THE GENETICS AND PLANT BREEDING OF ALLIUM CROPS - Onions and other Vegetable Alliums - page 82

inhibiting the plant-damaging enzymes produced by invading fungi or by
detoxifying fungal toxins. An example of the latter type is the gene encoding the
enzyme oxalic oxidase, which produces an enzyme that breaks down the fungal
toxin oxalic acid to carbon dioxide and hydrogen peroxide. This stops oxalic acid
decreasing tissue pH which, in turn, prevents the fungal pathogenic enzymes
from working effectively. This gene has been introduced into other plant species
and found to inhibit fungal invasion. It has now been introduced into onions
using Agrobacterium -mediated transformation (Eady et al ., 2005). Oxalic acid is
the toxin produced by Sclerotium cepivorum , the causal agent of onion white rot,
when it invades roots. Another intriguing possibility for preventing this
intractable disease is to use 'gene silencing' techniques to 'switch off ' the genes
responsible for producing the volatile sulfur compounds in allium roots.
Sclerotium cepivorum sclerotia are stimulated to germinate by the release of these
substances in soil. A root-specific alliinase enzyme has been identified and, by
'silencing' the gene coding for this enzyme, the release of the sulfur compound
germination signal to the disease fungus might be prevented, thereby avoiding
infection (Eady, 2002).
An interesting example of durable resistance to a pest is provided by that of
onion to Thrips tabaci , the onion thrip, probably the most severe pest of the crop
worldwide. Resistance was found in a cultivar with a wide angle of divergence of
the innermost leaves, and in which successive leaf sheaths elongate beyond the
older ones that enclose them. The pest normally shelters and proliferates in the
crevice between the youngest leaves, so this morphological adaptation denies
the pest its usual habitat. Another external change, 'glossy foliage' - caused by
a lack of wax on the outside of the leaf - confers thrips resistance. This is caused
by a single recessive gene allele. However, plants with this characteristic have
increased susceptibility to downy mildew and purple blotch disease.
This illustrates another general problem in developing resistance. Sometimes
the properties conferring resistance may increase susceptibility to other pests or
pathogens, as in the last example, and sometimes they may conflict with other
requirements for the crop. For example, studies on resistance to attack by the
onion fly indicated that onion varieties containing high concentrations of volatile
sulfur compounds were most susceptible to attack (Soni and Ellis, 1990). The fly
locates onions by these compounds. However, these compounds are those that
confer the flavour and pungency to onions, so a conflict between pest resistance
and a requirement for a strongly flavoured onion can be foreseen.
Transgenic shallot and garlic plants have been engineered to be resistant to
the beet army worm Spodoptera exigua , an important tropical pest. The resistant
plants contain a protein toxic to the pest that is coded by a gene derived from
Bacillus thuringiensis . This gene has been incorporated into the allium
chromosomes using the Agrobacterium -mediated genetic transformation of
callus tissue cultures of shallot and garlic. Normal plants containing the novel
protein have resulted and been shown to be resistant to the pest (see Fig. 3.7b;
Zheng et al ., 2004, 2005).