Drought and salinity are the two biggest abiotic
problems farmers face. Rice is one of the main food crops
worldwide, but of the total area of land on which rice is grown, 30%
contain enough salt to stunt or prevent rice growth, and 20% regularly
experience drought (Lane, 2002). The two problems often go
hand-in-hand. Farmers must irrigate their crops to provide enough
water for acceptable yields--in India, approximately 55% of rice lands
are dependent on rain, and drought regularly limits rice
production--but, over time, irrigation leads to the build-up of salt in
the soil (Raj, 2002).
A Nigerian farmer irrigates his newly-planted
wheat fields. Photo by Robert Grossman. *Permission pending
for Agricultural Development (IFAD, 2004)*
Interestingly enough, researchers developing salt-
and drought-tolerant plants have found that resistance to the two
stresses also seems to go hand-in-hand. Several methods of
conferring resistance to drought and salinity have been identified:
Gaxiola et al. examined the Arabidopsis AVP1 gene encoding
vacuolar H+-pyrophosphatase, a proton pump that transports
ions in and out of plant cell vacuoles. When overexpressed, the H+-pump
increased solute accumulation in the vacuoles of plant leaf tissue,
which led to increased water retention. Because of the increased
uptake of solutes like sodium and potassium ions, Arabidopsis plants
overexpressing the AVP1 gene showed greater resistance to salt
and drought than wild-type plants (Gaxiola et al, 2001).
Xu et al. introduced the barley late embryogenesis abundant (LEA)
protein gene HVA1 into rice. LEA proteins are highly
accumulated in embryos at the late stage of seed development. In
rice, expression of the HVA1 gene led to accumulation of the HVA1
protein in both leaves and roots of rice plants, similar to the effects
of the AVP1 gene in Arabidopsis. Second-generation
transgenic rice plants showed increased tolerance to salinity and
drought, as well as maintaining higher growth rates under stress
conditions than wild-type rice plants (Xu et al, 1996).
Microbiologists at the University of Illinois at Urbana-Champaign
identified several "osmoprotectants," compounds that shield proteins and
membranes from the effects of dehydration in some plants. When
expressed in transgenic tobacco, these compounds conferred some degree
of drought-resistance, although they did not make a significant
difference in the field (Kaufman, 2002).
Cornell University scientists have developed the most promising method
of conferring tolerance to salinity and drought. Adding the E.
coli gene encoding trehalose, a disaccharide of glucose, to rice
plants enabled the rice to withstand drought conditions, high salinity,
and cold temperatures, as well as increasing growth rates under normal
conditions due to an elevated capacity for photosynthesis.
Trehalose is not thought to accumulate to detectable levels in most
plants, but is found in desiccation-tolerant "resurrection plants" and
bacteria, fungi, and invertebrates under abiotic stress. The
transgenic rice accumulated trehalose at levels three to ten times that
of non-transgenic plants, conferring very high levels of tolerance to
salinity and drought and increasing the rate of photosynthesis, which
could prove extremely beneficial to farmers seeking to increase their
yields (Garg et al, 2002).
Metals in the Soil
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