Johnson J. F., Allan D. L., Vance C. P.
Department of Soil Science, (J.F.J., D.L.A.), and United States Department of Agriculture-Agricultural Research Service, Department of Agronomy and Plant Genetics (C.P.V.), University of Minnesota, St. Paul, Minnesota 55108-6028.
Plant Physiol. 1994 Feb;104(2):657-665. doi: 10.1104/pp.104.2.657.
Proteoid roots develop in Lupinus albus L. in response to nutrient stress, especially P. Proteoid roots excrete citrate and thus increase the availability of P, Fe, and Mn in the rhizosphere. In an effort to understand citrate synthesis and organic acid metabolism in proteoid roots of lupin, we have evaluated in vitro enzyme activities of citrate synthase (CS), malate dehydrogenase (MDH), and phosphoenolpyruvate carboxylase (PEPC) in proteoid and normal roots of plants grown with or without P. Organic acid concentrations, respiration rates, and dark 14CO2-labeling patterns were also determined. The in vitro specific activities of CS, MDH, and PEPC and in vivo dark 14CO2 fixation were higher in proteoid roots compared to normal roots, particularly under P stress. Western blot analysis showed that PEPC enzyme protein was more highly expressed in -P proteoid roots compared to other tissues. The majority of the fixed 14C was found in organic acids, predominantly malate and citrate. A larger fraction of citrate was labeled in P- stressed proteoid roots compared to other root tissue. Respiration rates of proteoid roots were 31% less than those of normal roots. The data provide evidence for increased synthesis of citrate in proteoid roots compared to normal roots, particularly under P stress. A portion of the carbon for citrate synthesis is derived from nonautotrophic CO2 fixation via PEPC in proteoid roots.
白羽扇豆在营养胁迫尤其是缺磷情况下会形成排根。排根会分泌柠檬酸，从而提高根际土壤中磷、铁和锰的有效性。为了了解白羽扇豆排根中柠檬酸合成及有机酸代谢情况，我们评估了在有磷和无磷条件下生长的植株的排根和正常根中柠檬酸合酶（CS）、苹果酸脱氢酶（MDH）和磷酸烯醇式丙酮酸羧化酶（PEPC）的体外酶活性。还测定了有机酸浓度、呼吸速率和暗条件下的14CO2标记模式。与正常根相比，排根中CS、MDH和PEPC的体外比活性以及体内暗条件下的14CO2固定率更高，尤其是在缺磷胁迫下。蛋白质免疫印迹分析表明，与其他组织相比，-P排根中PEPC酶蛋白的表达水平更高。大部分固定的14C存在于有机酸中，主要是苹果酸和柠檬酸。与其他根组织相比，缺磷胁迫下的排根中有更大比例的柠檬酸被标记。排根的呼吸速率比正常根低31%。这些数据证明，与正常根相比，排根中柠檬酸的合成增加，尤其是在缺磷胁迫下。柠檬酸合成的一部分碳源自排根中通过PEPC进行的非自养CO2固定。
