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2 Acta Biochimica Polonica

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Co-inoculation with two non-infectious cDNA copies of potato spindle tuber viroid (PSTVd) leads to the appearance of novel fully infectious variants.

100%

Podstolski W. , Góra-Sochacka A. , Zagórski W.

2005

tom 52

nr 1

87-98

Potato spindle tuber viroid (PSTVd) is one of the smallest (about 360 nt) infectious plant agents. It is composed of a single-stranded circular non-coding RNA molecule. In the course of previous passage experiments with two intermediate PSTVd variants I2 and I4, three non-infectious clones (I2-50, I4-37 and I4 VI-17) were found. When inoculated separately as cDNAs on tomato "Rutgers" test plants these variants did not induce any visible disease symptoms and did not produce progeny. The presence of such non-infectious variants raises several questions about their origin and biology and to answer them, mixed co-infections with cDNA copies of two non-infectious variants (I2-50, I4-37) were performed. PSTVd infection was observed in seven out of 30 inoculated plants. The progeny isolated from three separate plants contained novel variants, together with the parental I2 and I4 sequences. It is conceivable that the appearance of repaired PSTVd molecules, clearly capable of cell-to-cell movement leading to the systemic infection, results from recombination events. An analysis of the recombinant molecules and comparison with databases identified the specific sites responsible for the restricted infectivity of the I2-50 and I4-37 PSTVd variants. In parallel experiments in which (+) strand PSTVd infectious transcripts were used, no recombinants were observed, and the original I2-50 and I4-37 non-infectious sequences were not detected in the progeny.

Genetic variability of potato spindle tuber viroid RNA replicon.

100%

Góra-Sochacka A. , Candresse T. , Zagórski W.

tom 48

nr 2

467-476

The genetic continuity of the potato spindle tuber viroid (PSTVd) genome was analysed after infection of tomato plants with cloned cDNAs of parental strains. During the six weeks of the experiment, several new sequence variants appeared. The sequence variants detected in the progeny population induced sequence-specific disease symptoms. The PSTVd genome therefore follows the pattern expected for typical pseudo-strains propagating in plants as a population of similar sequences. Assessing further the replicon continuity, a PSTVd cDNA mutant with a deletion in the central conserved region was constructed and proven to be non-infectious. Surprisingly, in a sub-population of potato transformants expressing the same deleted PSTVd RNA an infectious viroid was detected. This suggests specific transcript conversion followed by recovery of the full-length pathogen genome.