Agriculture Reference
In-Depth Information
different metabolic pathways, enzymes and other factors. Many of the genes involved
may simply modify the expression of resistance rather than affect resistance
per se
.
6.2.12
Tolerance
This is covered in Chapter 7 but is included here as there have been many instances where
resistance and tolerance have been confused. Resistance is the ability of a plant to reduce
growth and multiplication of a pathogen, whereas tolerance is the ability of a plant to
yield despite the presence and growth of a pathogen on the plant. These two traits may be
inherited independently of each other, as in the case of cereal cyst nematodes in cereals,
which are characterised by having just a single generation in a year. Where growth of a
pathogen is continuous or where a nematode has multiple cycles of growth in a season,
then resistance and tolerance become less independent of each other.
6.3
In developing resistant varieties, plant breeders need access to effective sources of
resistance. These may already be present within current varieties and breeding lines in the
breeding programs or from similar programs in other regions or countries. Where these
are not adequate, the next source of variation comes from national or international germ-
plasm collections where landraces, closely related wild relatives and distantly related
species may be found that represent a wider range of useful variation for the trait being
sought. Finally, new variation can be generated in a crop using mutation or by genetic
engineering using genes identifi ed from other species or newly created versions (alleles)
of existing genes or by modifi ed expression of gene activity.
Sources of resistance
6.3.1
Landraces and wild relatives
Landraces are collections made from fi elds where the farmer is unlikely to have intro-
duced modern varieties. These landraces are often quite mixed and will have been locally
selected over a long period of time and may therefore contain genetic variation which is not
present in modern breeding programs, providing useful adaptation to that environment.
Wild relatives, which include crop ancestors, are species that in nature remain largely
genetically isolated from the crop species, but which can be hybridised with the crop
plant to allow the transfer of a required trait without overwhelming problems of hybrid
sterility. Depending on the degree of relatedness, the genetic transfer may involve from
two to several rounds of crossing and selection to obtain the required trait in a background
suitable for commercial production. Because cultivated wheats will have been selected
from a wider population in an earlier age, wild wheat relatives will often contain a much
greater degree of genetic variation for most traits.
Where the environments and circumstances in which collections of landraces and wild
relatives were made have been carefully documented, it is possible to target accessions
that are more likely to reveal useful variation for the trait being sought. Collections may
for example be sampled based on the latitude, altitude, soil pH, soil salinity, rainfall zone,
farming system or pathogen exposure of the collection site.
International collection centres, which may carry many thousands of accessions of a
particular crop species, are increasingly organising their collections such that core groups
