*Summary At harvest, sunflower (Helianthus annuus L.) seeds are dormant and unable to germinate at temperatures below 15 � C. Seed storage in the dry state, known as after-ripening, is associated with an alleviation of embryonic dormancy allowing subsequent germination at suboptimal temperatures. To identify the process by which dormancy is broken during after-ripening, we focused on the role of reactive oxygen species (ROS) in this phenomenon. After-ripening entailed a progressive accumulation of ROS, namely superoxide anions and hydrogen peroxide, in cells of embryonic axes. This accumulation, which was investigated at the cellular level by electron microscopy, occurred concomitantly with lipid peroxidation and oxidation (carbonylation) of specific embryo proteins. Incubation of dormant seeds for 3 h in the presence of hydrogen cyanide (a compound that breaks dormancy) or methylviologen (a ROS-generating compound) also released dormancy and caused the oxidation of a specific set of embryo proteins. From these observations, we propose a novel mechanism for seed dormancy alleviation. This mechanism involves ROS production and targeted changes in protein carbonylation patterns.

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ROS production and protein oxidation as a novel mechanism for seed dormancy alleviation.

Massive seed consumption is typical in many deserts. The “escape” or “protection” strategies of seed dispersal are important, as they prevent massive seed consumption. The more extreme the desert, the more unpredictable the low amounts and distribution of the rains as well as the beginning and length of the season or seasons with rains. Seeds, which have the highest resistance to extreme environmental conditions, develop during germination into seedlings, which are the most sensitive. Therefore, germination of parts of the seed population at their respective proper times spreads the risk over time and is thus very important for plant survival, especially in those plants inhabiting the more extreme deserts. Each of the plant species studied was found to have its own survival strategies of seed dispersal and germination. At least two extreme seed-dispersal and germination strategies have been observed: 1) the “escape” seed dispersal and “opportunistic” portioned seed-germination strategies, such as inSchismus arabicas andSpergularia
 diandra, and 2) the “cautious” portioned dispersal by rain of the protected seeds, such as inAsteriscus hierochunticus and portioned dispersal and rapid germination strategies such as inBlepharis spp. The fate of future generations, as far as the germinability of seeds of some species is concerned, depends on the influences of maternal and environmental factors when the seeds are still on the mother plant, mostly during the final stage of seed maturation, as inTrigonella arabica. It may even depend on the position of the caryopsis from which the mother plant originated, as inAegilops geniculata.
 The dry post-maturation conditions and the environmental factors during seed wetting and germination may also affect the percentage of seed germination, as inSchismus arabicus.

Strategies of seed dispersal and germination in plants inhabiting deserts

The physiological dormancy of sunflower (Helianthus annuus) embryos can be overcome during dry storage (after-ripening) or by applying exogenous ethylene or hydrogen cyanide (HCN) during imbibition. The aim of this work was to provide a comprehensive model, based on oxidative signaling by reactive oxygen species (ROS), for explaining the cellular mode of action of HCN in dormancy alleviation. Beneficial HCN effect on germination of dormant embryos is associated with a marked increase in hydrogen peroxide and superoxide anion generation in the embryonic axes. It is mimicked by the ROS-generating compounds methylviologen and menadione but suppressed by ROS scavengers. This increase results from an inhibition of catalase and superoxide dismutase activities and also involves activation of NADPH oxidase. However, it is not related to lipid reserve degradation or gluconeogenesis and not associated with marked changes in the cellular redox status controlled by the glutathione/glutathione disulfide couple. The expression of genes related to ROS production (NADPHox, POX, AO1, and AO2) and signaling (MAPK6, Ser/ThrPK, CaM, and PTP) is differentially affected by dormancy alleviation either during after-ripening or by HCN treatment, and the effect of cyanide on gene expression is likely to be mediated by ROS. It is also demonstrated that HCN and ROS both activate similarly ERF1, a component of the ethylene signaling pathway. We propose that ROS play a key role in the control of sunflower seed germination and are second messengers of cyanide in seed dormancy release.

http://www.seedbiology.de/pdf/Oracz2009.pdf

The Mechanisms Involved in Seed Dormancy Alleviation by Hydrogen Cyanide Unravel the Role of Reactive Oxygen Species as Key Factors of Cellular Signaling during Germination

Seed Research-Past,Present and Future.- Use of Transgenic Plants for Studies of Seed-Specific Gene Expression.- Impact of Simulated Drought Stress on Protein Body Cycle in Radicles of Developing and Germinating Cotton Seeds.- Synthetic Seed for Clonal Production of Crop Plants.- Synthesis of Acid-Soluble,ABA-Inducible Proteins in Wheat Embryos from Dormant Grain.- Hormones, Genetic Mutants and Seed Development.- Membrane Behavior in Drought and Its Physiological Significance.- The Basis of Recalcitrant Seed Behaviour: Cell Biology of the Homoiohydrous Seed Condition.- Phytin Synthesis and Deposition.- Calcium-Regulated Metabolism in Seed Germination.- Transport and Targeting of Proteins to Protein Storage Vacuoles (Protein Bodies) in Developing Seeds.- Biochemical Adaptations to Anoxia in Rice and Echinochloa Seeds.- Some Aspects of Metabolic Regulation of Seed Germination and Dormancy.- Phytochrome and Sensitization in Germination Control.- The Role of Light and Nitrate in Seed Germination.- Water-Impermeable Seed Coverings as Barriers to Germination.- Control of Germination and Early Development in Parasitic Angiosperms.- Factors Eliciting the Germination of Photoblastic Kalanchoe Seeds.- Factors Influencing the Efficacy of Dormancy-Breaking Chemicals.- Physiological Mechanisms Involved in Seed Priming.- Germination Research Towards the Nineties: A Summary and Prognosis.

Recent advances in the development and germination of seeds

Seed dormancy, defined as the inability to germinate under favourable conditions, is controlled by abscisic acid (ABA) and gibberellins (GAs). Phytohormone signalling interacts with reactive oxygen species (ROS) signalling regarding diverse aspects of plant physiology and is assumed to be important in dormancy alleviation. Using dormant barley grains that do not germinate at 30 °C in darkness, we analysed ROS content and ROS-processing systems, ABA content and metabolism, GA-responsive genes and genes involved in GA metabolism in response to hydrogen peroxide (H₂O₂) treatment. During after-ripening, the ROS content in the embryo was not affected, while the antioxidant glutathione (GSH) was gradually converted to glutathione disulphide (GSSG). ABA treatment up-regulated catalase activity through transcriptional activation of HvCAT2. Exogenous H₂O₂ partially alleviated dormancy although it was associated with a small increase in embryonic ABA content related to a slight induction of HvNCED transcripts. H₂O₂ treatment did not affect ABA sensitivity but up-regulated the expression of HvExpA11 (GA-induced gene), inhibited the expression of HvGA2ox3 involved in GA catabolism and enhanced the expression of HvGA20ox1 implicated in GA synthesis. In barley, H₂O₂ could be implicated in dormancy alleviation through activation of GA signalling and synthesis rather than repression of ABA signalling.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2011.02298.x

Crosstalk between reactive oxygen species and hormonal signalling pathways regulates grain dormancy in barley.

Dormancy of three cultivars of oat seeds (Avena sativa L.)

Some aspects of metabolic regulation of cereal seed germination and dormancy

Dormant dehusked Avena sativa L. (cv. Moyencourt) caryopses germinated easily at 10°C, but their germination was poor at 25 and 30°C. Ethanol overcame their dormancy and allowed germination at the last two temperatures when given continuously at concentrations ranging from 50 to 200 mm, or at higher concentrations at the beginning of imbibition. Ethanol also promoted germination in hypoxia. Other alcohols which were good substrates for alcohol dehydrogenase (ADH: EC 1.1.1.1), such as butanol-1, propanol-1 and 2-propen-1-ol, had the same stimulatory effect as ethanol, whereas alcohols which could not be oxidized by ADH (propanol-2 and methanol) did not improve germination. Salicylhydroxamic acid (SHAM) did not alter the stimulation of germination induced by ethanol, but 4-methylpyrazole (4-MP, an inhibitor of ADH) completely abolished this stimulation. The improvement of germination by alcohols which were good substrates for ADH was always associated with an increase in oxygen uptake by caryopses, whereas alcohols which were not ADH substrates did not enhance respiration. The stimulation of oxygen uptake induced by ethanol resulted in a decrease in CO2/O2 ratio and was suppressed by 4-MP, but ATP level in embryos was not modified until after the germination of caryopses. All the results obtained seem to demonstrate that the stimulatory action of ethanol on germination of dormant oat caryopses requires its metabolism through ADH and involves an activation of glycolysis and the Krebs cycle.

S. Lecat

Papers

Dormancy of three cultivars of oat seeds (Avena sativa L.)

Some aspects of metabolic regulation of cereal seed germination and dormancy

Stimulation of germination of dormant oat (Avena sativa L.) seeds by ethanol and other alcohols

Effects of gibberellic acid on the germination of dormant oat (Avena sativa L.) seeds as related to temperature, oxygen, and energy metabolism