- Volume 77, Issue 11, 1996

The three Coleus blumei viroids (CbVd) CbVd 1-RL, CbVd 2-RL and CbVd 3-RL occur naturally in plants of Coleus blumei cultivar (cv.) ‘Ruhm von Luxemburg’. CbVd 2-RL is a viroid chimera made up of the right half of CbVd 1-RL and the left half of CbVd 3-RL. Using the cDNAs of the left half of CbVd 1-RL and the right half of CbVd 3-RL, ‘inverse’ viroid chimeras (CbVd-Ch1 and CbVd-Ch2) were constructed in vitro as dimeric cDNA units. The in vitro-generated cDNA-transcripts of CbVd-Ch1 and CbVd-Ch2 were used to inoculate plants of Coleus blumei cv. ‘Ruhm von Luxemburg’ harbouring CbVd 1-RL and CbVd 3-RL, leading to the emergence of new viroid-like RNA replicons, named CbVd A-1 and CbVd A-2, that hitherto have not been found in Coleus plants. The sequences of CbVd A-1 and CbVd A-2 are not identical to the corresponding sequences of the cDNA-transcripts of CbVd-Ch1 and CbVd-Ch2, but show a deletion of a segment at the border of the two original halves as well as two U to A transitions. When the cDNA-transcripts of CbVd-Ch1 and CbVd-Ch2 were used to inoculate plants of Coleus blumei cv. ‘Scarlet Dragonfly’ that were either viroid-free or infected with CbVd 1-RL and CbVd 3-RL, no new viroid-like RNAs emerged. However, the cDNA-transcripts of CbVd A-1 and CbVd A-2 are infectious by mechanical inoculation of viroid-free plants of cv. ‘Scarlet Dragonfly’, thus proving their viroid nature.

Pea enation mosaic virus (PEMV) and the causal agents of groundnut rosette disease are diverse examples of disease complexes involving two RNA species, one of which is related to the genomes of luteoviruses and the other to those of umbraviruses. In both complexes, these viral RNA components may be supplemented with satellite RNAs that are dependent on the umbravirus component for replication and systemic movement, and on the luteovirus component for encapsidation and vector transmission. Sequence analysis identified regions of similarity between the satellites of groundnut rosette virus (GRV) and PEMV, particularly at the 5′ and 3′ termini and around duplicate sequence repeats present in each satellite RNA. The umbravirus GRV and the umbravirus-like PEMV RNA-2 were each able to support the replication and systemic spread of homologous and heterologous satellites. The presence of the PEMV satellite in infections with GRV had no effect on symptom expression in Nicotiana spp. or in Arachis hypogaea. Likewise, in Pisum sativum, the GRV satellite had no effect on the symptoms induced by PEMV. However, the intense yellow blotch symptoms induced in Nicotiana benthamiana by the YB3 GRV satellite in conjunction with GRV were also manifested when PEMV was the helper. Although PEMV RNA-1 was capable of supporting the encapsidation and aphid transmission of the GRV satellite, no evidence was obtained that the essential role of the GRV satellite in the aphid transmission of GRV could be supplied by the PEMV satellite. These data further strengthen the hypothesis of an evolutionary relationship between PEMV and the luteovirus-umbravirus complexes.

The movement proteins (MP) of cowpea mosaic virus and cauliflower mosaic virus (CaMV) are associated with tubular structures in vivo which participate in the transmission of virus particles from cell to cell. Both proteins have been expressed in plant protoplasts and insect cells. In all cases, immunofluorescent histochemistry showed that the MPs accumulate intracellularly as tubular extensions projecting from the cell surface. Additionally, electron microscopy revealed intracellular MP aggregates in CaMV MP-expressing cells. The data presented establish common features for the tubule-forming MPs: no other virus gene products are required for tubule formation and unique plant components (e.g. plasmodesmata) are not essential for tubule synthesis.