362
September 1998
Family: Avsunviroidae
Genus: Pelamoviroid
Species: Peach latent mosaic viroid
Acronym: PLMVd


Peach latent mosaic viroid

R. Flores
Instituto de Biología Molecular y Celular de Plantas (UPV-CSIC), Universidad Politécnica de Valencia, Camino de Vera 14, 46022, Valencia, Spain

C. Hernández
Instituto de Biología Molecular y Celular de Plantas (UPV-CSIC), Universidad Politécnica de Valencia, Camino de Vera 14, 46022, Valencia, Spain

G. Llácer
Departamento de Citricultura y Otros Frutales, Instituto Valenciano de Investigaciones Agrarias, 46113 Moncada, Valencia, Spain

J. C. Desvignes
Centre Technique Interprofessionel des Fruits et Légumes, Centre de Lanxade, BP 21, F-24130 Prigonrieux, France

Contents

Introduction
Main Diseases
Geographical Distribution
Host Range and Symptomatology
Strains
Transmission by Vectors
Transmission through Seed
Transmission by Grafting
Transmission by Dodder
Serology
Nucleic Acid Hybridization
Relationships
Stability in Sap
Purification
Properties of Particles
Particle Structure
Particle Composition
Properties of Infective Nucleic Acid
Molecular Structure
Genome Properties
Satellites
Relations with Cells and Tissues
Ecology and Control
Notes
References
Acknowledgements
Figures

Introduction

Disease first described by Desvignes (1976, 1980). It has many characteristics in common with those described more than 50 years ago for the diseases named peach mosaic (Hutchins et al., 1937; Stout, 1939), peach calico (Blodgett, 1944) and peach blotch (Willison, 1946) in North America, and peach yellow mosaic in Japan (Kishi et al., 1973), although a direct comparison is not generally possible because their inciting agents have never been identified and the typical isolates have been lost. The viroid nature of the causal infectious agent of peach latent mosaic was established by Flores & Llácer (1988) and by Flores et al. (1990).

A viroid specific to peach and peach hybrids consisting of a circular single-stranded RNA of 336-339 nt. Transmitted by graft propagation, mechanically and to a lesser extent by aphids. Widely distributed around the world.

Main Diseases

Infects peach (Prunus persica) usually without inducing obvious leaf symptoms, but very rarely it induces alterations on the peach foliage: yellow-creamy mosaic or white patterns (peach calico), chlorotic mosaic (peach blotch), or small chlorotic leaves with necrotic margins (Fig.1). The first signs of disorder become apparent 2 years after planting: delay of 4-6 days in foliation, flowering and ripening; irregularly shaped, flattened, colourless fruits with cracked sutures (Fig.2) and swollen stones; pink broken lines on the rose-white petals in warm temperatures (Fig.3); bud necrosis; open habit; and rapid ageing of the trees (Fig.4). Under conditions not well defined, some strains induce furrows in the wood (stem pitting) (Fig.5).

Geographical Distribution

Recorded in many peach-growing countries particularly in those growing American or Japanese cultivars: USA, Japan, China, and the Mediterranean area: France, Spain, Italy, Greece and Northern Africa (Algeria and Morocco) (Desvignes, 1986; Albanese et al., 1992; Skrzeczkowski, et al., 1996).

Host Range and Symptomatology

The only known host is peach (Prunus persica) and peach hybrids (almond x peach, plum x peach, etc); other Prunus species are immune (Desvignes, 1982, 1986). Recent claims indicate that it can also infect other Prunus species (Hadidi et al., 1997; Faggioli et al., 1997) but we have not been able to confirm these results. Transmitted by grafting, budding and experimentally by razor-slashing.

Diagnostic and assay species

Prunus persica (peach) cv. GF-305. Seedlings grown in the greenhouse are first inoculated by chip budding (10 replicates) with material from the trees to be analysed, and approximately 2 months later they are challenge inoculated with a severe isolate (D168). Plants previously infected with a latent strain of PLMVd do not display the characteristic chlorotic mosaic of the severe isolate. This selected severe strain is not totally stable (some plants revert to a symptomless condition) and is maintained in the greenhouse by re-inoculating it monthly. This cross-protection assay between the two types of PLMVd strains (Desvignes, 1976) has allowed the study and control of the disease in France.

PLMVd can also be detected by molecular methods including polyacrylamide gel electrophoresis (Flores & Llácer, 1988; Flores et al., 1990), dot-blot hybridization with radioactive and non- radioactive probes (Ambrós et al., 1995; Loreti et al., 1995), and RT-PCR (Shamloul et al., 1995) with primers derived from the reference sequence (Hernández & Flores, 1992). By molecular hybridization, Ambrós et al. (1995) have shown the presence of PLMVd in several Japanese peach cultivars displaying the characteristic symptoms of peach yellow mosaic disease, suggesting that it is the cause of this disease. However, by RT-PCR Shamloul et al. (1995) have been unable to detect PLMVd in some American isolates of peach mosaic so that peach latent mosaic and peach mosaic should be considered as two distinct diseases incited by a viroid and another pathogen, presumably a virus, respectively.

Propagation species

Prunus persica is the only host from which the viroid has been purified.

Strains

Most isolates do not incite symptoms on peach seedlings in the greenhouse. However, some isolates (D168) induce a general chlorotic mosaic, sometimes necrotic, or rarely a yellow mosaic, and others stem pitting (Desvignes, 1980). Minor differences in nucleotide sequence have been observed between two different complementary DNA clones from a French isolate (Hernández & Flores, 1992) and with respect to an Italian isolate (Shamloul et al., 1995).

Transmission by Vectors

PLMVd is transmitted by aphids (Myzus persicae) (Desvignes, 1981; Flores et al., 1992) but not by pollen or mites (Desvignes, 1981).

Transmission through Seed

No seed transmission reported (Desvignes, 1986; Flores et al., 1992).

Relationships

PLMVd exhibits very limited sequence similarities with other viroid and viroid-like satellite RNAs and does not have any of the central conserved regions characteristic of most typical viroids. However, PLMVd has the elements required to form in the RNAs of both polarities the hammerhead structures proposed to act in the in vitro self-cleavage of avocado sunblotch viroid (ASBVd) (Hutchins et al., 1986), chrysanthemum chlorotic mottle viroid (CChMVd) (Navarro & Flores, 1997) and some satellite RNAs (Bruening, 1989; Symons, 1989). Plus and minus RNA transcripts of PLMVd containing the hammerhead structures display self-cleavage during transcription and after purification. These results support the inclusion of PLMVd in a viroid family represented by ASBVd, whose members are characterized by their ability to self-cleave in vitro and very probably in vivo through hammerhead structures (Hernández & Flores, 1992).

Stability in Sap

No information.

Purification

The viroid can be purified from infected leaves or young fruits of peach (Flores & Llácer 1988; Flores et al., 1990). The tissue is extracted with buffer-saturated phenol, and the nucleic acids are partially purified by chromatography on non-ionic cellulose (CF-11). Polysaccharides can be removed by treatment with 2-methoxyethanol and subsequent recovery of the nucleic acids by precipitation with cetyltrimethylammonium bromide. Viroid circular molecules are obtained by two consecutive steps of polyacrylamide gel electrophoresis under non-denaturing and denaturing conditions, and final elution from the gel.

Properties of Infective Nucleic Acid

A covalently closed single-stranded RNA. The sequence of the reference variant of 337 residues has a base composition (G:A:C:U) of 27.0 : 23.7 : 25.5 : 23.7 which adopts a branched conformation (Fig.6) in the proposed secondary structure of minimum free energy (Hernández & Flores, 1992).

Relations with Cells and Tissues

The viroid can be isolated from leaves, fruits, stems and roots of infected peach plants (Flores et al., 1992), both from those that are symptomlessly infected and from those displaying typical disease symptoms.

Ecology and Control

The main control measure is the use of viroid-free planting material. The agent can be eliminated from infected peach trees, although with some difficulty, by growing plants in hot air at 37 °C for 35-45 days, followed by propagation of the tips (Desvignes, 1986).

References

  1. Albanese, Giunchedi, La Rosa, & Poggi-Pollini, Acta Hort. 309: 331, 1992.
  2. Ambrós, Desvignes, Llácer & Flores, Acta Hort. 386: 515, 1995.
  3. Blodgett, Phytopathology 34: 650, 1944.
  4. Bruening, Methods Enzymol. 180: 546, 1989.
  5. Desvignes, Acta Hort. 67: 315, 1976.
  6. Desvignes, Acta phytopath. Acad. Sci. hung. 15: 183, 1980.
  7. Desvignes, 1er Colloque Rech. Fruitières - Bordeaux, 263, 1981.
  8. Desvignes, Acta Hort. 130: 89, 1982.
  9. Desvignes, Acta Hort. 193: 51, 1986.
  10. Faggioli, Loreti, & Barba, Pl. Dis. 81: 423, 1997.
  11. Flores, & Llácer, Acta Hort. 235: 325, 1988.
  12. Flores, Hernández, Desvignes, & Llácer, Res. Virol. 141: 109, 1990.
  13. Flores, Hernández, Avinent, Hermoso, Llácer, Juárez, Arregui, Navarro, & Desvignes, Acta Hort. 309: 325, 1992.
  14. Hadidi, Giunchedi, Shamloul, Poggi-Pollini, & Amer, Pl. Dis. 81: 154, 1997.
  15. Hernández, & Flores, Proc. natn. Acad. Sci. U.S.A. 89: 3711, 1992.
  16. Hutchins, Bodine, & Thornberry, Circ. U.S. Dept. Agric. 427, 1937.
  17. Hutchins, Rathjen, Forster, & Symons, Nucleic Acids Res. 14: 3627, 1986.
  18. Kishi, Takanashi, & Abiko, Bull. hort. Res. Stn. A. 12: 197, 1973.
  19. Loreti, Faggioli, & Barba, Acta Hort. 386: 560, 1995.
  20. Navarro, & Flores, Proc. natn. Acad. Sci. U.S.A. 94: 11262, 1997.
  21. Stout, Bull. Dept. Agric. Calif. 28: 117, 1939.
  22. Shamloul, Minafra, Hadidi, Giunchedi, Waterworth, & Allam, Acta Hort. 386: 522, 1995.
  23. Skrzeczkowski, Howell, & Mink, Pl. Dis. 80: 823, 1996.
  24. Symons, Trends Biochem. Sci. 14: 445, 1989.
  25. Willison, Phytopathology 36: 273, 1946.


Figure 1

Variability of the foliar symptoms induced by PLMVd (severe strain D168) on GF-305 peach seedlings. From left to right: leaves 1-3 showing blotching and mosaic, leaf 4 chlorotic creamy calico, leaf 5 chlorosis with necrotic margins.

Figure 2

Uneven coloured fruits with cracked sutures on peach cv. Springtime.

Figure 3

Pink broken lines in the rose-white petals of peach cv. Top Crest.

Figure 4

Peach latent mosaic disease on peach cv. Springcrest.

Figure 5

Stem pitting on peach cv. Royal Glory.

Figure 6

Primary and proposed secondary structure of lowest free energy for the reference sequence variant of PLMVd (M83545). Plus and minus self-cleavage domains are delimited by flags, the 13 conserved residues present in most of the hammerhead structures are indicated by bars, and the self-cleavage sites are shown by arrows. Solid and open symbols refer to plus and minus polarities respectively (adapted with modifications from Hernández & Flores, 1992).