Complete nucleotide sequence and genome organization of a Cactus virus X strain from Hylocereus undatus (Cactaceae).

The complete nucleotide sequence of a strain of Cactus virus X (CVX-Hu) isolated from Hylocereus undatus (Cactaceae) has been determined. Excluding the poly(A) tail, the sequence is 6614 nucleotides in length and contains seven open reading frames (ORFs). The genome organization of CVX is similar to that of other potexviruses. ORF1 encodes the putative viral replicase with conserved methyltransferase, helicase, and polymerase motifs. Within ORF1, two other ORFs were located separately in the +2 reading frame, we call these ORF6 and ORF7. ORF2, 3, and 4, which form the "triple gene block" characteristic of the potexviruses, encode proteins with molecular mass of 25, 12, and 7 KDa, respectively. ORF5 encodes the coat protein with an estimated molecular mass of 24 KDa. Sequence analysis indicated that proteins encoded by ORF1-5 display certain degree of homology to the corresponding proteins of other potexviruses. Putative product of ORF6, however, shows no significant similarity to those of other potexviruses. Phylogenetic analyses based on the replicase (the methyltransferase, helicase, and polymerase domains) and coat protein demonstrated a closer relationship of CVX with Bamboo mosaic virus, Cassava common mosaic virus, Foxtail mosaic virus, Papaya mosaic virus, and Plantago asiatica mosaic virus.

First Report of Cactus virus X on Hylocereus undatus (Cactaceae) in Taiwan.

Hylocereus undatus Britt. & Rose (Cactaceae), commonly known as pitaya, produces edible fruits with red thorny peel and sweet white pulp containing numerous small soft seeds. In recent years, this fruit crop has become increasingly important in Taiwan. During a survey of diseases of pitaya, some plants were found with systemic mild mottling on the stems. A virus was mechanically transmitted that caused necrotic local lesions on Chenopodium amaranticolor and chlorotic lesions on C. quinoa. This virus also caused necrotic lesions with chlorotic halos on Gomphrena globosa and small chlorotic spots followed by systemic infection in Celosia argentea. Back inoculation from C. quinoa by sap transmission caused mild mottling on pitaya similar to that observed on the naturally infected plants and thus confirmed that the agent was the cause of the mottle symptom. Electron microscopic examination of negatively stained extracts from diseased plants revealed a flexuous rod-shaped virus with a length of 480 to 520 nm. Purified viral particles contained a single major protein of approximately 26 KDa as estimated by SDS polyacrylamide gel electrophoresis. In immunodiffusion tests, this virus reacted with antiserum to Cactus X virus (CVX) (ATCC #PVAS245), but did not react with antisera to Bamboo mosaic or Papaya mosaic viruses. These results establish the identity of the virus causing mottle disease on H. undatus as a strain of CVX.