Citrus tristeza virus: a pathogen that changed the course of the citrus industry

Abstract

Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) is the causal agent of devastating epidemics that changed the course of the citrus industry. Adapted to replicate in phloem cells of a few species within the family Rutaceae and to transmission by a few aphid species, CTV and citrus probably coevolved for centuries at the site of origin of citrus plants. CTV dispersal to other regions and its interaction with new scion varieties and rootstock combinations resulted in three distinct syndromes named tristeza, stem pitting and seedling yellows. The first, inciting decline of varieties propagated on sour orange, has forced the rebuilding of many citrus industries using tristeza-tolerant rootstocks. The second, inducing stunting, stem pitting and low bearing of some varieties, causes economic losses in an increasing number of countries. The third is usually observed by biological indexing, but rarely in the field. CTV polar virions are composed of two capsid proteins and a single-stranded, positive-sense genomic RNA (gRNA) of approximately 20 kb, containing 12 open reading frames (ORFs) and two untranslated regions (UTRs). ORFs 1a and 1b, encoding proteins of the replicase complex, are directly translated from the gRNA, and together with the 5' and 3'UTRs are the only regions required for RNA replication. The remaining ORFs, expressed via 3'-coterminal subgenomic RNAs, encode proteins required for virion assembly and movement (p6, p65, p61, p27 and p25), asymmetrical accumulation of positive and negative strands during RNA replication (p23), or suppression of post-transcriptional gene silencing (p25, p20 and p23), with the role of proteins p33, p18 and p13 as yet unknown. Analysis of genetic variation in CTV isolates revealed (1) conservation of genomes in distant geographical regions, with a limited repertoire of genotypes, (2) uneven distribution of variation along the gRNA, (3) frequent recombination events and (4) different selection pressures shaping CTV populations. Measures to control CTV damage include quarantine and budwood certification programmes, elimination of infected trees, use of tristeza-tolerant rootstocks, or cross protection with mild isolates, depending on CTV incidence and on the virus strains and host varieties predominant in each region. Incorporating resistance genes into commercial varieties by conventional breeding is presently unfeasible, whereas incorporation of pathogen-derived resistance by plant transformation has yielded variable results, indicating that the CTV-citrus interaction may be more specific and complex than initially thought. A deep understanding of the interactions between viral proteins and host and vector factors will be necessary to develop reliable and sound control measures.

Figure 1

Tristeza decline, stem pitting and seedling yellows symptoms induced by Citrus tristeza virus (CTV) in different varieties and scion/rootstock combinations. (A) Quick decline syndrome in a sweet orange tree propagated on sour orange rootstock. (B) Different stages of decline of three trees of the same scion/rootstock combination, in comparison with non‐declined neighbour trees (dark green colour). (C) Severe stem pitting symptoms in the trunk of a grapefruit on Poncirus trifoliata rootstock (this grapefruit was from one of the first imports from South Africa into Argentina; Museum of the INTA experiment station at Concordia, Entre Rìos, Argentina). (D) Small‐sized fruits from a grapefruit tree on Poncirus trifoliata rootstock severely affected by stem pitting, in comparison with a normal grapefruit (left). (E) Stunted growth and small yellow leaves in a sour orange seedling inoculated with a seedling yellows isolate of CTV (left), in comparison with a similar plant inoculated with a mild non‐seedling yellows isolate (right). (F) Vein clearing in a young leaf of Citrus macrophylla (upper) and vein corking in a leaf of Mexican lime (lower), inoculated with a mild and a severe CTV isolate, respectively. (G) Stem pitting in the trunk of CTV‐infected Mexican lime seedlings grown in the greenhouse. (H) Transgenic Mexican lime leaf expressing p23 (left) and a similar leaf from a CTV‐infected non‐transgenic lime CTV (right).

Figure 2

Organization and expression of the Citrus tristeza virus (CTV) genomic RNA. (A) Outline of the CTV genome. Boxes indicate ORFs, and proteins encoded by different ORFs are indicated. In ORFs 1a and 1b: PRO‐I and PRO‐II are papain‐like proteases; MT, methyl transferase; HEL, helicase; and Rdrp, RNA‐dependent RNA polymerase domains. CPm and CP are the minor and major coat proteins, and UTR indicates untranslated region. (B) Main RNA species produced for replication and expression of the genome. Black lines indicate positive stranded, and grey lines negative stranded, genomic or subgenomic (sg) RNAs. LMTs are low‐molecular‐weight tristeza sgRNAs. Arrowheads mark aproximate positions of the controller elements inducing formation of sgRNAs.

Figure 3

Neighbour‐joining tree calculated with the complete nucleotide sequence of the genomic RNA from nine CTV isolates, using 1000 bootstrap replicates. Branch length indicates genetic distance. The three clusters obtained include severe isolates inducing seedling yellows (SY) and stem pitting in sweet orange and/or grapefruit (SP) (T318A, SY568R, NuagA and VT), mild non‐SY, non‐SP isolates (T30 and T385) and a group of isolates with intermediate characteristics (T36, Qaha and Mexican).