Advances in Small Isometric Multicomponent ssDNA Viruses Infecting Plants

Abstract

Multicomponent ssDNA plant viruses were discovered during 1990s. They are associated with bunchy top, yellowing and dwarfing diseases of several economic plants under family Musaceae, Leguminosae and Zingiberaceae. In the current plant virus taxonomy, these viruses are classified under the family Nanoviridae containing two genera, Nanovirus and Babuvirus. The family Nanoviridae was created with five members in 2005 and by 2010, it has expanded with four additional members. The viruses are distributed in the tropical and subtropical regions of Asia, Australia, Europe and Africa. The viruses are not sap or seed transmissible and are naturally transmitted by aphid vector in a persistent manner. The genome is consisted of several circular ssDNAs of about 1 kb each. Up to 12 DNA components have been isolated from the diseased plant. The major viral proteins encoded by these components are replication initiator protein (Rep), coat protein, cell-cycle link protein, movement protein and a nuclear shuttle protein. Each ssDNA contains a single gene and a noncoding region with a stable stem and loop structure. Several Rep encoding components have been reported from each virus, only one of them designated as master Rep has ability to control replication of the other genomic components. Infectivity of the genomic DNAs was demonstrated only for two nanoviruses, Faba bean necrotic yellows virus and Faba bean necrotic stunt virus (FBNSV). A group of eight ssDNA components of FBNSV were necessary for producing disease and biologically active progeny viruses. So far, infectivity of genomic components of Babuvirus has not been demonstrated.

Keywords: ABTV; BBTV; Babuvirus; CBTV; CFDV; FBNSV; FBNYV; MDV; Nanovirus; PNYDV; SCSV; ssDNA virus.

Fig. 1

Global distribution of multicomponent ssDNA plant viruses. b Banana bunchy top virus (BBTV) (b1 Australia, b2 Fiji, b3 Tonga, b4 Samoa, b5 Vietnam, b6 Indonesia, b7 Taiwan, b8 Philippines, b9 Sri Lanka, b10 India, b11 Pakistan, b12 China; b13 Egypt, b14 Burundi, b15 Angola, b16 Gabon, b17 Hawaii); a Abaca bunchy top virus (Philippines), ca Cardamom bushy dwarf virus (India), sSubterranean clover stunt virus (Australia), m Milk vetch dwarf virus (MDV) (m1: Japan; m2: China), fFaba bean necrotic yellows virus (FBNYV) (f1 Spain, f2 Morocco, f3 Tunisia, f4 Egypt, f6 Sudan, f7 Ethiopia, f8 Jordon, f9 Syria, f10 Azerbaijan), cf Coconut foliar decay virus (CFDV) (Vanuatu), p Pea necrotic yellow dwarf virus (Germany), fs Faba bean necrotic stunt virus (fs1 Morocco, fs2 Ethiopia)

Fig. 2

Genome organization of multicomponent ssDNA plant viruses. M-Rep master Rep, CP coat protein MP movement protein, NSP nuclear shuttle protein, C cell-cycle linking protein, U1–U3 unknown function; * stem and loop structure, solid triangle TATA box, dot poly-A signal. In Babuvirus, hatched box indicate major common region. No ORF was identified in U3 component of Abaca bunchy top virus. a Nanovirus, b Babuvirus

Fig. 3

Disease symptoms of multicomponent ssDNA plant viruses. aMilk vetch dwarf virus (MDV) in faba bean showing yellowing; b Cardamom bushy dwarf virus in large cardamom showing excessive stunting and proliferation of lateral shoots; cBanana bunchy top virus (BBTV) in banana showing bunchy crown; d Coconut foliar decay virus (CFDV) in coconut (Malayan Red Dwarf) showing yellowing of fronds; eFaba bean necrotic yellows virus (FBNYV) in faba bean showing yellowing and stunting; f FBNYV in faba bean leaves showing marginal chlorosis and necrosis. (Source of figures: BBTV on banana was contributed by Dr. VK Baranwal, Indian Agricultural Research Institute, India; MDV on faba bean by Dr. Y Sano, Niigata University, Japan; FBNYV on faba bean by Dr. SG Kumari, ICARDA, Syria; CFDV on coconut by Dr. JW Randles, University of Adelaide, Australia)