There are two different approaches for developing resistance
via Agrobacterium-mediated transformation. The first is to identify
citrus or sexually compatible relatives that are resistant to the virus.
Although CTV can replicate in protoplasts of resistant varieties, the virus
lacks effective cell-to-cell and long-distance movement within resistant plants
(25). The genes responsible for this type of citrus packing machine CTV resistance could
then be identified and used to transform commercially important varieties (27).
Although the putative resistance genes from P. trifoliata have been
identified (10) and mapped (11, 12, 13), the recent discovery of CTV strains
that can overcome this resistance (9) has made this approach less attractive.
The second approach for developing resistance via Agrobacterium-mediated
transformation is to induce a natural resistance mechanism known as
post-transcriptional gene silencing (PTGS) in the plants. PTGS can be induced
by the introduction of citrus packing machine specific transgenes
into a plant genome. The plant recognizes the introduced sequences as foreign
and subsequently degrades the transcripts of the gene and any other sequence
that shares significant nucleotide homology, including pathogenic viral
sequences. This technique has been successfully employed in several crops, most
successfully in papaya, to produce plants that are resistant to some strains of
papaya ringspot virus (22, 34).
A
great deal of effort has been devoted to the development of transgenic
CTV-resistant citrus (58). The first attempts used functional coat protein
genes from mild and severe strains of CTV to produce transgenic plants (18).
Most transgenic lines that resulted were as susceptible to CTV as the
non-transgenic controls. However, some transgenic lines exhibited resistance in
the form of immunity or as a significant delay in the development of
symptoms. These resistant plants had variable levels of coat protein
expression, ranging from undetectable to relatively high, suggesting that PTGS
and/or other resistance mechanisms may be involved. Although citrus packing machine it
is unclear how genetically identical plants of the same transgenic line may
display variable resistance in their phenotypes, it was proposed that
epigenetic effects were involved (18, 19). Additional transgenic citrus plants
have since been created using functional CTV coat protein genes (32, 38), but
their resistance to CTV has yet to be reported. Other studies have shown that
citrus plants encoding untranslatable coat protein transgenes were found not to
be resistant to CTV (17, 24).
Other genes from CTV have been used to generate transgenic
citrus plants in the hopes of producing virus resistant plants. Febres et al.
(24) transformed grapefruit with RdRp gene sequences, but found that all were
susceptible to CTV. When truncated untranslatable p23 genes were introduced
into citrus plants, different transgenic lines displayed variable resistance
phenotypes similar to those citrus packing machine previously described
for coat protein gene sequences (19). For any given line, some plants were
immune, some had delayed development of disease symptoms, and others were
susceptible to the virus. Similar results were also obtained when p23 and
3’-UTR nucleotide sequences were linked and introduced to citrus plants in an
inverted-repeat configuration (4) or using only the 3’-UTR (23). These studies
suggest it may be possible to generate CTV-resistant citrus plants using the
PTGS mechanism.
Acknowledgements
The authors thank Wayne
Borth and Fred Brooks of UH-CTAHR for their thoughtful reviews of this
manuscript. Mission:
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Purpose, Scope and Intended
Audience:
This guide was designed to provide an
overview of the cultivation, chemistry, botany, history, folklore
and uses of Melissa officinalis. It
was written to accommodate a variety of audiences, providing basic citrus packing machine
information appropriate for beginners
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those interested in pursuing individual research or developing an
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