The development of clustered tertiary lateral roots (proteoid roots) and the expression of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in roots were studied in white lupin (Lupinus albus L.) grown with either 1 mM P (+P-treated) or without P (-P-treated). The +P-treated plants initiated fewer clustered tertiary meristems and the emergence of these meristems was delayed compared with -P-treated plants. Proteoid root zones could be identified 9 d after emergence in both P treatments. Amounts of PEPC mRNA, PEPC specific activity, and enzyme protein were greater in proteoid roots than in normal roots beginning at 1 O, 12, and 14 d after emergence, respectively. The increases in PEPC mRNA, PEPC enzyme, and PEPC specific activity suggest that this enzyme is in part under transcriptional regulation. Recovery of organic acids from root exudates coincided with the increases in PEPC specific activity. The -P-treated plants exuded 40-, 20-, and 5-fold more citrate, malate, and succinate, respectively, than did +P-treated plants. Data presented support the hypothesis that white lupin has concerted regulation of proteoid root development, transcriptional regulation of PEPC, and biosynthesis of organic acids for exudation in response to P deficiency.Proteoid root formation is enhanced by P deficiency in white lupin (Lupinus albus L.) (Gardner et al., 1981; Marschner et al., 1986; Dinkelaker et al., 1989; Johnson et al., 1994; Braum, 1995). The amount of citrate exuded by proteoid roots ranges from 6 to 23% of total plant dry weight (Dinkelaker et al., 1989; Braum, 1995). These amounts are phenomenal when compared with amounts in alfalfa (Medicago sativa L.), which exudes the equivalent of 0.3% of the total dry weight as citrate when P deficient (Lipton et al., 1987). Citrate exudation into the rhizosphere increases the P available to the plant by mobilizing the sparingly soluble mineral P and, possibly, organic P sources (Gardner et al

Proteoid roots develop in Lupinus albus 1. 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 i n vitro enzyme activities of citrate synthase (CS), malate dehydrogenase (MDH), and phosphoenolpyruvate carboxylase (PEPC) i n proteoid and normal roots of plants grown with or without P. Organic acid concentrations, respiration rates, and dark ''COz-labeling patterns were also determined. The in vitro specific activities of CS, MDH, and PEPC and in vivo dark "COZ fixation were higher i n 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 Pstressed 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 COz fixation via PEPC in proteoid roots.

When white lupin (Lupinus albus 1.) is subjected to P deficiency lateral root development is altered and densely clustered, tertiary lateral roots (proteoid roots) are initiated. These proteoid roots exude large amounts of citrate, which increases P solubilization. In the current study plants were grown with either 1 mM P (+P-treated) or without P (-P-treated). Shoots or roots of intact plants from both P treatments were labeled independently with 14C0, to compare the relative contribution of C fixed in each with the C exuded from roots as citrate and other organic acids. About 25-fold more acid-stable 14C, primarily in citrate and malate, was recovered in exudates from the roots of -P-treated plants compared with +P-treated plants. l h e rate of in vivo C fixation in roots was about 4-fold higher in -P-treated plants than in +P-treated plants. Evidente from labeling intact shoots or roots indicates that synthesis of citrate exuded by -P-treated roots is directly related to nonphotosynthetic C fixation in roots. C fixed in roots of -P-treated plants contributed about 25 and 34% of the C exuded as citrate and malate, respectively. Nonphotosynthetic C fixation in white lupin roots is an integral component in the exudation of large amounts of citrate and malate, thus increasing the P available to the plant.