Carbon Dioxide Fixation by Lupin Root Nodules

Abstract

In vivo C02 fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels have been measured in lupin (Lupinus angustifolius L.) root nodules of various ages. Both activities were greater in nodule tissue than in either primary or secondary root tissue, and increased about 3-fold with the onset of N2 fixation. PEP carboxylase activity was predominantly located in the bacteroid-containing zone of mature nodules, but purified bacteroids contained no activity. Partially purified PEP carboxylases from nodules, roots, and leaves were identical in a number of kinetic parameters. Both in vivo CO2 fixation activity and in vitro PEP carboxylase activity were significantly correlated with nodule acetylene reduction activity during nodule development. The maximum rate of in vivo CO2 fixation in mature nodules was 7.9 nmol hour-1 mg fresh weight-', similar to rates of N2 fixtion and reported values for amino acid translocation.The results suggest that the oxaloacetete used as the primary "carbon skeleton" acceptor for ammonia assimilation and amino acid synthesis in lupin nodules is provided via the PEP carboxylase reaction rather than through the tricarboxylic acid cyde. The source of PEP is presumably glycolysis, while the major source of CO2 is inferred to be respirtion.The symbiotic association which develops when legume roots are infected with an appropriate strain of Rhizobium is able to convert atmospheric N2 into ammonia. The ammonia is then assimilated into amino acids which are used for plant growth (1).The pathway for ammonia assimilation has been elucidated for lupin and other legumes (9,11,12).In lupin, asparagine is the main compound exported from the nitrogen-fixing nodule tissue, together with smaller amounts of aspartate, glutamine, glutamate, and threonine (11,13 (4), as are also the root and nodule enzymes for ammonia assimilation (9, 11-13). In addition, it has been shown that PEP carboxylase is present in the root nodules of broad bean, and that bean nodules take up CO2 (8).In this paper we present results showing that both in vitro PEP carboxylase activity and in vivo C02 fixation are found in detached lupin root nodules at rates suggesting that PEP carboxylase may be the main source of oxaloacetate for ammonia assimilation.MATERIALS AND METHODS Lupinus angustifolius L. cv. Bitter Blue, inoculated with Rhizobium strain NZP 2257, was grown and harvested as previously described (14).For each experiment, detached nodules from at least three plants were mixed and divided into a sufficient number of samples of 50 to 200 mg fresh wt for separate duplicate measurements of acetylene reduction and PEP carboxylase activity and quadruplicate measurements of CO2 fixation.Acetylene reduction activity was assayed as before (14). In vivo C02 fixation was measured by incubating detached nodules at 25 C in a 25-ml Erlenmeyer flask containing a small piece of moist filter paper. The assay was initiated by the injection of 100 ,ll of 14CO2 (Radiochemical Centre, Amersham; specific radioactivity 5-10 Ci/mol) ...

Phosphorus Stress-Induced Proteoid Roots Show Altered Metabolism in Lupinus albus

“…The protocol for the in vivo C 0 2 fixation assay for excised roots was modified from that used by Christeller et al (1977) and Maxwell et al (1984). Excised root tissue (350 mg fresh weight) was labeled with 7.4 X 104 Bq of aqueous NaH14C03 (2.1 X 109 Bq mmol-'; ICN Radiochemicals, Irvine, CA) for 20 min at 23OC.…”

Routes of pyruvate synthesis in phosphorus‐deficient lupin roots and nodules

“…In amide-exporting legumes, this route via PEPc is vital to provide C skeletons for the assimilation of NH 4 + (Christeller et al ., 1977;Vance et al ., 1985;Vance & Snapp, 1986). Furthermore, PEPc provides a portion of the C for both malate and aspartate synthesis (Romanov et al ., 1985).…”

Carbon Dioxide Fixation in Roots and Nodules of Alnus glutinosa

“…This is especially true in nodules of amide-exporting plants like lupin (3). Davies (7) has suggested that PEP carboxylase may function in duo with malate dehydrogenase as a pH stat for regulating intracellular pH.…”

“…Roots and nodules of N2-fixing leguminous plants such as lupins (3,16), peas (19,20), broad beans (15), garden beans (5), cowpeas (16), and soybeans (4) possess an active system for assimilating CO2. Although lesser activities of other carboxylating activities have been reported (8,15), PEP' carboxylase appears to be the primary enzyme responsible for CO2 fixation in these tissues (22).…”