Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/18/2002
Publication Date: 1/1/2003
Citation: UDHE-STONE, C., GILBERT, G., JOHNSON, J.M., LITJENS, R., ZINN, K.E., TEMPLE, S.J., VANCE, C.P., ALLAN, D. ACCLIMATION OF WHITE LUPIN TO PHOSPHORUS DEFICIENCY INVOLVES ENHANCED EXPRESSION OF GENES RELATED TO ORGANIC ACID METABOLISM. PLANT AND SOIL. 2003. V. 248. P. 99-116. Interpretive Summary: Phosphorus is an important mineral nutrient, which is frequently deficient in mineral soils. White lupin, a legume plant adapts to phosphorus deficiency by developing short, densely clustered lateral roots. These specialized roots discharge large amounts of citric and malic acid, which are organic acids. These acids improve the plants ability to absorb phosphorus from the soil. The enhanced release of these acids from phosphorus deficient white lupin roots is accompanied by increased activity of the enzymes phosphoenolpyruvate carboxylase and malate dehydrogenase. These enzymes have a role in the production of organic acids. Using genetic engineering methods, we found that the enzymes phosphoenolpyruvate carboxylase and malate dehydrogenase are made at a higher rate in the specialized roots of phosphorus deficient white lupin plants compared to roots of plant grown with adequate phosphorus. Taken together, the results suggest that adaptation of white lupin to low phosphorus changes the activity of plant genes involved in making and using carbon compounds to aid in the release of citrate and malic acid. The results of these experiments will help scientists better understand how plants adapt to phosphorus stress. Information from these experiments may also be useful for developing ways to improve how plants absorb and use phosphorus. Improvement in phosphorus uptake by plants could reduce the need for phosphorus fertilizer, which is a potential source of ground water pollution. Phosphorus fertilizer production and application are energy consuming, which can limit use of phosphorus fertilizer in developing countries.
Technical Abstract: White lupin adapts to phosphorus deficiency (-P) by the development of short, densely clustered lateral roots called proteoid (or cluster) roots. These specialized plant organs display increased exudation of citric and malic acid. The enhanced exudation of organic acids from P stressed white lupin roots is accompanied by increased in vitro phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH) activity. Here we report the cloning of full-length white lupin PEPC and MDH cDNAs. RNA blot analysis indicates enhanced expression of these genes in -P proteoid roots, placing higher gene expression at the site of organic acid exudation. Correspondingly, macroarray analysis of about 1250 ESTs (expressed sequence tags) revealed induced expression of genes involved in organic acid metabolism in -P proteoid roots. In situ hybridization revealed that PEPC and MDH were both expressed not only in mature rootlets but also in emerging proteoid rootlets. A C3 PEPC protein was partially purified from proteoid roots of P-deficient white lupin. Native and subunit Mr were determined to be 440 kD and 110 kD, respectively. Citrate and malate were effective inhibitors of in vitro PEPC activity at pH 7. Addition of ATP partially relieved inhibition of PEPC by malate, but had little effect on citrate inhibition. Taken together, the results presented here suggest that adaptation of white lupin to low P involves modified expression of plant genes involved in carbon metabolism.