Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: July 1, 1998
Publication Date: N/A
Technical Abstract: Acid soils are a severe impediment to the growth of crops in many areas of the world. Under low soil pH conditions, aluminum (Al) in soil becomes soluble and phytotoxic. The target of Al is the root tip and plants suffering from Al toxicity have reduced root systems and diminished capacity for water and nutrient uptake. Tolerance to Al is increased in roots that secrete organic acids, which chelate and block Al uptake into cells. To elevate organic acid synthesis, alfalfa cDNAs for phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH), two enzymes critical for organic acid accumulation, were introduced into alfalfa and tobacco under the control of the CaMV 35S promoter. The activity of MDH was measured spectrophotometrically in tobacco leaf tissue and alfalfa root tissue. Four out of 14 transgenic tobacco plants and 3 out of 9 transgenic alfalfa plants had MDH activities higher than control plants. Aluminum sensitivity was measured after exposing roots to a nutrient solution containing 100 micromolar AlK(SO4)2 at pH 4.1 for 24 h by staining with hematoxylin, a compound that binds Al to form a purple complex. Roots from alfalfa and tobacco plants with increased MDH activity had little or no reaction with hematoxylin indicating negligible Al uptake. Acidification of the rhizosphere was assayed visually by overlaying roots with an agarose gel containing the pH indicator bromocresol purple. Tolerance of plants to growth in acidic nutrient solutions and soil containing Al will be presented. Constitutive expression of the MDH cDNA in both transgenic alfalfa and tobacco plants appears to increase tolerance of plants to Al in acidic conditions.