Metals in the Soil
Aluminum is a metal toxic to plant and animal
cells. As mentioned previously, strongly acidic soils often
contain high levels of aluminum compounds (Plucknett and Smith, 1982).
Over 40% of the world's arable lands are acidic; in these acid soils,
the aluminum ion Al3+ exists in soil solution and is toxic to
plant root growth and function (Jones and Kochian, 1997). Aluminum
toxicity is the primary factor limiting crop production on these acid
soils. Researchers have not yet determined why aluminum is toxic;
some suggest that Al3+ ions affect phospholipids and proteins
in cell membranes (Ezaki et al., 2000). Two groups of researchers
have found confer aluminum resistance to crop plants:
Ezaki et al. examined nine aluminum-induced genes
from tobacco, wheat, Arabidopsis, and yeast. Transgenic
Arabidopsis lines overexpressing these genes were tested in root
elongation assays observing root growth in the presence of aluminum,
cadmium, copper, sodium, and zinc. Resistance was determined by
root growth relative to wild-type, non-transgenic Arabidopsis.
Expression in Arabidopsis of four of the nine genes yielded
phenotypically normal plants with a degree of resistance to aluminum;
one of these four genes also increased resistance to copper and sodium.
Although the levels of resistance were not very high, and differences in
resistance were observed over a narrow window of aluminum
concentrations, the results were statistically significant and
reproducible. The researchers concluded that overexpression of any
of the four genes can protect plant cells against aluminum toxicity
(Ezaki et al., 2000).
An ongoing Canadian project called the Functional Genomics of Abiotic
Stress in Wheat and Canola Crops examined aluminum stress in canola and
its distant relative Arabidopsis. The project studied the effects
of aluminum stress using proteomics, the study of the protein complement
of a genome. About 100 microtubule-associated proteins have been
identified in the plant cytoskeleton and root rhizosphere, the region of
the soil in contact with the roots; its composition is affected by root
activity. These microtubule-associated proteins respond to
aluminum stress and may help protect plants against aluminum toxicity
(Functional Genomics, 2002).
Drought and Salinity
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