Agriculture Reference
In-Depth Information
in crops of this nature using a single, low-dose, rapidly soil-degradable, modern
herbicide rather than the complex multi-herbicide pre- and post-emergence
programmes presently needed. Insect resistance has also been engineered into
shallot and garlic (see below). Another potentially useful transformation has
been achieved by introducing an alliinase 'antisense' gene into onion. These
partially block the expression of the alliinase enzyme responsible for producing
flavour and aroma (see Fig. 8.2) and should therefore impact on the level of the
flavour compounds (Eady, 2002).
A better technique for blocking the action of specific genes, or 'gene
silencing', is RNA interference (RNAi), which is based on a powerful natural
mechanism for seeking and destroying deleterious transcribed RNA, including
viruses. This has been applied to develop a 'tearless' onion by suppressing the
production of the enzyme lachrymatory factor synthase (LFS) that catalyses
the reaction that produces the volatile lachrymatory factor when bulbs are cut
(see Chapter 8; Eady, 2007).
Because genetic transformation of onion is still difficult and takes 1 year
from transformation to bulb production, the LFS gene-silencing construct was
first developed using tobacco plants, which are more amenable to genetic
transformation. Transgenic tobacco plants producing LFS were developed, and
a second transformation with an RNAi construct directed against the LFS-
producing RNA was introduced and shown to suppress LFS production in the
tobacco 'model' system. The LFS-suppressing genetic construct shown to work
in transgenic tobacco was then introduced to onion. The introduced RNAi
system seeks, dices and hence destroys specifically the RNA coding for LFS.
In this targeted way the production of the irritating lachrymatory factor is
suppressed and, rather than being lost as LF, precursors should be diverted to
forming more of the other flavour compounds beneficial to health (see Chapter
8). The RNAi system shows great promise for manipulating the genetic system
of onions to enhance the production of nutritionally desirable secondary
metabolites and for developing resistance to virus disease, as well as being a
powerful tool for fundamental investigations of metabolic pathways in the
plants (Eady, 2007).
The growing public and political pressure to minimize the use of pesticides has led
to increasing emphasis on the breeding of cultivars with resistance or tolerance
to disease and pest attack. Notable success in resistance breeding was achieved in
controlling onion pink root, a soil-borne disease of warm regions. Early studies
indicated the presence of a single recessive allele conferring resistance to this
disease in Bermuda-type cultivars. This was successfully transferred to other
sweet types suitable for the southern USA. Later research has indicated
susceptibility to other strains of the fungus in resistant cultivars (Pike, 1986).
