Cambial Activity and Distribution of Rubber in Guayule (Parthenium argentatum)

Vascular cambium in Guayule, a rubber producing Mexican shrubof Asteraceae family is non-storied. Cambial activity variesperiodically, and the vascular cambium and its immediate derivativesdo not contain rubber. However, as the xylem and phloem parenchymacells derived from the vascular cambium age, rubber depositionstarts from the cell periphery along the walls and later towardstheir cell lumen. Though the sieve tubes and companion cellsof phloem contain no rubber, all parenchyma cells of xylem andphloem, show the presence of rubber, though its amount varies.However, certain lignified xylem ray cells and lignified pithcells are devoid of rubber accumulation. Microfluorescence studiesshow that the epithelial, phloem ray parenchyma, cortical andpith cells, in descending order, have the highest to lowestrubber content. The size and number of rubber particles observedin the parenchyma cells are greatest during the period of cambialdormancy than in an active cambial period Cambium, guayule, rubber     

3-羟-3甲基戊二酰辅酶A还原酶及他汀类药物

3-羟-3甲基戊二酰辅酶A(HMG—CoA)还原酶是胆固醇合成的限速酶,它是胆固醇代谢的最重要的酶之一。HMG—CoA还原酶的抑制剂——他汀类药物是目前广泛用于临床的降脂药,它不但可降低血浆胆固醇水平,还可防止动脉粥样硬化。     

高等植物的3-羟基-3-甲基戊二酰辅酶A还原酶

介绍了植物3-羟基-3-甲基戊二酰辅酶A还原酶(HMGR)的结构和调控,并简略讨论了HMGR调控与植物类异戊二烯途径的关系.     

Expression of the Hevea brasiliensis (H.B.K.) Mull. Arg. 3-Hydroxy-3-Methylglutaryl-Coenzyme A Reductase 1 in Tobacco Results in Sterol Overproduction

A genomic fragment encoding one (HMGR1) of the three 3-hydroxy-3-methylglutaryl coenzyme A reductases (HMGRs) from Hevea brasiliensis (H.B.K.) Mull. Arg. (M.-L. Chye, C.-T. Tan, N.-H. Chua [1992] Plant Mol Biol 19: 473-484) was introduced into Nicotiana tabacum L. cv xanthi via Agrobacterium transformation to study the influence of the hmg1 gene product on plant isoprenoid biosynthesis. Transgenic plants were morphologically indistinguishable from control wild-type plants and displayed the same developmental pattern. Transgenic lines showed an increase in the level of total sterols up to 6-fold, probably because of an increased expression level of hmg1 mRNA and a corresponding increased enzymatic activity for HMGR, when compared with the level of total sterols from control lines not expressing the hmg1 transgene. In addition to the pathway end products, campesterol, sitosterol, and stigmasterol, some biosynthetic intermediates such as cycloartenol also accumulated in transgenic tissues. Most of the overproduced sterols were detected as steryl-esters and were likely to be stored in cytoplasmic lipid bodies. These data strongly support the conclusion that plant HMGR is a key limiting enzyme in phytosterol biosynthesis.     

Gland Cells in Resin Canal Epithelia in Guayule (Parthenium argentatum) in Relation to Resin and Rubber Production

Schizogenous resin canals develop in the pith and cortex ofthe primary stem tissue in guayule (Parthenium argentatum Gray).In secondary tissue concentric rings of resin canals are producedfrom derivatives of the vascular cambium. Both resin and rubberaccumulate in the epithelial cells of the canals. These havethe characteristics of gland cells. Resin is secreted into thecanals and rubber accumulates in the surrounding parenchymacells as well as the gland cells, especially in winter. Younggland cells contain modified plastids and smooth tubular endoplasmicreticulum. These organelles probably accommodate the compartmentalizedsteps of the isoprenoid biosynthetic pathway leading to theproduction of isopentenyl pyrophosphate. As these ultrastructuralcharacteristics only exist in young gland cells of the currentseason's growth they seem to be the sole source of the precursorsfor both resin and rubber formation. Parthenium argentatum, guayule, resin canals, gland cells, plastids, smooth endoplasmic reticulum, rubber, resin, epithelial cells, ultrastructure     

Occurrence of 3-Hydroxy-3-methyl-glutaryl Coenzyme A Reductase in Sweet Potato

Illumination of the dark-grown Euglena gracilis, both the wild-green type and a permanently bleached mutant, for 4 hr at 2,000 lux caused about 6-fold increase of the cellular content of total l-ascorbic acid. The increase was mainly due to an increase of reduced-form l-ascorbic acid. From the action spectrum only blue light was found to be effective for the increase. Darkening stopped the increase and reillumination started a renewed increase. The activity of l-gulono-γ-lactone dehydrogenase, catalyzing the last step of l-ascorbic acid biosynthesis, was also increased two fold by illumination for 2 hr, and was changed in parallel to that of the cellular content of l-ascorbic acid depending on the presence or absence of illumination. The augmentation of l-ascorbic acid formation was markedly inhibited by various inhibitors and uncouplers, but not by dichlorophenyldimethylurea. The results in sum suggest that the light-dependent increase of l-ascorbic acid formation in E. gracilis is not primarily associated with photosynthesis, but is apparently related to the adaptation of the dark-grown cells to the illuminated state.     

A novel cDNA from Parthenium argentatum Gray enhances the rubber biosynthetic activity in vitro

Natural rubber (cis-1,4-polyisoprene) is an isoprenoid compound produced exclusively in plants by the action of rubber transferase. Despite a keen interest in revealing the mechanisms of rubber chain elongation and chain length determination, the molecular nature of rubber transferase has not yet been identified. A recent report has revealed that a 24 kDa protein tightly associated with the small rubber particles of Hevea brasiliensis, therefore designated small rubber particle protein (SRPP), plays a positive role in rubber biosynthesis. Since guayule (Parthenium argentatum Gray) produces natural rubber similar in size to H. brasiliensis, it is of critical interest to investigate whether guayule contains a similar protein to the SRPP. A cDNA clone has been isolated in guayule that shares a sequence homology with the SRPP, thus designated guayule homologue of SRPP (GHS), and the catalytic function of the protein was characterized. Sequence analysis revealed that the GHS is highly homologous in several conserved regions to the SRPP (50% identity). In vitro functional analysis of the recombinant protein overexpressed in E. coli revealed that the GHS plays a positive role in isopentenyl diphosphate incorporation into high molecular weight rubbers as SRPP does. These results indicate that guayule and Hevea rubber trees contain a protein that is similar in its amino acid sequence and plays a role in isopentenyl diphosphate incorporation in vitro, implying that it contributes to the enhancement of rubber biosynthetic activity in rubber trees.     

3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Activity in Ochromonas malhamensis: A System to Study the Relationship between Enzyme Activity and Rate of Steroid Biosynthesis

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the key regulatory enzyme of the isoprenoid pathway, was found to be predominantly microsomal in Ochromonas malhamensis, a chrysophytic alga. Detection of HMG-CoA reductase requires the presence of 1% bovine serum albumin during cell homogenization, and the activity is stimulated by the presence of Triton X-100. The enzyme has a pH optimum of 8.0 and an absolute requirement for NADPH. When grown in 10 micromolar mevinolin, a competitive inhibitor of HMG-CoA reductase, O. malhamensis shows a 10- to 15-fold increase in HMG-CoA reductase activity (after washing) with little or no effect on cell growth rate. Cultures can be maintained in 10 micromolar mevinolin for months. O. malhamensis produces a large amount (1% dry weight) of poriferasterol, a product of the isoprenoid pathway. The addition of 10 micromolar mevinolin initially blocked poriferasterol biosynthesis by >90%; within 2 days the rate of synthesis returned to normal levels. Immediately after mevinolin was washed from the 2-day culture, there was a transient 2.5-fold increase in the rate of poriferasterol biosynthesis. The rate of poriferasterol biosynthesis and the level of HMG-CoA reductase activity both fell to control levels within hours.     

Impairment of Photorespiratory Carbon Flow into Rubber by the Inhibition of the Glycolate Pathway in Guayule (Parthenium argentatum Gray)

Cut shoots of guayule (Parthenium argentatum Gray) were treated with four inhibitors of the glycolate pathway (α-hydroxypyridinemethanesulfonic acid; isonicotinic acid hydrazide, glycine hydroxamate, and amino-oxyacetate, AOA) in order to evaluate the role of photorespiratory intermediates in providing precursors for the biosynthesis of rubber. Photorespiratory CO₂ evolution in guayule leaves was severely inhibited by AOA. Application of each of the four inhibitors has resulted in a significantly decreased incorporation of ¹⁴C into rubber fractions suggesting that the glycolate pathway is involved in the biosynthesis of rubber in guayule. However, the application of each of the glycolate pathway inhibitors showed no significant effect on photosynthetic CO₂ fixation in the leaves. The inhibitors individually also reduced the incorporation of labeled glycolate, glyoxylate, and glycine into rubber, while the incorporation of serine and pyruvate was not affected. The effective inhibition of incorporation of glycolate pathway intermediates in the presence of AOA was due to an inhibition of glycine decarboxylase and serine hydroxymethyltransferase. It is concluded that serine is a putative photorespiratory intermediate in the biosynthesis of rubber via pyruvate and acetyl coenzyme A.     

Soil Microbial Responses to Temporal Variations of Moisture and Temperature in a Chihuahuan Desert Grassland

Global climate change models indicate that storm magnitudes will increase in many areas throughout southwest North America, which could result in up to a 25% increase in seasonal precipitation in the Big Bend region of the Chihuahuan Desert over the next 50 years. Seasonal precipitation is a key limiting factor regulating primary productivity, soil microbial activity, and ecosystem dynamics in arid and semiarid regions. As decomposers, soil microbial communities mediate critical ecosystem processes that ultimately affect the success of all trophic levels, and the activity of these microbial communities is primarily regulated by moisture availability. This research is focused on elucidating soil microbial responses to seasonal and yearly changes in soil moisture, temperature, and selected soil nutrient and edaphic properties in a Sotol Grassland in the Chihuahuan Desert at Big Bend National Park. Soil samples were collected over a 3-year period in March and September (2004-2006) at 0-15 cm soil depth from 12 3 x 3 m community plots. Bacterial and fungal carbon usage (quantified using Biolog 96-well micro-plates) was related to soil moisture patterns (ranging between 3.0 and 14%). In addition to soil moisture, the seasonal and yearly variability of soil bacterial activity was most closely associated with levels of soil organic matter, extractable NH(4)-N, and soil pH. Variability in fungal activity was related to soil temperatures ranging between 13 and 26 degrees C. These findings indicate that changes in soil moisture, coupled with soil temperatures and resource availability, drive the functioning of soil-microbial dynamics in these desert grasslands. Temporal patterns in microbial activity may reflect the differences in the ability of bacteria and fungi to respond to seasonal patterns of moisture and temperature. Bacteria were more able to respond to moisture pulses regardless of temperature, while fungi only responded to moisture pulses during cooler seasons with the exception of substantial increased magnitudes in precipitation occurring during warmer months. Changes in the timing and magnitude of precipitation will alter the proportional contribution of bacteria and fungi to decomposition and nitrogen mineralization in this desert grassland.     

Distribution, Development and Structure of Resin Ducts in Guayule (Parthenium argentatum Gray)

The distribution, development and structure of resin ducts inguayule (Parthenium argentatum Gray), the second best sourceof natural rubber, have been studied. Resin ducts are widelydistributed in stem, root, leaf, petiole and peduncle. The ductsin the primary tissues are initiated schizogenously and theirfurther development is schizolysigenous. The ducts in the cortexof the root do not have a well-defined epithelium. Ducts developedfrom the vascular cambium are initiated and develop schizogenously.Both resin and rubber are produced in the epithelial cells ofresin ducts. While resin is secreted into the duct lumen, rubberis stored within these cells. Epithelial cells store more thanneighbouring parenchyma cells. Guayule, rubber, resin, ducts, epithelial cells     

Evaluation of the cytotoxicity, cytostaticity and genotoxicity of argentatins A and B from Parthenium argentatum (Gray)

Argentatins A and B are abundant triterpenes present in Parthenium argentatum. Both compounds have shown cytotoxic properties on K562, MCF-7, PC-3, HCT-15 and U251 human cancer cell lines. Furthermore the cytotoxic, cytostatic and genotoxic effects of the argentatins on proliferating lymphocytes were evaluated using cytokinesis-block micronucleus test. Argentatin A had no cytostatic properties, but it was cytotoxic for proliferating lymphocytes at a concentration of 25 microM (P < 0.005). On the other hand, argentatin B showed significant cytostatic effects (P < 0.001) at concentrations of 5 to 25 microM and it did not show cytotoxic effects at the same concentrations. Neither argentatin showed genotoxic effects in terms of micronucleus frequency in human lymphocytes. According to these results the argentatins are not able to cause injury on DNA by clastogenic or aneugenic mechanisms.     

Cholesterol Biosynthesis and Its Regulation in Dissociated Cell Cultures of Fetal Rat Brain: Developmental Changes and the Role of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase

Primary cultures of cells dissociated from fetal rat brain were utilized to define the developmental changes in cholesterol biosynthesis and the role of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in the regulation of these changes. Cerebral hemispheres of fetal rats of 15-16 days of gestation were dissociated mechanically into single cells and grown in the surface-adhering system. Cholesterol biosynthesis, studied as the rate of incorporation of [14C]acetate into digitonin-precipitable sterols, was shown to exhibit two distinct increases in synthetic rates, a prominent increase after 6 days in culture and a smaller one after 14 days in culture. Parallel measurements of HMG-CoA reductase activity also demonstrated two discrete increases in enzymatic activity, and the quantitative and temporal aspects of these increases were virtually identical to those for cholesterol synthesis. These data indicate that cholesterol biosynthesis undergoes prominent alterations with maturation and suggest that these alterations are mediated by changes in HMG-CoA reductase activity. The timing of the initial prominent peak in both cholesterol biosynthesis and HMG-CoA reductase activity at 6 days was found to be the same as the timing of the peak in DNA synthesis, determined as the rate of incorporation of [3H]thymidine into DNA. The second, smaller peak in reductase activity and sterol biosynthesis at 14 days occurred at the time of the most rapid rise in activity of the oligodendroglial enzyme, 2':3'-cyclic nucleotide 3'-phosphohydrolase (CNP). These latter observations suggest an intimate relationship of the sterol biosynthetic pathway with cellular proliferation and with oligodendroglial differentiation in developing mammalian brain.     

Seasonal Patterns of Nitrate Reductase and Nitrogenase Activities in Phaseolus vulgaris L

The patterns of nitrate reductase activity (NRA) in the leaves (in vivo assay) and root nodule nitrogenase activity (C₂H₂ reduction) were investigated throughout the season in field-grown Phaseolus vulgaris plants.     

Properties of 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase in Villous and Crypt Cells of the Rat Small Intestine

Some properties of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase in microsomes of villous and crypt cells from the jejunal and ileal epithelia of rats fed commercial pellet were studied. The optimum pH of the microsomal reductase from villi and crypts was 7.0~7.2 and the Km for HMG-CoA was 41.7 µm. The reductase specifically required dithiothreitol for its activity. The activity was higher in ileal populations than in the jejunum. Responses of the reductase in the villous fraction to feeding cholesterol and taurocholate in combination or cholestyramine resembled those observed in crypt cells. Thus, the properties of microsomal HMG-CoA reductase in villous and crypt cells from the small intestine are similar each other, and they are possibly the same enzyme.     

A Novel Red Clover Hydroxycinnamoyl Transferase Has Enzymatic Activities Consistent with a Role in Phaselic Acid Biosynthesis

Red clover (Trifolium pratense) leaves accumulate several μmol g⁻¹ fresh weight of phaselic acid [2-O-(caffeoyl)-l-malate]. Postharvest oxidation of such o-diphenols to o-quinones by endogenous polyphenol oxidases prevents breakdown of forage protein during storage. Forage crops like alfalfa (Medicago sativa) lack both polyphenol oxidase and o-diphenols, and breakdown of their protein upon harvest and storage results in economic losses and release of excess nitrogen into the environment. Understanding how red clover synthesizes o-diphenols such as phaselic acid will help in the development of forage crops utilizing this natural system of protein protection. A possible pathway for phaselic acid biosynthesis predicts a hydroxycinnamoyl transferase (HCT) capable of forming caffeoyl and/or p-coumaroyl esters with malate. Genes encoding two distinct HCTs were identified in red clover. HCT1 shares more than 75% amino acid identity with a number of well-characterized shikimate O-HCTs implicated in monolignol biosynthesis. HCT2 shares only 34% amino acid sequence identity with HCT1 and has limited sequence identity to any previously identified HCT. Expression analyses indicate that HCT1 mRNA accumulates to 4-fold higher levels in stems than in leaves, whereas HCT2 mRNA accumulates to 10-fold higher levels in leaves than in stems. Activity assays of HCT1 and HCT2 proteins expressed in Escherichia coli indicate that HCT1 transfers caffeoyl or p-coumaroyl moieties from a coenzyme A-thiolester to shikimate but not malate, whereas HCT2 transfers caffeoyl or p-coumaroyl moieties from a coenzyme A-thiolester to malate but not shikimate. Together, these results indicate that HCT1 is involved in monolignol biosynthesis and HCT2 is a novel transferase likely involved in phaselic acid biosynthesis.In contrast to many other forage legumes (e.g. alfalfa [Medicago sativa]; Jones et al., 1995), red clover (Trifolium pratense) accumulates relatively high levels of the phenylpropanoid o-diphenol phaselic acid [2-O-(caffeoyl)-l-malic acid; hereafter referred to as caffeoyl-malate or phaselic acid] in its leaves (Hatfield and Muck, 1999; Winters et al., 2008). In red clover, upon cellular disruption, phaselic acid and other o-diphenols are readily oxidized by a soluble polyphenol oxidase (PPO) to produce their corresponding o-quinones (Hatfield and Muck, 1999; Sullivan et al., 2004). The formation of such o-quinones by PPO, and the subsequent secondary reactions of these quinones, are most often associated with browning of fresh fruits and vegetables (Steffens et al., 1994), which has a negative impact on perceived quality. When preserved by ensiling, however, oxidation of o-diphenols by PPO in red clover prevents degradation of protein during storage (Sullivan et al., 2004; Sullivan and Hatfield, 2006). Although alfalfa lacks significant levels of both PPO activity and o-diphenol compounds in its leaves, red clover''s natural system of protein protection has been transferred to this forage legume by expressing a red clover PPO transgene in alfalfa and exogenously adding o-diphenol PPO substrates to the resulting tissues or tissue extracts (Sullivan et al., 2004; Sullivan and Hatfield, 2006). Because ruminant animals poorly utilize degraded protein, adaptation of the PPO system to alfalfa and other forage crops would have substantial positive economic and environmental impacts (Sullivan and Hatfield, 2006). Unfortunately, lack of system components in these forage crops, especially the o-diphenol PPO substrates, presents a challenge to practical adaptation of this natural system of protein preservation. Consequently, understanding how red clover is able to accumulate o-diphenols such as phaselic acid will be a key step to adapt the PPO/o-diphenol system to a wide range of economically important forage crops.The biosynthetic pathways whereby red clover synthesizes and accumulates phaselic acid and other o-diphenols have not been defined. However, in the Brassicaceae, hydroxycinnamoyl esters with malic acid can be made via the action of sinapoyl-Glc:malate sinapoyltransferase (SMT; EC 2.3.1), which is capable of transferring a hydroxycinnamoyl moiety from a hydroxycinnamoyl-Glc ester to a malic acid acceptor. In Arabidopsis (Arabidopsis thaliana), SNG1 (for sinapoylglucose accumulator 1), which encodes the enzyme, has been shown to be responsible for the accumulation of sinapoylmalate in seeds and leaves (Lehfeldt et al., 2000). An SMT from radish (Raphanus sativus), presumably the homolog of the Arabidopsis SNG1 gene product, has been purified to apparent homogeneity and characterized (Grawe et al., 1992). The purified enzyme is capable of utilizing sinapoyl-, feruloyl-, caffeoyl-, and to a lesser extent p-coumaroyl-Glc esters to form the corresponding malic acid esters, suggesting that it is responsible for the accumulation of these esters in vivo. In contrast, in many plants, formation of certain hydroxycinnamoyl esters is often mediated by a member of the BAHD transferase family (D''Auria, 2006) that utilize a CoA thiolester hydroxycinnamoyl donor. Some of the best characterized of these hydroxycinnamoyl transferases (HCTs) are those associated with the biosynthesis of monolignols (Hoffmann et al., 2003, 2004; Shadle et al., 2007). These are capable of transferring p-coumaroyl or caffeoyl moieties from the respective CoA thiolesters to form 5-O-esters with shikimic acid or, to a lesser extent, 3-O-esters with quinic acid. Separable enzymatic activities capable of transferring a p-coumaroyl moiety to either shikimate/quinate or to 4′-hydroxyphenyllactate in basil (Ocimum basilicum) peltate gland extracts have been identified, although genes encoding these activities have not been cloned (Gang et al., 2002). Niggeweg et al. (2004) used gene-silencing experiments to definitively demonstrate that a hydroxycinnamoyl-CoA:quinate hydroxycinnamoyl transferase (HQT) is responsible for chlorogenic acid accumulation in the Solanaceae. Although phaselic acid biosynthesis in red clover could be via a pathway utilizing SMT, lack of an apparent SNG1 homolog in a collection of red clover EST sequences derived from leaves and young plants suggests otherwise (see “Discussion”). Therefore, pathways in red clover for the biosynthesis of phaselic acid utilizing one or more BAHD family transferase (Fig. 1) should be considered. In these proposed pathways, Phe would be converted to p-coumaroyl-CoA by the sequential action of Phe ammonia lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate:CoA ligase (4CL). The action of one or more specific HCTs and one or more p-coumarate 3′-hydroxylases (C3Hs) would then result in the formation of phaselic acid.Open in a separate windowFigure 1.Possible pathways for phaselic acid biosynthesis in red clover. Proposed pathway enzymes for the production of phaselic acid include PAL, 4CL, hydroxycinnamoyl:shikimate transferase (HCT-S), hydroxycinnamoyl:malate transferase (HCT-M), and C3H. The branch point at p-coumaroyl-CoA represents two alternative pathways. For simplicity, not all reactants and products are shown.Existing literature suggests that C3H enzymes, which are cytochrome P450 enzymes (CYP98A subfamily), do not directly hydroxylate p-coumaric acid to caffeic acid but rather act on p-coumaroyl ester derivatives. For example, the enzyme from Arabidopsis hydroxylates shikimic and quinic acid esters of p-coumaric acid but only poorly or not at all p-coumaric acid or its Glc or CoA esters (Schoch et al., 2001; Franke et al., 2002). Thus, one model of phaselic acid biosynthesis is the formation of 2-O-(p-coumaroyl)-l-malic acid (hereafter referred to as p-coumaroyl-malate) by a HCT and its subsequent hydroxylation by a C3H enzyme capable of utilizing the malic acid ester as a substrate (Fig. 1, bottom, red pathway). An alternative model would require at least two HCT activities for phaselic acid biosynthesis (Fig. 1, top, blue pathway). The first activity would form a substrate suitable for hydroxylation (e.g. p-coumaroyl-shikimate, since several characterized C3H enzymes appear to favor this substrate [Schoch et al., 2001; Franke et al., 2002; Gang et al., 2002; Morant et al., 2007]). Following hydroxylation to the caffeoyl derivative by a C3H, the first HCT activity could synthesize caffeoyl-CoA via its reverse reaction (Hoffmann et al., 2003; Niggeweg et al., 2004). A second HCT activity would then transfer the caffeoyl moiety to malic acid to form phaselic acid. Both pathways predict a transferase capable of transferring a hydroxycinnamoyl moiety (either p-coumaroyl or caffeoyl) to malic acid. Also, these pathways are consistent with the observation that, at least in vitro, several characterized HCT enzymes are capable of transfer reactions utilizing either p-coumaroyl- or caffeoyl-CoA (Hoffmann et al., 2003; Niggeweg et al., 2004). The identification and characterization of two distinct HCTs from red clover, one of which has properties consistent with a role in phaselic acid biosynthesis, are reported here.     

Partial purification and characterization of 3-hydroxy-3-methylglutaryl coenzyme a reductase from the leaves of guayule (Parthenium argentatum)

3-Hydroxy-3-methylglutaryl coenzyme A reductase has been isolated and was partially purified from the leaves of Parthenium argentatum. The enzyme was found to be associated both with the cytosol and the chloroplasts. Ten mM dithiothreitol was essential to prevent loss of activity. Optimum activities of cytosolic and chloroplastic fractions were observed at pH 7.0 and 7.5 respectively. Preincubation of the reaction mixtures with CoA, acetyl-CoA, σ-phenanthroline and iodoacetamide resulted in the progressive loss of enzyme activity. 3-Hydroxybutyrate and mevalonate also inhibited the enzyme. The Michaelis constants of the enzyme for HMG-CoA and NADPH were 0.25 and 0.31 mM respectively for the cytosolic enzyme, while those for the chloroplastic enzyme were 0.018 and 0.42 mM respectively. Inhibition studies indicated that hydroxybutyrate was a competitive inhibitor with respect to HMG-CoA. The inhibition of mevalonate was competitive with HMG-CoA and non-competitive with NADPH.     

A floristic-structural gradient related to land forms in the southern Chihuahuan Desert

Abstract. The purposes of this study were to elucidate the floristic and structural characteristics of the vegetationin the Mapimi subdivision of the Chihuahuan Desert, Mexico, and to relate them to environmental variation. The main question addressed was: How does floristic composition, total species richness and life-form species richness vary in relation to environmental change? 154 sites, randomly selected and stratified over seven landscape units, were analyzed. Results showed the existence of a land form gradient along which vegetation types were ordered. Species richness varied along the gradient, the richest land form was bajada, indicating that the maximum species richness did not occur at one extreme of the water availability gradient but in a moderately poor situation. The lowest species richness was found in the playa land form. Cover-based life form spectra varied significantly with land forms, while presence-absence based life form spectra did not. It is suggested that this may be a consequence of the relatively young age of this desert.