late (post-symptomatic) stages of Etrog citron infected with CEVd were investigated using a citrus cDNA microarray. MaSigPro analysis was performed and, on the basis of gene expression profiles as a function of the time after infection, the differentially expressed genes were classified into five clusters. FatiScan analysis revealed significant enrichment of functional categories for each cluster, indicating that viroid infection triggers important changes in chloroplast, cell wall, peroxidase and symporter activities.