RNA as a signalling molecule - Intercellular Communication in Plants - page 57

specifically, in plants. The aim is to provide the reader with a basic knowledge of
this process, and its biological relevance, together with key notions that are relevant
to the non-cell autonomous nature of the phenomenon. More specific aspects about
the mechanism and roles of RNA silencing can be found in recent reviews (Voinnet,
2001, 2002; Hutvagner & Zamore, 2002b; Bartel, 2004).
3.3.1.1 Co-suppression in petunia
Shortly after the discovery of the Ti plasmid of Agrobacterium tumefaciens , molecu-
lar biologists exploited transgenesis to modify agronomic traits of plants. However,
for any given transgene construct, a variable fraction of the primary transformants
did not express the product of the introduced gene, despite its stable integration into
the plant genome. A now classical example of this phenomenon was provided more
than 10 years ago by studies of transgenic petunias (Napoli et al. , 1990). These plants
had been engineered to carry extra copies of the gene encoding the enzyme chalcone
synthase (CHS), which is involved in synthesis of anthocyanins, the pigments that
account for the purple colour of petals. However, although the aim of those experi-
ments was to produce petunia plants with deep purple corolla, some transgenic lines
had flowers with completely white petals, phenocopying a CHS knockout. Molecu-
lar analysis of these peculiar lines indicated that the white petal phenotype resulted
from elimination of both the transgene and the corresponding endogenous CHS
mRNAs, a phenomenon coined 'co-suppression'. Seminal nuclear run-on experi-
ments showed that the reduction in CHS mRNA levels was at the steady state, rather
than the transcriptional level (Napoli et al. , 1990; Vanderkrol et al. , 1993). Moreover,
only RNAs identical in sequence to the introduced CHS transgene were targeted by
this 'post-transcriptional gene silencing' (PTGS). Later on, many examples of co-
suppression involving various transgene-transgene or transgene-endogenous gene
combinations were described in plants and a related process called quelling was
also reported in the filamentous fungus Neurospora crassa (Cogoni et al. , 1996;
Vaucheret et al. , 1998).
3.3.1.2 Double-stranded RNA: trigger molecule of RNA silencing
In 1998, experiments carried out in Caenorhabditis elegans led to the discovery that
exogenous delivery of double-stranded (ds)RNA into the body cavity of the worm
induced a highly sequence-specific RNA degradation mechanism targeted against
any cellular mRNA that shared sequence homology with the introduced dsRNA
molecules (Fire et al. , 1998). There was little or no effect with either sense or an-
tisense single-stranded (ss)RNA. This process, referred to as RNA interference or
RNAi, was later found to occur in many other eukaryotes including fruitfly, mouse
and human (Billy et al. , 2001; Elbashir et al. , 2001a,b). Obviously, the effects
of RNAi were highly reminiscent of those of co-suppression and this prompted
plant scientists to re-evaluate the potential of dsRNA as a possible trigger of the
RNA degradation process involved. It was indeed found that a significant propor-
tion of plants exhibiting co-suppression carried transgene loci that were organized
in inverted repeats, with the potential to produce dsRNA-like transcripts through