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developing transgenic lines, the availability of
specific intellectual property, and public opposi-
tion to the deployment of the technology. New
insights concerning plant quantitative resistance,
plant basal defense mechanisms (Lacombe et al.
2010), and mechanisms and applications of RNA
interference (Nowara et al. 2010; Wulff et al.
2011; Yin et al. 2011), however, are likely to
lead to some practical applications in the fore-
seeable future. As more disease resistance alle-
les are cloned, it may be increasingly feasible to
use these alleles via direct gene transfer among
maize lines. Transfer of genes within different
members of the Poaceae can also be effective:
the maize NBS-LRR gene
Rxo1
protects rice
against
Xanthomonas oryzae
pv.
oryzae
(Zhao
et al. 2005).
Another potential impact of transgenics in
maize is the reduction of mycotoxins due to pest
resistance provided by transgenic expression of
toxins from
Bacillus thuringiensis
(Bt). Bt maize
is thought to decrease mycotoxin accumulation
because insect damage provides an entry point
for the fungus and limiting insect damage in
turn decreases mycotoxin accumulation (Dowd
2001). Lower levels of fumonisin have been asso-
ciated with the use of Bt maize (Hammond et al.
2004). In 2004, the reduction of fumonisin and
deoxynivalenol damage through the use of Bt
maize was estimated to have an annual economic
impact of $17 million (Wu et al. 2004). Bt maize
has been shown to reduce aflatoxin accumula-
tion when insect pressure is high (Williams et al.
2002; Williams et al. 2005; Windham et al. 1999;
Wu 2006).
ease resistance in maize. As a result, the genetic
architecture and biology of resistance is bet-
ter understood, but the challenge remains to
translate this knowledge into improved disease
resistance in maize varieties.
Understanding the biology of disease resis-
tance can inform the search for genes effec-
tive in conditioning resistance, and thus con-
tribute to harnessing genetic diversity for crop
improvement. Loci conferring resistance to
multiple pathogens are of particular inter-
est and have been identified, but are rela-
tively rare in maize. The mechanisms under-
lying such resistance and whether there are
associated pleiotropic effects affecting other
important agronomic traits, such as yield, are
thus of fundamental interest. A knowledge of the
multiple functions of defense-related genes can
thus inform breeding decisions. Understanding
the genetic architecture and biochemical path-
ways that underlie disease resistance will provide
a route by which to do so.
References
Abalo G, Tongoona P, Derera J, Edema R. 2009. A compar-
ative analysis of conventional and marker-assisted selec-
tion methods in breeding maize streak virus resistance in
maize.
Crop Science
49(2):509-520.
Ali ML, Taylor JH, Jie L, Sun G, William M, Kasha KJ,
Reid LM, Pauls KP. 2005. Molecular mapping of QTLs
for resistance to Gibberella ear rot, in corn, caused by
Fusarium graminearum
.
Genome
48(3):521-533.
Asea G, Vivek BS, Lipps PE, Pratt RC. 2011. Genetic gain
and cost efficiency of marker-assisted selection of maize
for improved resistance to multiple foliar pathogens.
Molecular Breeding
:1-13.
Balint-Kurti PJ, Johal GS. 2009. Maize disease resistance.
In:
Handbook of maize: its biology
. editors: Bennetzen
JL, Hake SC, Springer New York, 229-250.
Balint-Kurti PJ, Yang JY, Van Esbroeck G, Jung J, Smith
ME. 2010. Use of a maize advanced intercross line
for mapping of QTL for northern leaf blight resistance
and multiple disease resistance.
Crop Science
50(2):
458-466.
Beckman PM, Payne GA. 1982. External growth, penetra-
tion, and development of
Cercospora zeae-maydis
in corn
leaves.
Phytopathology
72(7):810-815.
Belcher AR, Zwonitzer JC, Cruz JS, Krakowsky MD,
Chung CL, Nelson R, Arellano C, Balint-Kurti PJ. 2012.
Conclusions
To meet increasing demand for maize, yield
constraints must be overcome. Diseases pose
an important constraint in many parts of the
world, including Asia, Sub-Saharan Africa, and
Latin America (Pingali 2001). A number of new
genomics-based resources have been developed
for public maize research in the past few years,
and have been employed to better elucidate dis-
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