Agriculture Reference
In-Depth Information
most effective methods of addressing water limitation problems—irrigation—unfortu-
nately is also one of the major causes of arable land degradation. It is estimated that 24.7
million acres of farmland worldwide is lost each year due to salinity buildup resulting
from over irrigation. In fact, salinity limits crops on 40% of the world's irrigated land
(25% in the United States). To address the extreme end of irrigation impact, Eduardo
Blumwald at the University of Calfornia at Davis (UC Davis) used AtNHX1, the most
abundant vacuolar Na+/H+ antiporter in Arabidopsis thaliana, which mediates the
transport of Na+ and K+ into the vacuole. By overexpressing this vacuolar Na+/H+
anti-porter transgenic tomatoes were able to grow, flower, and produce fruit in the pres-
ence of 200 mM sodium chloride (Sottosanto et al. 2007). Arcadia Biosciences has now
introduced this gene into economically important crops.
It is estimated that water stress is the most important variable in determining crop
yield and can explain approximately 80% variance in yields (Shin et al. 2009). Even at a
more moderate level of water stress, it is estimated that about seventy to eighty million
acres in the United States suffer yield losses annually (Kramer 1980). The most criti-
cal time for water stress is near pollination and flowering, where yields with or without
irrigation can vary by up to 100%. In dry land, production yields can be cut in half in
the absence of irrigation. At this time about 15% of US maize acres are irrigated; an esti-
mated 20 million acres in the United States would benefit from a drought tolerance gene
that gives a 10% yield increase. The trait would also allow shifting of high-value crops
into production on more marginal land.
The physiological responses of plants to water stress and their relative importance for
crop productivity vary with species, soil type, nutrients, and climate. On a global basis,
about one-third of potential arable land suffers from inadequate water supply, and the
yields of much of the remainder are periodically reduced by drought. Transcription fac-
tors are some of the most versatile tools being employed in developing stress-tolerant
plants. One of the most versatile classes of transcription factors involved in environ-
mental response is the DREB (dehydration-responsive element binding protein),
which is involved in the biotic stress-signaling pathway. These transcription factors can
activate as many as twelve resistant functional genes relying on DRE members of cis
regulation under adverse conditions. For instance, rd29, cor15, and rd17 cause proline
content to rise to enable plants to improve in many resistances, such as drought, freez-
ing, and salt tolerance (Agarwal et al. 2006). It has been possible to engineer stress toler-
ance in transgenic plants by manipulating the expression of DREBs. One isolated from
Arabidopsis has improved drought tolerance, increasing productivity by at least twofold
during severe water stress. DroughtGard maize will be the first commercially available
transgenic drought-tolerant crop. Hybrid seed sold under this trademark will combine a
novel transgenic trait (based on the bacterial cspB gene, an RNA chaperone, which help
to maintain normal physiological performance under stress by binding and unfolding
RNA molecules so that they can function normally) with Monsanto's optimized conven-
tional breeding program. In field trials using this approach, maize yields have increased
under water stress by up to 30% (Castiglioni et al. 2008). The yield gain of this variety
under drought appears to occur due to slowing of growth specifically under drought
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