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

Seed priming with gas plasma-activated water in Ethiopia's "orphan" crop tef (Eragrostis tef)

MAIN CONCLUSION

Seed priming with gas plasma-activated water results in an increased ageing resilience in Eragrostis tef grains compared to a conventional hydropriming protocol. Tef (Eragrostis tef) is a cereal grass and a major staple crop of Ethiopia and Eritrea. Despite its significant importance in terms of production, consumption, and cash crop value, tef has been understudied and its productivity is low. In this study, tef grains have undergone different priming treatments to enhance seed vigour and seedling performance. A conventional hydropriming and a novel additive priming technology with gas plasma-activated water (GPAW) have been used and tef grains were then subjected to germination performance assays and accelerated ageing. Tef priming increases the germination speed and vigour of the grains. Priming with GPAW retained the seed storage potential after ageing, therefore, presenting an innovative environmental-friendly seed technology with the prospect to address variable weather conditions and ultimately food insecurity. Seed technology opens new possibilities to increase productivity of tef crop farming to achieve a secure and resilient tef food system and economic growth in Ethiopia by sustainable intensification of agriculture beyond breeding.

Seed Longevity and Ageing: A Review on Physiological and Genetic Factors with an Emphasis on Hormonal Regulation

Upon storage, seeds inevitably age and lose their viability over time, which determines their longevity. Longevity correlates with successful seed germination and enhancing this trait is of fundamental importance for long-term seed storage (germplasm conservation) and crop improvement. Seed longevity is governed by a complex interplay between genetic factors and environmental conditions experienced during seed development and after-ripening that will shape seed physiology. Several factors have been associated with seed ageing such as oxidative stress responses, DNA repair enzymes, and composition of seed layers. Phytohormones, mainly abscisic acid, auxins, and gibberellins, have also emerged as prominent endogenous regulators of seed longevity, and their study has provided new regulators of longevity. Gaining a thorough understanding of how hormonal signalling genes and pathways are integrated with downstream mechanisms related to seed longevity is essential for formulating strategies aimed at preserving seed quality and viability. A relevant aspect related to research in seed longevity is the existence of significant differences between results depending on the seed equilibrium relative humidity conditions used to study seed ageing. Hence, this review delves into the genetic, environmental and experimental factors affecting seed ageing and longevity, with a particular focus on their hormonal regulation. We also provide gene network models underlying hormone signalling aimed to help visualize their integration into seed longevity and ageing. We believe that the format used to present the information bolsters its value as a resource to support seed longevity research for seed conservation and crop improvement.

Relationship between different physiological processes of Tomato seedlings exposed to acid mine water Uncovered using correlation analysis

This study was conducted to assess the correlation between growth response, phytoaccumulation factor of different tissues, and elemental composition in tomato seedlings exposed to acid mine water (AMW). In pairwise correlation determinations values of plant height, stem diameter, seed germination indices (radicle length, final germination percentage (FGP), emergency rate index (ERI), vigour index (VI), germination percentage (G%) and germination rate index (GRI)) and the elemental compositions (Cd, Cr, Cu, Ni and Zn) in the different plant tissues, root (root accumulation factor = RAF), stem (stem translocation factor = STF) and leaves (leaf translocation factor = LTF) were selected for the relationship determinations. Pearson correlation coefficients were calculated and revealed the relationships between the paired parameters. The study concluded that the strongly correlated physiological parameters were jointly co-ordinated in tomato seedlings exposed to AMW.

Transcription Factor DOF4.1 Regulates Seed Longevity in Arabidopsis via Seed Permeability and Modulation of Seed Storage Protein Accumulation

Seed longevity is modulated by multiple genetic factors in Arabidopsis thaliana. A previous genome-wide association study using the Elevated Partial Pressure of Oxygen (EPPO) aging assay pinpointed a genetic locus associated with this trait. Reverse genetics identified the transcription factor DOF4.1 as a novel seed longevity factor. dof4.1 loss-of-function plants generate seeds exhibiting higher germination after accelerated aging assays. DOF4.1 is expressed during seed development and RNAseq data show several putative factors that could contribute to the dof4.1 seed longevity phenotype. dof4.1 has reduced seed permeability and a higher levels of seed storage proteins mRNAs (cruciferins and napins) in developing seeds, as compared to wild-type seeds. It has been reported that mutant lines defective in cruciferins or napins present reduced seed longevity. The improved longevity of dof4.1 is totally lost in the quadruple mutant dof4.1 cra crb crc, but not in a dof4.1 line depleted of napins, suggesting a prominent role for cruciferins in this process. Moreover, a negative regulation of DOF4.1 expression by the transcription factor DOF1.8 is suggested by co-inoculation assays in Nicotiana benthamiana. Indeed, DOF1.8 expression anticorrelates with that of DOF4.1 during seed development. In summary, modulation of DOF4.1 levels during seed development contributes to regulate seed longevity.

Physiological and molecular aspects of seed longevity: exploring intra-species variation in eight Pisum sativum L. accessions

Conservation of plant genetic diversity is fundamental for crop improvement, increasing agricultural production and sustainability, especially in the face of climatic changes. Although seed longevity is essential for the management of seed banks, few studies have, so far, addressed differences in this trait among the accessions of a single species. Eight Pisum sativum L. (pea) accessions were investigated to study the impact of long-term (approximately 20 years) storage, aiming to reveal contrasting seed longevity and clarify the causes for these differences. The outstanding seed longevity observed in the G4 accession provided a unique experimental system. To characterize the biochemical and physical status of stored seeds, reactive oxygen species, lipid peroxidation, tocopherols, free proline and reducing sugars were measured. Thermoanalytical measurements (thermogravimetry and differential scanning calorimetry) and transmission electron microscopy combined with immunohistochemical analysis were performed. The long-lived G4 seeds neither consumed tocopherols during storage nor showed free proline accumulation, as a deterioration hallmark, whereas reducing sugars were not affected. Thermal decomposition suggested a biomass composition compatible with the presence of low molecular weight molecules. Expansion of heterochromatic areas and reduced occurrence of γH2AX foci were highlighted in the nucleus of G4 seeds. The longevity of G4 seeds correlates with the occurrence of a reducing cellular environment and a nuclear ultrastructure favourable to genome stability. This work brings novelty to the study of within-species variations in seed longevity, underlining the relevance of multidisciplinary approaches in seed longevity research.

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.

Ageing beautifully: can the benefits of seed priming be separated from a reduced lifespan trade-off?

Germination performance is affected following seed exposure to a combination of temperature fluctuations and cycles of hydration and dehydration. This has long been exploited in a seed technology termed priming, which increases germination speed and seedling vigour, but these benefits have often been associated with effects on seed lifespan, or longevity, with conflicting evidence for positive and negative effects. Seed longevity is a key seed trait influencing not only the storage of commercial stocks but also in situ and ex situ seed conservation. In the context of increasingly variable environmental conditions faced by both crops and wild species, this has led to renewed interest in understanding the molecular factors that underlie priming. Here, we provide an overview of the literature relating to the effect of priming on seed lifespan, and catalogue the different parameters used for priming treatments and their consequences on longevity for a range of species. Our current limited understanding of the molecular basis for priming effects is also outlined, with an emphasis on recent advances and promising approaches that should lead towards the application and monitoring of the priming process in a less empirical manner.

Transcriptome analysis in osmo-primed tomato seeds with enhanced longevity by heat shock treatment

Seed priming is widely used in commercial seeds and its main function is to accelerate and synchronize seed germination. Undesirably, primed seeds show reduced longevity and treatments like heat shock have been shown to improve longevity in primed seeds. Nonetheless, the effect of heat shock treatment on primed seeds at the mRNA level is not known. Thus, the aim of this work was to investigate the effect of heat shock treatment on the longevity of primed tomato (Solanum lycopersicum) seeds at the physiological and transcriptome levels. Tomato seeds were primed and dried (control). Alternatively, primed seeds were subjected to heat shock treatment (38 °C/32 % relative humidity) before drying. Germination, vigor and longevity were evaluated. Transcriptome analysis was performed by RNA sequencing (RNA-seq) from biological samples collected immediately after priming and another samples collected from primed seeds followed by the heat shock treatments. The gene expression was validated by quantitative real time PCR (RT-qPCR). We showed that applying heat shock treatment after priming increased germination speed, enhanced seed longevity and preserved the vigor during storage of primed tomato seeds. Through transcriptome analysis, 368 differentially expressed genes were identified, from which 298 genes were up-regulated and 70 were down-regulated. We showed the increase of mRNA levels of HEAT SHOCK FACTOR-like and HEAT SHOCK PROTEIN-like chaperone genes, suggesting the involvement of the proteins coded by these transcripts in the enhancement of longevity in primed tomato seeds. The heat shock treatment after priming enhances and preserves the vigor of tomato primed seeds during storage. In addition, improves seed longevity through the increase in the expression of transcripts related to protection by response to stress.

Spectral Response Assessment of Moss-Dominated Biological Soil Crust Coverage Under Dry and Wet Conditions

Biological soil crusts (BSCs) are a major functional vegetation unit, covering extensive parts of drylands worldwide. Therefore, several multispectral indices have been proposed to map the spatial distribution and coverage of BSCs. BSCs are composed of poikilohydric organisms, the activity of which is sensitive to water availability. However, studies on dry and wet BSCs have seldom considered the mixed coverage gradient that is representative of actual field conditions. In this study, in situ spectral data and photographs of 136 pairs of dry and wet plots were collected to determine the influence of moisture conditions on BSC coverage detection. Then, BSC spectral reflectance and continuum removal (CR) reflectance responses to wetting were analyzed. Finally, the responses of four commonly used indices (i.e., normalized difference vegetation index (NDVI); crust index (CI); biological soil crust index (BSCI); and band depth of absorption feature after CR in the red band, (BD_red)), calculated from in situ hyperspectral data resampled to two multispectral data channels (Landsat-8 and Sentinel-2), were compared in dry and wet conditions. The results indicate that: (i) on average, the estimated BSC coverage using red-green-blue (RGB) images is 14.98% higher in wet than in dry conditions (P < 0.001); (ii) CR reflectance features of wet BSCs are more obvious than those of dry BSCs in both red and red-edge bands; and (iii) NDVI, CI, and BSCI for BSC coverage of 0%–60% under dry and wet conditions are close to those of dry and wet bare sand, respectively. NDVI and BD_red cannot separate dead wood and BSC with low coverage. This study demonstrates that low-coverage moss-dominated BSC is not easily detected by the four indices. In the future, remote-sensing data obtained during the rainy season with red and red-edge bands should be considered to detect BSCs.

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.

A Genome-Scale Metabolic Model of Soybean (Glycine max) Highlights Metabolic Fluxes in Seedlings

Until they become photoautotrophic juvenile plants, seedlings depend upon the reserves stored in seed tissues. These reserves must be mobilized and metabolized, and their breakdown products must be distributed to the different organs of the growing seedling. Here, we investigated the mobilization of soybean (Glycine max) seed reserves during seedling growth by initially constructing a genome-scale stoichiometric model for this important crop plant and then adapting the model to reflect metabolism in the cotyledons and hypocotyl/root axis (HRA). A detailed analysis of seedling growth and alterations in biomass composition was performed over 4 d of postgerminative growth and used to constrain the stoichiometric model. Flux balance analysis revealed marked differences in metabolism between the two organs, together with shifts in primary metabolism occurring during different periods postgermination. In particular, from 48 h onward, cotyledons were characterized by the oxidation of fatty acids to supply carbon for the tricarboxylic acid cycle as well as production of sucrose and glutamate for export to the HRA, while the HRA was characterized by the use of a range of imported amino acids in protein synthesis and catabolic processes. Overall, the use of flux balance modeling provided new insight into well-characterized metabolic processes in an important crop plant due to their analysis within the context of a metabolic network and reinforces the relevance of the application of this technique to the analysis of complex plant metabolic systems.

Responses of seed germination and shoot metabolic profiles of maize (Zea maysL.) to Y2O3nanoparticle stress

The potential risks of rare-earth nanoparticles (RENPs) to plants in the environment are attracting increasing attention due to their wide-spread application. In this regard, little is known about the effects of Y₂O₃ NPs as an important member of RENPs on crop plants. Seed germination is vulnerable to environmental stress, which determines the growth and yield of crops. Here, maize seeds were exposed to a Y₂O₃ NP suspension (0–500 mg L⁻¹) in the dark for 6 days. It was found that the Y₂O₃ NPs had no significant effect on the germination rates (>93%) in all treatments, but they could reduce seed vitality, delay germination, and inhibit seedling growth in a dose-dependent manner. Further, the inhibition effect of Y₂O₃ NPs on root elongation was much stronger than that on shoot elongation. Meanwhile, the activities of peroxidase (POD) and catalase (CAT) in shoots were enhanced with the increase in the Y₂O₃ NP concentration. A high-concentration (≥300 mg L⁻¹) of Y₂O₃ NPs induced a significant increase in the malondialdehyde (MDA) level in shoots compared to the control, indicating that the membrane lipid peroxidation and permeability were enhanced. ¹H NMR-based analysis showed that the polar metabolic profiles were altered significantly after treatment with 0, 10, and 500 mg L⁻¹ of Y₂O₃ NPs, but there was no marked alteration observed for the non-polar metabolic profiles. The polar metabolites (e.g., sugars, amino acids, and most organic acids) showed a dose-dependent increase to Y₂O₃ NP stress, indicating that the metabolic pathways of carbohydrate metabolism, the tricarboxylic acid cycle (TCA), and amino acid synthesis were disturbed. There were significantly positive correlations found among the metabolites related with the antioxidant response and osmotic adjustment. The simultaneous accumulation of these metabolites possibly indicated the adaptation of the seedlings to stress at the cost of retarding glycolysis, TCA, and protein synthesis. The retarded effects finally inhibited the apparent growth of the seedlings. These findings reveal the phytotoxicity of Y₂O₃ NPs and provide physiological and biochemical and molecular-scale perspectives on the response of seedlings to stress.

A hypothetic model for the adaptation of maize to Y₂O₃ NPs stress during seed germination.

Functional characterization of BnHSFA4a as a heat shock transcription factor in controlling the re-establishment of desiccation tolerance in seeds

Desiccation tolerance (DT) is the crucial ability of seeds to resist desiccation. However, the regulatory mechanisms of seed DT are not fully understood. In this study, two heat shock cis-elements (HSEs) were identified in the Brassica napus galactinol synthase (BnGolS1) promoter and shown to bind the heat shock transcription factor A4a (BnHSFA4a). Transcriptional expression of BnHSFA4a was induced at the early stage of DT acquisition, prior to increased BnGolS1 activity and galactinol production. Ectopic overexpression of BnHSFA4a (oxBnHSFA4a) in Arabidopsis enhanced DT, particularly during DT re-establishment. OxBnHSFA4a up-regulated the expression of GolS1, GolS2, and raffinose synthase 2 (BnRS2) in Arabidopsis and increased the enzymatic activity of GolS and RS and the concentration of raffinose family oligosaccharides (RFOs). Additionally, the overexpression lines exhibited increased antioxidant abilities. In contrast, the Arabidopsis mutant athsfa4a was more sensitive to dehydration, showing decreases in the efficiency of DT re-establishment, RFO contents, and oxidation resistance. Complementation analysis indicated that DT was rescued in athsfa4a/BnHSFA4a seeds to similar levels compared with those of Col-0. Taken together, these results indicated that BnHSFA4a probably functions in the regulation of GolS expression and activity, and activation of the antioxidative system and other stress response factors to improve DT.

Germline Development and Fertilization Mechanisms in Maize

Maize is the most important agricultural crop used for food, feed, and biofuel as well as a raw material for industrial products such as packaging material. To increase yield and to overcome hybridization barriers, studies of maize gamete development, the pollen tube journey, and fertilization mechanisms were initiated more than a century ago. In this review, we summarize and discuss our current understanding of the regulatory components for germline development including sporogenesis and gametogenesis, the progamic phase of pollen germination and pollen tube growth and guidance, as well as fertilization mechanisms consisting of pollen tube arrival and reception, sperm cell release, fusion with the female gametes, and egg cell activation. Mechanisms of asexual seed development are not considered here. While only a few molecular players involved in these processes have been described to date and the underlying mechanisms are far from being understood, maize now represents a spearhead of reproductive research for all grass species. Recent development of essentially improved transformation and gene-editing systems may boost research in this area in the near future.

Late seed maturation: drying without dying

Besides the deposition of storage reserves, seed maturation is characterized by the acquisition of functional traits including germination, desiccation tolerance, dormancy, and longevity. After seed filling, seed longevity increases up to 30-fold, concomitant with desiccation that brings the embryo to a quiescent state. The period that we define as late maturation phase can represent 10-78% of total seed development time, yet it remains overlooked. Its importance is underscored by the fact that in the seed production chain, the stage of maturity at harvest is the primary factor that influences seed longevity and seedling establishment. This review describes the major events and regulatory pathways underlying the acquisition of seed longevity, focusing on key indicators of maturity such as chlorophyll degradation, accumulation of raffinose family oligosaccharides, late embryogenesis abundant proteins, and heat shock proteins. We discuss how these markers are correlated with or contribute to seed longevity, and highlight questions that merit further attention. We present evidence suggesting that molecular players involved in biotic defence also have a regulatory role in seed longevity. We also explore how the concept of plasticity can help understand the acquisition of longevity.

ABI5 Is a Regulator of Seed Maturation and Longevity in Legumes

The preservation of our genetic resources and production of high-quality seeds depends on their ability to remain viable and vigorous during storage. In a quantitative trait locus analysis on seed longevity in Medicago truncatula, we identified the bZIP transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5). Characterization of Mt-abi5 insertion mutant seeds revealed that both the acquisition of longevity and dormancy were severely impaired. Using transcriptomes of developing Mt-abi5 seeds, we created a gene coexpression network and revealed ABI5 as a regulator of gene modules with functions related to raffinose family oligosaccharide (RFO) metabolism, late embryogenesis abundant (LEA) proteins, and photosynthesis-associated nuclear genes (PhANGs). Lower RFO contents in Mt-abi5 seeds were linked to the regulation of SEED IMBIBITION PROTEIN1 Proteomic analysis confirmed that a set of LEA polypeptides was reduced in mature Mt-abi5 seeds, whereas the absence of repression of PhANG in mature Mt-abi5 seeds was accompanied by chlorophyll and carotenoid retention. This resulted in a stress response in Mt-abi5 seeds, evident from an increase in α-tocopherol and upregulation of genes related to programmed cell death and protein folding. Characterization of abi5 mutants in a second legume species, pea (Pisum sativum), confirmed a role for ABI5 in the regulation of longevity, seed degreening, and RFO accumulation, identifying ABI5 as a prominent regulator of late seed maturation in legumes.

Inference of Longevity-Related Genes from a Robust Coexpression Network of Seed Maturation Identifies Regulators Linking Seed Storability to Biotic Defense-Related Pathways

Seed longevity, the maintenance of viability during storage, is a crucial factor for preservation of genetic resources and ensuring proper seedling establishment and high crop yield. We used a systems biology approach to identify key genes regulating the acquisition of longevity during seed maturation of Medicago truncatula. Using 104 transcriptomes from seed developmental time courses obtained in five growth environments, we generated a robust, stable coexpression network (MatNet), thereby capturing the conserved backbone of maturation. Using a trait-based gene significance measure, a coexpression module related to the acquisition of longevity was inferred from MatNet. Comparative analysis of the maturation processes in M. truncatula and Arabidopsis thaliana seeds and mining Arabidopsis interaction databases revealed conserved connectivity for 87% of longevity module nodes between both species. Arabidopsis mutant screening for longevity and maturation phenotypes demonstrated high predictive power of the longevity cross-species network. Overrepresentation analysis of the network nodes indicated biological functions related to defense, light, and auxin. Characterization of defense-related wrky3 and nf-x1-like1 (nfxl1) transcription factor mutants demonstrated that these genes regulate some of the network nodes and exhibit impaired acquisition of longevity during maturation. These data suggest that seed longevity evolved by co-opting existing genetic pathways regulating the activation of defense against pathogens.

Evidence for water deficit-induced mass increases of raffinose family oligosaccharides (RFOs) in the leaves of three Craterostigma resurrection plant species

The leaves of the resurrection plant Craterostigma plantagineum accumulate sucrose during dehydration, via a conversion from the unusual C8 ketose-sugar 2-octulose. However, raffinose family oligosaccharides (RFOs) have been shown to be major photosynthetic products in this plant. The tetrasaccharide stachyose is the major phloem-mobile carbohydrate and is used as a carbon store in roots. It has been suggested that this carbon store is remobilized during rehydration, presumably for cellular repair processes. We examined the effects of water deficit on the leaf water-soluble carbohydrate profiles of three Craterostigma species. Apart from the classical 2-octulose-to-sucrose interconversion, there was a strong water deficit-associated mass increase of RFOs up to the pentasaccharide verbascose. However, the activities of three dedicated RFO biosynthetic enzymes (raffinose, stachyose, and verbascose synthase) was not correlated with RFO accumulation, suggesting that biosynthetic enzyme activities measured in the early stages of water-deficit were sufficient to synthesize enough galactinol and lead to RFO accumulation in the leaves. Our findings are suggestive of RFOs providing additional carbohydrate-based stress protection to the leaves of these plants during the desiccated state.

Metabolic patterns associated with the seasonal rhythm of seed survival after dehydration in germinated seeds of Schismus arabicus

BACKGROUND

Seed of Shismus arabicus, a desert annual, display a seasonal tolerance to dehydration. The occurrence of a metabolic seasonal rhythm and its relation with the fluctuations in seed dehydration tolerance was investigated.

RESULTS

Dry seeds metabolism was the least affected by the season, while the metabolism of germinated and dehydrated seeds exhibit distinct seasonal patterns. Negative associations exist between amino acids, sugars and TCA cycle intermediates and seed survival, while positive relations exist with seed germination. In contrast, associations between the level of secondary metabolites identified in the dehydrated seeds and survival percentage were evenly distributed in positive and negative values, suggesting a functional role of these metabolites in the establishment of seed dehydration tolerance.

CONCLUSION

Our results indicate the occurrence of metabolic biorhythms in germinating and dehydrating seeds associated with seasonal changes in germination and, more pronouncedly, in seed dehydration tolerance. Increased biosynthesis of protective compounds (polyphenols) in dehydrating seeds during the winter season at the expenses of central metabolites likely contributes to the respective enhanced dehydration tolerance monitored.

Benefits of rice seed priming are offset permanently by prolonged storage and the storage conditions

Seed priming is a commercially successful practice, but reduced longevity of primed seeds during storage may limit its application. We established a series of experiments on rice to test: (1) whether prolonged storage of primed and non-primed rice seeds for 210 days at 25°C or −4°C would alter their viability, (2) how long primed rice seed would potentially remain viable at 25°C storage, and (3) whether or not post-storage treatments (re-priming or heating) would reinstate the viability of stored primed seeds. Two different rice cultivars and three priming agents were used in all experiments. Prolonged storage of primed seeds at 25°C significantly reduced the germination (>90%) and growth attributes (>80%) of rice compared with un-stored primed seeds. However, such negative effects were not observed in primed seeds stored at −4°C. Beneficial effects of seed priming were maintained only for 15 days of storage at 25°C, beyond which the performance of primed seeds was worse even than non-primed seeds. The deteriorative effects of 25°C storage were related with hampered starch metabolism in primed rice seeds. None of the post-storage treatments could reinstate the lost viability of primed seeds suggesting that seeds become unviable by prolonged post-priming storage at 25°C.

The ecophysiology of seed persistence: a mechanistic view of the journey to germination or demise

Seed persistence is the survival of seeds in the environment once they have reached maturity. Seed persistence allows a species, population or genotype to survive long after the death of parent plants, thus distributing genetic diversity through time. The ability to predict seed persistence accurately is critical to inform long-term weed management and flora rehabilitation programs, as well as to allow a greater understanding of plant community dynamics. Indeed, each of the 420000 seed-bearing plant species has a unique set of seed characteristics that determine its propensity to develop a persistent soil seed bank. The duration of seed persistence varies among species and populations, and depends on the physical and physiological characteristics of seeds and how they are affected by the biotic and abiotic environment. An integrated understanding of the ecophysiological mechanisms of seed persistence is essential if we are to improve our ability to predict how long seeds can survive in soils, both now and under future climatic conditions. In this review we present an holistic overview of the seed, species, climate, soil, and other site factors that contribute mechanistically to seed persistence, incorporating physiological, biochemical and ecological perspectives. We focus on current knowledge of the seed and species traits that influence seed longevity under ex situ controlled storage conditions, and explore how this inherent longevity is moderated by changeable biotic and abiotic conditions in situ, both before and after seeds are dispersed. We argue that the persistence of a given seed population in any environment depends on its resistance to exiting the seed bank via germination or death, and on its exposure to environmental conditions that are conducive to those fates. By synthesising knowledge of how the environment affects seeds to determine when and how they leave the soil seed bank into a resistance-exposure model, we provide a new framework for developing experimental and modelling approaches to predict how long seeds will persist in a range of environments.

Physiological aspects of raffinose family oligosaccharides in plants: protection against abiotic stress

Abiotic stresses resulting from water deficit, high salinity or periods of drought adversely affect plant growth and development and represent major selective forces during plant evolution. The raffinose family oligosaccharides (RFOs) are synthesised from sucrose by the subsequent addition of activated galactinol moieties donated by galactinol. RFOs are characterised as compatible solutes involved in stress tolerance defence mechanisms, although evidence also suggests that they act as antioxidants, are part of carbon partitioning strategies and may serve as signals in response to stress. The key enzyme and regulatory point in RFO biosynthesis is galactinol synthase (GolS), and an increase of GolS in expression and activity is often associated with abiotic stress. It has also been shown that different GolS isoforms are expressed in response to different types of abiotic stress, suggesting that the timing and accumulation of RFOs are controlled for each abiotic stress. However, the accumulation of RFOs in response to stress is not universal and other functional roles have been suggested for RFOs, such as being part of a carbon storage mechanism. Transgenic Arabidopsis plants with increased galactinol and raffinose concentrations had better ROS scavenging capacity, while many sugars have been shown in vitro to have antioxidant activity, suggesting that RFOs may also act as antioxidants. The RFO pathway also interacts with other carbohydrate pathways, such as that of O-methyl inositol (OMI), which shows that the functional relevance of RFOs must not be seen in isolation to overall carbon re-allocation during stress responses.

A regulatory network-based approach dissects late maturation processes related to the acquisition of desiccation tolerance and longevity of Medicago truncatula seeds

In seeds, desiccation tolerance (DT) and the ability to survive the dry state for prolonged periods of time (longevity) are two essential traits for seed quality that are consecutively acquired during maturation. Using transcriptomic and metabolomic profiling together with a conditional-dependent network of global transcription interactions, we dissected the maturation events from the end of seed filling to final maturation drying during the last 3 weeks of seed development in Medicago truncatula. The network revealed distinct coexpression modules related to the acquisition of DT, longevity, and pod abscission. The acquisition of DT and dormancy module was associated with abiotic stress response genes, including late embryogenesis abundant (LEA) genes. The longevity module was enriched in genes involved in RNA processing and translation. Concomitantly, LEA polypeptides accumulated, displaying an 18-d delayed accumulation compared with transcripts. During maturation, gulose and stachyose levels increased and correlated with longevity. A seed-specific network identified known and putative transcriptional regulators of DT, including ABSCISIC ACID-INSENSITIVE3 (MtABI3), MtABI4, MtABI5, and APETALA2/ ETHYLENE RESPONSE ELEMENT BINDING PROTEIN (AtAP2/EREBP) transcription factor as major hubs. These transcriptional activators were highly connected to LEA genes. Longevity genes were highly connected to two MtAP2/EREBP and two basic leucine zipper transcription factors. A heat shock factor was found at the transition of DT and longevity modules, connecting to both gene sets. Gain- and loss-of-function approaches of MtABI3 confirmed 80% of its predicted targets, thereby experimentally validating the network. This study captures the coordinated regulation of seed maturation and identifies distinct regulatory networks underlying the preparation for the dry and quiescent states.

Content of zinc, iron and their absorption inhibitors in Nicaraguan common beans (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.), the staple crop of Nicaragua, provides protein and nonhaem iron, but inhibitors such as phytate may prevent absorption of iron and zinc by the consumer. Warehouses in Nicaragua do not have controlled atmospheres, so beans are exposed to high temperatures and humidities that may accelerate quality loss. To evaluate the impact of 6months of storage on quality, four national accessions of common bean were submitted to two treatments, a conventional warehouse with uncontrolled temperature and humidity, and accelerated ageing at 40°C and 75% RH. Iron content was 61-81mg/kg of which 3-4% was bioavailable, and zinc content was 21-25mg/kg, of which 10-12% was bioavailable. Bioavailability generally increased in storage, significantly so in year-old INTA Linea 628 in accelerated ageing. The concentration of phytate was 8.6-9.6mg/g and it contained 54-63% of the total phosphorus. Improvement in bioavailability of divalent cations is needed.

Expression of a GALACTINOL SYNTHASE gene is positively associated with desiccation tolerance of Brassica napus seeds during development

Desiccation tolerance of seeds is positively correlated with raffinose family oligosaccharides (RFOs). However, RFOs' role in desiccation tolerance is still a matter of controversy. The aim of this work was to monitor the accumulation of RFO during acquisition of desiccation tolerance in rapeseed (Brassica napus L.). Rapeseeds become desiccation tolerant at 21-24d after flowering (DAF), and the time was coincident with an accumulation of raffinose and stachyose. A gene encoding galactinol synthase (GolS; EC2.4.1.123), involved in RFO biosynthesis, was cloned and functionally characterized. Enzymatic properties of recombinant galactinol synthase were also determined. Accumulation of BnGOLS-1 mRNA in developing rapeseeds was concomitant with dry weight deposition and the acquisition of desiccation tolerance, and was concurrent with the formation of raffinose and stachyose. The physiological implications of BnGOLS-1 expression patterns in developing seeds are discussed in light of the hypothesized role of RFOs in seed desiccation tolerance.

Quantitative trait loci analysis reveals a correlation between the ratio of sucrose/raffinose family oligosaccharides and seed vigour in Medicago truncatula

Seed vigour is important for successful establishment and high yield, especially under suboptimal environmental conditions. In legumes, raffinose oligosaccharide family (RFO) sugars have been proposed as an easily available energy reserve for seedling establishment. In this study, we investigated whether the composition or amount of soluble sugars (sucrose and RFO) is part of the genetic determinants of seed vigour of Medicago truncatula using two recombinant inbred line (RIL) populations. Quantitative trait loci (QTL) mapping for germination rate, hypocotyl and radicle growth under water deficit and nutritional stress, seed weight and soluble sugar content was performed using RIL populations LR1 and LR4. Seven of the 12 chromosomal regions containing QTL for germination rate or post-germinative radicle growth under optimal or stress conditions co-located with Suc/RFO QTL. A significant negative correlation was also found between seed vigour traits and Suc/RFO. In addition, one QTL that explained 80% of the variation in the ratio stachyose/verbascose co-located with a stachyose synthase gene whose expression profile in the parental lines could explain the variation in oligosaccharide composition. The correlation and co-location of Suc/RFO ratio with germination and radicle growth QTL suggest that an increased Suc/RFO ratio in seeds of M. truncatula might negatively affect seed vigour.

Hyperprolinemic larvae of the drosophilid fly, Chymomyza costata, survive cryopreservation in liquid nitrogen

The larva of the drosophilid fly, Chymomyza costata, is probably the most complex metazoan organism that can survive submergence in liquid nitrogen (-196 °C) in a fully hydrated state. We examined the associations between the physiological and biochemical parameters of differently acclimated larvae and their freeze tolerance. Entering diapause is an essential and sufficient prerequisite for attaining high levels of survival in liquid nitrogen (23% survival to adult stage), although cold acclimation further improves this capacity (62% survival). Profiling of 61 different metabolites identified proline as a prominent compound whose concentration increased from 20 to 147 mM during diapause transition and subsequent cold acclimation. This study provides direct evidence for the essential role of proline in high freeze tolerance. We increased the levels of proline in the larval tissues by feeding larvae proline-augmented diets and found that this simple treatment dramatically improved their freeze tolerance. Cell and tissue survival following exposure to liquid nitrogen was evident in proline-fed nondiapause larvae, and survival to adult stage increased from 0% to 36% in proline-fed diapause-destined larvae. A significant statistical correlation was found between the whole-body concentration of proline, either natural or artificial, and survival to the adult stage in liquid nitrogen for diapause larvae. Differential scanning calorimetry analysis suggested that high proline levels, in combination with a relatively low content of osmotically active water and freeze dehydration, increased the propensity of the remaining unfrozen water to undergo a glass-like transition (vitrification) and thus facilitated the prevention of cryoinjury.

Sugar signalling and antioxidant network connections in plant cells

Sugars play important roles as both nutrients and regulatory molecules throughout plant life. Sugar metabolism and signalling function in an intricate network with numerous hormones and reactive oxygen species (ROS) production, signalling and scavenging systems. Although hexokinase is well known to fulfil a crucial role in glucose sensing processes, a scenario is emerging in which the catalytic activity of mitochondria-associated hexokinase regulates glucose-6-phosphate and ROS levels, stimulating antioxidant defence mechanisms and the synthesis of phenolic compounds. As a new concept, it can be hypothesized that the synergistic interaction of sugars (or sugar-like compounds) and phenolic compounds forms part of an integrated redox system, quenching ROS and contributing to stress tolerance, especially in tissues or organelles with high soluble sugar concentrations.

Time-resolved spectral measurements of delayed luminescence from a single soybean seed: effects of thermal damage and correlation with germination performance

Delayed luminescence from a single dry soybean seed was investigated in both spectral and time domains, under different excitation wavelengths. Emission spectra were collected, under 337 nm laser excitation, from native and artificially deteriorated seeds and the time-dependence of different spectral components was analyzed in detail. The single seed viability was evaluated through observation of germination properties after imbibition and compared with different parameters related to the luminescence kinetics. The significant correlation found between single seed delayed luminescence parameters and germination capability strongly validates the connection of this phenomenon with the functional state of the system and suggests the development of a non-invasive technique for seed quality determination.

Non-reducing sugar levels in beech (Fagus sylvatica) seeds as related to withstanding desiccation and storage

Levels of sucrose and raffinose family oligosaccharides (RFOs) (raffinose and stachyose) were determined in beech (Fagus sylvatica L.) seeds during development, maturation, desiccation and storage. An increase in RFOs and a marked decrease in the S:(R+St) ratio (i.e. mass ratio of sucrose to the sum of RFOs) were observed at the time of desiccation tolerance (DT) acquisition by seeds. In seeds stored at -10 degrees C through 1, 4, 7, and 12 years, changes in sucrose, raffinose and stachyose levels and in alpha-galactosidase activity were noted. The S/R+St ratio and alpha-galactosidase activity significantly increased in seeds after 7 and 12 years of storage, when a marked decrease in viability, measured as germination capacity, was recorded. Germination capacity was found to be strongly correlated with sucrose content, the S:(R+St) ratio, and alpha-galactosidase activity. A strong positive correlation was found between germination capacity and stachyose content. The results clearly indicated that the composition of RFOs in beech seeds is closely related to DT acquisition and seed viability during storage.

Seed dehydration and the establishment of desiccation tolerance during seed maturation is altered in the Arabidopsis thaliana mutant atem6-1

The end of orthodox seed development is typified by a developmentally regulated period of dehydration leading to the loss of bulk water from the entire structure. When dehydration occurs, the cytoplasm condenses and intracellular components become more crowded, providing an environment amenable to numerous undesirable interactions that can lead to protein aggregation, denaturation and organelle-cell membrane fusion. Acquisition of desiccation tolerance, or the ability to withstand these very low water potentials and consequent molecular crowding, has been correlated with the accumulation of various protective compounds including proteins and sugars. Among these are the late embryogenesis abundant (LEA) proteins, a diverse class of highly abundant, heat-stable proteins that accumulate late in embryo maturation coincident with the acquisition of desiccation tolerance. Previous work led us to hypothesize that the protein ATEM6, one of the two Arabidopsis thaliana group 1 LEA proteins, is involved in regulating the rate at which water is lost from the maturing embryo; homozygous atem6-1 mutants display premature dehydration of seeds at the distal end of the silique. Here we demonstrate that rehydrated, mature seeds from atem6-1 mutant plants lose more water during subsequent air drying than wild-type seeds, consistent with a role for ATEM6 protein in water binding/loss during embryo maturation. In addition, and possibly as a result of premature dehydration, mutant seeds along the entire length of the silique acquire desiccation tolerance earlier than their wild-type counterparts. We further demonstrate precocious, and perhaps elevated, expression of the other A. thaliana group 1 LEA protein, ATEM1, that may compensate for loss or ATEM6 expression. However, this observation could also be consistent with acceleration of the entire normal maturation program in atem6-1 mutant embryos. Interestingly, ATEM6 protein does not appear to be required in mature seeds for viability or efficient germination.

Sucrose, sucrosyl oligosaccharides, and oxidative stress: scavenging and salvaging?

In nature, no single plant completes its life cycle without encountering environmental stress. When plant cells surpass stress threshold stimuli, chemically reactive oxygen species (ROS) are generated that can cause oxidative damage or act as signals. Plants have developed numerous ROS-scavenging systems to minimize the cytotoxic effects of ROS. The role of sucrosyl oligosaccharides (SOS), including fructans and the raffinose family oligosaccharides (RFOs), is well established during stress physiology. They are believed to act as important membrane protectors in planta. So far a putative role for sucrose and SOS during oxidative stress has largely been neglected, as has the contribution of the vacuolar compartment. Recent studies suggest a link between SOS and oxidative defence and/or scavenging. SOS might be involved in stabilizing membrane-associated peroxidases and NADPH oxidases, and SOS-derived radicals might fulfil an intermediate role in oxido-reduction reactions taking place in the vicinity of membranes. Here, these emerging features are discussed and perspectives for future research are provided.

[Metabolic control of seed germination]

We have used proteomics to better characterize germination and early seedling vigor in sugarbeet. Our strategy includes (1) construction of proteome reference maps for dry and germinating seeds of a high-vigor reference seed lot; (2) investigation of the specific tissue accumulation of proteins (root, cotyledon, perisperm); (3) investigation of changes in protein expression profiles detected in the reference seed lot subjected to different vigor-modifying treatments, e.g. aging and/or priming. More than 1 000 sugarbeet seed proteins have been identified by LC/MS-MS mass spectrometry (albumins, globulins and glutelins have been analyzed separately). Due to the conservation of protein sequences and the quality of MS sequencing (more than 10 000 peptide sequences have been obtained), the success rate of protein identification was on the average of 80%. This is to our knowledge the best detailed proteome analysis ever carried out in seeds. The data allowed us to build a detailed metabolic chart of the sugarbeet seed, generating new insights into the molecular mechanisms determining the development of a new seedling. Also, the proteome of a seed-storage tissue as the perisperm is described for the first time.

Enzymatic breakdown of raffinose oligosaccharides in pea seeds

Both alkaline and acidic alpha-galactosidases (alpha-D: -galactoside galactohydrolases, E.C.3.2.1.22) isolated from various plant species have been described, although little is known about their co-occurrence and functions in germinating seeds. Here, we report on the isolation of two cDNAs, encoding for alpha-galactosidases from maturing and germinating seeds of Pisum sativum. One was identified as a member of the acidic alpha-galactosidase of the family 27 glycosyl hydrolase cluster and the other as a member of the family of alkaline alpha-galactosidases, which are highly homologous to seed imbibition proteins (SIPs). PsGAL1 transcripts, encoding for the ACIDIC alpha-GALACTOSIDASE, were predominately expressed during seed maturation and acidic enzyme activities were already present in dry seeds, showing little changes during seed germination. Compartmentation studies revealed that acidic alpha-galactosidases were located in protein storage vacuoles (PSVs). PsAGA1, encoding for the ALKALINE alpha-GALACTOSIDASE, was only expressed after radicle protrusion, when about 50% of RFOs have already been broken down. RFO breakdown was markedly decreased when the translation of the alkaline enzyme was inhibited, providing evidence that PsAGA1 indeed functioned in RFO degradation. Based on these data, we present an integrated model of RFO breakdown by two sequentially active alpha-galactosidases in pea seeds.

Assessment of chronic gastritis in pet dogs and its relation with helicobacter-like organisms

The aim of this study was to evaluate the prevalence of chronic gastritis in pet dogs, to determine the histopathologic changes of gastric mucosa and, to determine its relationship with canine gastric Helicobacter infection. Sixty percent (n = 18), 27% (n = 8) and 13% (n = 4) of the examined stomachs showed normal, congested and erosive gastric mucosa respectively. Histopathologic examination was confirmed the presence of chronic gastritis in 40% of dogs (n = 12). Lymphocytic-plasmacytic gastritis was the most common type of chronic gastritis. Gastric Helicobacter was detected in cytological examination of 26 out of 30 dogs (86.6%) but in the PCR analysis, 93% of gastric samples were positive for GHLO. There was no significant relation between the presence of Helicobacters and chronic gastritis (p>0.05). Follicular gastritis was detected in 12 cases (40%) and there was also no significant correlation between its presence and GHLO's infection (p>0.05). In conclusion, chronic gastritis can be considered as a prevalent disease especially in dogs. Nutritional and environmental factors as well as individual immune response may have role in induction of chronic gastritis, but the clinical significance of these histopathologic changes should be evaluated.

Protection of liposomes against fusion during drying by oligosaccharides is not predicted by the calorimetric glass transition temperatures of the dry sugars

Sugars play an important role in the desiccation tolerance of most anhydrobiotic organisms. It has been shown in previous studies that different structural families of oligosaccharides have different efficacies to interact with phospholipid headgroups and protect membranes from solute leakage during drying. Here, we have compared three families of linear oligosaccharides (fructans (inulins), malto-oligosaccharides, manno-oligosaccharides) for their chain-length dependent protection of egg phosphatidylcholine liposomes against membrane fusion. We found increased protection with chain length up to a degree of polymerization (DP) of 5 for malto-oligosaccharides, and a decrease for inulins and manno-oligosaccharides. Differential scanning calorimetry measurements showed that for all sugars the glass transition temperature (T_g) increased with DP, although to different degrees for the different oligosaccharide families. Higher T_g values resulted in reduced membrane fusion only for malto-oligosaccharides below DP5. Contrary to expectation, for inulins, manno-oligosaccharides and malto-oligosaccharides of a DP above five, fusion increased with increasing T_g, indicating that other physical parameters are more important in determining the ability of different sugars to protect membranes against fusion during drying. Further research will be necessary to experimentally define such parameters.

Trehalose As a “Chemical Chaperone”

Trehalose is a disaccharide of glucose that is found at high concentrations in a wide variety of organisms that naturally survive drying in nature. Many years ago we reported that this molecule has the remarkable ability to stabilize membranes and proteins in the dry state. A mechanism for the stabilization rapidly emerged, and it was sufficiently attractive that a myth grew up about trehalose as a universal protectant and chemical chaperone. Many of the claims in this regard can be explained by what is now known about the physical properties of this interesting sugar. It is emerging that these properties may make it unusually useful in stabilizing intact cells in the dry state.

Inhibition of raffinose oligosaccharide breakdown delays germination of pea seeds

Raffinose family oligosaccharides (RFOs) are almost ubiquitous in seeds and have been hypothesized to constitute an important energy source during germination. To test this hypothesis we applied a specific alpha-galactosidase inhibitor (1-deoxygalactonojirimycin, DGJ) to germinating pea seeds, resulting in a complete blocking of RFO breakdown. The germination rates of DGJ-treated seeds dropped drastically to about 25% of controls two days after imbibition. Similarly, the activities of the key enzymes in the galactose salvage pathway galactokinase, UDP-galactose pyrophosphorylase and UDP-galactose 4'-epimerase, were also significantly lower in seeds treated with the inhibitor. The inhibitory effect on germination could be relieved by galactose but only partially by sucrose, indicating that galactose, in addition to providing easily available energy for growth, may also be an important component of the sugar signaling pathway during germination. Taken together our study, for the first time, provides clear evidence that RFOs play an important role for early germination.

A predicted N-terminal helical domain of a Group 1 LEA protein is required for protection of enzyme activity from drying

Late embryogenesis abundant (LEA) proteins have been repeatedly implicated in the acquisition of desiccation tolerance in angiosperm seed embryos. However, the mechanism(s) by which protection occurs is not well understood. While the Group 1 LEA proteins are predicted to be largely unordered in solution, there is strong evidence that upon drying these proteins undergo a structural transition that leads to an increase in alpha-helical content. Several studies also suggest there is a direct interaction between Group 1 LEA proteins and other molecules in the cytoplasm that may be critical for the establishment of desiccation tolerance during embryo maturation. We have produced a recombinant Group 1 LEA protein and show that it is capable of protecting the enzyme lactate dehydrogenase from the deleterious effects of drying. We have also evaluated the ability of various altered recombinant Group 1 LEA proteins to protect in the same assay. Our results suggest that the highly conserved 20 amino acid Group 1 LEA signature motif is not required for protection in our in vitro assay. However, introduction of two juxtaposed proline residues into an N-terminal helical domain predicted to exist in the hydrated structure significantly compromises the ability of the recombinant protein to provide protection from drying. These results suggest that the N-terminal domain of Group 1 LEA proteins may be important for proper folding during dehydration.

Changes in lipid status and glass properties in cotyledons of developing sunflower seeds

Biochemical events involved in the acquisition of germinability and storability during orthodox seed development are well documented; however, the roles played by the physical organization of lipids and water are poorly characterized. The aim of this work was to determine, using a thermodynamic approach, whether changes in thermal properties of lipid reserves, and intracellular glasses might play a role in sunflower (Helianthus annuus L.) seed development. Triacyglycerols (TAGs) accumulated in cotyledons until the end of seed filling, which occurred 42 days after anthesis (DAA). Further seed development, leading to mature seed at 58 DAA, was mainly associated with an enlargement of lipid bodies without significant changes either in the lipid content or in their composition. When cooled to -100 degrees C, lipid reserves from cotyledons of mature seeds displayed alpha and beta' polymorphic crystalline structures; however, the ability to form alpha crystals, which was an indicator of lipid purity, progressively appeared during seed development. Characteristics of lipid melting confirmed that seed maturation drying was associated with changes in TAG physical organization. Cotyledon development was associated with an increase in the temperature of glass to rubber transition (Tg), thus suggesting a decrease in molecular mobility during maturation drying. This phenomenon was concomitant with an increase in raffinose content. Our results demonstrate that physical characteristics of lipid reserves and glasses of sunflower cotyledons are developmentally regulated and might play a role in acquisition of seed germinability and storability.

Development and composition of the seeds of nine genotypes of the Medicago truncatula species complex

The seed development and composition of Medicago truncatula Gaertn., the new model plant for grain legumes, was studied using nine genotypes of the species complex: M. truncatula-Medicago littoralis (M. truncatula). The seed development of M. truncatula was very similar to that of other legumes, the only notable exception being the presence, in the mature seed, of an endosperm layer that is absent in grain legumes. During early embryogenesis and until mid-maturation, transient storage of starch occurred in the seed coat and embryo. This temporary storage probably contributed to the early development of the embryo and reserve synthesis. During maturation the synthesis and accumulation of proteins and oil took place at quasi-constant rates. Conversely oligosaccharides, mainly stachyose, were synthesised only during late maturation and at the beginning of desiccation. Proteins represented the major class of storage compounds and their average amino acid composition was found to be very close to that of pea and robust in various environmental conditions. Similar compositions between the two species and other grain legumes were also found for the fatty acids and the soluble sugars; most of these characters varied depending on the various environmental conditions used for seed production. All these similarities fully justify the use of M. truncatula as a model plant for genomic approaches to grain legume improvement. The major difference between M. truncatula seeds and European grain legume seeds resides in the nature of their carbon storage namely triacylglycerides for M. truncatula and starch for pea and faba bean.

Seasonal patterns of growth, dehydrins and water-soluble carbohydrates in genotypes of Dactylis glomerata varying in summer dormancy

BACKGROUND AND AIMS

Summer dormancy in perennial grasses has been studied inadequately, despite its potential to enhance plant survival and persistence in Mediterranean areas. The aim of the present work was to characterize summer dormancy and dehydration tolerance in two cultivars of Dactylis glomerata (dormant 'Kasbah', non-dormant 'Oasis') and their hybrid using physiological indicators associated with these traits.

METHODS

Dehydration tolerance was assessed in a glasshouse experiment, while seasonal metabolic changes which produce putative protectants for drought, such as carbohydrates and dehydrins that might be associated with summer dormancy, were analysed in the field.

KEY RESULTS

The genotypes differed in their ability to survive increasing soil water deficit: lethal soil water potential (Psi(s)) was -3.4 MPa for 'Kasbah' (although non-dormant), -1.3 MPa for 'Oasis', and -1.6 MPa for their hybrid. In contrast, lethal water content of apices was similar for all genotypes (approx. 0.45 g H(2)O g d. wt(-1)), and hence the greater survival of 'Kasbah' can be ascribed to better drought avoidance rather than dehydration tolerance. In autumn-sown plants, 'Kasbah' had greatest dormancy, the hybrid was intermediate and 'Oasis' had none. The more dormant the genotype, the lower the metabolic activity during summer, and the earlier the activity declined in spring. Decreased monosaccharide content was an early indicator of dormancy induction. Accumulation of dehydrins did not correlate with stress tolerance, but dehydrin content was a function of the water status of the tissues, irrespective of the soil moisture. A protein of approx. 55 kDa occurred in leaf bases of the most dormant cultivar even in winter.

CONCLUSIONS

Drought avoidance and summer dormancy are correlated but can be independently expressed. These traits are heritable, allowing selection in breeding programmes.

Gene expression programs during Brassica oleracea seed maturation, osmopriming, and germination are indicators of progression of the germination process and the stress tolerance level

During seed maturation and germination, major changes in physiological status, gene expression, and metabolic events take place. Using chlorophyll sorting, osmopriming, and different drying regimes, Brassica oleracea seed lots of different maturity, stress tolerance, and germination behavior were created. Through careful physiological analysis of these seed lots combined with gene expression analysis using a dedicated cDNA microarray, gene expression could be correlated to physiological processes that occurred within the seeds. In addition, gene expression was studied during early stages of seed germination, prior to radicle emergence, since very little detailed information of gene expression during this process is available. During seed maturation expression of many known seed maturation genes, such as late-embryogenesis abundant or storage-compound genes, was high. Notably, a small but distinct subgroup of the maturation genes was found to correlate to seed stress tolerance in osmoprimed and dried seeds. Expression of these genes rapidly declined during priming and/or germination in water. The majority of the genes on the microarray were up-regulated during osmopriming and during germination on water, confirming the hypothesis that during osmopriming, germination-related processes are initiated. Finally, a large group of genes was up-regulated during germination on water, but not during osmopriming. These represent genes that are specific to germination in water. Germination-related gene expression was found to be partially reversible by physiological treatments such as slow drying of osmoprimed seeds. This correlated to the ability of seeds to withstand stress.

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.

Analysis of natural allelic variation of Arabidopsis seed germination and seed longevity traits between the accessions Landsberg erecta and Shakdara, using a new recombinant inbred line population

Quantitative trait loci (QTL) mapping was used to identify loci controlling various aspects of seed longevity during storage and germination. Similar locations for QTLs controlling different traits might be an indication for a common genetic control of such traits. For this analysis we used a new recombinant inbred line population derived from a cross between the accessions Landsberg erecta (Ler) and Shakdara (Sha). A set of 114 F9 recombinant inbred lines was genotyped with 65 polymerase chain reaction-based markers and the phenotypic marker erecta. The traits analyzed were dormancy, speed of germination, seed sugar content, seed germination after a controlled deterioration test, hydrogen peroxide (H2O2) treatment, and on abscisic acid. Furthermore, the effects of heat stress, salt (NaCl) stress, osmotic (mannitol) stress, and natural aging were analyzed. For all traits one or more QTLs were identified, with some QTLs for different traits colocating. The relevance of colocation for mechanisms underlying the various traits is discussed.

The preservation of liposomes by raffinose family oligosaccharides during drying is mediated by effects on fusion and lipid phase transitions

Raffinose family oligosaccharides (RFO) have been implicated as protective agents in the cellular dehydration tolerance, especially of many plant seeds. However, their efficacy in stabilizing membranes during dehydration has never been systematically investigated. We have analyzed the effects of sucrose, raffinose, stachyose, and verbascose on liposome stability during air-drying. With increasing degree of polymerization (DP), the RFO were progressively better able to stabilize liposomes against leakage of aqueous content and against membrane fusion after rehydration. Indeed, there was a very tight linear correlation between fusion and leakage for all RFO. These data indicate that increased protection of liposomes against leakage with increasing DP is due to better protection against fusion. This is in accord with the higher glass transition temperature of the longer chain oligosaccharides. Further evidence for the influence of glass transitions on membrane stability in the dry state was provided by experiments testing the temperature dependence of membrane fusion. During incubation at temperatures up to 95 degrees C for 2 h, fusion increased less with temperature in the presence of higher DP sugars. This indicates that RFO with a higher glass transition temperature are better able to protect dry membranes at elevated temperatures. In addition, Fourier-transform infrared (FTIR) spectroscopy showed a reduction of the gel to liquid-crystalline phase transition temperature of dry liposomes in the presence of all investigated sugars. However, the RFO became slightly less effective with increasing chain length, again pointing to a decisive role for preventing fusion. A direct interaction of the RFO with the lipids was indicated by a strong effect of the sugars on the phosphate asymmetric stretch region of the infrared spectrum.