Comparison of RNA extraction methods for RT-PCR detection of Coconut cadang-cadang viroid variant in orange spotting oil palm leaves

Abstract

Coconut cadang-cadang viroid (CCCVd) is a lethal disease that has devastated the coconut industry in the Philippines. Recently, a CCCVd variant associated with orange spotting of foliage, which is a phenomenon reported in the Malaysian oil palm industry, occurring as an isolated palm with no similar symptoms on the adjacent neighbouring palms. The presence of this viroid in oil palms is difficult to detect as it is found in very low concentrations. Thus, an efficient RNA extraction method for isolating the CCCVd variant and to detect it through reverse transcriptase polymerase chain reaction (RT-PCR) using specific primers for CCCVd was required. Leaf samples from symptomatic and asymptomatic oil palms were collected from various locations in Malaysia, namely, Negeri Sembilan, Selangor and Perak. The samples were subjected to three RNA extraction methods – natrium chloride EDTA Tris-HCl mercaptoethanol extraction (NETME), polyethylene glycol extraction (PEG) and cetyltrimethylammonium bromide extraction (CTAB). The quantification of the RNA, based on optical density (OD) and concentration, showed that the CTAB extraction was the best, followed by the NETME extraction. The RNA extraction using PEG resulted in poor yields and had the lowest purity. The viroid was detected in all of the samples extracted via CTAB and NETME by RT-PCR with reduced starting material compared with PEG extracted RNA. A BLAST analysis indicated that the viroid sequences are highly conserved and shared a 93% sequence identity with the CCCVd246 variant from oil palm. Phylogenetic analysis showed that the variant belonged to an outgroup from other cocadviroids found in monocots.