Agriculture Reference
In-Depth Information
Insect resistance and tolerance has been shown to be generally quan-
titative and polygenic [85]. Several traits were shown to specifi cally de-
ter the herbivory of insect pests including changes in both epidermal and
chemical composition, including leaf glossiness, cuticular wax, trichome
density, and hormone production. Phenotypes that exhibit glossy leaves
have shown increased resistance to insect feeding in several crop species,
such as cabbage, soybean, common bean, maize, and sorghum [86-89]. Pi-
coaga et al. [86] found that
Brassica
sp. with glossy leaf phenotypes were
more resistant to insect herbivory. Eigenbrode and Espelie [87] suggested
that reduced concentration and chemical composition of epicuticular lip-
ids, common to glossy leaves, was the primary reason behind the increased
crop resistance to insect infestation, ultimately effecting pest movement,
feeding, and oviposition. Chemical composition of epicuticular lipids can
be an important factor in deterring insect herbivory, crop devastation, and
yield loss. It has been illustrated that aphid resistance is correlated to high
concentrations of triacontarial (C
30
) in alfalfa and β-amyrin in raspberries
[87,90,91]. Several studies have implied that pesticides and other agricul-
tural chemicals can affect the epicuticular lipid composition, reducing the
ability of the crop to resist insect attack [92,93]. To date, multiple genes
have been identifi ed for the gloss leaf trait in cabbage, and 18 loci have
been mapped using mutants of maize and sorghum [88,94,95]. Although
leaf waxes can vary with crop age and be infl uenced by the production
environment, they provide an avenue for breeders to develop varieties that
are suited for specifi c agroecosystems while improving pest resistance,
yield loss, and overall sustainability of low-input systems.
The most noted of changes to epidermal tissues leading to increased
pest resistance is trichome density and morphology. Although not as ef-
fective against large or heavy insect pests, increased trichome density has
been shown to alter the behavior of several small but economical agricul-
tural pests. The defensive role of trichome density has been examined in
several crop species. Diamondback moth resistance in
Arabidopsis
, green
mite and mealybug resistance in cassava crops, and cabbage white but-
terfl y larvae resistance in
Brassica
sp. were all associated with increased
trichome density on both upper and lower leaf surfaces [84,96,97]. In ad-
dition, trichome morphology has also been shown to play an important
role in limiting pest establishment within a crop. Sorghum genotypes with
