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The accumulative low temperature of 5–7 in the fall and winter of the Chihuahuan Desert induces rubber biosynthesis in Parthenium argentatum Gray (guayule) and the enzyme activities of 3-hydroxy-3-methylglutaryl-CoA reductase and rubber transferase. The conclusion that the induced rates of 3-hydroxy-3-methylglutaryl-CoA reductase and rubber transferase account, in part, for the induced rate of rubber biosynthesis is incomplete. The objective of this Short Communication was to determine if the induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase supported the rate of rubber biosynthesis as determined by the rate isopentenyl pyrophosphate polymerization to rubber polymers catalyzed by rubber transferase. The rates of these 2 enzymes were measured in the bark of lower stems of guayule from July to December in plants transplanted to the field in the Chihuahuan Desert in May. At the peak of the low temperature induced rubber formation the induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase of 29.9 nmol MVA h −1 g fr wt −1 was significantly lower than the induced rate of rubber transferase of 357.5 nmol IPP h −1 g fr wt −1 . The rate of 3-hydroxy-3-methylglutaryl-CoA reductase was 8.3% of the theoretical rate of rubber formation based on the rate of rubber transferase at saturating concentrations of isopentenyl pyrophosphate Mg 2+ and dimethylallyl pyrophosphate initiator in vivo . A new interpretation of the induced developmental rate curves of 3-hydroxy-3-methylglutaryl-CoA reductase and rubber transferase is that the low temperature induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase limits rubber biosynthesis in guayule. The new calculations support the conclusion that isopentenyl pyrophosphate from an alternate source, other than the mevalonic acid pathway may additionally supply isopentenyl pyrophosphate to rubber transerase for polymerization into natural rubber. [ABSTRACT FROM AUTHOR]

Abstract: This review discusses our research at Texas A&M University, demonstrating that the accumulative hours of low temperature of the fall and winter of the Chihuahuan Desert stimulates rubber biosynthesis in the indigenous rubber plant, Parthenium argentatum Gray (guayule), by inducing the activities of new enzymes and forming undifferentiated cortical parenchyma with enhanced rubber biosynthesis. Two-month-old guayule seedlings were transplanted to field plots in the Chihuahuan Desert in May. The bark from stems was analyzed for enzymatic activities from June to June, and stem cross-sections were analyzed by electron microscopy from September and January. During the high temperatures of the summer months, rubber transferase activity was low. The activity of 3-hydroxy-3-methylglutaryl-CoA reductase was high in June during membrane formation for seedling growth returning to low values during July and August. The activity of both enzymes increased in September, October, November and December with increases in hours of low temperature and rubber biosynthesis. Northern blots with tomato HMGR-1 cDNA probe demonstrated 3-hydroxy-3-methylglutaryl-CoA reductase mRNA was high in June, low in July and August, with increasing amounts in September–October–November, with increasing accumulated hours of low temperature and rubber biosynthesis. Electron photomicrographs of stem cross-sections in the maturation zone of the stem meristems sampled in September showed cortical parenchyma with a large central vacuole, tonoplast and a thin layer of cytosol embedded with a nucleus and only an occasional rubber particle, mitochondria and chloroplast. Electron photomicrographs of stem cross-sections in the maturation zone of 10-month-old plants sampled in January, after the exposure to the fall and winter low temperatures, showed undifferentiated cortical parenchyma lacking a vacuole with central cytosol embedded with a large population of rubber particles, an extensive number of mitochondria and chloroplasts, a nucleus, RER, Golgi apparatus and the presence of micro vacuoles. The dense cytosol and organelles equip the undifferentiated cortical parenchyma with the capacity to support a high rate of rubber biosynthesis. In September electron photomicrographs of cross-sections at the base of the stems showed mature cortical parenchyma with minimal rubber particles. Electron photomicrographs of the cross-sections at the stem base taken in January showed undifferentiated cortical parenchyma filled with rubber particles in a central cytosol. [Copyright &y& Elsevier]

Abstract: The low temperatures of the Chihuahuan Desert stimulate rubber biosynthesis in guayule plants, primarily by promoting rubber-producing cortical parenchyma cells and inducing the activity of enzymes in the pathway of rubber biosynthesis. Further progress in increasing rubber production in guayule requires a better understanding of the ultrastructure of the organelles in the rubber producing cortical parenchyma. The objective of this study was to investigate the ultrastructure of the organelles in the rubber producing cortical parenchyma. The electron photomicrographs show that the rubber-producing cortical parenchyma, in contrast to mature cortical parenchyma in stems not exposed to the low temperatures, contain a high population of rubber particles, an abundant number of mitochondria, chloroplasts, a well-structured nucleus, rough endoplasmic reticulum and Golgi apparati. The double membrane nucleus is important in the low temperature induction of rubber-forming enzymes: rubber transferase and 3-hydroxy-3-methylglutaryl coenzyme A reductase. Photographic enlargements of the population of mitochondria show well-developed inner membrane folding that functions in ATP production to support the energy requirements of rubber biosynthesis. Electron photomicrographs show dividing chloroplasts that increase the number of plastids in the rubber-producing parenchyma. The well-structured grana and stroma lamellae may produce energy for the stroma production of carbon precursors of rubber. Photographic enlargements show a cortical parenchyma with an active Golgi apparatus, producing an abundant number of vesicles. Features of this electron photomicrograph is the fusion of a Golgi vesicle with a rubber particle, suggesting the transport of Golgi-processed glycolipids and glycoproteins to the rubber particles, and the rubber particles extending into the central cytosol still attached to the RER indicating an earlier origin of the rubber particle from the RER. The electron photomicrographs of the ultrastructure of rubber-producing cortical parenchyma in guayule stems support the conclusion that these cells contain an abundant number of organelles relating to the low temperature stimulated rubber synthesis. [Copyright &y& Elsevier]

Abstract: Exposure of guayule plants to low temperature promotes the development of rubber-producing undifferentiated cortical parenchyma. Electron photomicrographs of cross-sections 1cm below the stem tip of guayule plants grown at 27–32°C days and 7°C nights with a 16h photoperiod for 6 months showed cortical parenchyma containing micro-vacuoles appressing a wide band of cytosol embedded with rubber particles and mitochondria against a thick 2° cell wall. The undifferentiated cortical parenchyma did not contain a central vacuole, tonoplast or parietal cytosol. Mature cortical parenchyma were developed at 1cm from the stem tip in plants grown in the greenhouse at 27–32°C days and 21–24°C nights with a 16h photoperiod for 6 months. These results demonstrates that the rubber-producing undifferentiated cortical parenchyma cells were promoted as a result of the exposure of the guayule plants to the low night temperature and not the result of plant age. Electron photomicrographs of cross-sections 3cm from the stem tips of plants grown through the high temperatures of the summer of the Chihuahuan Desert and sampled in September showed mature cortical parenchyma with a large defined central vacuole, tonoplast and a thin layer of parietal cytosol. The cortical parenchyma in a cross-section of stems 3cm from the tip from plants exposed to the high temperature of the summer and low temperature of the fall and winter of the desert showed immature cells with a high population of rubber particles in the cytosol that nearly fill the cell. There was a small central space in the cytosol indicating an early stage of the development of a central vacuole, but no tonoplast or parietal cytosol were present. High magnification photomicrographs showed the cytosol was disorganized and digested. Cortical parenchyma in cross-sections at the base of the stems of plants growing through the high summer temperatures of the desert and sampled in September showed a well developed central vacuole with tonoplast and a thin layer of parietal cytosol but only a few rubber particles. In contrast, the cortical parenchyma at the base of the stems of plants exposed to high temperature of the summer and low fall and winter temperatures of the desert, although more mature than parenchyma 3cm from the tips of the stems, did not contain a central vacuole, tonoplast or parietal cytosol. The developing central vacuole in the center of the cell was filled with rubber particles in a digested cytosol. Measurements of rubber synthesis in the field grown guayule plants from September to March increased 97.4% while growth of the plants from August to March increased 81.1%, demonstrating that rubber synthesis increases as new growth and new rubber-producing cells were produced. [Copyright &y& Elsevier]