Characterization of molecular mobility in seed tissues: an electron paramagnetic resonance spin probe study

Molecular dynamics of seed priming at the crossroads between basic and applied research

The potential of seed priming is still not fully exploited. Our limited knowledge of the molecular dynamics of seed pre-germinative metabolism is the main hindrance to more effective new-generation techniques. Climate change and other recent global crises are disrupting food security. To cope with the current demand for increased food, feed, and biofuel production, while preserving sustainability, continuous technological innovation should be provided to the agri-food sector. Seed priming, a pre-sowing technique used to increase seed vigor, has become a valuable tool due to its potential to enhance germination and stress resilience under changing environments. Successful priming protocols result from the ability to properly act on the seed pre-germinative metabolism and stimulate events that are crucial for seed quality. However, the technique still requires constant optimization, and researchers are committed to addressing some key open questions to overcome such drawbacks. In this review, an update of the current scientific and technical knowledge related to seed priming is provided. The rehydration-dehydration cycle associated with priming treatments can be described in terms of metabolic pathways that are triggered, modulated, or turned off, depending on the seed physiological stage. Understanding the ways seed priming affects, either positively or negatively, such metabolic pathways and impacts gene expression and protein/metabolite accumulation/depletion represents an essential step toward the identification of novel seed quality hallmarks. The need to expand the basic knowledge on the molecular mechanisms ruling the seed response to priming is underlined along with the strong potential of applied research on primed seeds as a source of seed quality hallmarks. This route will hasten the implementation of seed priming techniques needed to support sustainable agriculture systems.

Seed Moisture Isotherms, Sorption Models, and Longevity

Seed moisture sorption isotherms show the equilibrium relationship between water content and equilibrium relative humidity (eRH) when seeds are either losing water from a hydrated state (desorption isotherm) or gaining water from a dry state (adsorption isotherm). They have been used in food science to predict the stability of different products and to optimize drying and/or processing. Isotherms have also been applied to understand the physiological processes occurring in viable seeds and how sorption properties differ in relation to, for example, developmental maturity, degree of desiccation tolerance, or dormancy status. In this review, we describe how sorption isotherms can help us understand how the longevity of viable seeds depends upon how they are dried and the conditions under which they are stored. We describe different ways in which isotherms can be determined, how the data are modeled using various theoretical and non-theoretical equations, and how they can be interpreted in relation to storage stability.

Mathematical modeling of nutritional, color, texture, and microbial activity changes in fruit and vegetables during drying: A critical review

Retention of quality attributes during drying of fruit and vegetables is a prime concern since the product's acceptability depends on the overall quality; particularly on the nutritional, color, and physical attributes. However, these quality parameters deteriorate during drying. Food quality changes are strongly related to the drying conditions and researchers have attempted to develop mathematical models to understand these relationships. A better insight toward the degradation of quality attributes is crucial for making real predictions and minimizing the quality deterioration. The previous empirical quality models employed kinetic modeling approaches to describe the quality changes and therefore, lack the realistic understanding of fundamental transport mechanisms. In order to develop a physics based mathematical model for the prediction of quality changes during drying, an in-depth understanding of research progress made toward this direction is indispensable. Therefore, the main goal of this paper is to present a critical review of the mathematical models developed and applied to describe the degradation kinetics of nutritional, color, and texture attributes during drying of fruit and vegetables and microbial growth model during storage. This review also presents the advantages and drawbacks of the existing models along with their industrial relevance. Finally, future research propositions toward developing physics-based mathematical model are presented.

Advances in Understanding of Desiccation Tolerance of Lichens and Lichen-Forming Algae

Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.

Metabolic Changes on the Acquisition of Desiccation Tolerance in Seeds of the Brazilian Native Tree Erythrina speciosa

Erythrina speciosa Andrews (Fabaceae) is a native tree of Atlantic forest from Southern and Southeastern Brazil. Although this species is found in flooded areas, it produces highly desiccation tolerant seeds. Here, we investigated the physiological and metabolic events occurring during seed maturation of E. speciosa aiming to better understand of its desiccation tolerance acquisition. Seeds were separated into six stages of maturation by the pigmentation of the seed coat. Water potential (WP) and water content (WC) decreased gradually from the first stage to the last stage of maturation (VI), in which seeds reached the highest accumulation of dry mass and seed coat acquired water impermeability. At stage III (71% WC), although seeds were intolerant to desiccation, they were able to germinate (about 15%). Desiccation tolerance was first observed at stage IV (67% WC), in which 40% of seeds were tolerant. At stage V (24% WC), all seeds were tolerant to desiccation and at stage VI all seeds germinated. Increased deposition of the arabinose-containing polysaccharides, which are known as cell wall plasticizers polymers, was observed up to stage IV of seed maturation. Raffinose and stachyose gradually increased in axes and cotyledons with greater increment in the fourth stage. Metabolic profile analysis showed that levels of sugars, organic, and amino acids decrease drastically in embryonic axes, in agreement with lower respiratory rates during maturation. Moreover, a non-aqueous fractionation revealed a change on the proportions of sugar accumulation among cytosol, plastid, and vacuoles between the active metabolism (stage I) and the dormant seeds (stage VI). The results indicate that the physiological maturity of the seeds of E. speciosa is reached at stage V and that the accumulation of raffinose can be a result of the change in the use of carbon, reducing metabolic activity during maturation. This work confirms that raffinose is involved in desiccation tolerance in seeds of E. speciosa, especially considering the different subcellular compartments and suggests even that the acquisition of desiccation tolerance in this species occurs in stages prior to the major changes in WC.

Longevity of Preserved Germplasm: The Temperature Dependency of Aging Reactions in Glassy Matrices of Dried Fern Spores

This study explores the temperature dependency of the aging rate in dry cells over a broad temperature range encompassing the fluid to solid transition (Tg) and well below. Spores from diverse species of eight families of ferns were stored at temperatures ranging from +45�C to approximately -176�C (vapor phase above liquid nitrogen), and viability was monitored periodically for up to 4,300 d (∼12 years). Accompanying measurements using differential scanning calorimetry (DSC) provide insights into structural changes that occur, such as Tg between +45 and -20�C (depending on moisture), and triacylglycerol (TAG) crystallization between -5 and -35�C (depending on species). We detected aging even at cryogenic temperatures, which we consider analogous to unscheduled degradation of pharmaceuticals stored well below Tg caused by a shift in the nature of molecular motions that dominate chemical reactivity. We occasionally observed faster aging of spores stored at -18�C (conventional freezer) compared with 5�C (refrigerator), and linked this with mobility and crystallization within TAGs, which probably influences molecular motion of dried cytoplasm in a narrow temperature range. Temperature dependency of longevity was remarkably similar among diverse fern spores, despite widely disparate aging rates; this provides a powerful tool to predict deterioration of germplasm preserved in the solid state. Future work will increase our understanding of molecular organization and composition contributing to differences in longevity.

Physical Methods for Seed Invigoration: Advantages and Challenges in Seed Technology

In the context of seed technology, the use of physical methods for increasing plant production offers advantages over conventional treatments based on chemical substances. The effects of physical invigoration treatments in seeds can be now addressed at multiple levels, ranging from morpho-structural aspects to changes in gene expression and protein or metabolite accumulation. Among the physical methods available, "magneto-priming" and irradiation with microwaves (MWs) or ionizing radiations (IRs) are the most promising pre-sowing seed treatments. "Magneto-priming" is based on the application of magnetic fields and described as an eco-friendly, cheap, non-invasive technique with proved beneficial effects on seed germination, vigor and crop yield. IRs, as γ-rays and X-rays, have been widely regarded as a powerful tool in agricultural sciences and food technology. Gamma-rays delivered at low dose have showed to enhance germination percentage and seedling establishment, acting as an actual 'priming' treatment. Different biological effects have been observed in seeds subjected to MWs and X-rays but knowledge about their impact as seed invigoration agent or stimulatory effects on germination need to be further extended. Ultraviolet (UV) radiations, namely UV-A and UV-C have shown to stimulate positive impacts on seed health, germination, and seedling vigor. For all mentioned physical treatments, extensive fundamental and applied research is still needed to define the optimal dose, exposition time, genotype- and environment-dependent irradiation conditions. Electron paramagnetic resonance has an enormous potential in seed technology not fully explored to monitor seed invigoration treatments and/or identifying the best suitable irradiation dose or time-point to stop the treatment. The present manuscript describes the use of physical methods for seed invigoration, while providing a critical discussion on the constraints and advantages. The future perspectives related to the use of these approaches to address the need of seed technologists, producers and trade markers will be also highlighted.

Understanding the molecular pathways associated with seed vigor

Farmers and growers are constantly looking for high quality seeds able to ensure uniform field establishment and increased production. Seed priming is used to induce pre-germinative metabolism and then enhance germination efficiency and crop yields. It has been hypothesized that priming treatments might also improve stress tolerance in germinating seeds, leaving a sort of 'stress memory'. However, the molecular bases of priming still need to be clarified and the identification of molecular indicators of seed vigor is nowadays a relevant goal for the basic and applied research in seed biology. It is generally acknowledged that enhanced seed vigor and successful priming depend on DNA repair mechanisms, activated during imbibition. The complexity of the networks of DNA damage control/repair functions has been only partially elucidated in plants and the specific literature that address seeds remains scanty. The DNA repair pathways hereby described (Nucleotide and Base Excision Repair, Non-Homologous End Joining, Homologous Recombination) play specific roles, all of them being critical to ensure genome stability. This review also focuses on some novel regulatory mechanisms of DNA repair (chromatin remodeling and small RNAs) while the possible use of telomere sequences as markers of aging in seed banks is discussed. The significant contribution provided by Electron Paramagnetic Resonance in elucidating the kinetics of seed aging, in terms of free radical profiles and membrane integrity is reported.

Detailed characterization of mechanical properties and molecular mobility within dry seed glasses: relevance to the physiology of dry biological systems

Slow movement of molecules in glassy matrices controls the kinetics of chemical and physical reactions in dry seeds. Variation in physiological activity among seeds suggests that there are differences in mobility among seed glasses. Testing this hypothesis is difficult because few tools are available to measure molecular mobility within dry seeds. Here, motional properties within dry pea cotyledons were assessed using dynamic mechanical analysis. The technique detected several molecular relaxations between -80 and +80°C and gave a more detailed description of water content-temperature effects on molecular motion than previously understood from studies of glass formation in seeds at glass transition (Tg). Diffusive movement is delimited by the α relaxation, which appears to be analogous to Tg. β and γ relaxations were also detected at temperatures lower than α relaxations, clearly demonstrating intramolecular motion within the glassy matrix of the pea cotyledon. Glass transitions, or the mechanical counterpart α relaxation, appear to be less relevant to seed aging during dry storage than previously thought. On the other hand, β relaxation occurs at temperature and moisture conditions typically used for seed storage and has established importance for physical aging of synthetic polymer glasses. Our data show that the nature and extent of molecular motion varies considerably with moisture and temperature, and that the hydrated conditions used for accelerated aging experiments and ultra-dry conditions sometimes recommended for seed storage give greater molecular mobility than more standard seed storage practices. We believe characterization of molecular mobility is critical for evaluating how dry seeds respond to the environment and persist through time.

Glass formation in plant anhydrobiotes: survival in the dry state

Anhydrobiotes can resist complete dehydration and survive the dry state for extended periods of time. During drying, cytoplasmic viscosity increases dramatically and in the dry state, the cytoplasm transforms into a glassy state. Plant anhydrobiotes possess large amounts of soluble non-reducing sugars and their state diagrams resemble those of simple sugar mixtures. However, more detailed in vivo measurements using techniques such as Fourier transform infrared spectroscopy and electron paramagnetic resonance spectroscopy reveal that these intracellular glasses are complex systems with properties quite different from those of simple sugar glasses. Intracellular glasses exhibit a high molecular packing and slow molecular mobility, resembling glasses made of mixtures of proteins and sugars, which potentially interact with additional cytoplasmic components such as salts, organic acids, and amino acids. Above the glass transition temperature, the cytoplasm of biological systems still exhibits a high stability and low molecular mobility, which could serve as an ecological advantage. All desiccation-tolerant organisms form glasses upon drying, but desiccation-sensitive organisms generally lose their viability during drying at water contents at which the glassy state has not yet been formed, suggesting that other factors are necessary for desiccation tolerance. Nevertheless, the formation of intracellular glasses is indispensable to survive the dry state. Storage stability of seeds and pollens is related to the molecular mobility and packing density of the intracellular glass, suggesting that the characteristic properties of intracellular glasses provide stability for long-term survival.

The Stability of lyophilized lipid/DNA complexes during prolonged storage

It is well known that excipients are required to protect nonviral vectors during the lyophilization process. The goal of this study is to describe the stability of lyophilized nonviral vector preparations on pharmaceutically relevant timescales and provide insight into the factors that govern long-term stability of vectors in the dried state. Lipid/DNA complexes were lyophilized in glucose, sucrose, or trehalose and stored for a period of up to 2 years at five different temperatures (-20, 4, 22, 40, 60 degrees C). We evaluated simultaneously the physico-chemical characteristics (size, zeta potential, ethidium bromide (EtBr) accessibility, supercoiled DNA content) and the ability of vector formulations to transfect COS-7 cells at different time intervals. In addition, a fluorescence assay was utilized to assess levels of ROS in the dried cake after storage. The physical state of each formulation was evaluated by determination of the glass transition temperature and residual moisture content, before and after storage. Results from our stability study show that a progressive degradation of lipid/DNA complexes occurs in terms of transfection rates, particle size, dye accessibility, and supercoil content, even when samples are stored at low temperatures (e.g., -20 degrees C). Furthermore, our preliminary results on the quantification of free radicals in rehydrated formulations emphasize the importance of developing strategies to prevent the formation of reactive oxygen species (ROS) during prolonged storage in the dried state.

Longevity of cryogenically stored seeds

Though cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual shelf life achieved under ultra-cold temperatures has not been addressed theoretically or empirically. Here, we report measurable changes in germination of dried seeds stored under liquid nitrogen conditions for >10 years. There was considerable variability in the extent of deterioration among species and accessions within a species. Aging time courses for lettuce seeds stored at temperatures between 50 and -196 degrees C were fit to a form of the Avrami equation to determine rate coefficients and predict half-life of accessions. A reduction in the temperature dependency on aging rate, determined as a break in the Arrhenius plot, occurred at about -15 degrees C, and this resulted in faster deterioration than anticipated from extrapolation of kinetics measured at higher temperatures. The break in Arrhenius behavior occurred at temperatures in between the glass transition temperature (28 degrees C) and the Kauzmann temperature (-42 degrees C) and also coincided with a major triacylglycerol phase change (-40 to -7 degrees C). In spite of the faster than anticipated deterioration, cryogenic storage clearly prolonged shelf life of lettuce seeds with half-lives projected as approximately 500 and approximately 3400 years for fresh lettuce seeds stored in the vapor and liquid phases of liquid nitrogen, respectively. The benefit of low temperature storage (-18 or -135 degrees C) on seed longevity was progressively lost if seeds were first stored at 5 degrees C. Collectively, these results demonstrate that lowering storage temperature progressively increases longevity of seeds. However, cryogenic temperatures were not sufficient to stop deterioration, especially if initial stages of aging were allowed to progress at higher storage temperatures. This work contributes to reliable assessments of the potential benefit and cost of different genebanking strategies.

Temperature dependency of molecular mobility in preserved seeds

Although cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual timescale for changes in viability has not been addressed theoretically or empirically. Molecular mobility within preserved biological materials provides a first approximation of the rate of deteriorative reactions that ultimately affect shelf-life. Here, temperature effects on molecular mobility in partially dried seeds are calculated from heat capacities, measured using differential scanning calorimetry, and models for relaxation of glasses based on configurational entropy. Based on these analyses, glassy behavior in seeds containing 0.07 g H(2)O/g dm followed strict Vogel-Tamman-Fulcher (VTF) behavior at temperatures above and just below the glass transition temperature (Tg) at 28 degrees C. Temperature dependency of relaxation times followed Arrhenius kinetics as temperatures decreased well below Tg. The transition from VTF to Arrhenius kinetics occurred between approximately 5 and -10 degrees C. Overall, relaxation times calculated for seeds containing 0.07 g H(2)O/g dm decreased by approximately eight orders of magnitude when seeds were cooled from 60 to -60 degrees C, comparable to the magnitude of change in aging kinetics reported for seeds and pollen stored at a similar temperature range. The Kauzmann temperature (T(K)), often considered the point at which molecular mobility of glasses is practically nil, was calculated as -42 degrees C. Calculated relaxation times, temperature coefficients lower than expected from VTF kinetics, and T(K) that is 70 degrees C below Tg suggest there is molecular mobility, albeit limited, at cryogenic temperatures.

Nanometer-resolved interfacial fluidity

Confined liquids can have properties that are poorly predicted from bulk parameters. We resolve with 0.5 nm resolution the nanoscale perturbations that interfaces cause on fluidity, in thin 3-methylpentane (3MP) films. The films of glassy 3MP are much less viscous at the vacuum-liquid interface and much more viscous at the 3MP-metal interface, compared to the bulk of the film. We find that the viscosity at the interfaces continuously returns to the bulk value over about a 3 nm distance. The amorphous 3MP films are constructed using molecular beam epitaxy on a Pt(111) substrate at low temperatures (<30 K). Ions are gently inserted at specific distances from the substrate with a 1 eV hydronium (D(3)O(+)) or Cs(+) ion beam. The voltage across the film, which is directly proportional to the position of the ions within the film, is monitored electrostatically as the film is heated at a rate of 0.2 K/s. Above the bulk glass transition temperature (T(g)) of 3MP (77 K), the ions are expected to begin to move down through the film. However, ion movement is observed at temperatures as low as 50 K near the vacuum interface, well below the bulk T(g). The fitted kinetics predict that at 85 K, the glass is about 6 orders of magnitude less viscous near the free interface compared to that of the bulk.

Mechanisms of seed ageing under different storage conditions for Vigna radiata (L.) Wilczek: lipid peroxidation, sugar hydrolysis, Maillard reactions and their relationship to glass state transition

Two primary biochemical reactions in seed ageing (lipid peroxidation and non-enzymatic protein glycosylation with reducing sugars) have been studied under different seed water contents and storage temperatures, and the role of the glassy state in retarding biochemical deterioration examined. The viability loss of Vigna radiata seeds during storage is associated with Maillard reactions; however, the contribution of primary biochemical reactions varies under different storage conditions. Biochemical deterioration and viability loss are greatly retarded in seeds stored below a high critical temperature (approximately 40 degrees C above glass transition temperature). This high critical temperature corresponds to the cross-over temperature (T(c)) of glass transition where molecular dynamics changes from a solid-like system to a normal liquid system. The data show that seed ageing slows down significantly, even before seed tissue enters into the glassy state.

DNA methylation analysis of a male reproductive organ specific gene (MROS1) during pollen development

Pollen grains of angiosperm plants represent a good model system for studies of chromatin structure and remodelling factors, but very little is known about the DNA methylation status of particular genes in pollen. In this study, we present an analysis of the DNA methylation patterns of the MROS1 gene, which is expressed in the late phases of pollen development in Silene latifolia (syn. Meladrium album). The genomic sequencing technique revealed similar DNA methylation patterns in leaves, binucleate pollen, and trinucleate pollen. Extremely high DNA methylation levels occurred in the CG dinucleotides of the upstream region (99%), whereas only a low level of CG methylation was observed in the transcribed sequence (7%). Low levels of methylation were also observed in asymmetric sequences (in both regions; 2% methylated). The results obtained in the MROS1 gene are discussed in consequence with the immunohistochemical data showing a hypermethylation of DNA in the vegetative nucleus.

Looking beyond sugars: the role of amphiphilic solutes in preventing adventitious reactions in anhydrobiotes at low water contents

Plants and animals that can survive dehydration accumulate high concentrations of disaccharides in their cells and tissues during desiccation. These sugars are necessary both for the depression of the membrane phase transition temperature of the dry lipid and for the formation of a carbohydrate glass. In the past decade, however, it has become clear that certain types of adventitious enzymatic reactions are possible at low water contents, which along with free-radical mediated damage, can cause hydrolysis of lipids and loss of membrane barrier function. Disaccharides do not necessarily prevent these types of reactions, which suggests that other compounds might also be necessary for protecting organisms from this type of degradation during anhydrobiosis. Arbutin, one possible example, accumulates in large quantities in certain resurrection plants and has been shown to inhibit phospholipase A(2) activity at low water contents. The direct effect of arbutin on membranes under stress conditions depends on the membrane lipid composition. It can serve a protective function during desiccation- or freeze/thaw-induced stress in the presence of nonbilayer-forming lipids or a disruptive function in their absence. Other possible amphiphiles, including certain naturally occurring flavonols, may serve as anti-oxidants and some might have similar lipid composition-dependent effects. Such compounds, therefore, are likely to be localized near specific membranes, where they might provide the greatest benefit at the least liability to the organism.

Influence of a stationary magnetic field on water relations in lettuce seeds. Part II: experimental results

An experimental study on water absorption by lettuce seeds previously treated in a stationary magnetic field of 0-10 mT is presented. A significant increase in the rate with which the seeds absorb water is observed in the interval 0-10 mT of magnetic treatment. An increment in the total mass of absorbed water in this interval is also observed. These results are consistent with the reports on the increase of germination rate of the seeds, and the theoretical calculation of the variations induced by magnetic fields in the ionic currents across the cellular membrane. The fields originate in changes in the ionic concentration and thus in the osmotic pressure which regulates the entrance of water to the seeds. The good correlation between the theoretical approach and experimental results provides strong evidence that the magnetic field alters the water relations in seeds, and this effect may be the explanation of the reported alterations in germination rate of seeds by the magnetic field.

Induction of desiccation tolerance in plant somatic embryos: how exclusive is the protective role of sugars?

Plant somatic embryos usually lack desiccation tolerance. They may acquire such a tolerance upon preculture in the presence of abscisic acid (ABA), followed by slow drying, but not fast drying. ABA causes torpedo-shaped somatic embryos to lose their chlorophyll, suspend growth, exhibit low rates of respiration, and maintain elevated sucrose contents. The subsequent slow drying leads to a partial conversion of sucrose into oligosaccharides and the expression of dehydrin transcripts. Slow-dried, desiccation-tolerant somatic embryos have stable membranes, retain their native protein secondary structure, and have a densely packed cytoplasmic glassy matrix. Fast-dried, desiccation-sensitive somatic embryos experience some loss of phospholipids and an increase in free fatty acids. Their proteins show signs of denaturation and aggregation, and the glassy matrix has reduced hydrogen bonding. The reduced conversion of sucrose into oligosaccharides appears not to underlie dehydration injury. Proteins in slow-dried somatic embryos, not pretreated with ABA, also show signs of denaturation, which might be attributed to low sugar contents. We conclude that by reducing cellular metabolism, ABA maintains high sugar contents. These sugars contribute to the stability of membranes, proteins, and the cytoplasmic glassy matrix, whereas slow drying permits a further fine tuning of this stability. Partitioning of endogenous amphiphiles from the cytoplasm into membranes during drying may cause membrane perturbance, although it might confer protection to membranes in the case of amphiphilic antioxidants. The perturbance appears to be effectively controlled in desiccation-tolerant systems but not in sensitive systems, for which we suggest dehydrins are responsible. In this context, the low desiccation tolerance in the presence of ample sugars is discussed.

Mechanisms of plant desiccation tolerance

Anhydrobiosis ("life without water") is the remarkable ability of certain organisms to survive almost total dehydration. It requires a coordinated series of events during dehydration that are associated with preventing oxidative damage and maintaining the native structure of macromolecules and membranes. The preferential hydration of macromolecules is essential when there is still bulk water present, but replacement by sugars becomes important upon further drying. Recent advances in our understanding of the mechanism of anhydrobiosis include the downregulation of metabolism, dehydration-induced partitioning of amphiphilic compounds into membranes and immobilization of the cytoplasm in a stable multicomponent glassy matrix.

Isolation and characterization of a D-7 LEA protein from pollen that stabilizes glasses in vitro

A heat-soluble protein present in substantial quantities in Typha latifolia pollen was purified to homogeneity. The protein was subjected to cyanogen bromide cleavage, and the peptides produced were separated by HPLC chromatography and sequenced. The two sequences determined were found to be related to the putative D76 LEA protein from Brassica napus seeds and one of them to the D-7 LEA protein from upland cotton. This suggests the pollen protein to be a member of the LEA group III family of proteins. The secondary structure of the protein in solution and in the dry state was investigated using Fourier transform IR spectroscopy. Whereas the protein in solution was highly unordered, being largely in a random coil conformation, the conformation was largely alpha-helical after fast drying. Slow drying reversibly led to both alpha-helical and intermolecular extended beta-sheet structures. When dried in the presence of sucrose, the protein adopted alpha-helical conformation, irrespective of drying rate. The effect of the protein on the stability of sucrose glasses was also investigated. The dehydrated mixture of sucrose and the LEA protein had higher glass transition temperatures and average strength of hydrogen bonding than dehydrated sucrose alone. We suggest that LEA proteins may play a role together with sugars in the formation of a tight hydrogen bonding network in the dehydrating cytoplasm, thus conferring long-term stability.

High critical temperature above T(g) may contribute to the stability of biological systems

In this study, we characterized the molecular mobility around T(g) in sugars, poly-L-lysine and dry desiccation-tolerant biological systems, using ST-EPR, (1)H-NMR, and FTIR spectroscopy, to understand the nature and composition of biological glasses. Two distinct changes in the temperature dependence of the rotational correlation time (tau(R)) of the spin probe 3-carboxy-proxyl or the second moment (M(2)) were measured in sugars and poly-L-lysine. With heating, the first change was associated with the melting of the glassy state (T(g)). The second change (T(c)), at which tau(R) abruptly decreased over several orders of magnitude, was found to correspond with the so-called cross-over temperature, where the dynamics changed from solid-like to liquid-like. The temperature interval between T(g) and T(c) increased in the order of sucrose < trehalose < raffinose </= staychose < poly-L-lysine < biological tissues, from 17 to >50 degrees C, implying that the stability above T(g) improved in the same order. These differences in temperature-dependent mobilities above T(g) suggest that proteins rather than sugars play an important role in the intracellular glass formation. The exceptionally high T(c) of intracellular glasses is expected to provide excellent long-term stability to dry organisms, maintaining a slow molecular motion in the cytoplasm even at temperatures far above T(g).

Is there a role for oligosaccharides in seed longevity? An assessment of intracellular glass stability

We examined whether oligosaccharides extend seed longevity by increasing the intracellular glass stability. For that purpose, we used a spin probe technique to measure the molecular mobility and glass transition temperature of the cytoplasm of impatiens (Impatiens walleriana) and bell pepper (Capsicum annuum) seeds that were osmo-primed to change oligosaccharide content and longevity. Using saturation transfer electron paramagnetic resonance spectroscopy, we found that the rotational correlation time of the polar spin probe 3-carboxy-proxyl in the cytoplasm decreased, together with longevity, as a function of increasing seed water content, suggesting that longevity may indeed be regulated by cytoplasmic mobility. Osmo-priming of the seeds resulted in considerable decreases in longevity and oligosaccharide content, while the sucrose content increased. No difference in the glass transition temperature was found between control and primed impatiens seeds at the same temperature and water content. Similarly, there was no difference in the rotational motion of the spin probe in the cytoplasm between control and primed impatiens and bell pepper seeds. We therefore conclude that oligosaccharides in seeds do not affect the stability of the intracellular glassy state, and that the reduced longevity after priming is not the result of increased molecular mobility in the cytoplasm.

Molecular mobility in the cytoplasm: an approach to describe and predict lifespan of dry germplasm

Molecular mobility is increasingly considered a key factor influencing storage stability of biomolecular substances, because it is thought to control the rate of detrimental reactions responsible for reducing the shelf life of, for instance, pharmaceuticals, food, and germplasm. We investigated the relationship between aging rates of germplasm and the rotational motion of a polar spin probe in the cytoplasm under different storage conditions using saturation transfer electron paramagnetic resonance spectroscopy. Rotational motion of the spin probe in the cytoplasm of seed and pollen of various plant species changed as a function of moisture content and temperature in a manner similar to aging rates or longevity. A linear relationship was established between the logarithms of rotational motion and aging rates or longevity. This linearity suggests that detrimental aging rates are associated with molecular mobility in the cytoplasm. By measuring the rotational correlation times at low temperatures at which experimental determination of longevity is practically impossible, this linearity enabled us to predict vigor loss or longevity. At subzero temperatures, moisture contents for maximum life span were predicted to be higher than those hitherto used in genebanks, urging for a reexamination of seed storage protocols.

Pulsed EPR spin-probe study of intracellular glasses in seed and pollen

EPR spectra of 3-carboxy-proxyl (CP) in dry biological tissues exhibited a temperature-dependent change in the principal value A'(zz) of the hyperfine interaction tensor. The A'(zz) value changed sharply at a particular temperature that was dependent on water content. At elevated water contents, the break occurred at lower temperatures and appeared to be associated with the melting of the cytoplasmic glassy state. To investigate the reason for the change in A'(zz), we employed echo-detected EPR (ED EPR) spectroscopy. The shape of the ED EPR spectrum revealed the presence of librational motion of the spin probe, a motion typically present in glassy materials. The similarities in temperature dependency of A'(zz) and librational motion of CP in pea seed axes indicated that the change in A'(zz) arose from librational motion. ED EPR measurements of CP as a function of water content in Typha latifolia pollen showed that librational motion decreased with decreasing water contents until a plateau or minimum was reached. ED EPR spectroscopy is a valuable technique for characterizing the relation between molecular motion and storage kinetics of dry seed and pollen.