An investigation into the potato leafroll virus problem in the Sandveld region, South Africa

Van Wyk, Lezan (2017-12)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Potato leafroll virus (PLRV) is responsible for significant yield losses in the South African (SA) potato industry. PLRV incidence in the Sandveld region has increased dramatically over the past 15 years. Enzyme-linked immunosorbent assay (ELISA) is used for routine testing by the SA Seed Potato Certification Scheme to diagnose PLRV infection, but many countries have changed to reverse transcription polymerase chain reaction (RT-PCR) for detection of PLRV because of its greater sensitivity. This project aimed to develop and validate a probe-based quantitative real-time reverse transcription PCR (RT-qPCR) to detect PLRV in potatoes and obtain an assessment of PLRV incidence in the Sandveld region, SA. This project also aimed to confirm infection in aphids and characterise aphid transmitted PLRV isolates by sequencing. Finally, this project aimed to apply a next-generation sequencing (NGS) technology to identify and characterise isolates, to compare non-coding 5’ and 3’ regions of the genome and lastly, to identify unknown viruses and other pathogens that possibly occur in potatoes in the Sandveld region. Suitable primers and a TaqMan probe were designed to develop a highly sensitive RT-qPCR detection method for PLRV. An amplified complementary DNA (cDNA) was cloned into a plasmid and used for assay quantification and validation. Thereafter, potato leaves were tested over a full calendar year and results were compared to vector pressure. Overall high infection levels were found, but in certain times of the year low infection levels were found due to low vector pressure. SA tubers were also tested with this method. This study indicates that the SA Potato Certification Scheme should reconsider the use of ELISA as the method for PLRV detection and replace it with the described RT-qPCR method. Secondly, the presence of PLRV in aphids was confirmed with RT-qPCR. A whole PLRV genome was amplified and sequenced after extraction from an infectious aphid. This generated whole PLRV genome was aligned in a data matrix with other whole genome sequences. Phylogenetic analysis of the whole genomes revealed that the aphid extracted PLRV isolate grouped with eight other SA isolates from the Sandveld region. Lastly, Ion Torrent was used to obtain information about further PLRV isolates present in the Sandveld region. Samples with low Cq values corresponded to a high number mapping, coverage and sequencing depth of small interfering RNAs (siRNAs). Three complete genomes were obtained by mapping siRNAs to the reference sequence, as de novo assembly could not obtain contigs longer than 700 nucleotides. Phylogenetic analysis of the whole genomes revealed that three of the samples grouped with an Australian isolate and seven SA isolates. The remaining isolate grouped with nine other SA isolates. Minor variation between upstream and downstream non-coding regions was seen. No other potato or unknown viruses were identified, but an unknown fungus was identified in all samples which needs further investigation.

AFRIKAANSE OPSOMMING: Aartappelrolbladvirus (PLRV) is verantwoordelik vir beduidende opbrengsverliese in die Suid-Afrikaanse (SA) aartappelbedryf. Voorkoms van PLRV in die Sandveld streek het oor die afgelope 15 jaar drasties vermeerder. “Enzyme-linked immunosorbent assay” (ELISA) word gebruik vir roetine toetsing deur die SA moersertifiseringskema om PLRV infeksies te diagnoseer, maar ander lande het verander na “reverse” transkripsie polimerase kettingreaksie (RT-PCR) vir die opsporing van PLRV weens sy beter sensitiwiteit. Die doel van hierdie projek was om ‘n peiler-gebaseerde kwantitatiewe “real-time reverse” transkripsie polimerase kettingreaksie (RT-qPCR) te ontwikkel en te valideer om PLRV in aartappels op te spoor en ‘n oorsig van PLRV voorkoms in die Sandveld streek, SA te kry. Hierdie projek het ook gemik om infeksie in plantluise op te spoor en die plantluis-oordraagbare-PLRV isolate te karakteriseer deur middel van volgordebepaling. Ten slotte het die projek “next generation sequencing” (NGS) aangewend om isolate te identifiseer en karakteriseer, nie-koderende genoom streke stroomopwaarts en stroomafwaarts te vergelyk en onbekende virusse en ander patogene te identifiseer wat moontlik in aartappels van die Sandveld streek voorkom. Geskikte inleiers en ‘n TaqMan peiler is ontwerp om ‘n hoogs-sensitiewe RT-qPCR deteksiemetode vir PLRV te ontwikkel. Geamplifiseerde komplementêre DNA (cDNA) is in ‘n plasmied gekloneer vir toets kwantifisering en validering. Aartappelblare getoets oor ‘n volle kalender jaar en die resultate was vergelyk met vektordruk. Oor die algemeen was hoë infeksievlakke gevind, maar daar was sekere tye van die jaar wat lae infeksievlakke gehad het weens lae vektordruk. SA moere is ook getoets met die metode. Die studie dui dus dat die SA moersertifiseringskema die keuse om ELISA te gebruik as metode vir PLRV deteksie moet heroorweeg en vervang met die beskryfde RT-qPCR metode. Tweedens is die teenwoordigheid van PLRV in plantluise bepaal met RT-qPCR. ‘n Heel PLRV genoom is geamplifiseer en sy volgorde bepaal na ekstraksie vanuit ‘n geinfekteerde plantluis. Die genereerde heel PLRV genoom is in ‘n datamatriks opgelyn met ander heelgenoom volgordes. Filogenetiese ontleding van die heel genome het getoon dat die PLRV isolaat wat uit die plantluis geekstrateer is, groepeer het met agt ander SA isolate van die Sandveld streek. Laastens is Ion Torrent gebruik om inligting oor verdere PLRV isolate in die Sandveld streek te verkry. Monsters met lae Cq-waardes het ooreengestem met hoë getalle klein interferende RNAs (siRNAs) kartering, volgordedekking en volgordediepte. Drie volledige genome is verkry deur verwysingsgenoom siRNA kartering omdat de novo samestelling nie kontigs langer as 700 nukleotides kon verkry nie. Filogenetiese ontleding van die volledige genome het bepaal dat drie isolate gegroepeer het met ‘n Australiaanse isolaat en sewe SA isolate. Die oorblywende isolaat het gegroepeer met nege ander SA isolate. Min betekenisvolle stroomop- en stroomafwaartse variasies in nie-koderende streke is gesien. Geen ander aartappel of onbekende virusse geïdentifiseer nie, behalwe ‘n onbekende swam wat geidentifiseer is in al die monsters wat verdere ondersoek benodig.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102903
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