Agriculture Reference
In-Depth Information
However, the trade-off between higher yield and yield stability is not a rigid one,
and post-Green Revolution, public breeding programs as well as commercial breed-
ing programs have met with some success in achieving greater yield stability while
breeding for higher yields (Cattivelli et al.
2008
; Traxler et al.
1995
). A 1998 survey
to assess gene pool enrichment in the US summarized the objectives of 280 breed-
ing projects, largely by public-sector breeders at the State Agricultural Experiment
Stations (SAES) associated with the state Land Grantre search universities and the
US Department of Agriculture's (USDA) Agricultural Research Service (ARS) in
the US (Frey
1998
). Out of the total number of breeding projects surveyed, 33
projects—representing just 12 %—reported an objective related to abiotic stress
tolerance. Of these 33 stress-tolerance projects, seven had not produced successful
results and five more were still uncertain. Considering breeding projects by type of
crop, emphasis on stress-tolerance objectives was highest among temperate fruit
and nut crops, with 14 out of 44 projects (32 %) related to improving cold hardiness,
winter hardiness, heat tolerance, or drought-tolerance—with these concentrated pri-
marily in blueberry, strawberry, and grape. Emphasis on stress-tolerance objectives
was also higher in forage grasses, where four out of 29 projects (14 %) were breed-
ing for drought and salt tolerance. By contrast, in grains, only four out of 52 (8 %)
of reported projects related to stress tolerance; all of which were in wheat. In the
majority of crop categories—including fiber crops, forage legumes, root crops, and
oilseeds—stress tolerance objectives were all but absent. While stress tolerance has
been targeted in breeding programs, it has been a minor emphasis relative to other
types of objectives, and more difficult to achieve.
1.2 Recent Advances at the Molecular Level
Over the last two decades, rapid advances in plant molecular biology have opened
up new opportunities to enhance stress tolerance while also increasing or at least
preserving mean yields. The tools of molecular biology have enabled the identifica-
tion of hundreds of genes involved in plant stress response and elucidated plants'
complex stress response mechanisms as well as the interrelationships amongst them.
This rapidly expanding knowledge base has enabled molecular breeding programs
and transgenic strategies for drought and stress tolerance. Such knowledge can be
used in molecular breeding programs to identify and bring multiple genes involved
in stress response into elite germ plasm (Araus et al.
2008
; Cattivelli et al.
2008
; Liu
and Chen
2010
; Salekdeh et al.
2009
; Sinclair
2011
; Jenks et al.
2007
). The molecu-
lar breeding approach has, for example, resulted in new varieties of hybrid maize
released for the North America 2011 growing season that are marketed specifically
as 'drought tolerant', including Syngenta's
Agrisure Artesian
maize, released in
Colorado, Kansas, and Nebraska (Syngenta rolling out drought-tolerant corn
2011
),
and Pioneer Hi-Bred's
Optimum AQUAmax
maize, released in Colorado, Kansas,
Nebraska, and Texas (Bennett
2011
).
