Plant Physiol. (1987) 84, 1447-1450 0032-0889/87/84/ 1447/04/$0 1.00/0

Impairment of Photorespiratory Carbon Flow into Rubber by the Inhibition of the Glycolate Pathway in Guayule ( argentatum Gray)' Received for publication September 12, 1986 and in revised form April 20, 1987 Downloaded from https://academic.oup.com/plphys/article/84/4/1447/6084770 by guest on 29 September 2021 A. RAMACHANDRA REDDY, M. SUHASINI, AND V. S. RAMA DAS* Department ofBotany, School ofBiological and Earth Sciences, Sri Venkateswara University, Tirupati 517 502, India

ABSTRACT in the formation of chloroplast terpenoids is of greater interest (9, 15, 23). The intent of the present investigation is to examine Cut shoots of guayule (Parthenium argentatum Gray) were treated the role of carbon from the photorespiratory cycle in providing with four inhibitors of the glycolate pathway (a-hydroxypyridinemetha- for nesulfonic acid; isonicotinic acid hydrazide, glycine hydroxamate, and precursors rubber biosynthesis in guayule. amino-oxyacetate, AOA) in order to evaluate the role of photorespiratory intermediates in providing precursors for the biosynthesis of rubber. MATERIALS AND METHODS Photorespiratory CO2 evolution in guayule leaves was severely inhibited Radiochemicals were obtained from either the Amersham by AOA. Application of each of the four inhibitors has resulted in a Corporation or from Bhabha Atomic Research Center, Bombay. significantly decreased incorporation of '4C into rubber fractions sug- The a-HPMS2 was obtained from Fluka, Switzerland, and was gesting that the glycolate pathway is involved in the biosynthesis of recrystallized twice from 25% before use. INH, GH, rubber in guayule. However, the application of each of the glycolate AOA, and all other biochemicals were purchased from Sigma. pathway inhibitors showed no significant effect on photosynthetic CO2 Guayule (Parthenium argentatum Gray) var 1591 were fixation in the leaves. The inhibitors individually also reduced the incor- grown in 30 cm pots under natural photoperiod. Plants received poration of labeled glycolate, glyoxylate, and glycine into rubber, while full solar irradiance for most of the day in an 11 h photoperiod. the incorporation of serine and pyruvate was not affected. The effective The maximum light intensity (PAR, 400-700 nm) available at inhibition of incorporation of glycolate pathway intermediates in the the top of the canopy was 1800 to 2000 zE m 2s-' on a clear presence of AOA was due to an inhibition of glycine decarboxylase and day. Daily maximum and minimum air temperatures had ranges serine hydroxymethyltransferase. It is concluded that serine is a putative 29 to 33°C and 17 to 22°C, respectively. The present investigation photorespiratory intermediate in the biosynthesis of rubber via pyruvate was carried out during the winter months (November to January) and acetyl coenzyme A. of 1985 to 1986. The average daily maximum and minimum temperatures during this period was 31 and 17°C, respectively. Two to 3-year-old plants were used in the present study. Anatomy ofstem sections showed that the majority were filled with rubber having been through two winters. Analysis of 2-year-old plants showed that they contained 5.8% (dry weight basis) rubber. The Much of the early work on biosynthesis of rubber and its cyclic nature ofrubber accumulation was evident, as significantly regulation has been made in the laticiferous systems of Hevea high rate of rubber accumulation was noticed during winter braziliensis (2, 3, 5). However, in the last decade, renewed months (November to January). research has been directed toward establishing guayule (Parthen- Photosynthesis. CO2 exchange rates in the leaves (usually ium argentatum Gray) as an additional source to . second or third from the top ofshoots) were measured according In contrast to Hevea , the major deposition of rubber in to Monson et al. (18), with an IRGA (Analytical Development guayule occurs in the parenchyma cells of the stem and root, Company Ltd., England) using open gas circuit system. A plex- while in the leaves the quantity of rubber has never exceeded iglass cuvette (4 x 4 x 0.5 cm) was used as the photosynthetic 0.5% (3, 14). Hence increased experimentation is essential to chamber enclosing the lower and upper surfaces of the leaf. Air establish the biosynthetic mechanism of rubber formation in entering into the photosynthetic chamber was humidified and guayule. This is expected to lead to a better understanding of maintained at constant temperature (30 ± 1VC) by bubbling various control mechanisms involved in rubber biosynthesis. through water. Air flow containing 340 Id/L CO2 and 21% 02 Even though recent studies have placed much emphasis on was adjusted to 0.4 L/min. After preillumination for 30 to 40 the role of chloroplasts in isoprenoid production ( 15), the exact min, CO2 depletion by leaves was monitored until a steady state nature of the precursor used for rubber formation is unknown. was recorded. Illumination was provided by halide flood lamp A direct carbon flow from the primary photosynthetic product behind a water screen. CO2 exchange rates for individual leaves to isoprenoid compounds has been shown to exist in the chlo- were repeated two to three times on different individual shoots. roplasts of higher plants (4, 6, 7). Among the several functions Photosynthetic Carbon Labeling and Rates of Photorespira- ofglycolate pathway in higherplants, an appreciable contribution 2 Abbreviations: HPMS, a-hydroxypyridinemethanesulfonic acid; 'Research support from the Department of Science and Technology INH, isonicotinic acid hydrazide; GH, glycine hydroxamate; AOA, grant 21(5)/83-STP II, Government of India, and United States Depart- amino-oxyacetate; SHMT, serine hydroxymethyl transferase; MOPS, ment of Agriculture grant FG-IN-576/In-SEA- 17 1 are acknowledged. morpholinopropanesulfonic acid. 1447 1448 REDDY ET AL. Physiol. Vol. 84, 1987 tion. Photosynthetic 14C labeling patterns in the leaves were serine containing 0.3 ,Ci [U-'4]serine. After 15 min the reaction followed in a closed system by supplying '4CO2 (NaH"'CO3, 58.1 was stopped by the addition of 0.5 ml 1 M sodium acetate (pH ,uCi ,mol-'). A cut shoot was placed in a large photosynthesis 5). Hydroxypyruvate reductase was assayed spectrophotomet- chamber at 30°C and flushed with a gas stream (340 ul L-'C02, rically by following the decrease in the absorbance at 340. The 21% 02, balance N2) at 1600 ME m 2s-' for 5 min. A small vial assay mixture (3 ml) contained 10 mm Hepes buffer (pH 7.0), containing NaH'4CO3 was placed in the chamber. The system 0.5 mM NADH, 15 mm hydroxypyruvate, and the enzyme was then closed and '4CO2 was generated by injecting few drops extract. of concentrated sulphuric acid. After 3 min of '4CO2 incorpora- Protein content was estimated according to Lowry et al. (17) tion, the leaves were quickly transferred to liquid N2 and then using BSA as standard. homogenized in 4 M HCO2H. The extracts were pooled, concen- trated and fractioned by ion-exchange and paper chromatogra- RESULTS phy as described by Atkins and Canvin (1). Photorespiratory rates of the leaves of guayule were measured by following CO2 The proportion of '4C incorporated into triose phosphates, Downloaded from https://academic.oup.com/plphys/article/84/4/1447/6084770 by guest on 29 September 2021 evolution into C02 free air (19). hexoses, glycine and serine after 3 min of photosynthesis by 14C Incorporation into Rubber. Uniform cut shoots were placed actively photosynthesizing (37 mg dm-2h-') guayule leaves is in a beaker containing either water which served as control or in given in Table I. The incorporation of carbon into amino acids, solutions of various inhibitors indicated in the tables and figures. particularly into senne (18.9%) was significantly high when The samples were exposed to light (1600 MuE m 2s-') at 30C. compared to glycine (8.2%). Activities of various enzymes in the 14C02 was supplied to leaves as described above. At appropriate leaf extracts of guayule involved in photorespiratory pathway time intervals cuttings were removed, rinsed quickly with dis- were assayed and presented in Table II. The activities of amino- tilled water, and dropped in boiling 80% ethanol. At the end of transferases, glycine decarboxylase and serine hydroxymethyl the 14CO2 fixation period, excess 14C02 was flushed into 4 N transferase were comparable to those reported for other higher KOH. The tissue was homogenized with 80% ethanol and dried plants. In order to elucidate the effects of different photorespir- in an oven at 70C. The dried tissue was powdered in a blender. atory inhibitors on rates of photosynthesis and the rates of CO2 The powder was placed in Whatman cellulose extraction thimble evolution in light were monitored in the leaves ofguayule (Table and extracted thrice with 80% ethanol followed by extraction III). The application of each of the inhibitors showed little effect with acetone for 8 h on a Soxhlet apparatus. The powder was on CO2 fixation in guayule leaves. However, among the four dried and extracted with hexane. The radioactivity in the hexane inhibitors tested, AOA was highly effective in inhibiting the extract was assayed using a Beckman LS 1800 scintillation photorespiratory CO2 release. The effects ofthe photorespiratory counter. To confirm the polymer extracted by hexane is '4C-cis- inhibitors on "4CO2 incorporation into rubber in guayule shoots polyisoprene the radioactive crude rubber fraction was subjected are depicted in Table IV. Again, AOA was highly effective in to ozonolysis and the products chromatographed by the proce- reducing the '4C-incorporation into rubber. Time course of 14C- dure of Park and Bonner (21). The resulting '4C-levulinic acid labeling of rubber has further confirmed the effect AOA which was isolated by chromatography and assayed for radioactivity indicated ca. 60% inhibition of label incorporation (Fig. 1). (21). Radiolabeled photorespiratory intermediates were effectively Administration of "'C-Labeled Precursors. The specific activ- incorporated into rubber (Table V, Fig. 2). The incorporation of ities of the labeled compounds used in the study were: [2-'4C] [2-`'C]pyruvate and L-[U-"4C]serine was highest followed by (2- +glycolate, 9.8 MACi Amol-'; sodium [2-'4C]glyoxylate, 5 MCi '4C) glycine (Table V). The incorporation from [-"'4C]glyoxylate ,Mmol-'; [l-'4C]glycine, 5.5 uCi usmol-'; [2-'4C]glycine, 20 MCi ,umol-'; L[U-'4C]serine, 6.5 MuCi Mmol '; sodium [2-"'C]pyru- Table I. Distribution ofRadioactivity Among Products of vate, 10.5 MCi Amol'1. To study the relative incorporations of Photosynthesisfor 3 min in '4C02 by Leaves ofGuayule the various labeled precursors, the excised ends of the cuttings Actively photosynthesizing (37 mg CO2 dm-2 leaf area h-') young and were immersed in a shallow glass beaker containing the solutions fully expanded leaves were used. "'C-labeling patterns were followed as with radioisotope with or without inhibitor to a depth of 3 cm. described in "Materials and Methods." The shoots were then exposed to light (1600 ME m 2s-') and a of Total continuous air stream was allowed over the leaf surface by using Product ~~Percent Product 14C Extracted a small electric fan placed a few feet away from the experimental setup. The extraction of rubber and assaying of radioactivity Triose phosphates 6.8 ± 0.65a were made as described above. Sucrose 47.2 ± 2.87 Enzyme Assays. Five g of leaf material were homogenized at Glucose and fructose 11.5 ± 0.90 5°C in 50 ml 0.1 M Hepes buffer (pH 7.5) which contained 1 mM Glycine 8.2 ± 0.54 DTT, 1% (w/v) PVP-40. The homogenate was centrifuged at Serine 18.9 ± 1.78 20,000g for 20 min, and the supernatant was used for enzyme Alanine 1.3 ± 0.01 assays. Others 6.0 ± 0.82 Glycolate oxidase was measured polarographically at 30°C a Average ofthree replications ± SE. (12). Glyoxylate reductase was assayed by following the oxidation of NADH (16). Glutamate:glyoxylate aminotransferase and Table II. Enzyme Activities in serine:glyoxylate aminotransferase were assayed according to ofGlycolate Pathway Leaves ofGuayule Rehfeld and Tolbert (22). Enzyme Activity Leaf mitochondria were isolated and purified on a Percoll/ Mgmol mg-' protein h-' sucrose gradient (24). The activities of glycine decarboxylase Glycolate oxidase 3.89 were determined by the method of Shingles et al. (24). SHMT ± 0.04a Glyoxylate was assayed according to Taylor and Weissbach (25) with a reductase 4.72 ± 0.06 detergent-treated mitochondrial fraction. Mitochondria were sus- Glutamate:glyoxylate aminotransferase 11.52 ± 0.18 Serine:glyoxylate aminotransferase 6.35 0.08 pended in a solution containing 20 mm Mops-KOH (pH 7.4), 1 + mM DTT, and 0.05% Triton X-100. The assay was carried out Glycine decarboxylase 6.20 ± 0.06 Serine hydroxymethyltransferase 2.50 ± 0.05 at 28°C in a total volume of 1 ml and contained 50 mm Mops- Hydroxypyruvate reductase 15.60 ± 1.09 KOH (pH 7.4), 20m' tetrahydrofolate, and mitochondrial fraction (50 Mg protein). The reaction was initiated with 5 mm a Average of three replications ± SE. PHOTORESPIRATORY INHIBITORS AND 14C INC,ORPORATION INTO RUBBER IN GUAYULE 1449 Table III. Effect ofGlycolate Pathway Inhibitors on Photosynthesis and decarboxylation (INH, GH, and AOA), AOA was highly effective Photorespiratory CO2 Evolution in Guayule Leaves in suppressing the incorporation ofglyoxylate and glycine (Table The excised shoots were incubated for I h in darkness in the inhibitor V). Figure 3 illustrates the inhibitory effect of AOA on the solutions at the concentrations indicated. The shoots used for control activities of glycine decarboxylase and SHMT. Glycine decar- experiments were incubated in distilled water. Photosynthetic rates were boxylase was more sensitive to AOA than SHMT. measured as described in "Materials and Methods." The rates of photo- respiration were determined as CO2 evolution into CO2-free air. DISCUSSION Experiment CO2 Fixation CO2 Evolution The results described here provide for the first time evidence mg CO2 dMn2 h1 for the role of photorespiratory intermediates in generating pre- Control 37.01 (0)- 5.19 ± 0.41 (0) cursors for rubber biosynthesis. These studies with photorespir- HPMS (10 mM) 33.36 3.34 ± 0.40 atory inhibitors indicate that a portion of the carbon is drained (10)b (36) Downloaded from https://academic.oup.com/plphys/article/84/4/1447/6084770 by guest on 29 September 2021 INH (10 mM) 34.69 (8) 2.56 ± 0.31 (51) from the photorespiratory carbon oxidation cycle into rubber. GH (5 mM) 33.16 (11) 3.88 ± 0.29 (25) The labeling pattern ofglycine and serine during photosynthesis AOA (0.1 mM) 34.05 (8) 2.02 ± 0.18 (61) was presumed to be a reference system for photorespiratory carbon oxidation activity (20, 26, 27). The high labeling ofserine a Average of three replications ± SE. b Figures in parentheses indi- observed in the present study is in agreement with the prelimi- cate the percent inhibition over control. nary findings of Keithly and Benedict (1 1). However, the signif- icance of high amounts of serine synthesis in guayule needs Table IV. Effect ofDifferent Inhibitors ofGlycolate Pathway on further investigaton. Incorporation of4C02 into Rubber Fractions The excised shoots were exposed to 14CO2 for 3 h in a closed photo- Table V. Effects ofInhibitors ofGlycolate Pathway on Incorporation of synthesis chamber as described in "Materials and Methods." At the end '4C-Labeled Glycolate, Glyoxylate, Glycine, Serine and Pyruvate into of the experiment the rubber was extracted into hexane fractions and Rubber Fractions in Detached Shoots ofGuayule assayed for the Most of the was radioactivity. radioactivity recovered The excised shoots were either from stem fractions while leaves contained insignificant amount of placed in distilled water or in inhibitor radioactive rubber. solution for I h and illuminated at 1600 uE m2s-' and later 10 MCi of each labeled compound was administered still in presence ofthe inhibitor '4C Incorporation into .h.t. in separate experiments. The incorporated radioactivity after 2 h expo- Hexane Fractions sure was assayed in the hexane fractions. The radioactivity was mostly cpm x 10~3g'fresh wt % recovered from stem fractions and only insignificant amount of radio- Control 6.29 ± 1.04a 0 activity was noticed in leaves. + HPMS (10 mM) 3.54 ± 0.85 44 '4C Incorporationa +INH(lOmM) 3.09±0.80 51 Compound No +HPMS +INH +GH +AOA + 4.12 ± 0.93 GH (5 mM) 35 inhibitor + AoA (0.1 mM) 2.05 ± 0.86 67 (1O mM) (1O mM) (5 mM) (0.1 mM) cpm x fresh wt a Average ofthree replications ± SE. 10-3g-' [2-`CJGlycolate 8.4* 1.7 3.9 3.0 2.9 [l-'4CJGlyoxylate 2.0 2.0 1.1 1.0 1.1 [2-'4ClGlyoxylate 9.3 8.9 4.6 3.9 3.1 [1-'4CJGlycine 4.2 4.0 1.9 1.0 1.2 mw [2-'4ClGlycine 10.6 10.2 5.0 4.2 3.0 [U-'4CJSerine 15.6 14.9 14.0 14.6 13.5 0 [2-14C]Pyruvate 20.2 19.8 18.2 19.1 18.5 coz ' Average ofthree experiments. ix 0~-

1'5 pyPs E a~.CZ w a: 0 co z =12 1;

-4 0 S z TIME (h ) z ;- FIG. 1. Effect of glycolate pathway inhibitors on time course of 14C- 6 labeling of rubber fractions in guayule shoots. The excised shoots were o C kept in inhibitor solutions (distilled water for control experiments) at the O u 9G A concentrations indicated in the figure. The shoots were to z 3 exposed '4CO2 e-- for various periods of time. Rubber was extracted into hexane fractions ,AX . A. and assayed for radioactivity. 4 0 40 80 120 1bO and [1-'4C]glycine was however low. The incorporation of label TIME (min) from [2-'4C]glycolate was inhibited by HPMS while "'C incor- FIG. 2. Effect of AOA on the time course of 14C incorporation from poration of radioactivity from gIyoxylate and glycine was sup- [2-'4C]pyruvate and [2-'4Clglycine into rubber fractions in guayule. Gua- pressed by INH, GH, and AOA. In contrast, the inhibitors had yule shoots were kept in solutions containing labeled pyruvate (pyr) or very little effect on the incorporation of labeled seine and glycine (gly) (with or without I mm AOA). Rubber was extracted into pyruvate into rubber. Among the three inhibitors of glycine hexane fractions and assayed for radioactivity. 1450 REDDY ET AL. Plant Physiol. Vol. 84, 1987 rubber biosynthesis. Our results also indicate that in presence of photorespiratory inhibitors, '4C02 may not be diverted entirely to hexose formation but some amount of carbon is directed for GLy- decarboxylaSC 0 isoprenoid biosynthesis via serine-acetyl CoA-IPP chain. It is conceivable that early precursors (not sucrose) of rubber are

- E 75 produced in the leaves of guayule (via glycolate, glycine and serine) which are then transported into stems for polymerization. .~'.- / ~~~~SHMT The results from this study highlight the role of photorespira- tory intermediates in providing precursors for isoprenoid biosyn- thesis in the leaves of guayule which thus implicate glycolate 00 metabolism in the production of secondary metabolites. Finally, the results clearly indicate a close association between ph6tores- piration and rubber biosynthesis in guayule. Downloaded from https://academic.oup.com/plphys/article/84/4/1447/6084770 by guest on 29 September 2021 w LITERATURE CITED 1. ATKINS CA, DT CANVIN 1971 Photosynthesis and CO2 evolution by leaf discs: 0 0.05 0.10 015 0.20 gas exchange, extraction and ion-exchange fractionation of '4C-labeled pho- Amino - oxyacetote (mM) tosynthetic products. Can J Bot 49: 1225-1234 2. ARCHER BL 1980 Polyisoprene. In EA Bell, BV Charlwood, eds, Encyclopedia FIG. 3. Effect of AOA concentration on glycine decarboxylase and of Plant Physiology, Vol 8, Secondary Plant Products. Springer-Verlag, SHMT in isolated leaf mitochondrial fractions of guayule. Berlin, pp 309-322 3. BENEDICT CR 1983 Biosynthesis of rubber. In JW Porter, SL Spurgeon, eds, Biosynthesis of Isoprenoid Compounds. John Wiley & Sons, New York, pp The inhibition ofphotorespiratory CO2 release in the presence 357-369 of the inhibitors was consistent with the decreased '4C-label 4. BICKEL H, G SCHULTZ 1976 Biosynthesis of plastoquinone and 0-carotene in incorporation into rubber (Table III and IV). However, the isolated chloroplasts. Phytochemistry 15: 1253-1255 application of each of the inhibitors showed no significant effect 5. BONNER J, JE VARNER 1965 Plant Biochemistry, Ed 2. Academic Press, New York, pp 665-692 on the photosynthetic CO2 fixation in guayule leaves and thus 6. GILLILAND MG, J VAN STADEN, )J MITCHELL 1985 CO2 fixation and rubber the results reported in the present investigation are significantly deposition in the bark and leaves of guayule during winter. J Plant Physiol different from those observed with other plants. It is presumed 121: 369-379 that the concentrations used in the present study can only inhibit 7. GOODWIN TW, El MERCER 1972 Introduction to Plant Biochemistry. Perga- of mon Press, Oxford glycolate pathway in guayule leaves and the concentrations 8. HILL HM, U ROGERS 1972 Bacterial origin of alkaline L-serine dehydratase in the inhibitors may not produce the intermediate metabolites of french beans. Phytochemistry 11: 9-18 glycolate pathway sufficiently enough to inhibit photosynthesis. 9. JENKINS CLD, LJ ROGERS, MW KEER 1983 Inhibition ofglycolate metabolism Studies with the 14-C-intermediates of photorespiratory cycle by amino-oxyacetate: effects on pigment formation in higher plants. Phyto- glycolate chemistry 22: 347-352 (Table V) have further confirmed that the pathway of 10. JONES CA, RA RASMUSSEN 1975 Production of isoprene by leaf tissue. Plant metabolism is intimately associated with the rubber biosynthesis. Physiol 55: 982-987 None of the inhibitors used had any effect on the incorporation 11. KEITHLY JH, CR BENEDICT 1985 The glycerate and glycolate pathways in of either serine or pyruvate into rubber fractions. It is believed guayule leaves. Plant Physiol 77: S-90 may be an important 12. KERR MW, D GROVES 1975 Purification and properties of glycolate oxidase that the serine production from glycine from Pisum sativum leaves. Phytochemistry 14: 359-362 step for the possible flow of photorespiratory carbon into the 13. LAWYER AL, I ZELITCH 1979 Inhibition of glycine decarboxylation and serine rubber biosynthetic chain. AOA and GH are the known inhibi- formation in tobacco by glycine hydroxamate and its effects on photorespir- tors of glycine decarboxylation and similarly INH has also been atory carbon flow. Plant Physiol 64: 706-711 only at high (13, 14. LLOYD FE 1911 Guayule (Parthenium argentatum Gray): a rubber plant of the known to inhibit the reaction concentrations . Carnegie Inst Wash Publ No. 139 27). The drastic inhibition in the incorporation of the labeled 15. LOOMIS WD, R CROTEAU 1980 Biochemistry of terpenoids In PK Stumpf, EE intermediates by glycine decarboxylase inhibitors suggest a pos- Conn, eds, The Biochemistry of Plants, Vol 4. Academic Press, New York, itive role of serine in providing early precursors for the biosyn- pp 363-418 16. LORD JM, MJ MERRETT 1970 The pathway ofglycolate utilization in Chlorella thesis of rubber in guayule. The incorporation of certain photo- pyrenoidosa. Biochem J 117: 929-937 respiratory intermediates into ,8-carotene via isoprene has been 17. LOWRY OH, NJ ROSEBROUGH, AL FARR, RJ RANDALL 1951 Protein measure- previously observed in greening seedlings and in the leaves ments with the Folin phenol reagent. J Biol Chem 193: 265-275 of Hamamelis virginiata (10, 23). The inhibition of glycine 18. MONSON RK, MA STIDHAM, GJ WILLIAMS, GE EDWARDS, EG URIBE 1982 Temperature dependence of photosynthesis in Agropogon Rydb. was in the activities of smithii I. decarboxylation by AOA due reduction Factors affecting net CO2 uptake in intact leaves and contribution from glycine decarboxylase and SHMT in isolated mitochondria (Fig. ribulose I,5-bisphosphate carboxylase measured in vivo and in vitro. Plant 3), glycine decarboxylase was significantly more sensitive than Physiol 69: 921-928 SHMT. The inhibitory action of other compounds on various 19. Moss DN 1966 Respiration of leaves in light and darkness. Crop Sci 6: 351- and 354 enzymes was not shown due to multiplicity of sites of action 20. OGREN WL 1984 Photorespiration: Pathways, regulation and modification. variable concentrations used for optimal rates of inhibition. Annu Rev Plant Physiol 35: 415-442 The data in the present study indicate the role of serine as the 21. PARK RB, J BONNER 1958 Enzymatic synthesis of rubber from mevalonic acid. major precursor for isoprenoid biosynthesis in guayule. The J Biol Chem 233: 340-343 22. REHFELD DW, NE TOLBERT 1972 Aminotransferases in peroxisomes from incorporation of serine into chloroplast terpenoids via acetyl spinach leaves. J Biol Chem 247: 4803-481 1 CoA has been shown to be due the cleavage of serine to pyruvate 23. SHAH SPJ, U ROGERS 1969 Compartmentation of terpenoid biosynthesis in mediated by serine dehydratase (8). The involvement of this green plants. Biochem J 114: 395-405 photorespiratory intermediate both in glycolate metabolism and 24. SHINGLES R, L WOODROW, B GRODZINSKI 1984 Effects of glycolate pathway in pea (Pisum to be based on the available intermediates on glycine decarboxylation and serine synthesis in isoprenoid biosynthesis is believed sativum L). Plant Physiol 74: 705-7 10 pools of the intermediate and the physiological concentrations 25. TAYLOR RT, H WEISSBACH 1965 Radioactive assay for serine hydroxymethyl required for optimizing the respective metabolic pathways in transferase. Anal Biochem 13: 80-84 vivo. 26. TOLBERT NE 1981 Metabolic pathways in peroxisomes and glyoxysomes. Annu indicate that eventhough Rev Biochem 50: 133-157 The observations in the present study 27. ZELITCH I 1979 Photorespiration: studies with whole tissues. In M Gibbs, E the stem portion of guayule is the major site of rubber accumu- Latzko, eds, Encyclopedia of Plant Physiology (New Series). Photosynthesis lation, the leaves play a major role in providing precursors for II, Vol 6. Springer-Verlag Heidelberg, pp 353-367