Bacilliform DNA-containing plant viruses in the tropics: commonalities within a genetically diverse group

Abstract

Plant viruses, possessing a bacilliform shape and containing double-stranded DNA, are emerging as important pathogens in a number of agricultural and horticultural crops in the tropics. They have been reported from a large number of countries in African and Asian continents, as well as from islands from the Pacific region. The viruses, belonging to two genera, Badnavirus and Tungrovirus, within the family Caulimoviridae, have genomes displaying a common plan, yet are highly variable, sometimes even between isolates of the same virus. In this article, we summarize the current knowledge with a view to revealing the common features embedded within the genetic diversity of this group of viruses.

Taxonomy: Virus; order Unassigned; family Caulimoviridae; genera Badnavirus and Tungrovirus; species Banana streak viruses, Bougainvillea spectabilis chlorotic vein banding virus, Cacao swollen shoot virus, Citrus yellow mosaic badnavirus, Dioscorea bacilliform viruses, Rice tungro bacilliform virus, Sugarcane bacilliform viruses and Taro bacilliform virus.

Microbiological properties: Bacilliform in shape; length, 60-900 nm; width, 35-50 nm; circular double-stranded DNA of approximately 7.5 kbp with one or more single-stranded discontinuities.

Host range: Each virus generally limited to its own host, including banana, bougainvillea, black pepper, cacao, citrus species, Dioscorea alata, rice, sugarcane and taro.

Disease symptoms: Foliar streaking in banana and sugarcane, swelling of shoots in cacao, yellow mosaic in leaves and stems in citrus, brown spot in the tubers in yam and yellow-orange discoloration and stunting in rice.

Useful websites: http://www.dpvweb.net.

Figure 1

Occurrence of badnaviruses and tungrovirus across the tropical regions of the world. Symbols given for each virus are as follows: Δ, Banana streak virus; ψ, Bougainvillea vein banding‐associated badnavirus; ◊, Cacao swollen shoot virus; ♠, Citrus yellow mosaic badnavirus; ♣, Pineapple bacilliform comosus virus and Pineapple bacilliform erectifolius virus; ♥, Piper yellow mottle virus; , Rice tungro bacilliform virus; , Sugarcane bacilliform virus; ♯, Taro bacilliform virus; ■, Dioscorea bacilliform virus.

Figure 2

Arrangement of motifs present within open reading frame (ORF) III polyprotein. Bold continuous lines indicate the motifs and broken lines indicate the gaps. The numbers above the lines indicate the approximate lengths of nonhomologous amino acid residues, and those below the lines indicate the approximate start and end positions. AP, aspartic protease; MP, movement protein; RNaseH, ribonucleaseH; RT, reverse transcriptase.

Figure 3

Organization of open reading frames (ORFs) in badnaviruses and tungrovirus. The homologous ORFs (arrows) are shaded in the same pattern, and their nucleotide positions along the genome are also indicated. The name of the virus and the National Center for Biotechnology Information (NCBI) accession number used for the analysis are shown on the left‐hand side, and the lengths of the genomes in nucleotides are indicated on the right‐hand side. The approximate nucleotide positions are scaled at the bottom. BSV‐GF, Banana streak virus cultivar Goldfinger; CMBV, Citrus yellow mosaic badnavirus; CSSV, Cacao swollen shoot virus; RTBV, Rice tungro bacilliform virus; SCBV‐Mor, Sugarcane bacilliform virus from Morocco; TaBV, Taro bacilliform virus.