Effects of photooxidation on membrane integrity in Salix nigra seeds

Ageing kinetics of fern chlorophyllous spores during dry storage is determined by its antioxidant potential and likely induced by photosynthetic machinery

Ageing in dry chlorophyllous propagules is leaded by photooxidation through the photosynthetic machinery, but why species differ in longevity and the ageing mechanisms of when light and oxygen are absent are unknown. We hypothesize that the cellular antioxidant capacity is key for the inter- and intra-specific differences in the ageing process. We have tested this hypothesis in chlorophyllous spores of two ferns. They were subjected to four different storage regimes resulting from light/dark and normoxia/hypoxia combinations. Lipophilic and hydrophilic antioxidants, reactive oxygen species (ROS), and photosynthetic pigments were analysed in parallel to germination and the recovery of Fv/Fm over a storage period of up to 22-months. We show that light and oxygen accelerate the ageing process, but their mechanisms (ROS, increase, antioxidant capacity decrease, loss of efficiency of the photosystem II, pigment degradation) appear the same under all conditions tested. The end of the asymptomatic phase of longevity, when a sudden drop of germination occurs, seems to be determined by a threshold in the depletion of antioxidants. Our results support the hypothesis that ageing kinetics in dry plant propagules is determined by the antioxidant system, but also suggests an active role of the photosynthetic machinery during ageing, even in darkness and hypoxia.

Identification of DNA Methylation Changes in European Beech Seeds during Desiccation and Storage

Ageing and deterioration of seeds is a major problem for the maintenance of seed quality and viability during long-term storage. Prediction of early stages of seed deterioration in order to point out the plantlets' regeneration time is a major challenge of successful storage. In preserved seeds, damages accumulate within cells at the rate mainly related to their moisture content and temperature of storage. Current research reveals global alterations in DNA methylation in lipid-rich intermediate seeds during desiccation and storage at various regimes covering nonoptimal and optimal conditions. We show for the first time that monitoring of 5-methylcytosine (m⁵C) level in seeds can be used as a truly universal viability marker regardless of postharvest category of seeds and their composition. For seeds stored up to three years, in varied conditions, moisture content, temperature, and time of storage had significant influence on seedling emergence and DNA methylation (p < 0.05). Similarities among lipid-rich intermediate and orthodox seeds regarding different reactions of embryonic axes and cotyledons to desiccation are newly revealed. Along with previous studies on seeds dramatically different in desiccation tolerance (recalcitrant vs. orthodox), results regarding lipid-rich seeds positioned in-between (intermediate) prove that maintaining global DNA methylation status is crucial for maintaining seed viability.

DNA Methylation as an Early Indicator of Aging in Stored Seeds of “Exceptional” Species Populus nigra L

Ex situ preservation of genetic resources is an essential strategy for the conservation of plant biodiversity. In this regard, seed storage is the most convenient and efficient way of preserving germplasm for future plant breeding efforts. A better understanding of the molecular changes that occur during seed desiccation and aging is necessary to improve conservation protocols, as well as real-time methods for monitoring seed quality. In the present study, we assessed changes in the level of genomic 5-methylcytosine (5mC) in seeds of Populus nigra L. by 2D-TLC. Epigenetic changes were characterized in response to several seed storage regimes. Our results demonstrate that P. nigra seeds represent an intermediate type of post-harvest behavior, falling between recalcitrant and orthodox seeds. This was also true for the epigenetic response of P. nigra seeds to external factors. A crucial question is whether aging in seeds is initiated by a decline in the level of 5mC, or if epigenetic changes induce a process that leads to deterioration. In our study, we demonstrate for the first time that 5mC levels decrease during storage and that the decline can be detected before any changes in seed germination are evident. Once P. nigra seeds reached an 8–10% reduction in the level of 5mC, a substantial decrease in germination occurred. The decline in the level of 5mC appears to be a critical parameter underlying the rapid deterioration of intermediate seeds. Thus, the measurement of 5mC can be a fast, real-time method for assessing asymptomatic aging in stored seeds.

Desiccation Tolerance in Chlorophyllous Fern Spores: Are Ecophysiological Features Related to Environmental Conditions?

Fern spores of most species are desiccation tolerant (DT) and, in some cases, are photosynthetic at maturation, the so-called chlorophyllous spores (CS). The lifespan of CS in the dry state is very variable among species. The physiological, biochemical, and biophysical mechanisms underpinning this variability remain understudied and their interpretation from an ecophysiological approach virtually unexplored. In this study, we aimed at fulfilling this gap by assessing photochemical, hydric, and biophysical properties of CS from three temperate species with contrasting biological strategies and longevity in the dry state: Equisetum telmateia (spore maturation and release in spring, ultrashort lifespan), Osmunda regalis (spore maturation and release in summer, medium lifespan), Matteuccia struthiopteris (spore maturation and release in winter, medium-long lifespan). After subjection of CS to controlled drying treatments, results showed that the three species displayed different extents of DT. CS of E. telmateia rapidly lost viability after desiccation, while the other two withstood several dehydration-rehydration cycles without compromising viability. The extent of DT was in concordance with water availability in the sporulation season of each species. CS of O. regalis and M. struthiopteris carried out the characteristic quenching of chlorophyll fluorescence, widely displayed by other DT cryptogams during drying, and had higher tocopherol and proline contents. The turgor loss point of CS is also related to the extent of DT and to the sporulation season: lowest values were found in CS of M. struthiopteris and O. regalis. The hydrophobicity of spores in these two species was higher and probably related to the prevention of water absorption under unfavorable conditions. Molecular mobility, estimated by dynamic mechanical thermal analysis, confirmed an unstable glassy state in the spores of E. telmateia, directly related to the low DT, while the DT species entered in a stable glassy state when dried. Overall, our data revealed a DT syndrome related to the season of sporulation that was characterized by higher photoprotective potential, specific hydric properties, and lower molecular mobility in the dry state. Being unicellular haploid structures, CS represent not only a challenge for germplasm preservation (e.g., as these spores are prone to photooxidation) but also an excellent opportunity for studying mechanisms of DT in photosynthetic cells.

Deterioration of willow seeds during storage

Willow (Salix spp.) seeds are able to tolerate desiccation, but differ from typical orthodox seeds in that they lose viability in a few days at room temperature, and in that the chloroplasts in embryo tissues do not dedifferentiate during maturation drying, thus retaining chlorophyll and maintaining intact their thylakoid membranes. In the present study, we investigated the damage generated in willow seeds during storage under appropriate conditions to exclude the eventual generation of reactive oxygen species by photooxidation. To this end, we measured different indicators of molecular damage, such as changes in the fatty acid profile, protein degradation, nuclease activities, and DNA damage, and evaluated normal germination and total germination in seeds stored for one, ten and sixteen years. We found: (i) a decrease in the fraction of unsaturated fatty acids; (ii) changes in the protein profile due to a decrease in protein solubility; (iii) activation of nucleases; and (iv) DNA fragmentation. Taken together, our findings identified programmed cell death as a key mechanism in seed deterioration during storage. We also found that, although the seeds maintained high percentages of total germination, the death program had already started in the seeds stored for ten years and was more advanced in those stored for sixteen years.

Genes for seed longevity in barley identified by genomic analysis on near isogenic lines

Genes controlling differences in seed longevity between 2 barley (Hordeum vulgare) accessions were identified by combining quantitative genetics "omics" technologies in near isogenic lines (NILs). The NILs were derived from crosses between the spring barley landraces L94 from Ethiopia and Cebada Capa from Argentina. A combined transcriptome and proteome analysis on mature, nonaged seeds of the 2 parental lines and the L94 NILs by RNA-sequencing and total seed proteomic profiling identified the UDP-glycosyltransferase MLOC_11661.1 as candidate gene for the quantitative trait loci on 2H, and the NADP-dependent malic enzyme (NADP-ME) MLOC_35785.1 as possible downstream target gene. To validate these candidates, they were expressed in Arabidopsis under the control of constitutive promoters to attempt complementing the T-DNA knockout line nadp-me1. Both the NADP-ME MLOC_35785.1 and the UDP-glycosyltransferase MLOC_11661.1 were able to rescue the nadp-me1 seed longevity phenotype. In the case of the UDP-glycosyltransferase, with high accumulation in NILs, only the coding sequence of Cebada Capa had a rescue effect.

A novel gene of Kalanchoe daigremontiana confers plant drought resistance

Kalanchoe (K.) daigremontiana is important for studying asexual reproduction under different environmental conditions. Here, we describe a novel KdNOVEL41 (KdN41) gene that may confer drought resistance and could thereby affect K. daigremontiana development. The detected subcellular localization of a KdN41/Yellow Fluorescent Protein (YFP) fusion protein was in the nucleus and cell membrane. Drought, salt, and heat stress treatment in tobacco plants containing the KdN41 gene promoter driving β-glucuronidase (GUS) gene transcription revealed that only drought stress triggered strong GUS staining in the vascular tissues. Overexpression (OE) of the KdN41 gene conferred improved drought resistance in tobacco plants compared to wild-type and transformed with empty vector plants by inducing higher antioxidant enzyme activities, decreasing cell membrane damage, increasing abscisic acid (ABA) content, causing reinforced drought resistance related gene expression profiles. The 3,3'-diaminobenzidine (DAB) and nitroblue tetrazolium (NBT) staining results also showed less relative oxygen species (ROS) content in KdN41-overexpressing tobacco leaf during drought stress. Surprisingly, by re-watering after drought stress, KdN41-overexpressing tobacco showed earlier flowering. Overall, the KdN41 gene plays roles in ROS scavenging and osmotic damage reduction to improve tobacco drought resistance, which may increase our understanding of the molecular network involved in developmental manipulation under drought stress in K. daigremontiana.

Seed Carotenoid and Tocochromanol Composition of Wild Fabaceae Species Is Shaped by Phylogeny and Ecological Factors

Carotenoids distribution and function in seeds have been very scarcely studied, notwithstanding their pivotal roles in plants that include photosynthesis and phytohormone synthesis, pigmentation, membrane stabilization and antioxidant activity. Their relationship with tocochromanols, whose critical role in maintaining seed viability has already been evidenced, and with chlorophylls, whose retention in mature seed is thought to have negative effects on storability, remain also unexplored. Here, we aimed at elucidating seed carotenoids relationship with tocochromanols and chlorophylls with regard to phylogenetic and ecological traits and at understanding their changes during germination. The composition and distribution of carotenoids were investigated in seeds of a wide range of wild species across the Fabaceae (the second-most economically important family after the Poaceae). Photosynthetic pigments and tocochromanols were analyzed by HPLC in mature dry seeds of 50 species representative of 5 subfamilies within the Fabaceae (including taxa that represent all continents, biomes and life forms within the family) and at key timepoints during seedling establishment in three species representative of distinct clades. Total-carotenoids content positively correlated with tocopherols in the basal subfamilies Detarioideae, Cercidoideae, and Dialioideae, and with chlorophylls in the Papilionoideae. Papilionoideae lacked tocotrienols and had the highest total-carotenoids, chlorophyll and γ-tocopherol contents. Interestingly, lutein epoxide was present in 72% of the species including several herbs from different subfamilies. Overall, species original from temperate biomes presented higher carotenoids and lower tocochromanols levels than those from tropical biomes. Also shrub species showed higher carotenoids content than herbs and trees. During germination, total content of photosynthetic pigments increased in parallel to changes in relative abundance of carotenoids: zeaxanthin and anteraxanthin decreased and β-carotene augmented. Notably, the highest contents of nutritionally valuable carotenoids were found in Papilionoideae subfamily to which all pulses of socio-economic importance belong. The major differences in carotenoids and tocochromanols composition across the Fabaceae are apparently related to phylogeny in conjunction with ecological traits such as biome and growth form.

Genetic Modification for Improving Seed Vigor Is Transitioning from Model Plants to Crop Plants

Although seed vigor is a complex physiological trait controlled by quantitative trait loci, technological advances in the laboratory are being translated into applications for enhancing seed vigor in crop plants. In this article, we summarize and discuss pioneering work in the genetic modification of seed vigor, especially through the over-expression of protein L-isoaspartyl methyltransferase (PIMT, EC 2.1.1.77) in seeds. The impressive success in improving rice seed vigor through the over-expression of PIMT provides a valuable reference for engineering high-vigor seeds for crop production. In recent decades, numerous genes/proteins associated with seed vigor have been identified. It is hoped that such potential candidates may be used in the development of genetically edited crops for a high and stable yield potential in crop production. This possibility is very valuable in the context of a changing climate and increasing world population.

Glutathione redox state, tocochromanols, fatty acids, antioxidant enzymes and protein carbonylation in sunflower seed embryos associated with after-ripening and ageing

BACKGROUND AND AIMS

Loss of seed viability has been associated with deteriorative processes that are partly caused by oxidative damage. The breaking of dormancy, a seed trait that prevents germination in unfavourable seasons, has also been associated with oxidative processes. It is neither clear how much overlap exists between these mechanisms nor is the specific roles played by oxygen and reactive oxygen species.

METHODS

Antioxidant profiles were studied in fresh (dormant) or after-ripened (non-dormant) sunflower (Helianthus annuus) embryos subjected to controlled deterioration at 40 °C and 75 % relative humidity under ambient (21 %) or high O2 (75 %). Changes in seed vigour and viability, dormancy, protein carbonylation and fatty acid composition were also studied.

KEY RESULTS

After-ripening of embryonic axes was accompanied by a shift in the thiol-based cellular redox environment towards more oxidizing conditions. Controlled deterioration under high O2 led to a faster loss of seed dormancy and significant decreases in glutathione reductase and glutathione peroxidase activities, but viability was lost at the same rate as under ambient O2. Irrespective of O2 concentration, the overall thiol-based cellular redox state increased significantly over 21 d of controlled deterioration to strongly oxidizing conditions and then plateaued, while viability continued to decrease. Viability loss was accompanied by a rapid decrease in glucose-6-phosphate-dehydrogenase, which provides NADPH for reductive processes such as required by glutathione reductase. Protein carbonylation, a marker of protein oxidation, increased strongly in deteriorating seeds. The lipid-soluble tocochromanols, dominated by α-tocopherol, and fatty acid profiles remained stable.

CONCLUSIONS

After-ripening, dormancy-breaking during ageing and viability loss appeared to be associated with oxidative changes of the cytosolic environment and proteins in the embryonic axis rather than the lipid environment. High O2 concentrations accelerated dormancy alleviation but, surprisingly, did not accelerate the rate of viability loss.

The hydroxyl radical in plants: from seed to seed

The hydroxyl radical (OH(•)) is the most potent yet short-lived of the reactive oxygen species (ROS) radicals. Just as hydrogen peroxide was once considered to be simply a deleterious by-product of oxidative metabolism but is now acknowledged to have signalling roles in plant cells, so evidence is mounting for the hydroxyl radical as being more than merely an agent of destruction. Its oxidative power is harnessed to facilitate germination, growth, stomatal closure, reproduction, the immune response, and adaptation to stress. It features in plant cell death and is a key tool in microbial degradation of plant matter for recycling. Production of the hydroxyl radical in the wall, at the plasma membrane, and intracellularly is facilitated by a range of peroxidases, superoxide dismutases, NADPH oxidases, and transition metal catalysts. The spatio-temporal activity of these must be tightly regulated to target substrates precisely to the site of radical production, both to prevent damage and to accommodate the short half life and diffusive capacity of the hydroxyl radical. Whilst research has focussed mainly on the hydroxyl radical's mode of action in wall loosening, studies now extend to elucidating which proteins are targets in signalling systems. Despite the difficulties in detecting and manipulating this ROS, there is sufficient evidence now to acknowledge the hydroxyl radical as a potent regulator in plant cell biology.

Impact of protective agents and drying methods on desiccation tolerance of Salix nigra L. seeds

Willow seeds are classified as orthodox, but they show some recalcitrant characteristics, as they lose viability in a few weeks at room temperature. The aim of this work was to improve the desiccation tolerance of willow seeds (Salix nigra L.), as a model of sensitive materials to dehydration, through imbibition in solutions and later vacuum (VD) or freeze-drying (FD). Imbibition was conducted with 45% w/v trehalose or polyethylene glycol 400 -PEG- or water prior to dehydration treatments. Water- and especially trehalose-imbibed seeds subjected to VD showed better germination capability with respect to the freeze-dried ones. Water crystallization was mainly responsible for the great loss of capability germination observed in water- or trehalose-imbibed seeds subjected to FD. PEG behavior was better when seeds were FD instead of VD. DSC thermograms of seeds allowed to identify two thermal transitions corresponding to lipids melting and to proteins denaturation. This last transition reveals information about proteins state/functionality. Dehydration of control and PEG- or water-imbibed seeds affected proteins functionality leading to lower germinability. In the case of trehalose-imbibed seeds subjected to VD, proteins maintained their native state along dehydration, and the seeds showed a great germination capacity for all the water content range. Germinated seeds showed higher luminosity (L*), greenness (a*) and yellowness (b*) values than not-germinated seeds independently of the employed agent. Present work reveals that the presence of adequate protective agents as well the dehydration method were the main critical factors involved in willow seed desiccation tolerance.

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.

Willow seedlings from photooxidized seeds accelerate cotyledon death and anticipate first leaf emergence: a histological and biochemical study following germination

In willow seeds, photooxidative damage is mainly restricted to the outer cotyledonary tissues, significantly reducing normal germination. Here we analyzed the damage generated in cotyledonary tissues and investigated whether the increase in reactive oxygen species (ROS) generation in seedlings from photooxidized seeds can affect the morphogenetic capacity of the shoot apical meristem. Seeds were photooxidized under different light intensities and the evolution of the damage during seedling growth was studied by light and transmission electron microscopies. The level of lipid peroxidation and changes in antioxidant capacity were measured following the time course of superoxide dismutase, catalase, ascorbate peroxidase and guaiacol peroxidase enzyme activities, and the effect of photooxidative stress on the genesis of new leaf primordia and lateral roots was examined. Early and active endocytosis and autophagy, changes in chloroplast morphology, as well as the accumulation and diffusion of ROS all play important roles in the early cell death observed in cotyledonary tissues. Following germination, seedlings from photooxidized seeds anticipated the emergence of first leaves, which complemented the altered functionality of the damaged cotyledons.

Fluctuation of oxidative stress indicators in Salix nigra seeds during priming

Salix nigra seeds subjected to increased humidification show a decrease in normal germination (NG) during early imbibition followed by a recovery in that parameter at increasing imbibition times. Since photo-oxidized seeds contain high levels of reactive oxygen species (ROS), it is possible to infer that the atypical decrease in NG is a consequence of a higher ROS mobilization at early imbibition and the subsequent recovery from an increase in antioxidant activity. In this study, several oxidative stress indicators were evaluated in photo-oxidized seeds subjected to priming. ROS production was studied using electronic spin resonance spectroscopy, spontaneous chemiluminescence (SCL), spectrophotometry (with XTT), and histochemical (with DAB and NBT) and cytochemical (with CeCl(3)) techniques. Four indicators of molecular damage were monitored: lipid peroxidation, pigment destruction, protein oxidation, and membrane integrity. Antioxidant activity was evaluated by changes in the enzymes SOD, CAT, APX, and POX. The results revealed that the decrease in NG at the beginning of priming occurs by an oxidative burst, as determined by increases in both SCL and superoxide anion radical (O2(·-)) Such oxidative burst generates lipid peroxidation, protein oxidation, and a decrease in both pigment content and enzyme activities. With increasing hydration, damages are progressively reversed and NG restored, which coincides with the increased activity of antioxidant defences. It is proposed that these novel observations regarding the occurrence of an oxidative burst are related to the high basal ROS levels and the high membrane content retained in the mature embryo tissues.