Molecular characterization and infectivity analysis of tomato leaf curl New Delhi virus isolates infecting potato

Optimization of a simple, low-cost one-step reverse transcription recombinase polymerase amplification method for real-time detection of potato virus A in potato leaves and tubers

Vegetative propagation of potatoes makes it possible for potato viruses to be transmitted through tubers. Potato virus A (PVA) is one of these viruses, which belongs to the Potyvirus genus in the Potyviridae family. Potato tuber yield can be reduced by 30-40% by PVA alone. Losses can be further exacerbated by potato virus X and/or potato virus Y infection. PVA is transmitted primarily by several species of aphids in non-persistent manner. With the aim of resolving this problem, we developed one-step reverse transcription-recombinase polymerase amplification (RT-RPA), a highly sensitive and cost-effective method for detecting PVA in both potato tubers and leaves. Detection and amplification are performed using isothermal conditions in this method. There was good amplification of the coat protein gene in PVA with all three primers tested. To conduct this study, a primer set that can amplify specific 185 base pair (bp) product was selected. PVA detection was optimized by 30-min amplification reactions, which showed no cross-reactivity with other potato viruses. A simple heating block or water bath was used to amplify PVA product using RT-RPA at a temperature range of 38-42 °C. In comparison to conventional reverse transcription-polymerase chain reaction (RT-PCR), the newly developed RT-RPA protocol exhibited high sensitivity for both potato leaves and tuber tissues. Using cellular paper-based simple RNA extraction procedure, the virus was detected in leaf samples as efficiently as purified total RNA. We also found that combining LiCl-based RNA precipitation with cellular paper discs allowed us to successfully optimize RNA extraction for one-step RT-RPA for detecting PVA in tubers. Tests using this simplified one-step RT-RPA method were successfully applied to 300 samples of both leaves and tubers from various potato cultivars. In our knowledge, this is the first report of an RT-RPA assay utilizing simple RNA obtained from either cellular disc paper or LiCl coupled with cellular disc paper to detect PVA. As a result, this method was equally sensitive and specific for detecting PVA in potatoes. The developed RT-RPA assay is more versatile, durable, and do not require highly purified RNA templates, thus providing an effective alternative to RT-PCR assays for screening of germplasm, certifying planting materials, breeding for virus resistance, and real-time monitoring of PVA.

Genome editing (CRISPR-Cas)-mediated virus resistance in potato (Solanum tuberosum L.)

Plant viruses are the major pathogens that cause heavy yield loss in potato. The important viruses are potato virus X, potato virus Y and potato leaf roll virus around the world. Besides these three viruses, a novel tomato leaf curl New Delhi virus is serious in India. Conventional cum molecular breeding and transgenics approaches have been applied to develop virus resistant potato genotypes. But progress is slow in developing resistant varieties due to lack of host genes and long breeding process, and biosafety concern with transgenics. Hence, CRISPR-Cas mediated genome editing has emerged as a powerful technology to address these issues. CRISPR-Cas technology has been deployed in potato for several important traits. We highlight here CRISPR-Cas approaches of virus resistance through targeting viral genome (DNA or RNA), host factor gene and multiplexing of target genes simultaneously. Further, advancement in CRISPR-Cas research is presented in the area of DNA-free genome editing, virus-induced genome editing, and base editing. CRISPR-Cas delivery, transformation methods, and challenges in tetraploid potato and possible methods are also discussed.