Plant Physiol. (1992) 99, 748-750 Received for publication October 28, 1991 0032-0889/92/99/0748/03/$01 .00/0 Accepted December 2, 1991

Communication A Developmental Analysis of the Enolase from Ricinus communis'

Jan A. Miernyk* and David T. Dennis Seed Biosynthesis Research Unit, U.S. Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research, Peoria, Illinois 61604 (J.A.M.); and Biology Department, Queen's University, Kingston Ontario, Canada K7L 3N6 (D. T.D.)

ABSTRACT clarified by centrifugation as previously described (11). In Enolase activity was measured in clarified homogenates of Ricinus, it is difficult to determine the precise date of polli- various tissues during the life cycle of the castor oil plant (Ricinus nation. Hence, a staging system was developed, based upon communis L. cv Baker 296). The proportions of total activity due seed size, endosperm fresh and dry weights, percentage of to the plastid and cytosolic isozymes were determined after total lipid, and seed coat morphology, similar to the system separation by ion-exchange chromatography. The contribution of described for developing cotton seeds (14). For germinating the plastid varied from more than 30% of the total at the endosperm, quiescent seeds were imbibed overnight in run- midpoint of endosperm development to less than 1% in mature ning tap water (day 0), then planted in trays containing leaves and roots. During endosperm development, enolase activ- vermiculite and placed ity increased to a peak coincident with the maximum rate of in a glasshouse at 28°C and ambient storage lipid accumulation, then decreased to nearly undetecta- light. To remove some pigments and to concentrate proteins, ble levels in the mature seed. Plastid enolase protein, measured developing seedling tissues or organelle fractions were homog- using an -linked immunosorbent assay, increased in par- enized in 10 volumes of -20°C acetone, then centrifuged and allel with the increase in activity but decreased less rapidly and the acetone decanted. The pellet was rehomogenized in 5 was still easily detectable in mature seeds. volumes of -20°C acetone and recentrifuged. The acetone powders were allowed to air dry in a fume hood and then stored desiccated at room temperature. Acetone powders were homogenized in a manner identical to that for native tissues, The developing endosperm of the COS2 has been used as a but in 10 volumes of buffer containing 2 mm DTT, 2 mM model system to study compartmentation of the glycolytic ascorbate, and 2% insoluble PVP. Low mol wt compounds pathway in plants (reviewed in ref. 4). In this tissue, all of the were removed by centrifugation of samples of clarified ho- glycolytic occur as distinct isozymes, present in both mogenates through 2.5-mL bed volume columns of Sephadex the cytosol and plastids. The results of surveys of other plant G-25, previously equilibrated with homogenization buffer, set systems have, in essence, agreed with this model (7, 13). Most in conical Corex tubes. Enzyme assays, separation of the research on the control ofglycolytic activity has addressed the enolase isozymes, immunochemical analyses, and all other acute, short-term regulatory properties of a few key enzymes: materials and methods have been previously described (1 1- , phosphofructokinase, and . There 13). The data for total endosperm enolase activity were ana- have been a few partial developmental analyses of glycolytic lyzed using the curve-fitting capability of the Sigmaplot 3.0 activity during COS maturation (3, 8, 17) or postgerminative graphics program from Jandel Scientific. The program em- catabolism (3), but there is not yet sufficient information to ploys the Marquardt-Levenberg algorithm to fit an equation propose a long-term control mechanism. Enolase (EC to the data by iterative nonlinear regression. 4.2.1.1 1) has been used as a probe with which to study various aspects of in the developing COS endosperm (1 1- RESULTS AND DISCUSSION 13). Herein we present a detailed developmental analysis of Ion-filtration chromatography using Sephadex A-25 at pH the activities of the enolase isozymes, and propose a possible 6.7 will separate the plastid and cytosolic isozymes of enolase control mechanism. from developing COS endosperm (11, 12). Although this method is rapid and has high resolution, relatively large MATERIALS AND METHODS amounts of starting material are required. For this reason, Developing COS (Ricinus communis L. cv Baker 296) ion-exchange chromatography using DEAE-Sephacel, equili- endosperm were selected, homogenized, and the homogenates brated under identical conditions, was used to separate the isozymes from tissues ofdeveloping castor oil plants. A typical ' Supported in part by a grant from the Natural Sciences and ion-exchange profile, from young, rapidly expanding leaves, Engineering Research Council of Canada. is presented in Figure 1. 2 Abbreviation: COS, castor oil seed. Total enolase activity, as a function of age during endo- 748 ENOLASE ISOZYMES 749

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I I 10 30 30 40 s00 2 4 Days After Flowering Days Of Germination Figure 2. Endosperm enolase during development, maturation, qui- 20 30 40 escence, and subsequent germination of castor oil seeds. Panels A Fraction Number and D are total enolase activity; panels B and E are the proportion of total activity that can be attributed to the plastid isozyme; panels C Figure 1. Anion-exchange chromatography, using DEAE-Sephacel, and F are plastid enolase protein in the same samples as presented of a clarified homogenate (A), a purified chloroplast fraction (B), and in panel A, as determined by ELISA. Values during germination are a cytosol fraction (C) prepared from young, rapidly expanding Ricinus means ± SE for 10 separate experiments. leaves and assayed for enolase activity. In each case, the acetone powder from approximately 5 g of tissue was used. The column was equilibrated with 20 mm imidazole, pH 6.7, containing 2 mm MgCI2. Actual enzyme activities in nmol min-1 fraction-' can be obtained by % ISOZYME multiplying ordinate values by 30 (A), 2.7 (B), or 21 (C). Recoveries TISSUE ENOLASE ACTIVITY DISTRIBUTION following chromatography were 107% (A), 93% (B), and 89%. (,umol / min I gfw) plastid cytosol 1 1.54 o 100 sperm development, maturation, quiescence, and subsequent 2 2.38 12 88 germination, is presented in Figure 2. Over a period of 9 3 1.57 18 82 months, endosperm were dissected from 100 developing seeds of various ages, individually homogenized, and the clarified 4 0.63 0 100 homogenates assayed for total enolase activity (Fig. 2, A and D) or plastid enolase protein (Fig. 2, C and F). During seed germination and postgerminative growth, endosperm from 10 seeds were selected daily and individually analyzed. 5 0.29 7 93 At regular developmental intervals, 3 to 5 g of dissected endosperm were homogenized and total enolase activity re- solved into the plastid and cytosolic isozymes by ion-exchange Figure 3. Total enolase activity and the percentage distribution of chromatography (Fig. 2, B and E). The peak endosperm the plastid and cytosolic isozymes in the various tissues of the castor activities of both total and plastid enolase occurred around oil plant. Tissues are: 1, mature leaves; 2, young, expanding leaves; 30 days after anthesis, during seed development, and after 3 3, fully expanded green cotyledons; 4, stems; 5, roots. to 4 d of postgerminative growth, coincident with peak levels of storage lipid accumulation and organelle membrane bio- genesis, respectively. Total enzyme activity per g fresh weight was determined for various tissues from Ricinus seedlings (Fig. 3). The highest 750 MIERNYK AND DENNIS Plant Physiol. Vol. 99, 1992 total activity and the greatest proportion of plastid activity on reciprocal Southern blots (2). Given the widespread occur- were found in young, rapidly expanding leaves. No plastid rences of plastid enolase isozymes (13), it seems less likely enolase activity was detectable in stems or mature leaves (Fig. that Arabidopsis is exceptional than that in this case inappro- 3), nor was plastid enolase protein detectable in these tissues priate methods of detection were employed. by ELISA (data not presented). The highest plastid enolase activity in the seedlings was found in the cotyledons (18%), ACKNOWLEDGMENT followed by young leaves (12%) and roots (7%). We have previously demonstrated that plastid glycolysis F.C. Felker provided artistic assistance with Figure 3. can provide pyruvate that is subsequently converted to acetyl- CoA by the plastid complex (10, 16). LITERATURE CITED This acetyl-CoA is the substrate for de novo fatty acid synthe- 1. Blakeley SD, Plaxton WC, Dennis DT (1990) Cloning and sis, which occurs exclusively within the plastids of higher characterization of a cDNA for the cytosolic isozyme of plant In agreement with this model, plastid enolase pyruvate kinase: the relationship between the plant and non- plant cells. plant enzyme. Plant Mol Biol 15: 665-669 activity is highest during the periods of most rapid storage 2. Blakeley SD, Plaxton WC, Dennis DT (1991) Relationship be- lipid accumulation during endosperm development (Fig. 2), tween the subunits of leucoplast pyruvate kinase from Ricinus or during periods of elevated membrane lipid synthesis (e.g. communis and a comparison with the enzyme from other postgerminative growth and leaf expansion) (Figs. 2 and 3). sources. Plant Physiol 96: 1283-1288 3. Botha FC, Dennis DT (1987) Phosphoglyceromutase activity and Using plastid enolase as the reporter, it appears that there is concentration in the endosperm ofdeveloping and germinating little if any plastid glycolysis in mature or fully differentiated Ricinus communis seeds. Can J Bot 65: 1908-1912 tissues. Acetyl-CoA is still necessary for membrane mainte- 4. Dennis DT, Miernyk JA (1982) Compartmentation of nonpho- nance at these developmental stages, but could be provided tosynthetic carbohydrate . Annu Rev Plant Physiol by alternative pathways (e.g. 9, 19). 33: 27-50 5. Eigenbrodt E, Fister P, Rubsamen H, Friis RR (1983) Influence Whereas plastid enolase followed a developmental program of transformation by Rous sarcoma virus on the amount, parallel to that ofstorage lipid accumulation (Fig. 2B), plastid phosphorylation and enzyme kinetic properties of enolase. enolase protein did not (Fig. 2C). Activity declined rapidly to EMBO J 2: 1565-1570 near zero levels in the mature, quiescent seed, whereas plastid 6. Ireland RJ, Dennis DT (1980) Isozymes of the glycolytic and pentose-phosphate pathways in storage tissues of different oil enolase protein declined more slowly and was still detectable seeds. Planta 149: 476-479 in the mature seed. It is possible that the loss of activity was 7. Ireland RJ, Dennis DT (1981) Isoenzymes of the glycolytic and due to some sort of posttranslational modification. pentose-phosphate pathway during the delopment ofthe castor It has been reported that enolases from various sources can oil seed. Can J Bot 59: 1423-1425 be phosphorylated, and that phosphorylation reduces catalytic 8. Lal SK, Johnson S, Conway T, Kelley PM (1991) Characteriza- tion of a maize cDNA that complements an enolase-deficient activity (5). Alternatively, phosphorylation could be a signal mutant of Escherichia coli. Plant Mol Biol 16: 787-795 for proteolysis (10). Proteolytic modification of plastid gly- 9. Masterson C, Wood C, Thomas DR (1990) L-Acetyl-carnitine, a colytic isozymes has been demonstrated (3, 15), and a modi- substrate for chloroplast fatty acid biosynthesis. Plant Cell fied protein could give a positive ELISA signal, whereas Environ 13: 767-771 catalytic activity was greatly reduced or absent. More detailed 10. Miernyk JA, Camp PJ, Randall DD (1985) Regulation of plant pyruvate dehydrogenase complexes. Curr Top Plant Biochem analyses than those reported here will be necessary to resolve Physiol 4: 175-190 the various possibilities. 11. Miernyk JA, Dennis DT (1982) Isozymes of the glycolytic path- Recently, enolase from higher plants has been addressed at way in developing castor oil seeds. Plant Physiol 69: 825-828 the molecular level (8, 18). The results of Southem analyses 12. Miernyk JA, Dennis DT (1984) Enolase isozymes from Ricinus in most enolase communis. Partial purification and characterization of the ofplant genomic DNA suggest that instances isozymes. Arch Biochem Biophys 233: 643-651 is encoded by a small multigene family (18), consistent with 13. Miernyk JA, Dennis DT (1987) Enolase isozymes from devel- the multiple enolase isozymes typically observed at the protein oping oilseeds: a survey. J Plant Physiol 128: 351-359 level (13). Arabidopsis and maize were exceptions in that 14. Miernyk JA, Trelease RN (1981) Role of malate synthase in when genomic Southern blots were probed with cytoplasmic citric acid synthesis by maturing cotton embryos. A proposal. Plant Physiol 67: 875-881 enolase cDNAs, only a single gene was detected (8, 18). 15. Plaxton WC (1991) Leucoplast pyruvate kinase from developing Based upon the results of Southern analyses plus an inabil- castor oil seeds. Characterization of the enzyme's degradation ity to measure enolase in purified chloroplasts, Van Der by a cysteine endopeptidase. Plant Physiol 97: 1334-1338 Straeten et al. ( 18) suggested that Arabidopsis plastids lack at 16. Randall DD, Miernyk JA, Fang TK, Budde RJA, Schuller KA least part (enolase) of the glycolytic pathway. Although this (1989) Regulation of the pyruvate dehydrogenase complexes in plants. Ann NY Acad Sci 573: 192-205 could be true, such a conclusion is at this point premature. 17. Simcox PD, Garland W, DeLuca V, Canvin DT, Dennis DT As we report herein, the ability to measure plastid enolase (1979) Respiratory pathways and fat synthesis in the develop- activity is dependent upon plant developmental state. Fur- ing castor oil seed. Can J Bot 57: 1008-1014 thermore, there is not always immunochemical cross-reactiv- 18. Van Der Straeten D, Rodrigues-Pousada RA, Goodman HM, ity between cytoplasmic and plastid isozymes of enolase (12) Van Montagu M (1991) Plant enolase: gene structure, expres- sion, and evolution. Plant Cell 3: 719-735 or other glycolytic enzymes (1, 2). Finally, there are instances 19. Zeiher CA, Randall DD (1991) Spinach leaf acetyl-CoA synthe- in which cDNAs for plastid and cytoplasmic glycolytic iso- tase. Purification and characterization. Plant Physiol 96: zymes have too little homology to generate positive signals 382-390