Genetically Improved Crops - The Oxford Topic of Food, Politics, and Society - page 70

stress such that existing soil moisture is saved for the critical period surrounding flower-
ing, resulting in less kernel abortion, higher harvest index, and greater yield (Castiglioni
et al. 2008).
Other approaches include modification of individual genes involved in stress response
and cell signaling. For example, drought-tolerant canola engineered to reduce the levels
PARP [poly (ADP-ribose) polymerase], a key stress-related protein in many organisms,
shows relative yield increases of up to 44% compared with control varieties. A subset of
the transcription factors—homeodomain leucine zipper proteins (HDZip)—play a role
in regulating adaptation responses including developmental adjustment to environ-
mental cues such as water stress in plants (Deng 2006). One of these effectors is absisic
acid (ABA), an important plant regulator controlling many environmental responses
including stomata movement—which is itself modulated by the DREB elements. Some
work is being done on modifying HDZip directly. Other work involves indirect mecha-
nisms, for example, down-regulating farnesyltransferase, a signaling system in the pro-
duction of absisic acid and stomata control, which results in stomata closure and water
retention.
Eduardo Blumwald is also working on modifying basic acid to enhance the tolerance
of plants to water deficit by delaying the drought-induced leaf senescence and abscis-
sion during the stress episode. This approach is now being introduced into rice, among
other crops. The work is being done in conjunction with Arcadia Biosciences. Bayer
CropScience, Pioneer Hi-Bred, BASF, and Dow, among others, are conducting research
on maize, cotton, canola, and rice to develop a new generation of stress-tolerant,
high-performance crop varieties. Clearly, stress-tolerant traits are of paramount impor-
tance in low-income countries, especially in sub-Saharan Africa and Asia. Major efforts
are already under way on this front. The partnership, known as Water Efficient Maize
for Africa (WEMA), was formed in response to a growing call by African farmers, lead-
ers, and scientists to address the devastating effects of drought on small-scale farmers
(Foundation 2007). Frequent drought leads to crop failure, hunger, and poverty. Climate
change only aggravates this situation.
On the other end of the spectrum of climate change impact is flooding due to chang-
ing rain patterns and rising sea levels. This is already a major cause of rice crop loss. It is
estimated that four million tons of rice are lost every year because of flooding; this loss is
equal to the amount of rice sufficient to feed thirty million people. Rice is not grown in
flooded fields through necessity but rather to control weeds. However, most rice variet-
ies die after more than three days of complete submergence. Researchers know of at least
one rice variety—accession number FR13A—that can tolerate flooding for longer peri-
ods, but conventional breeding failed to create a cultivar that was acceptable to farm-
ers. The Ronald Laboratory at UC Davis cloned the submergence tolerance (Sub1) locus
from this resistance variety using a map-based cloning approach (Jung 2010). The Sub1
locus encodes three putative transcription regulators, one of which (Sub1A-1) increases
dramatically in response to oxygen deprivation in Sub1 seedlings while Sub1C levels
decrease. Transgenic lines overexpressing the Sub1A-1 gene have been introgressed into
a submergence-intolerant line and display enhanced submergence tolerance.