II.3. BRASSICA CROPS (BRASSICA SPP.) – 233
Varieties with single race resistance have been developed, but the multi-race
pathogenicity of these fungi has made it difficult to breed varieties with long-lasting
resistance. However, it is anticipated that with the location of resistance genes on
marker-saturated genome maps, breeders will be able to bring together multiple resistance
genes from both within and outside the genus that will provide long-lasting disease
resistance.Breeding improved varieties
Introduction
The objective of all plant breeding programmes is to produce plants of greater value
to the producer, the industry and the consumer. The objective is achieved by building on
past advances, through the incorporation of desirable traits that impart increased yield,
pest resistance, superior quality and/or utility to new varieties. To accomplish the task,
many related disciplines are essential including genetics, biotechnology, agronomy,
cytology, chemistry, pathology, entomology, physiology and statistics. Within the
biotechnology component, gene transfer and the production of transgenic varieties has
attracted public attention but the discipline is much broader and includes, among others,
tissue culture, protoplast fusion, dihaploid production, gene identification and cloning.
The essential requirement for success is genetic variation for the trait or traits of
interest. The breeder will normally search for the desired trait within adapted genotypes
and then the crop’s world germplasm collection. If it is not present within the species but
present in a related species, interspecific and intergeneric crosses and/or protoplast fusion
may be attempted. If those approaches fail, induced mutation may be explored. Generally
gene transfer, because of regulatory hurdles, is the last resort.
Valuable, new gene-controlled traits are added with each improved variety.
The breeder evaluates the need and the genetic variability available and stacks desirable
traits, be they large or small advances, into the genetic base that previous breeders have
built. Gene stacking is the very essence of plant breeding. Breeding techniques vary with
the crop being bred and its mode of pollination and reproduction. Among the commercial
Brassica crops, both self-compatible and self-incompatible species are present so that a
wide array of techniques are employed, as described below, depending on the species and
the trait or traits to be introduced.
The application of conventional genetic manipulation in plants can have major
beneficial impact on the nutritional quality and quantity of the world’s food supply.
A very successful example, described below, is the conversion of Brassica oilseed crops
from a problematic commodity to the high-quality productive crop we now define as
canola.
Lipids not only make our food taste better but are required dietary ingredients.
They are essential cell membrane components, regulating cell permeability and are
responsible for vitamin transport as well as the starting point for hormone biosynthesis.
Oils and fats are predominantly (~98%) triacylglycerols (TAGs) that consist of a three-
carbon chain with fatty acids attached to each carbon. The fatty acid composition of an oil
determines its value, use and nutritional worth.
Oils from B. juncea, B. rapa and later B. napus have been part of the Asian diet for
centuries, but in Europe and the Americas they are relatively recent edible oil additions.
Prior to and during the Second World War, rapeseed oil was primarily used as a lubricant
for steam engines and as a lamp oil, but following the war, B. napus and B. rapa oils