Agriculture Reference
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checkpoints ensuring, for instance, that macromolecules are not mistargeted. The
presence of a finite number of mRNAs in cucurbit phloem sap suggests the existence
of a selective recognition and transport mechanism able to discriminate between
cell-autonomous and non-cell autonomous CC-SE transcripts. Moreover, the find-
ing that only a subset of the mRNAs found in cucurbit phloem sap was detected
in apices of heterografted plants (Ruiz-Medrano et al. , 1999) suggests that such
discrimination process may also operate at the level of phloem exit. The mechanism
and components of this elusive 'surveillance system' still remain to be identified.
3.2
Intercellular movement of viroids
3.2.1
What are viroids?
For decades, viruses were considered the smallest biological entity, a paradigm
remaining undisputed until the discovery of viroids, a term coined by Diener to
name the agent responsible for the Potato spindle tuber (PST) disease (Diener,
1971). Following from their discovery many studies provided compelling evidence
that, far from being just a curiosity, viroids were in fact a novel class of subviral
pathogens that accounted for some of the most devastating diseases of plants. For
instance, the cadang-cadang viroid (CCCVd) has been directly implicated in the
death of more than 20 million coconut trees in Southeast Asia.
On the basis of some of their autocatalytic properties, viroids are considered as
possible relics of the RNA world (Diener, 2001). Isolation, cloning and sequencing
procedures have led to the identification of more than 30 different species (Flores
et al. , 2003; Pelchat et al. , 2003) that are classified in two families: the Pospivi-
roidae , whose type member is PSTVd; and the Avsunviroidae , represented by avo-
cado sun blotch viroid (ASBVd). Viroids are single-stranded, non-coding and cova-
lently closed RNA molecules, which adopt in most cases a quasi-rod-like secondary
structure. Viroid genomes range in size from 246 to 401 nucleotides, which is ap-
proximately 10-fold smaller than the smallest known viral RNA. Their replication
is either nuclear ( Pospiviroidae )orchloroplastic ( Avsunviroidae ), and involves an
asymmetric ( Pospiviroidae )orsymmetric ( Avsunviroidae ) rolling-circle mechanism
with three catalytic steps that are briefly described below.
The first step involves rolling-circle transcription of the monomeric circular
RNA by a cellular RNA polymerase to produce oligomeric strands. For PSTVd
and presumably other members of the Pospiviroidae , the enzyme involved is the
DNA-dependent RNA polymerase II (Muhlbach & Sanger, 1979; Rackwitz et al. ,
1981; Schindler & Mulbach, 1992) whereas this reaction is performed by a nuclear-
encoded homologue of a choloroplastic RNA polymerase in the case of Avsunvi-
roidae (Flores & Semancik, 1982; Navarro et al. , 2000). The second step involves
precise cleavage of the oligomeric transcripts to release linear monomeric forms
of the viroid. For Avsunviroidae , this step is mediated by a hammerhead ribozyme
embedded in both polarity strands of the viroid (Hutchins et al. , 1986; Hernandez
et al. , 1992; Navarro & Flores, 1997). In the case of Pospiviroidae ,itisthought
that cleavage is mediated by a host-encoded RNase, which recognizes a specific
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