Desiccation of imbibed and germinating pea axes causes a partial reversal of germination events

During the first 20 h of imbibition, the viability of ungerminated pea seeds was progressively impaired by desiccation, although by no more than about 15%. However, a drastic reduction in radicle growth and viability resulted if the seeds were imbibed for 24 h or longer before dehydration to 5% moisture content (but not if drying was to 10% moisture content or greater). Therefore, pea seeds undergo a transition from a desiccation-tolerant to a desiccation-intolerant state between 20 and 24 h after the start of imbibition. Seeds initially imbibed for 8, 12, and 16 h and desiccated to their original moisture content completed germination in a shorter time than control (once-imbibed) seeds. However, a residual effect of the treatment was noted, in that the total imbibition time required before germination was longer in those seeds that had been subjected to desiccation between periods of imbibition. Hence, events occurring prior to water loss are not completely stable to desiccation and are partially reversed. Axes from germinating seeds interrupted by a desiccation treatment retained some of their capacity to synthesize proteins. This metabolic event thus responded to desiccation in a similar manner to a physiological parameter, i.e., the rate of germination.

Seed development in Ricinus communis cv. Hale (castor bean). III. Pattern of storage protein and phytin accumulation in the endosperm

The development of the endosperm of castor bean seed from its initial free nuclear state through to the end of maturation is presented. An investigation of the pattern of reserve accumulation in the endosperm at the light microscopy level revealed that the accumulation of soluble and insoluble storage proteins, and of phytin, does not occur simultaneously in all cells of the developing storage organ. Rates of reserve accumulation also vary among regions of the endosperm. Storage protein and phytin accumulation are initiated in a region midway between the periphery and central lumen of the endosperm by the early cotyledon stage of seed development. Afterwards, reserve deposition occurs more intensely in the proximal and more peripheral regions than in the distal and internal regions. A wave of reserve accumulation, or protein body maturation, proceeds from the more peripheral and the proximal regions to the more internal and distal regions as development continues. The last cells to complete reserve deposition are those in regions lying close to the endosperm lumen (into which the cotyledons have expanded) and the outermost two cell layers of the endosperm.

Reprogramming of Protein Synthesis from a Developmental to a Germinative Mode Induced by Desiccation of the Axes of Phaseolus vulgaris

Immature seeds of Phaseolus vulgaris cv Taylor's Horticultural removed from the pod at 32 days of development do not germinate unless first subjected to desiccation. Our results show that premature drying not only redirects metabolism from a developmental to a germination program but it does so permanently, thus effecting an irreversible switch. This is shown by in vitro protein synthesis, and analysis of poly(A)(+) mRNA with a cDNA probe specific for phaseolin message. For example, the pattern of proteins synthesized in vitro by the mRNA fraction from fresh and prematurely dried axes show strong similarities; on the other hand, the mRNA population from rehydrated axes code for a different set of proteins. Also, the message for phaseolin is preserved following the normal maturation process and premature desiccation of seeds. Following rehydration of immature seeds at the desiccation-tolerant stage, this message is no longer detectable in the axes.

Changes in germination and respiratory potential of embryos of dormant Grand Rapids lettuce seeds during long-term imbibed storage, and related changes in the endosperm

Grand Rapids lettuce (Lactuca sativa L.) seeds were stored in an imbibed state for up to two years. Embryos dissected from stored seeds showed a progressive loss with time in their ability to germinate on polyethylene glycol (PEG) solutions. Little germination of dissected embryos from one-month imbibed seeds occurred on-6 bar PEG but only after four months of storage did the dissected embryos fail to germinate on-4 bar PEG. After two years storage 30% of dissected embryos still were able to germinate on-2 bar PEG. This loss of germination potential, which may be a symptom of the development of an embryo dormancy, could be reversed by N(6)-benzyladenine (BA) and red light (R) applied together or separately to dissected embryos. Two weeks of chilling of 12-month imbibed seeds restored sensitivity to R and a 48-h BA pretreatment prior to R resulted in germination rates similar to those of seeds emerging from primary dormancy. There was loss of embryo control of endo-β-mannanase activity after two weeks of storage even though the endosperms themselves retained their capacity for enzyme synthesis for six more weeks. Eventually, then, endo-β-mannanase synthesis is not possible because of inherent changes in both the embryo and endosperm, although each tissue undergoes changes at its own rate. Oxygen uptake by embryos dissected from two-month imbibed seeds did not increase to the same extent as embryos dissected from freshly imbibed seeds. In intact seeds germinating from a skotodormant state, oxygen uptake increased at a time coincident with radicle protrusion, but did not achieve the levels of uptake of those seeds germinating from a primary dormant state. The decline in uptake of oxygen by secondary dormant seeds is the result of a lowered respiratory capability of the embryo itself, rather than of changes in permeability of the surrounding structures.

Induction of heat shock protein messenger RNA in maize mesocotyls by water stress, abscisic Acid, and wounding

Exposure of the excised growing region of the mesocotyl of young corn seedlings to heat shock stimulated the production of specific heat shock proteins and the intensification of synthesis of two proteins with a molecular weight of approximately 70,000. Water stress and abscisic acid also stimulated synthesis of these 70,000-dalton proteins, and other unique proteins distinct from those induced by heat shock. Growing tissues of intact corn mesocotyls exposed to heat shock, water stress, or abscisic acid accumulated mRNA species homologous to a cloned genomic probe of the 5' end of the 70,000-dalton Drosophila heat shock protein gene. Since cut segments of the mesocotyl under unstressed conditions produced a similar mRNA, we suggest that the hsp 70 gene is activated in corn by a variety of diverse stresses. Production of the mRNA is rapid, but transient, being induced within 3 hours of the imposition of the stress, but declining after reaching a maximum at 9 hours.

Membrane Organization of the Desiccation-Tolerant Moss Tortula ruralis in Dehydrated States

Membrane organization of the desiccation tolerant moss Tortula ruralis was studied in several intensely dehydrated states (75% relative humidity [RH], 90% RH, plasmolysis in molar salt, freezing to -20 degrees C) by (31)P nuclear magnetic resonance and ultrastructural analyses. Both methods revealed that even at 75% RH (-400 bars), the moss cellular membranes retained extended phospholipid bilayers. Ultrastructural analyses of the fully hydrated moss showed an extensive proliferation of membrane vesicles in the endoplasmic reticulum. During dehydration, these vesicles form layers of membrane under the plasmalemma and in some cases appear to fuse with the surface membrane. This suggests that these vesicles may serve as a reservoir of membranes to accommodate for membrane surface area changes during desiccation and subsequent rehydration.

A comparative study of the insoluble storage proteins and the lectins of seeds of the Euphorbiaceae

The major storage protein within seeds of the Euphorbiaceae is the 11S crystalloid, which is only completely soluble in buffer or salt solutions if sodium dodecylsulphate or urea is present. Prior to this study, only the storage proteins of the castor bean had been characterized. The nonreduced crystalloid protein complex in all species tested has a molecular weight of 50 000 – 55 000, and in reduced form the proteins migrate on polyacrylamide gels as two distinct groups of polypeptides, one in the molecular weight range 20 000 – 25 000 and the other in the 29 000 – 35 000 range. In this respect the proteins have the general characteristics of those of castor bean, but only the proteins of Jatropha gossypifolia show striking similarities. Within any one genus, the storage proteins appear to be more or less identical (e.g., Manihot spp.) or show distinct differences (e.g., Euphorbia spp.). The soluble lectin proteins of J. gossypifolia have very similar haemagglutination properties to those of castor bean lectins, and the glycoproteins of both species separate similarly on polyacrylamide gels. Few other species contain glycoproteins or lectins that can cause agglutination.

Plant Desiccation and Protein Synthesis : VI. Changes in Protein Synthesis Elicited by Desiccation of the Moss Tortula ruralis are Effected at the Translational Level

Upon rehydration of the moss Tortula ruralis following desiccation at a rapid or slow rate, there is increasing utilization of newly synthesized-poly(A)(+) RNA for protein synthesis. Initially, poly(A)(+) RNA conserved in the dry moss is associated with polysomes, but by 2 hours of rehydration there is an overwhelming recruitment of newly synthesized poly(A)(+) RNA, at the expense of conserved messages. In rehydrated moss, there is a marked synthesis in vivo of new proteins, which are separable by two-dimensional electrophoresis, and identifiable by fluorography. These new proteins, termed rehydration proteins, are synthesized after both rapid and slow desiccation, but their synthesis persists longer after rapid desiccation. The protein patterns obtained following in vitro translation of bulk RNA from hydrated, desiccated, and rehydrated moss were qualitatively identical. Thus the differences in protein patterns observed in vivo must result from preferential selection of specific mRNAs from the same pool, which is indicative of control of protein synthesis at the translational level. The implications of these observations in relation to the response of the moss to drying in its natural environment are discussed.

Synthesis of the crystalloid protein complex in vivo in the endosperm of developing castor bean seeds

The synthesis of subunit polypeptides of the crystalloid protein complex has been examined in endosperm from developing castor bean (Ricinus communis L. cv Hale) seeds. Pulse-label and -chase studies in vivo have shown that synthesis initially involves the formation of high molecular weight precursors (50 to 60 kilodaltons) comprising peptide-linked acidic and basic polypeptides. Precursor processing involves the posttranslational cleavage of the peptide bond to yield authentic and polypeptides. This processing has a half-life of 35 to 40 minutes and is preceded by a 45- to 60-minute lag period. Both precursor and subunit polypeptides are shown to exhibit similar molecular weight and pI heterogeneity, and this is suggested to be due to the expression of a multigene family.

Plant Desiccation and Protein Synthesis : V. Stability of Poly (A) and Poly (A) RNA during Desiccation and Their Synthesis upon Rehydration in the Desiccation-Tolerant Moss Tortula ruralis and the Intolerant Moss Cratoneuron filicinum

Upon desiccation of gametophytes of the desiccation-tolerant moss Tortula ruralis preexisting pools of poly(A)(-) RNA (rRNA) remain inact, regardless of the speed at which desiccation is achieved. Preexisting poly(A)(+) RNA pools (mRNA) are unaffected by slow desiccation but are substantially reduced during rapid desiccation. Poly(A)(-) RNA involved in protein synthesis is also unaffected by desiccation, whereas the levels of polysomal poly(A)(+) RNA in rapid- and slow-dried moss closely reflect the state of the protein synthetic complex in these dried samples.Poly(A)(-) RNA pools, both total and polysomal, are also stable during the rehydration of both rapid- and slow-dried moss. The total poly(A)(+) RNA pool decreases upon rehydration, but this reduction is simply an expression of the normal turnover of poly(A)(+) RNA in this moss. Analysis of polysomal fractions during rehydration reveals the continued use of conserved poly(A)(+) RNA for protein synthesis. The rate of synthesis of poly(A)(+) RNA upon rehydration appears to depend upon the speed at which prior desiccation is administered. Rapidly dried moss synthesizes poly(A)(+) RNA at a faster rate, 60 to 120 minutes after the addition of water, than does rehydrated slowly dried moss. Recruitment of this RNA into the protein synthetic complex also follows this pattern. Comparative studies involving the aquatic moss Cratoneuron filicinum are used to gain an insight into the relevance of these findings with respect to the cellular mechanisms associated with desiccation tolerance.

Subcellular distribution of phytin in the endosperm of developing castor bean: a possibility for its synthesis in the cytoplasm prior to deposition within protein bodies

Studies using light and electron microscopy, and energy-dispersive X-ray analysis have allowed us to identify phytin particles within the cytoplasm of the developing endosperm of castor bean (Ricinus communis L.). These particles are present at the time of the formation of globoid particles within the protein bodies, but they are absent from mature tissue with fully formed protein bodies. We suggest that phytin is formed initially in the cytoplasm (perhaps in association with the cisternal endoplasmic reticulum) before being transported to the protein bodies, wherein it condenses to form the globoid.

Plant Desiccation and Protein Synthesis. IV. RNA Synthesis, Stability, and Recruitment of RNA into Protein Synthesis during Desiccation and Rehydration of the Desiccation-Tolerant Moss, Tortula ruralis

Upon rehydration of desiccated Tortula ruralis, RNA synthesis is immediately resumed; this resumption is quicker in moss recovering from slow drying than from rapid drying. Newly synthesized RNA enters the protein synthetic complex almost immediately upon rehydration, reaching control steady state levels within 2 hours after slow drying and 6 hours after rapid drying. RNA synthesized in the 1st hour following the readdition of water enters into polysomes much earlier after slow drying than after rapid drying. The RNA components of the protein synthetic complex are stable to desiccation at either slow or rapid speeds, although more so following the former drying regime. Immediately upon rehydration, these conserved RNA are readily utilized for protein synthesis, and continue to be so at least 4 hours thereafter. Hence, the speed of desiccation affects the rate at which RNA is synthesized upon subsequent rehydration, as well as the mode of utilization of that RNA.

Long-term storage of dormant Grand Rapids lettuce seeds in the imbibed state: physiological and metabolic changes

Grand Rapids lettuce (Lactuca sativa L.) seeds retained their viability for up to ten months when maintained in the fully imbibed state on moist filter paper at 25°C in darkness. The ability of red light (R) to promote their germination was essentially lost within one week from the start of imbibition; sensitivity to gibberellic acid was retained for two weeks longer. Seeds which did not respond to either treatment had entered the state of secondary dormancy (skotodormancy). This could be relieved at all times by a combination of benzyladenine and R applied to the intact seed, or by isolation of the embryo and incubation on water. Protein synthesis increased initially following imbibition but declined after 72 h to a constant low level. Respiration declined over the first week of storage in the imbibed state to a much-reduced steady level. Cotyledonary lipid declined between four and ten months of storage but the axial lipid remained unchanged. Sucrose in the embryo increased after five months, but no changes in glucose, galactose, fructose or mannose were found. The total N content of the cotyledons declined over the first three months of storage in the imbibed state, with a concomitant rise in axial N; the latter declined slowly thereafter. Basal α-galactosidase (EC 3.2.1.22) activity decreased over seven months, but the phytochrome-induced component could not be raised by a 15-min R treatment even after one month. Germination induced by R and benzyladenine was achieved at later times without a rise in α-galactosidase levels.

An analysis of the subunit structure of the crystalloid protein complex from castor bean endosperm

Chromatographic and electrophoretic studies have shown that the subunits of the crystalloid protein, isolated from mature castor bean (Ricinus communis L. cv Hale) seed endosperm protein bodies, are heterogeneous with molecular weights in the range 49 to 53.5 kilodaltons (kD), and are quantitatively in unequal amounts. Each subunit comprises an alphabeta polypeptide pair which are reduced by 2-mercaptoethanol in two subgroups with molecular weights in the 29 to 34 kD and 20.5 to 23.5 kD ranges. Subunits and corresponding polypeptide pairs are also seen to be heterogeneous in pI following isoelectric focusing. In general, large polypeptides are acidic (pI 4.8-6.2) and small polypeptides basic (pI 7.4-9.4), although overlap of some isoelectric isomers does occur, notably in polypeptides derived from subunits which are quantitatively present in smaller amounts.

A role for α-galactosidase in the degradation of the endosperm cell walls of lettuce seeds, cv. Grand Rapids

Isolated endosperms of Grand Rapids lettuce (Lactuca sativa L.) seeds undergo extensive cell-wall degradation and sugars are released into the surrounding incubation medium. One sugar so released is galactose. α-Galactosidase (EC 3.2.122) is present at the same level in both dry and imbibed isolated endosperms and is responsible for the release of galactose. However, this enzyme does not act upon the native endosperm cell wall, but requires first its partial hydrolysis and the production of oligomers by the action of endo-β-mannanase (EC 3.2.1.787). Galactose is then cleaved from these oligomers, allowing their further subsequent hydrolysis by endo-β-mannanase. Thus α-galactosidase and endo-β-mannanase act cooperatively to effect the hydrolysis of the lettuce endosperm cell walls.

SO2-Induced changes in the polyribosomal profiles of the gametophyte of the moss tortula ruralis

Aqueous SO2 (0.5-2.0 mM), in short-term exposure experiments (30-120 min.), causes a rapid loss of integrity of the polyribosomes in the moss Tortula ruralis. This effect is concentration- and exposure time-dependent and appears to be unattributable to an SO2-induced pH change.

Desiccation of Axes of Phaseolus vulgaris during Development of a Switch from a Development Pattern of Protein Synthesis to a Germination Pattern

Immature seeds of Phaseolus vulgaris removed from the pod at 32 days of development do not germinate unless first subjected to a desiccation treatment. This change from development to germination caused by premature drying is mirrored in the pattern of protein synthesis by the axes. Rehydrated axes from 32-day-developed seeds cease to synthesize proteins that are uniquely associated with development, but instead synthesize some proteins that are similar to those made in the germinating axes from mature dry seeds. Desiccation of 22-day-developed seeds does not lead to their germination, nor does it cause a switch from a developmental to a germination mode of protein synthesis by the axes. It is proposed that desiccation plays a role in permanently suppressing developmental protein synthesis and in inducing germination protein synthesis.

Deposition of Matrix and Crystalloid Storage Proteins during Protein Body Development in the Endosperm of Ricinus communis L. cv. Hale Seeds

Protein bodies within the endosperm of castor bean (Ricinus communis L. cv. Hale) seeds arise from numerous small vacuoles which progressively become filled with storage protein, of which the crystalloid proteins make up approximately 70%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) shows that the crystalloids are a family of at least four proteins which reduce to two complementary groups after 2-mercaptoethanol treatment. The matrix, which comprises the remainder, has two major components, the soluble albumins and the lectins. The lectins are the only glycoproteins within the mature protein body. Both cytochemical staining and SDS-PAGE indicate that the synthesis of the crystalloid and the majority of matrix proteins begins some 20 days after pollination. Additionally, the crystalloid proteins are synthesized concurrently, whereas there is temporal variation in the synthesis of matrix proteins.

Stability and Synthesis of Phospholipids during Desiccation and Rehydration of a Desiccation-Tolerant and a Desiccation-Intolerant Moss

The fatty acid composition of the phospholipids from the desiccation-tolerant moss Tortula ruralis (Hedw.) Gaertn, Meyer and Scherb and the desiccation-intolerant moss Cratoneuron filicinum has been determined. No changes in composition occur in either moss as a consequence of rapid drying, but, after slow drying, there is a decline in some unsaturated fatty acids. Upon rehydration of T. ruralis after slow drying, these acids decline further; however, within 105 minutes, they regain the same levels as those in undesiccated controls. A smaller and more transient decline occurs after rapid desiccation. Most phospholipid unsaturated fatty acids decrease during rehydration of C. filicinum, and their levels are not recovered. After both rapid and slow drying of T. ruralis, acetate and glycerol are incorporated into the phospholipid fraction, although de novo synthesis, alone, might not account for the increase in unsaturated fatty acids upon rehydration. Very little acetate or glycerol is incorporated during rehydration of C. filicinum. Loss of unsaturated fatty acids from the phospholipids of T. ruralis does not appear to be associated with increased lipoxygenase activity. Furthermore, there is little correlation between the extent of peroxidation of fatty acids due to desiccation and changes in the phospholipid fraction.

Responses of the moss Tortula ruralis to desiccation treatments. I. Effects of minimum water content and rates of dehydration and rehydration

Samples of the moss Tortula ruralis were desiccated either rapidly or slowly to different tissue water contents to determine the limits of desiccation tolerance. Experimental samples were rehydrated rapidly by contact with wet filter paper, and gas exchange, chlorophyll content, electrolyte efflux, and linear growth rate were compared with those of control samples. Drying to 0.30–0.089 g H2O∙g dry weight−1 did not significantly affect the moss. Slow drying to 0.058–0.008 g H2O∙g dry weight−1 caused temporary increases in dark respiration and electrolyte leakage, and a slight inhibition of growth. Rapid drying of the same water contents caused visible injury, reduced total chlorophyll and the ratio of chlorophyll a:b, greatly enhanced electrolyte efflux, and severely inhibited gross photosynthesis and linear growth. The damaging effects of rapid drying could be eliminated either by partial desiccation for 1–3 h before rapid drying, or by placing the dry moss in a 100% relative humidity (RH) atmosphere for 1–5 h before rehydration.Tortula ruralis demonstrated greater drought tolerance than many other bryophytes, algae, seeds, and desert angiosperms, and is clearly capable of surviving any drought which might occur in its natural habitat.

Responses of the moss Tortula ruralis to desiccation treatments. II. Variations in desiccation tolerance

Variations in the apparent desiccation tolerance of the moss Tortula ruralis were traced to several causes. When the moss was kept continuously hydrated for several days, desiccation tolerance tended to decrease ("dehardening"). Conversely, daily episodes of drying and rehydration induced hardening so that the moss could tolerate rapid drying to a tissue water content of 0.008 g∙g dry weight−1.Different clumps of moss showed very different degrees of desiccation tolerance. The least tolerant samples suffered severe damage after rapid drying, whereas the most tolerant suffered little damage, and did not deharden during 3 days continuous hydration.

Mobilisation of the major stored reserves in the embryo of fenugreek (Trigonella foenum-graecum L., Leguminosae), and correlated enzyme activities

Changes in total nitrogen, soluble amino nitrogen, lipid and phytate contents, and in the activities of proteinase (pH 7.0), isocitrate lyase and phytase were followed in the endosperm, cotyledons, and axis during germination of fenugreek seeds and subsequent growth of the seedlings. The endosperm is comprised largely of cell-wall galactomannans: the majority of the seed total nitrogen, lipid and phytate (5%, 8%, 0.44% of seed dry weight respectively) is localised within the cotyledons as stored reserves. Germination is completed after 10-14 h from the start of imbibition, but the major reserves are not mobilised during the first 24 h. Then the total nitrogen content of the cotyledons starts to decrease and that of the axis increases; there is a concomitant accumulation of soluble amino nitrogen in both cotyledons and axis. An increase in proteinase activity in the cotyledons correlates well with the depletion of total nitrogen therein. Depletion of lipid and phytate reserves in the different seed tissues constitutes a late event, occurring after 50 h from the start of imbibition, and is coincident with the final disintegration of the endosperm tissue. The depletion of phytate and stored lipids is accompanied by an increase in phytase and isocitrate lyase activity. It appears that the products of lipid hydrolysis are converted by gluconeogenesis to serve as the major source of sugars for the growing axis after the endosperm galactomannan has been completely mobilised.

Red-light- and gibberellic-acid-enhanced α-galactosidase activity in germinating lettuce seeds, cv. Grand Rapids : Control by the axis

Dry lettuce (Lactuca sativa L.) "seeds" (achenes) contain α-galactosidase (EC 3.2.122) at a level which is maintained in the imbibed dormant state in darkness. Both red light (R) and gibberellic acid promote an increase in enzyme activity several hours prior to the completion of germination. Germination and enzyme activity are not essentially linked, however, for the latter can increase while the former is inhibited. α-Galactosidase activity increases within the cotyledons and the endosperm following R stimulation, but the axis is essential to perceive the stimulus and to promote and maintain the increase in enzyme activity. A diffusible factor (or factors) is produced by and-or released from irradiated axes, and it migrates to the cotyledons (and possibly endosperm) to promote the increase in α-galactosidase activity. Gibberellic acid, particularly in the presence of benzyladenine, can replace the requirement for irradiated axes.

Protein synthesis and phospholipids in soybean axes in response to imbibitional chilling

The responses of two cultivars of soybean (Merr.) to a chilling treatment (4 C for first hour of imbibition) were compared. The germination of cv. Biloxi was unaffected by the treatment, while the germination of cv. Fiskeby was reduced. The phospholipid fatty acids of dry axes of the two cultivars were very similar, and, thus, could not be correlated with their responses to chilling. The fatty acid composition of chilling-tolerant Biloxi did not change over a subsequent 23-hour warm incubation, but there was a marked reduction in the unsaturated fatty acids of chilling-sensitive Fiskeby after 12 hours, which may be a symptom of deterioration. Protein synthesis in both cultivars was reduced by the chilling treatment. Redrying of Biloxi axes up to 18 hours after the onset of imbibition had no effect on their germination upon rehydration. Germination of Fiskeby axes was reduced by redrying after 8 hours of imbibition. After 7 months of dry storage of intact seeds, the sensitivity of the axes to chilling was retested. Biloxi axes had become chilling-sensitive, while the germination of Fiskeby axes was reduced to zero by the chilling treatment. A hypothesis is presented that imbibitional chilling sensitivity is an indication of reduced vigor, axes with a high vigor can tolerate the stress, while those without cannot.