Plum pox virus and sharka: a model potyvirus and a major disease

Abstract

Taxonomic relationships: Plum pox virus (PPV) is a member of the genus Potyvirus in the family Potyviridae. PPV diversity is structured into at least eight monophyletic strains.

Geographical distribution: First discovered in Bulgaria, PPV is nowadays present in most of continental Europe (with an endemic status in many central and southern European countries) and has progressively spread to many countries on other continents.

Genomic structure: Typical of potyviruses, the PPV genome is a positive-sense single-stranded RNA (ssRNA), with a protein linked to its 5' end and a 3'-terminal poly A tail. It is encapsidated by a single type of capsid protein (CP) in flexuous rod particles and is translated into a large polyprotein which is proteolytically processed in at least 10 final products: P1, HCPro, P3, 6K1, CI, 6K2, VPg, NIapro, NIb and CP. In addition, P3N-PIPO is predicted to be produced by a translational frameshift.

Pathogenicity features: PPV causes sharka, the most damaging viral disease of stone fruit trees. It also infects wild and ornamental Prunus trees and has a large experimental host range in herbaceous species. PPV spreads over long distances by uncontrolled movement of plant material, and many species of aphid transmit the virus locally in a nonpersistent manner.

Sources of resistance: A few natural sources of resistance to PPV have been found so far in Prunus species, which are being used in classical breeding programmes. Different genetic engineering approaches are being used to generate resistance to PPV, and a transgenic plum, 'HoneySweet', transformed with the viral CP gene, has demonstrated high resistance to PPV in field tests in several countries and has obtained regulatory approval in the USA.

Figure 1

Typical symptoms induced by Plum pox virus on a domestic plum leaf (A), domestic plum fruits (B), premature domestic plum fruit drop (C), an apricot fruit (D), an apricot stone (E), peach fruits (F), a peach leaf (G) and Japanese plum leaves (H). (A, B, D, E and F) were kindly supplied by Dr M. A. Cambra, Centro de Protección Vegetal y Certificación, Diputación General de Aragón, Montañana‐Zaragoza, Spain.

Figure 2

Genomic map of Plum pox virus. The long open reading frame (ORF) is represented by a rectangular box divided into viral products by solid black lines. PIPO ORF, translatable with a frameshift, is indicated by a grey box below the P3 region. Cleavage sites recognized by the indicated proteinases are signalled by arrows. The terminal protein (VPg) is represented as a black ellipse.

Figure 3

Phylogenetic and recombination analysis of Plum pox virus (PPV) strains. Phylogenetic tree of representative PPV isolates belonging to the known PPV strains (left). The genomic organization and recombination history of the corresponding PPV strains are shown on the right. The phylogenetic tree was reconstructed with the neighbour‐joining technique from full‐length nucleotide sequences and bootstrap (1000 replicates) was used to evaluate branch validity. The following sequences were used: PPV‐An (unpublished sequence; Palmisano et al., 2012); PPV‐T (EU734794); PPV‐M (AJ243957); PPV‐Rec (AY028309); PPV‐D (AY912057); PPV‐EA (DQ431465); PPV‐C (HQ840518); PPV‐CR (KC020126). For PPV‐W, two isolates were used to reflect differences in recombination history between members of this strain: LV‐145bt (HQ670748) and W3174 (AY912055), which is marked by an asterisk. For the right panel, strains are colour coded and arrows mark the recombination breakpoints identified. The 5′ genome portion in PPV‐M, PPV‐D and PPV‐Rec, affected by an ancestral recombination, is boxed and the colour of the affected region in PPV‐M is modified from that of the parental PPV‐D to reflect its divergence posterior to the recombination event.