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PubMed1999

Embryo-specific gene expression in microspore-derived embryos of brassica napus. An interaction between abscisic acid and jasmonic acid1,2

The induction of napin and oleosin gene expression in Brassica napus microspore-derived embryos (MDEs) was studied to assess the possible interaction between abscisic acid (ABA) and jasmonic acid (JA). Napin and oleosin transcripts were detected sooner following treatment with ABA than JA. Treatment of MDEs with ABA plus JA gave an additive accumulation of both napin and oleosin mRNA, the absolute amount being dependent on the concentration of each hormone. Endogenous ABA levels were reduced by 10-fold after treatment with JA, negating the possibility that the observed additive interaction was due to JA-induced ABA biosynthesis. Also, JA did not significantly increase the uptake of [3H-ABA] from the medium into MDEs. This suggests that the additive interaction was not due to an enhanced carrier-mediated ABA uptake by JA. Finally, when JA was added to MDEs that had been treated with the ABA biosynthesis inhibitor fluridone, napin mRNA did not increase. Based on these results with the MDE system, it is possible that embryos of B. napus use endogenous JA to modulate ABA effects on expression of both napin and oleosin. In addition, JA could play a causal role in the reduction of ABA that occurs during late stages of seed development.

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PubMed1999

Lysophosphatidic acid acyltransferase from coconut endosperm mediates the insertion of laurate at the sn-2 position of triacylglycerols in lauric rapeseed oil and can increase total laurate levels

Expression of a California bay laurel (Umbellularia californica) 12:0-acyl-carrier protein thioesterase, bay thioesterase (BTE), in developing seeds of oilseed rape (Brassica napus) led to the production of oils containing up to 50% laurate. In these BTE oils, laurate is found almost exclusively at the sn-1 and sn-3 positions of the triacylglycerols (T.A. Voelker, T.R. Hayes, A.C. Cranmer, H.M. Davies [1996] Plant J 9: 229-241). Coexpression of a coconut (Cocos nucifera) 12:0-coenzyme A-preferring lysophosphatitic acid acyltransferase (D.S. Knutzon, K.D. Lardizabal, J.S. Nelsen, J.L. Bleibaum, H.M. Davies, J.G. Metz [1995] Plant Physiol 109: 999-1006) in BTE oilseed rape seeds facilitates efficient laurate deposition at the sn-2 position, resulting in the acccumulation of trilaurin. The introduction of the coconut protein into BTE oilseed rape lines with laurate above 50 mol % further increases total laurate levels.

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PubMed1966

Fatty Acid accumulation in maturing flaxseeds as influenced by environment.

The effects of temperature and light on boll and shoot maturity and on the accumulation of fatty acids in developing seeds of flax (Linum usitatissimum L.) were determined in controlled environments. Palmitic and linoleic acids decreased but linolenic increased in percent as seed formation progressed. In the same period, oleic acid increased in percent in 1 variety and decreased in another. Increased temperatures hastened these changes and resulted in decreased iodine value of the oil at maturity.Calculated on a weight basis (mg per 1000 seeds), all 5 major fatty acids increased during seed formation. Increased temperatures initially accelerated the accumulation of all fatty acids, but the period of net fatty acid synthesis was eventually shortened in comparison with cooler temperatures. At 15 degrees and 20 degrees , linolenic acid accumulation closely paralleled the rate of boll maturation, measured by boll moisture content; at 30 degrees linolenic accumulation ceased before maturation could be detected.A photoperiod of 20 hours accelerated plant maturity resulting in decreased seed weight in comparison with a photoperiod of 16 hours. Eight hour photoperiod favored late blossoming and depressed seed weight, oil content, and fatty acid content. Weights of linoleic and linolenic acids were high in both the 16 and 20 hour photoperiods.Linolenic acid was reduced in percent and weight per 1000 seeds at light intensities of 1200 ft-c as compared with 2700 ft-c.

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PubMed1970

Mobilization and Incorporation of Photoassimilated C by Growing Vegetative and Reproductive Tissues of Adult Pinus resinosa Ait. Trees.

Variations in translocation and incorporation of (14)C-labeled photosynthate in actively growing vegetative and reproductive tissues of 20-year-old Pinus resinosa trees with time were investigated. Five hours after exposure of 1-year-old needles on second order branches to (14)CO(2), 90% of the recoverable activity was still in the needles. The 1-year-old bark showed a small amount of activity, but terminal sinks had accumulated little (14)C. After 24 hours, activity in the 1-year-old needles decreased to 59% while actively growing terminal and lateral shoots and 2nd year cones had accumulated considerable tracer. Whereas activity in the 1-year-old bark increased, recovery from the 1-year-old wood was only 1.8%. After 1 week, relative activity in the 1-year-old needles had declined to 27%. Recovery of (14)C from various sinks was high and in the following order: 2nd year cones > terminal needles > lateral needles > terminal internode > lateral internodes > 1-year-old wood. Ninety per cent of the (14)C recovered after 5 hours was in the ethanol-soluble form, but by the end of 1 week only half was in the soluble fraction. Incorporation of tracer into insoluble compounds was high in expanding internodes and in 1-year-old xylem, and low in 2nd year cones. Total activity in the terminal internode generally declined toward the distal end while specific radioactivity increased. Only 5% of the (14)C recovered from cones was localized in developing seeds, but specific radioactivity of seeds was higher than in the cone scales and axis.

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PubMed1971

Influence of Temperature and Seed Ripening on the in-vivo Incorporation of CO(2) into the Lipids of Oat Grains (Avena sativa L.).

To elucidate the influence of growth temperature and of stage of maturity on lipid synthesis in seeds, oat plants (Avena sativa nuda L., variety NOS) were fed with (14)CO(2) at different stages after flowering, and the (14)C-incorporation into the grain lipids was determined at 2, 24, and 48 hours after the end of (14)CO(2)-application. By changing growth temperature from 12 C to 28 C after the application of (14)CO(2) to intact plants, a higher (14)C-labeling of saturated fatty acids was found at the higher temperature. At 28 C, palmitic and stearic acids contained 23% and 9% respectively of total fatty acid-(14)C shortly after the (14)CO(2)-application, whereas at 12 C the corresponding values were 19% and 4%, respectively. Within 2 days (14)C-activity of saturated fatty acids decreased at both temperatures, but to a lesser degree at 28 C. The higher (14)C-labeling of saturated fatty acids and its lower decrease within 2 days at 28 C clearly show a direct influence of temperature on fatty acid biosynthesis in oat grains.At all stages of grain growth, oleic acid had the highest (14)C-activity of all fatty acids shortly after the (14)CO(2)-application. However, (14)C activity of oleic acid rapidly decreases in favor of linoleic acid. With increasing maturity, the intensity of lipid synthesis in the grains decreases; simultaneously, the relative amount of (14)C-saturated fatty acids increases primarily at the expense of (14)C-oleic acid. These tendencies, which were observed in oat plants grown at day temperatures of 12 C during seed development, seem to be paralleled by lipid synthesis in younger grains grown at day temperatures of 28 C. This indicates an indirect influence of growth temperature on lipid synthesis in oat grains during maturation.

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PubMed1979

Identification of Cultivar Differences in Seed Polypeptide Composition of Peanuts (Arachis hypogaea L.) by Two-Dimensional Polyacrylamide Gel Electrophoresis.

Seed polypeptides from several cultivars of peanut (Arachis hypogaea L.) have been compared by means of a two-dimensional polyacrylamide gel electrophoresis. Protein was extracted from the defatted peanut meal by homogenizing in 5 millimolar K(2)CO(3)-9.5 molar urea. After addition of Nonidet P-40 (2%, v/v) and dithiothreitol (0.5%, w/v) the solution was centrifuged at 25,000 g. This procedure led to solubilization of more than 95% of the total protein. The clear supernatant fraction was then subjected to two-dimensional polyacrylamide gel electrophoresis, employing isoelectric focusing in the first dimension and electrophoresis in presence of sodium dodecyl sulfate in the second. After examining several cultivars, it was possible to construct a composite map to include all of the polypeptide species found among all of the cultivars examined. At least 74 major and between 100 and 125 minor components were detectable by Coomassie blue staining. The majority of these had isoelectric points between pH 4.4 and 8.0, and molecular weights between 16,000 and 75,000. Several different cultivars have been compared using this method and it has been shown that considerable variation exists among the major polypeptides present. The method should prove valuable for analyzing different genotypes and selecting varieties with a particular storage protein make-up, as well as for following compositional changes that occur during seed development and germination.

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PubMed1979

Malate synthase activity in cotton and other ungerminated oilseeds: a survey.

Extracts from several species and varieties of ungerminated cotton seeds plus homogenates from 18 other oilseeds (representing 11 different families) were examined for malate synthase and isocitrate lyase activity. Malate synthase activities in the various cotton seeds ranged from 35 to 129% of the units per dry seed weight found in Deltapine 16 cotton. For other oilseeds, the range was from 0.3 to 58% of Deltapine 16 cotton. Castor bean (Ricinus communis L.) had the least activity per mg dry weight (12-fold lower than the next lowest species), while Pima cotton (Gossypium barbadense L.) had the highest level (8.53 units). On a per seed basis, these values were 15 and 747 nanomoles per minute.Malate synthase activity was measurable in all seed types examined, whereas isocitrate lyase could not be detected in any of the seeds. We suggest that synthesis of malate synthase during seed development is universal among oilseeds in the absence of glyoxylate-cycle-associated isocitrate lyase activity.

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PubMed1979

Subcellular distribution of gluconeogenetic enzymes in germinating castor bean endosperm.

The intracellular distribution of enzymes capable of catalyzing the reactions from oxaloacetate to sucrose in germinating castor bean endosperm has been studied by sucrose density gradient centrifugation. One set of glycolytic enzyme activities was detected in the plastids and another in the cytosol. The percentages of their activities in the plastids were less than 10% of total activities except for aldolase and fructose diphosphatase. The activities of several of the enzymes present in the plastids seem to be too low to account for the in vivo rate of gluconeogenesis whereas those in the cytosol are quite adequate. Furthermore, phosphoenolypyruvate carboxykinase, sucrose phosphate synthetase, and sucrose synthetase, which catalyze the first and final steps in the conversion of oxaloacetate to sucrose, were found only in the cytosol. It is deduced that in germinating castor bean endosperm the complete conversion of oxaloacetate to sucrose and CO(2) occurs in the cytosol. The plastids contain some enzymes of the pentose phosphate pathway, pyruvate dehydrogenase and fatty acid synthetase in addition to the set of glycolytic enzymes. This suggests that the role of the plastid in the endosperm of germinating castor bean is the production of fatty acids from sugar phosphates, as it is known to be in the endosperm during seed development.

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PubMed1979

Ribulose Bisphosphate Carboxylase and Proteolytic Activity in Wheat Leaves from Anthesis through Senescence.

Changes in ribulose bisphosphate carboxylase (RuBPCase) and proteolytic activity were followed in the flag leaf and second leaf of field-grown winter wheat (cv. Arthur). These changes were followed in relation to changes in leaf chlorophyll, protein, and photosynthesis, and seed development. Levels of RuBPCase were determined by rocket immunoelectrophoresis as described previously (Wittenbach 1978 Plant Physiol 62: 604-608). RuBPCase constituted 40 to 45% of the total soluble protein in the flag leaf and an even higher percentage of the soluble protein in the second leaf. This ratio remained unchanged until senescence when RuBPCase protein was apparently lost at a faster rate than total soluble protein. No change in the specific activity of RuBPCase on either a milligram protein or RuBPCase basis was observed until senescence. A close correlation existed among the various indices of senescence in the field, namely, the decline in chlorophyll, protein, photosynthesis, and RuBPCase activity. In addition, proteinase activity increased with the onset of senescence. These enzymes readily degraded RuBPCase, exhibiting a pH optimum of 4.8 to 5.0 and a temperature optimum of 50 C. Proteinase activity was modified by sulfydryl reagents suggesting the presence of sulfydryl groups at or near the active sites.

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PubMed1981

Sucrose uptake by developing soybean cotyledons.

Sucrose uptake by excised developing soybean cotyledons shows a biphasic dependence on sucrose concentration. At concentrations less than about 50 millimolar external sucrose, uptake can be described as a carrier-mediated process, with a K(m) of 8 millimolar. At higher external sucrose concentrations, a linear dependence becomes apparent, which suggests the participation of a nonsaturable component in total uptake. Sucrose absorption is dependent on the presence of an electrochemical potential gradient for protons since agents interfering with the generation or maintenance of this gradient (NaN(3) or carbonylcyanide-m-chlorophenyl hydrazone) decrease sucrose transport to a level at or below that predicted from the operation of the noncarrier-mediated process alone. The saturable component of sucrose uptake is also sensitive to the sulfhydryl-modifying compounds N-ethylmaleimide and p-chloro-mercuribenzenesulfonate. The thiol-reducing agent diethioerythritol reverses fully the p-chloro-mercuri-benzenesulfonate inhibition, but not that of N-ethyl maleim de. Sucrose transport is sensitive to external pH, being decreased at high pH(0). Since sucrose-induced depolarization of the membrane potential and carrier-mediated sucrose influx show similar pH-dependence, inhibitor sensitivity, and values of K(m) for sucrose, a sucrose/proton contransport process appears to operate in developing soybean cotyledon cells. Measurement of free space and intracellular sucrose concentrations in vivo suggests that the carrier-mediated process is fully saturated and that sucrose transport may be limiting for sucrose accumulation by the developing seed.

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PubMed1983

Effect of pod removal on leaf photosynthesis and soluble protein composition of field-grown soybeans.

Well nodulated, field-grown soybeans (Glycine max [L.] Merr. var Williams) were depodded just prior to seed development and near mid pod-fill. Both treatments caused a considerable increase in leaf dry weight, suggesting continued photosynthate production following pod removal. Moreover, depodding had a marked effect on leaf soluble protein without affecting total proteolytic activity. Early depodding caused a 50% increase in leaf protein, and both early and late depodding caused the retention of protein for several weeks following the decline in control leaves. But despite this retention of protein, leaves of depodded plants showed no difference in the onset of the irreversible decline in photosynthesis. Therefore, although depodding delayed the loss of leaf chlorophyll and protein, it did not delay the onset of functional leaf senescence and in fact, actually appeared to enhance the rate of decline in photosynthesis. There was a good correlation between the irreversible decline in ribulose bisphosphate carboxylase (activity and amount) and that of photosynthesis. In contrast, the correlation did not seem as good between stomatal closure and the onset of the irreversible decline in photosynthesis. The reason total soluble protein remained high following depodding while carboxylase, which normally comprised 40% of the soluble protein, declined was because several polypeptides increased in amounts sufficient to offset the loss of carboxylase. This change in leaf protein composition indicates a change in leaf function; this is discussed in terms of other recent findings.

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PubMed1984

Storage Protein Composition of Soybean Cotyledons Grown In Vitro in Media of Various Sulfate Concentrations in the Presence and Absence of Exogenous l-Methionine.

Immature soybean (Glycine max L. Merrill cv Provar) cotyledons were grown aseptically for 6 days in complete culture medium with zero, deficient (17 micromolar), sufficient (1.5 millimolar), or supraoptimal (7.5 millimolar) levels of sulfate. Some cotyledons at each sulfate concentration were supplemented with l-methionine. No sulfate or 17 micromolar sulfate were inadequate for growth and protein accumulation, but all the major subunits of the 7S and 11S storage protein fractions were detected. The ratio of 11S to 7S proteins was <1.0. Addition of 8.4 millimolar methionine overcame the restriction of cotyledon growth and protein accumulation in the sulfate-deficient media, and the ratio of 11S to 7S proteins was significantly increased. The amino acid compositions of the 7S and 11S fractions from sulfate-sufficient cotyledons and from sulfate-deficient cotyledons were not significantly different. There was no difference in fresh weight or total protein accumulation in cotyledons grown in 1.5 millimolar or 7.5 millimolar sulfate. At 7.5 millimolars sulfate, the 11S to 7S ratio was significantly increased, and the amount of beta-subunit in the 7S fraction decreased. At all sulfate levels supplemented with methionine, the 11S to 7S ratio was greater than 1, and no beta-subunit was detected in the 7S fraction. Supplemental methionine in media of any sulfate concentration increased growth and protein methionine content to a greater extent than high (7.5 millimolar) sulfate only. Adding supraoptimal sulfate is not equivalent to supplementing with methionine. Results of this study of in vitro growth are compared to results of studies of seed development on intact plants supplied with various sulfur concentrations.

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PubMed1984

Pyruvate orthophosphate dikinase of c(3) seeds and leaves as compared to the enzyme from maize.

Pyruvate orthophosphate dikinase (PPDK) was found in various immature seeds of C(3) plants (wheat, pea, green bean, plum, and castor bean), in some C(3) leaves (tobacco, spinach, sunflower, and wheat), and in C(4) (maize) kernels. The enzyme in the C(3) plants cross-reacts with rabbit antiserum against maize PPDK. Based on protein blot analysis, the apparent subunit size of PPDK from wheat seeds and leaves and from sunflower leaves is about 94 kdaltons, the same as that of the enzyme from maize, but is slightly less (about 90 kdaltons) for the enzyme from spinach and tobacco leaves. The amount of this enzyme per mg of soluble protein in C(3) seeds and leaves is much less than in C(4) leaves. PPDK is present in kernels of the C(4) plant, Zea mays in amounts comparable to those in C(4) leaves.Regulatory properties of the enzyme from C(3) tissues (wheat) are similar to those of the enzyme from C(4) leaves with respect to in vivo light activation and dark inactivation (in leaves) and in vivo cold lability (seeds and leaves).Following incorporation of (14)CO(2) by illuminated wheat pericarp and adjoining tissue for a few seconds, the labeled metabolites were predominantly products resulting from carboxylation of phosphoenolpyruvate, with lesser labeling of compounds formed by carboxylation of ribulose 1,5-bisphosphate and operation of the reductive pentose phosphate cycle of photosynthesis. PPDK may be involved in mechanisms of amino acid interconversions during seed development.

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PubMed1986

Limitations on Leaf Nitrate Reductase Activity during Flowering and Podfill in Soybean.

The objective of this study was to identify factors which limit leaf nitrate reductase (NR) activity as decline occurs during flowering and beginning seed development in soybean (Glycine max [L.] Merr. cv Clark). Level of NR enzyme activity, level of reductant, and availability of NO(3) (-) as substrate were evaluated for field-grown soybean from flowering through leaf senescence. Timing of reproductive development was altered within one genotype by (a) exposure of Clark to an artificially short photoperiod to hasten flowering and podfill, and (b) the use of an early flowering isoline. Nitrogen (N) was soil-applied to selected plots at 500 kilograms per hectare as an additional variable. Stem NO(3) (-) concentration and in vivo leaf NR activity were significantly correlated (R(2) = 0.69 with nitrate in the assay medium and 0.74 without nitrate in the medium at P = 0.001) across six combinations of reproductive and soil N-treatment. The supply of NO(3) (-) from the root to the leaf tissue was the primary limitation to leaf NR activity during flowering and podfill. Levels of NR enzyme and reductant were not limiting to leaf NR activity during this period.

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PubMed1986

Sucrose Transport and Phloem Unloading in Stem of Vicia faba: Possible Involvement of a Sucrose Carrier and Osmotic Regulation.

Stems of Vicia faba plants were used to study phloem unloading because they are hollow and have a simple anatomical structure that facilitates access to the unloading site. After pulse labeling of a source leaf with (14)CO(2), stem sections were cut and the efflux characteristics of (14)C-labeled sugars into various buffered solutions were determined. Radiolabeled sucrose was shown to remain localized in the phloem and adjacent phloem parenchyma tissues after a 2-hour chase. Therefore, sucrose leakage from stem segments prepared following a 75-minute chase period was assumed to be characteristic of phloem unloading. The efflux of (14)C assimilates from the phloem was enhanced by 1 millimolar p-chloromercuribenzene sulfonic acid (PCMBS) and by 5 micromolar carbonyl cyanide m-chlorophenly hydrazone (CCCP). However, PCMBS inhibited and CCCP enhanced general leakage of nonradioactive sugars from the stem segments. Sucrose at concentrations of 50 millimolar in the free space increased efflux of [(14)C]sucrose, presumably through an exchange mechanism. This exchange was inhibited by PCMBS and abolished by 0.2 molar mannitol. Increasing the osmotic concentration of the efflux medium with mannitol reduced [(14)C]sucrose efflux. However, this inhibition seems not to be specific to sucrose unloading since leakage of total sugars, nonlabeled sucrose, glucose, and amino acids from the bulk of the tissue was reduced in a similar manner. The data suggest that phloem unloading in cut stem segments is consistent with passive efflux of sucrose from the phloem to the apoplast and that sucrose exchange via a membrane carrier may be involved. This is consistent with the known conductive function of the stem tissues, and contrasts with the apparent nature and function of unloading in developing seeds.

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PubMed1987

Heat stress enhances phytohemagglutinin synthesis but inhibits its transport out of the endoplasmic reticulum.

In this study we examined the effect of heat stress (up to 6 hours at 43 degrees C) on the biosynthesis and transport of phytohemagglutinin (PHA) in cotyledons of developing seeds of the common bean, Phaseolus vulgaris. Heat stress resulted in a decrease of total protein synthesis and an enhancement of the synthesis of heat shock proteins and PHA. Pulse chase experiments showed that a considerable proportion of the newly synthesized PHA was present in the endoplasmic reticulum (ER)/Golgi fraction and did not readily chase-out. Analysis with endoglycosidase H showed that the oligosaccharide sidechains of PHA were almost entirely in the high mannose configuration, indicating that most of the newly synthesized PHA was in the ER. However, some of the PHA became fucosylated at 43 degrees C, indicating fucosyltransferase activity. That the biosynthesis and secretion of fucosyl-containing cell wall polymers proceeded normally at 43 degrees C provided evidence that certain Golgi functions (i.e. transport to the cell wall) remained unaffected by heat stress. The ER obtained from these heat stress cotyledons had a greater density (1.16 g. cm(-3) at 43 degrees C instead of 1.14 g.cm(-3) at 22 degrees C) in sucrose gradients. Ultrastructural observations showed that the width of the lumen of the ER cisternae had increased from 20 nanometers at 22 degrees C to 60 to 80 nanometers at 43 degrees C; the lumen was filled with electrondense material presumed to be protein. The experiments are interpreted as evidence that heat stress imposes a block in the transport of PHA out of the ER. Whether heat stress affects the ER itself or alters the conformation of PHA, thereby preventing its transport, is not clear.

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PubMed1989

Mannose analog 1-deoxymannojirimycin inhibits the Golgi-mediated processing of bean storage glycoproteins.

The asparagine-linked oligosaccharide chains of glycoproteins can be processed to form a wide variety of structures. The Golgi complex is the main compartment involved in this processing. In mammalian cells the first enzyme acting along the Golgi processing pathway is mannosidase I, whose action is a prerequisite for any further processing and which is inhibited by the mannose analog 1-deoxymannojirimycin (dMM). To have insights into the processing pathway in plant cells, we have studied the in vivo effect of dMM on the processing of the bean (Phaseolus vulgaris) storage proteins phaseolin and phytohemagglutinin, two well characterized plant glycoproteins. Cotyledons obtained from developing seeds were labeled with radioactive leucine, glucosamine, or fucose in the presence or absence of dMM. Treatment with dMM fully inhibited the acquisition of resistance to endo-beta-N-acetylglucosaminidase H by phaseolin and phytohemagglutinin and the incorporation of fucose into protein. Furthermore, the apparent molecular weight of the polypeptides of phaseolin and phytohemagglutinin synthesized in dMM-treated cotyledons was consistent with the exclusive presence of oligommanose oligosaccharide chains which had not been processed in the Golgi complex. The inhibition of processing did not prevent exit from the Golgi complex, and most probably the storage proteins were correctly targeted to the protein bodies as indicated by the post-translational polypeptide cleavage of phaseolin. These results indicate that the action of a mannosidase is the first obligatory step of Golgi-mediated processing also in a plant cell and, together with data obtained in other laboratories on the in vitro specificity of glycosidases and glycosyltransferases present in the Golgi complex of plant cells, support the hypothesis that the key early reactions in Golgi-mediated processing are similar if not identical in plants and mammals.

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PubMed1990

Heat Shock Proteins and Their mRNAs in Dry and Early Imbibing Embryos of Wheat.

Two-dimensional gels of in vitro translation products of mRNAs isolated from quiescent wheat (Triticum aestivum) embryos demonstrate the presence of mRNAs encoding heat shock proteins (hsps). There were no detectable differences in the mRNAs found in mature embryos from field grown, from 25 degrees C growth chamber cultivated, or from plants given 38 degrees C heat stresses at different stages of seed development. The mRNAs encoding several developmentally dependent (dd) hsps were among those found in the dry embryos. Stained two-dimensional gels of proteins extracted from 25 degrees C growth chamber cultivated wheat embryos demonstrated the presence of hsps, including dd hsps. A study of the relationship of preexisting hsp mRNAs and the heat shock response during early imbibition was undertaken. Heat shocks (42 degrees C, 90 minutes) were administered following 1.5, 16, and 24 hours of 25 degrees C imbibition. While the mRNAs encoding the low molecular weight hsps decayed rapidly upon imbibition, the mRNAs for dd hsps persisted longer and were still detectable following 16 hours of imbibition. After 1.5 hours of imbibition, the mRNAs for the dd hsps did not accumulate in response to heat shock, even though the synthesis of the proteins was enhanced. Thus, an applied heat shock appeared to lead to the preferential translation of preexisting dd hsp mRNAs. The mRNAs for the other hsps, except hsp 70, were newly transcribed at all of the imbibition times examined. The behavior of the hsp 70 group of proteins during early imbibition was examined by RNA gel blot analysis. The mRNAs for the hsp 70 group were detectable at moderate levels in the quiescent embryo. The relative level of hsp 70 mRNA increased after the onset of imbibition at 25 degrees C and remained high through 25.5 hours of prior imbibition. The maximal levels of these mRNAs at 25 degrees C was reached at 17.5 hours of imbibition. Heat shock caused modest additional accumulation of hsp70 mRNA at later imbibition times.

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