The protective role of selenium in recalcitrant Acer saccharium L. seeds subjected to desiccation

The chances in the redox priming of nondormant recalcitrant seeds by spermidine

The problems posed by seed sensitivity to desiccation and aging have motivated the development of various techniques for mitigating their detrimental effects. The redox priming of seeds in antioxidant solution to improve their postharvest performance is one of the approaches. Spermidine (Spd) was tested as an invigorating solution on nondormant recalcitrant (desiccation sensitive) seeds of the silver maple (Acer saccharinum L.). The treatment resulted in an 8-10% increase in germination capacity in seeds subjected to mild and severe desiccation, while in aged seeds stored for six months, no significant change was observed. The cellular redox milieu, genetic stability, mitochondrial structure and function were investigated to provide information about the cellular targets of Spd activity. Spd improved the antioxidative capacity, especially the activity of catalase, and cellular membrane stability, protected genome integrity from oxidative damage and increased the efficiency of mitochondria. However, it also elicited a hydrogen peroxide burst. Therefore, it seems that redox priming in nondormant seeds that are highly sensitive to desiccation, although positively affected desiccated seed performance, may not be a simple solution to reinvigorate stored seeds with a low-efficiency antioxidant system.

Linkages between soil, crop, livestock, and human selenium status in Sub-Saharan Africa: a scoping review

Selenium (Se) is essential for human health, however, data on population Se status and agriculture-nutrition-health linkages are limited in sub-Saharan Africa (SSA). The scoping review aims to identify linkages between Se in soils/crops, dietary Se intakes, and livestock and human Se status in SSA. Online databases, organisational websites and grey literature were used to identify articles. Articles were screened at title, abstract and full text levels using eligibility criteria. The search yielded 166 articles from which 112 were excluded during abstract screening and 54 full text articles were assessed for eligibility. The scoping review included 34 primary studies published between 1984 and 2021. The studies covered Se concentrations in soils (n = 7), crops (n = 9), animal tissues (n = 2), livestock (n = 3), and human Se status (n = 15). The evidence showed that soil/crop Se concentrations affected Se concentration in dietary sources, dietary Se intake and biomarkers of Se status. Soil types are a primary driver of human Se status and crop Se concentration correlates positively with biomarkers of Se dietary status. Although data sets of Se concentrations exist across the food system in SSA, there is limited evidence on linkages across the agriculture-nutrition nexus. Extensive research on Se linkages across the food chain is warranted.

Voyage of selenium from environment to life: Beneficial or toxic?

Selenium (Se), a naturally occurring metalloid, is an essential micronutrient for life as it is incorporated as selenocysteine in proteins. Although beneficial at low doses, Se is hazardous at high concentrations and poses a serious threat to various ecosystems. Due to this contrasting 'dual' nature, Se has garnered the attention of researchers wishing to unravel its puzzling properties. In this review, we describe the impact of selenium's journey from environment to diverse biological systems, with an emphasis on its chemical advantage. We describe the uneven distribution of Se and how this affects the bioavailability of this element, which, in turn, profoundly affects the habitat of a region. Once taken up, the subsequent incorporation of Se into proteins as selenocysteine and its antioxidant functions are detailed here. The causes of improved protein function due to the incorporation of redox-active Se atom (instead of S) are examined. Subsequently, the reasons for the deleterious effects of Se, which depend on its chemical form (organo-selenium or the inorganic forms) in different organisms are elaborated. Although Se is vital for the function of many antioxidant enzymes, how the pro-oxidant nature of Se can be potentially exploited in different therapies is highlighted. Furthermore, we succinctly explain how the presence of Se in biological systems offsets the toxic effects of heavy metal mercury. Finally, the different avenues of research that are fundamental to expand our understanding of selenium biology are suggested.

Micronutrient seed priming: new insights in ameliorating heavy metal stress

Plants need to survive with changing environmental conditions, be it different accessibility to water or nutrients, or attack by insects or pathogens. Few of these changes, especially heavy metal stress, can become more stressful and needed strong countermeasures to ensure survival of plants. Priming, a pre-sowing hydration treatment, involves pre-exposure of plants to an eliciting component which enhance the plant's tolerance to later stress events. By considering the role of micronutrients in aiding plants to cope up under adverse conditions, this review addresses various aspects of micronutrient seed priming in attenuating heavy metal stress. Priming using micronutrients is an adaptive strategy that boosts the defensive capacity of the plant by accumulating several active or inactive signaling proteins, which hold considerable importance in signal amplification against the triggered stimulus. Priming induced 'defence memory' persists in both present generation and its progeny. Therefore, it is considered a promising approach by seed technologist for commercial seed lots to enhance the vigour in terms of seed germination potential, productivity and strengthening resistance response against metalloid stress. The present review provides an overview regarding the potency of priming with micronutrient to ameliorate harmful effects of heavy metal stress, possible mechanism how attenuation is accomplished, role of priming in enhancing crop productivity and inducing defence memory against the metalloid stress stimulus.

Mechanistic insights derived from re-establishment of desiccation tolerance in germinating xerophytic seeds: Caragana korshinskii as an example

Germplasm conservation strongly depends on the desiccation tolerance (DT) of seeds. Xerophytic seeds have strong desiccation resistance, which makes them excellent models to study DT. Although some experimental strategies have been applied previously, most methods are difficult to apply to xerophytic seeds. In this review, we attempted to synthesize current strategies for the study of seed DT and provide an in-depth look at Caragana korshinskii as an example. First, we analyze congenital advantages of xerophytes in the study of seed DT. Second, we summarize several strategies used to study DT and illustrate a suitable strategy for xerophytic species. Then, based on our previous studies work with C. korshinskii, a feasible technical strategy for DT re-establishment is provided and we provide illustrate some special molecular mechanisms seen in xerophytic seeds. Finally, several steps to unveil the DT mechanism of xerophytic seeds are suggested, and three scientific questions that the field should consider are listed. We hope to optimize and utilize this strategy for more xerophytic species to more systematically decipher the physiological and molecular processes of seed DT and provide more candidate genes for molecular breeding.

Cadmium in plants: uptake, toxicity, and its interactions with selenium fertilizers

Cd is the third major contaminant of greatest hazard to the environment after mercury and lead and is considered as the only metal that poses health risks to both humans and animals at plant tissue concentrations that are generally not phytotoxic. Cd accumulation in plant shoots depends on Cd entry through the roots, sequestration within root vacuoles, translocation in the xylem and phloem, and Cd dilution within the plant shoot throughout its growth. Several metal transporters, processes, and channels are involved from the first step of Cd reaching the root cells and until its final accumulation in the edible parts of the plant. It is hard to demonstrate one step as the pivotal factor to decide the Cd tolerance or accumulation ability of plants since the role of a specific transporter/process varies among plant species and even cultivars. In this review, we discuss the sources of Cd pollutants, Cd toxicity to plants, and mechanisms of Cd uptake and redistribution in plant tissues. The metal transporters involved in Cd transport within plant tissues are also discussed and how their manipulation can control Cd uptake and/or translocation. Finally, we discuss the beneficial effects of Se on plants under Cd stress, and how it can minimize or mitigate Cd toxicity in plants.

Selenium supplementation alleviates cadmium-induced damages in tall fescue through modulating antioxidant system, photosynthesis efficiency, and gene expression

Selenium (Se) is beneficial for plant growth under different stressful conditions. In this study, we investigated the protective effects of Se supply from Cd-induced damages in tall fescue under Cd stress. Tall fescue seedlings (40 days old) were treated with Cd (30 mg/L, as CdSO₄·8/3 H₂O) and Se (0.1 mg/L, as Na₂SeO₃) individually and in combination using 1/2 Hoagland's solution system for 7 days. Various physiological parameters, photosynthetic behaviors, and gene expressions were measured. The results showed that Cd-stressed plants displayed obvious toxicity symptoms such as leaf yellowing, decreasing plant height, and root length. Cd stress significantly increased the malondialdehyde (MDA) content and electrolyte leakage (EL), and remarkably reduced the chlorophyll and soluble protein content, antioxidant enzyme activities, and photosynthetic efficiency. Cd stress significantly inhibited the expression of two photosynthesis-related genes (psbB and psbC), but not psbA. In addition, it significantly inhibited the expression of antioxidant system-related genes such as ChlCu/ZnSOD, CytCu/ZnSOD, GPX, and pAPX, but significantly increased the expression of GR. However, Se improved the overall physiological and photosynthetic behaviors of Cd-stressed plants. Se significantly enhanced the chlorophyll and soluble protein content and CAT and SOD activities, but decreased MDA contents, EL, and Cd content and translocation in tall fescue under Cd stress. Furthermore, under Cd stress, Se increased the expression of psbA, psbB psbC, ChlCu/ZnSOD, CytCu/ZnSOD, GPx, and PAPx. The result suggests that Se alleviated the deleterious effects of Cd and improved Cd resistance in tall fescue through upregulating the antioxidant system, photosynthesis activities, and gene expressions.

Comparative proteomic analysis of pepper (Capsicum annuum L.) seedlings under selenium stress

Selenium (Se) is an essential trace element for human and animal health. Se fertilizer has been used to increase the Se content in crops to meet the Se requirements in humans and animals. To address the challenge of Se poisoning in plants, the mechanisms underlying Se-induced stress in plants must be understood. Here, to elucidate the effects of Se stress on the protein levels in pepper, we used an integrated approach involving tandem mass tag labeling, high performance liquid chromatography fractionation, and mass spectrometry-based analysis. A total of 4,693 proteins were identified, 3,938 of which yielded quantitative information. Among them, the expression of 172 proteins was up-regulated, and the expression of 28 proteins was down-regulated in the Se/mock treatment comparison. According to the above data, we performed a systematic bioinformatics analysis of all identified proteins and differentially expressed proteins (DEPs). The DEPs were most strongly associated with the terms “metabolic process,” “posttranslational modification, protein turnover, chaperones,” and “protein processing in endoplasmic reticulum” according to Gene Ontology, eukaryotic orthologous groups classification, and Kyoto Encyclopedia of Genes and Genomes enrichment analysis, respectively. Furthermore, several heat shock proteins were identified as DEPs. These results provide insights that may facilitate further studies on the pepper proteome expressed downstream of the Se stress response. Our data revealed that the responses of pepper to Se stress involve various pathways.

Foliar application of selenium for protection against the first stages of mycotoxin infection of crop plant leaves

BACKGROUND

The aim of this study was to investigate whether the application of selenium (Se) ions directly to the leaf surface can protect plants against infection by the fungal toxin zearalenone (ZEA). The experiments were performed for the most common and agronomically important crops such as wheat, oat, and barley (both tolerant and sensitive varieties) because mycotoxin accumulation in plants is the cause of many diseases in animals and people.

RESULTS

ZEA at a concentration of 10 µmol L^-1 either alone or in combination with Se (5 µmol L^-1 Na₂ SeO₄ ) was applied to the second leaf of seedlings. Visualization of leaf temperature profiles by infrared thermography demonstrated a decrease in temperature at the location of ZEA infection that was more noticeable in sensitive genotypes. The presence of Se significantly suppressed changes at the site of ZEA application in all tested plants, especially the tolerant genotypes. Microscopic observations confirmed that foliar administration of ZEA resulted in its penetration to deeper localized cells and that damage induced by ZEA (mainly to chloroplasts) decreased after Se application. Analyses of antioxidant enzymes demonstrated the involvement of Se in antioxidation mechanisms, in particular by activating SOD and CAT under ZEA-induced stress conditions.

CONCLUSION

The foliar application of Se to seedling leaves may be a non-invasive method of protecting crops against the first steps of ZEA infection. © 2018 Society of Chemical Industry.

The effect of a doubled glutathione level on parameters affecting the germinability of recalcitrant Acer saccharinum seeds during drying

Approximately 20% of plant species, including silver maple (Acer saccharinum L.), produce seeds that are sensitive to desiccation, which is reflected in their poor storage potential and viability. In the search for a compound that can improve seed recalcitrance, freshly harvested seeds were soaked in either 2.5 mM reduced glutathione (GSH) or water and desiccated to comparable water levels of 55-20%. We examined the impact of a doubled endogenous level of glutathione on the seed germination capacity, the activity of enzymes involved in glutathione metabolism, the cell membrane components and integrity, reactive oxygen species, and ascorbate levels. GSH treatment resulted in slower dehydration and a higher germination capacity. The increased glutathione was mainly consumed by glutathione S-transferase, leading to more efficient detoxification, and by dehydroascorbate reductase (DHAR), accelerating the ascorbate regeneration. As a result, the cellular environment became more reduced, and protection of the membrane structures was enhanced. The ameliorated membrane integrity was manifested via a lower electrolyte leakage and a lower lipid peroxide level despite the higher level of hydrogen peroxide (H₂O₂) detected in the GSH-treated seeds. The degradation of phospholipids (PLs) was less intense and related to the phosphatidylinositol (PI) level, which is the precursor of the phospholipase D cofactor, whereas in water-soaked seeds, PL degradation was promoted by H₂O₂. The germination capacity of the dehydrated seeds depended primarily on the level of H₂O₂, lipid hydroxyperoxides, electrolyte leakage, GSH, the half-cell reduction potential of glutathione, PI, and the activity of DHAR and γ-glutamylcysteine synthetase. Interestingly, H₂O₂ affected all of the parameters. The germination of GSH-boosted seeds was strongly impacted by the pool of ascorbate, the half-cell reduction potential of ascorbate, and the glutathione peroxidase activity. In general, germination was DHAR activity-dependent. A strong negative correlation was detected in the water-soaked seeds, whereas a strong positive correlation was detected in the GSH-treated seeds. The enhanced level of glutathione likely improved the efficiency of the ascorbate-glutathione cycle, confirming its effect on seed germinability after dehydration.

Protective role of selenium against chromium stress involving metabolites and essential elements in Brassica juncea L. seedlings

The present study aimed at the potential role of selenium in providing protection to plants subjected to chromium toxicity. The study was carried out on 15-day-old seedlings of Brassica juncea raised in the solutions of Cr (300 µM) and Se (2, 4 and 6 µM), both alone and in combinations under controlled laboratory environment. The effects were studied on growth, plant metabolites (involved in osmotic homeostasis and stress protection), and essential elements. The results showed that the exposure of B. juncea seedlings to 300 µM Cr led to an increase in the contents of total sugars, reducing sugars, non-reducing sugars, total phenols and flavonoids. However, a significant decline in growth characteristics, the contents of proteins and free amino acids was observed. The essential elements (Na, K, Ca, Mg, C, H, N) also decreased in response to Cr. Se application in binary combinations, on the other hand, aided in improving seed germination (19%), root (88.3%) and shoot (18.2%) lengths. It also helped to increase the contents of sugars [total (16.3%), reducing (21.6%) and non-reducing (15.2%)], phenols (36.7%) and flavonoids (27.4%), thereby aiding in alleviating the phytotoxicity of Cr. The profiling of polyphenols and amino acids, and histological study of phenols supported the above results. The contents of essential elements also showed a significant increase, while Cr uptake was observed to decline by Se supplementation. The observations from the present study indicate that Se has the ability to influence primary and secondary metabolism, improve mineral nutrition and reduce Cr uptake in B. juncea seedlings to combat the Cr phytotoxicity and enhance the tolerance against stress.

An Overview of Selenium Uptake, Metabolism, and Toxicity in Plants

Selenium (Se) is an essential micronutrient for humans and animals, but lead to toxicity when taken in excessive amounts. Plants are the main source of dietary Se, but essentiality of Se for plants is still controversial. However, Se at low doses protects the plants from variety of abiotic stresses such as cold, drought, desiccation, and metal stress. In animals, Se acts as an antioxidant and helps in reproduction, immune responses, thyroid hormone metabolism. Selenium is chemically similar to sulfur, hence taken up inside the plants via sulfur transporters present inside root plasma membrane, metabolized via sulfur assimilatory pathway, and volatilized into atmosphere. Selenium induced oxidative stress, distorted protein structure and function, are the main causes of Se toxicity in plants at high doses. Plants can play vital role in overcoming Se deficiency and Se toxicity in different regions of the world, hence, detailed mechanism of Se metabolism inside the plants is necessary for designing effective Se phytoremediation and biofortification strategies.

Indications of Selenium Protection against Cadmium and Lead Toxicity in Oilseed Rape (Brassica napus L.)

The present study investigated the beneficial role of selenium (Se) in protecting oilseed rape (Brassica napus L.) plants from cadmium (Cd+2) and lead (Pb+2) toxicity. Exogenous Se markedly reduced Cd and Pb concentration in both roots and shoots. Supplementation of the medium with Se (5, 10, and 15 mg kg-1) alleviated the negative effect of Cd and Pb on growth and led to a decrease in oxidative damages caused by Cd and Pb. Furthermore, Se-enhanced superoxide free radicals ([Formula: see text]), hydrogen peroxide (H2O2), and lipid peroxidation, as indicated by malondialdehyde accumulation, but decreased superoxide dismutase and glutathione peroxidase activities. Meanwhile, the presence of Cd and Pb in the medium affected Se speciation in shoots. The results suggest that Se could alleviate Cd and Pb toxicity by preventing oxidative stress in oilseed rape plant.

Essential and Beneficial Trace Elements in Plants, and Their Transport in Roots: a Review

The essentiality of 14 mineral elements so far have been reported in plant nutrition. Eight of these elements were known as micronutrients due to their lower concentrations in plants (usually ≤100 mg/kg/dw). However, it is still challenging to mention an exact number of plant micronutrients since some elements have not been strictly proposed yet either as essential or beneficial. Micronutrients participate in very diverse metabolic processes, including from the primary and secondary metabolism to the cell defense, and from the signal transduction to the gene regulation, energy metabolism, and hormone perception. Thus, the attempt to understand the molecular mechanism(s) behind their transport has great importance in terms of basic and applied plant sciences. Moreover, their deficiency or toxicity also caused serious disease symptoms in plants, even plant destruction if not treated, and many people around the world suffer from the plant-based dietary deficiencies or metal toxicities. In this sense, shedding some light on this issue, the 13 mineral elements (Fe, B, Cu, Mn, Mo, Si, Zn, Ni, Cl, Se, Na, Al, and Co), required by plants at trace amounts, has been reviewed with the primary focus on the transport proteins (transporters/channels) in plant roots. So, providing the compiled but extensive information about the structural and functional roles of micronutrient transport genes/proteins in plant roots.

Selenium (Se) improves drought tolerance in crop plants--a myth or fact?

Climate change has emerged as one of the most complex challenges of the 21st century and has become an area of interest in the past few decades. Many countries of the world have become extremely vulnerable to the impacts of climate change. The scarcity of water is a serious concern for food security of these countries and climate change has aggravated the risks of extreme events like drought. Oxidative stress, caused by a variety of active oxygen species formed under drought stress, damages many cellular constituents, such as carbohydrates, lipids, nucleic acids and proteins, which ultimately reduces plant growth, respiration and photosynthesis. Se has become an element of interest to many biologists owing to its physiological and toxicological importance. It plays a beneficial role in plants by enhancing growth, reducing damage caused by oxidative stress, enhancing chlorophyll content under light stress, stimulating senesce to produce antioxidants and improving plant tolerance to drought stress by regulating water status. Researchers have adopted different strategies to evaluate the role of selenium in plants under drought stress. Some of the relevant work available regarding the role of Se in alleviating adverse effect of drought stress is discussed in this paper.

Exogenously applied selenium reduces oxidative stress and induces heat tolerance in spring wheat

Heat stress (HS) is a worldwide threat to productivity of wheat, especially in arid and semiarid regions of the world. Earlier studies suggested the beneficial effects of selenium (Se) on the growth of some crop species grown under stressful environments. In the present study, we assessed whether Se application could increase antioxidative potential, and thus enhance tolerance to heat in wheat at the sensitive stage i.e., heading stage. At the heading stage, after foliar application of sodium selenate solutions (0, 2 and 4 mg Se L(-1)), the plants of wheat cultivars, namely Chakwal-97 (drought tolerant) and Faisalabad-08 (drought sensitive), were subjected to HS (38 ± 2 °C). The HS significantly altered antioxidative potential, affected growth, photosynthetic pigments and grain yield in both cultivars. Exogenous application of low (2 mg L(-1)) Se increased chlorophyll a and total chlorophyll contents and modulated the growth of wheat plants under HS. However, high concentration (4 mg L(-1)) of Se was much more effective in increasing grains per spike and grain yield in heat stressed plants of both wheat cultivars. Exogenous Se increased both enzymatic (catalase and ascorbate peroxidase activities) and non-enzymatic (carotenoids, anthocyanins and ascorbic acid contents) antioxidants while decreased oxidants (hydrogen peroxide and malondialdehyde contents) under HS in both wheat cultivars. In conclusion, foliar application of Se (4 mg L(-1)) was much more effective in mitigating the deleterious effects of HS on grain yield of wheat plants. The results suggested that Se-mediated up-regulation of antioxidative system (both enzymatic and non-enzymatic) helped the wheat plants to increase fertility, and hence avoid reduction of grain yield under HS.

Involvement of Selenium in Protective Mechanisms of Plants under Environmental Stress Conditions – Review

In recent years there has been growing interest in selenium (Se) as an important micronutrient not only for animals and humans but also for plants. In particular, its protective effect in plants exposed to stress conditions has been suggested. In spite of many studies, the mechanism of Se action is not fully understood. In this review, possible ways of interaction of Se with stress factors leading to optimal growth and development of plants are presented. As the majority of experiments have focused on the effects of Se application under stress conditions induced by heavy metals, special attention is paid to the results obtained in such studies. Changes of physiological and biochemical properties of plant cells, with particular regard to the influence of Se on the activation of enzymatic and non-enzymatic antioxidants under this stress, are summarized. Experiments in which Se was used in some other environmental stresses (drought, UV, cold and high temperature) are also cited. On the basis of the presented literature it is suggested that a positive effect of Se depends on both its doses and on chosen plant genotypes and is mainly connected with activation of antioxidative defense in plant cells.

Selenium (Se) Regulates Seedling Growth in Wheat under Drought Stress

Selenium (Se) is an essential micronutrient with a range of physiological and antioxidative properties. Reports regarding effect of Se application on plants growth and development are not consistent. The identification of effective Se dose and application method is crucial for better understanding of Se translocation within crop plants under drought stress. The present study aimed at investigating the role of Se supplementation in improving the drought tolerance potential of wheat at early growth stages. Two wheat genotypes (Kohistan-97 and Pasban-90) were grown in plastic pots (8 × 12 cm) in green/wire-house experiments. Results demonstrated that the growth and biomass of seedlings increased at high Se foliar concentrations and decreased at low and high Se fertigation levels. The seedlings exhibited the highest values for plant height stress tolerance index (PHSI), root length stress tolerance index (RLSI), dry matter stress tolerance index (DMSI), and fresh matter stress tolerance indices (FMSI) at Se fertigation level of 7.35 μM, whereas Se foliar treatment of 7.06 μM resulted in maximum values for these indices. The seedlings foliarly sprayed with Se maintained higher DMSI and FMSI than those fertigated with Se which suggests that Se foliar spray is more effective than Se fertigation for improving drought tolerance.

Selenium alleviates cadmium toxicity by preventing oxidative stress in sunflower (Helianthus annuus) seedlings

The present study investigated the possible mediatory role of selenium (Se) in protecting plants from cadmium (Cd) toxicity. The exposure of sunflower seedlings to 20μM Cd inhibited biomass production, decreased chlorophyll and carotenoid concentrations and strongly increased accumulation of Cd in both roots and shoots. Similarly, Cd enhanced hydrogen peroxides content and lipid peroxidation as indicated by malondialdehyde accumulation. Pre-soaking seeds with Se (5, 10 and 20μM) alleviated the negative effect of Cd on growth and led to a decrease in oxidative injuries caused by Cd. Furthermore, Se enhanced the activities of catalase, ascorbate peroxidase and glutathione reductase, but lowered that of superoxide dismutase and guaiacol peroxidase. As important antioxidants, ascorbate and glutathione contents in sunflower leaves exposed to Cd were significantly decreased by Se treatment. The data suggest that the beneficial effect of Se during an earlier growth period could be related to avoidance of cumulative damage upon exposure to Cd, thus reducing the negative consequences of oxidative stress caused by heavy metal toxicity.

Implications of the lack of desiccation tolerance in recalcitrant seeds

A suite of interacting processes and mechanisms enables tolerance of desiccation and storage (conservation) of orthodox seeds in the dry state. While this is a long-term option under optimized conditions, dry orthodox seeds are not immortal, with life spans having been characterized as short, intermediate and long. Factors facilitating desiccation tolerance are metabolic "switch-off" and intracellular dedifferentiation. Recalcitrant seeds lack these mechanisms, contributing significantly to their desiccation sensitivity. Consequently, recalcitrant seeds, which are shed at high water contents, can be stored only in the short-term, under conditions not allowing dehydration. The periods of such hydrated storage are constrained by germination that occurs without the need for extraneous water, and the proliferation of seed-associated fungi. Cryopreservation is viewed as the only option for long-term conservation of the germplasm of recalcitrant-seeded species. This is not easily achieved, as each of the necessary procedures imposes oxidative damage. Intact recalcitrant seeds cannot be cryopreserved, the common practice being to use excised embryos or embryonic axes as explants. Dehydration is a necessary procedure prior to exposure to cryogenic temperatures, but this is associated with metabolism-linked injury mediated by uncontrolled reactive oxygen species generation and failing anti-oxidant systems. While the extent to which this occurs can be curtailed by maximizing drying rate (flash drying) it cannot be completely obviated. Explant cooling for, and rewarming after, cryostorage must necessarily be rapid, to avoid ice crystallization. The ramifications of desiccation sensitivity are discussed, as are problems involved in cryostorage, particularly those accompanying dehydration and damage consequent upon ice crystallization. While desiccation sensitivity is a "fact" of seed recalcitrance, resolutions of the difficulties involved germplasm conservation are possible as discussed.

Implications of the lack of desiccation tolerance in recalcitrant seeds

A suite of interacting processes and mechanisms enables tolerance of desiccation and storage (conservation) of orthodox seeds in the dry state. While this is a long-term option under optimized conditions, dry orthodox seeds are not immortal, with life spans having been characterized as short, intermediate and long. Factors facilitating desiccation tolerance are metabolic "switch-off" and intracellular dedifferentiation. Recalcitrant seeds lack these mechanisms, contributing significantly to their desiccation sensitivity. Consequently, recalcitrant seeds, which are shed at high water contents, can be stored only in the short-term, under conditions not allowing dehydration. The periods of such hydrated storage are constrained by germination that occurs without the need for extraneous water, and the proliferation of seed-associated fungi. Cryopreservation is viewed as the only option for long-term conservation of the germplasm of recalcitrant-seeded species. This is not easily achieved, as each of the necessary procedures imposes oxidative damage. Intact recalcitrant seeds cannot be cryopreserved, the common practice being to use excised embryos or embryonic axes as explants. Dehydration is a necessary procedure prior to exposure to cryogenic temperatures, but this is associated with metabolism-linked injury mediated by uncontrolled reactive oxygen species generation and failing anti-oxidant systems. While the extent to which this occurs can be curtailed by maximizing drying rate (flash drying) it cannot be completely obviated. Explant cooling for, and rewarming after, cryostorage must necessarily be rapid, to avoid ice crystallization. The ramifications of desiccation sensitivity are discussed, as are problems involved in cryostorage, particularly those accompanying dehydration and damage consequent upon ice crystallization. While desiccation sensitivity is a "fact" of seed recalcitrance, resolutions of the difficulties involved germplasm conservation are possible as discussed.

The involvement of the mitochondrial peroxiredoxin PRXIIF in defining physiological differences between orthodox and recalcitrant seeds of two Acer species

Norway maple (Acer platanoides L., orthodox) and sycamore (Acer pseudoplatanus L., recalcitrant) belong to the same genus and grow under similar climatic conditions, but their seeds differ in their tolerance to desiccation. The initial water content (WC) of the seeds used in this study was 50%, and they were dried to 40, 20 and 7%. The mitochondrial peroxiredoxin IIF (PRXIIF) was identified in seeds of both species by immunoblotting. Semiquantitative RT-PCR analyses indicated that the transcript level of PRXIIF in both types of seeds increased during different stages of desiccation and was higher in seeds of Norway maple than in sycamore. General proteome analyses showed important differences between orthodox and recalcitrant seeds. In sycamore seeds that had been desiccated to a 7% WC, the number of protein spots and the levels of those spots were lower than in desiccation-tolerant Norway maple seeds. Post-translational modifications of PRXIIF in seeds at a 50% WC were detected via 2D electrophoresis and subsequent western blot analysis. The detected shift in the pI values (± 0.3) in A. pseudoplatanus was possibly caused by phosphorylation because several potential phosphorylation sites were predicted in silico for that protein. The gene and amino acid sequences were obtained and aligned with known sequences of other plant PRXIIF genes and proteins. High values of sequence identity were noted between the PRXIIF protein sequences of Acer species, Populus trichocarpa Torr. & A. Gray and Arabidopsis thaliana (L.) Heynh. The involvement of PRXIIF in defining the physiological differences between desiccation-tolerant and desiccation-sensitive Acer seeds is discussed in the context of its role in mitochondrial redox homeostasis.

Global changes in DNA methylation in seeds and seedlings of Pyrus communis after seed desiccation and storage

The effects of storage and deep desiccation on structural changes of DNA in orthodox seeds are poorly characterized. In this study we analyzed the 5-methylcytosine (m(5)C) global content of DNA isolated from seeds of common pear (Pyrus communis L.) that had been subjected to extreme desiccation, and the seedlings derived from these seeds. Germination and seedling emergence tests were applied to determine seed viability after their desiccation. In parallel, analysis of the global content of m(5)C in dried seeds and DNA of seedlings obtained from such seeds was performed with a 2D TLC method. Desiccation of fresh seeds to 5.3% moisture content (mc) resulted in a slight reduction of DNA methylation, whereas severe desiccation down to 2-3% mc increased DNA methylation. Strong desiccation of seeds resulted in the subsequent generation of seedlings of shorter height. A 1-year period of seed storage induced a significant increase in the level of DNA methylation in seeds. It is possible that alterations in the m(5)C content of DNA in strongly desiccated pear seeds reflect a reaction of desiccation-tolerant (orthodox) seeds to severe desiccation. Epigenetic changes were observed not only in severely desiccated seeds but also in 3-month old seedlings obtained from these seeds. With regard to seed storage practices, epigenetic assessment could be used by gene banks for early detection of structural changes in the DNA of stored seeds.