Biology Reference
In-Depth Information
The molecular structure of chitinases produced by the plant and of chiti-
nase inhibitors produced by the pathogen may also be subject to such
selective interaction. Gene-for-gene interactions have been demonstrated
for many fungal pathogens, several bacterial and viral pathogens and their
hosts, and a few nematodes and insects and their hosts (Thompson 1994).
The relationship between the rust,
Melampsora lini
, and its host,
Linum mar-
ginale
, discussed earlier, follows this gene-for-gene pattern (Thrall and
Burdon 2003).
In other cases, resistance of plants to pathogens has a polygenic basis
(Simms 1996). Polygenic systems tend to show continuous variation in
degree of resistance by the host to pathogens of a certain degree of path-
ogenicity. Up to 14 genes with additive effects have been reported in dif-
ferent plants. Resistance of grain plants to several fungi appears to be of
this type. Corresponding patterns of virulence of pathogens is poorly
understood.
Rust fungi provide some of the most dramatic examples of rapid evo-
lutionary capability. Rust fungi are extraordinarily diverse, consisting of
about 5,000 species classified in about 150 genera. Wheat leaf rust (
Puc-
cinia triticina
) and stem rust (
P. g r aminis
), wheat and barley stripe rust (
P. stri-
iformis
), and oat crown rust (
P. coronata
), together with other cereal grain
rusts, have followed their hosts to all parts of the agricultural world.These
rusts all show high evolutionary adaptability to grain varieties bearing
new patterns of genetic resistance.
Grain rust fungi have very complicated life cycles (Kolmer 1996).The
full life cycle of wheat leaf rust, for example, has five stages involving
wheat and, in the Old World, a second host plant, meadow rue (
Thalictrum
spp.) (Kolmer and Liu 2000). Diploid teliospores formed on wheat plants
in the fall overwinter. These germinate in spring and undergo meiosis,
producing haploid basidiospores. Basidiospores engage in a sexual cycle
on meadow rue in which different mating types combine and produce
aeciospores that contain two haploid nuclei. These spores disperse and
infect the primary host, wheat. In wheat, the rust produces masses of ure-
diospores, also containing two haploid nuclei, that can infect other wheat
plants in a cycle repeated many times during the growing season. In many
areas, including North America, wheat leaf rust does not reproduce sex-
ually but only asexually by means of urediospores. These can be carried
by wind over great distances and are responsible for a progression of rust
infections that move northward from the southern United States and
Mexico into Canada during the spring and summer. In 1981, wind-car-
ried urediospores introduced a virulent strain of
Puccinia triticina
to New
