From intracellular signaling networks to cell death: the dual role of reactive oxygen species in seed physiology

Reactive Oxygen Species (ROS) are continuously produced during seed development, from embryogenesis to germination, but also during seed storage. ROS play a dual role in seed physiology behaving, on the one hand, as actors of cellular signaling pathways and, on the other hand, as toxic products that accumulate under stress conditions. ROS, provided that their amount is tightly regulated by the balance between production and scavenging, appear now as being beneficial for germination, and in particular to act as a positive signal for seed dormancy release. Such an effect might result from the interplay between ROS and hormone signaling pathways thus leading to changes in gene expression or in cellular redox status. We also propose that changes in ROS homeostasis would play a role in perception of environmental factors by seeds during their germination, and thus act as a signal controlling the completion of germination. However, uncontrolled accumulation of ROS is likely to occur during seed aging or seed desiccation thus leading to oxidative damage toward a wide range of biomolecules and ultimately to necroses and cell death. We present here the concept of the "oxidative window for germination", which restricts the occurrence of the cellular events associated with germination to a critical range of ROS level, enclosed by lower and higher limits. Above or below the "oxidative window for germination", weak or high amounts of ROS, respectively, would not permit progress toward germination.     

Seed birth to death: dual functions of reactive oxygen species in seed physiology

Background Reactive oxygen species (ROS) are considered to be detrimental to seed viability. However, recent studies have demonstrated that ROS have key roles in seed germination particularly in the release of seed dormancy and embryogenesis, as well as in protection from pathogens.Scope This review considers the functions of ROS in seed physiology. ROS are present in all cells and at all phases of the seed life cycle. ROS accumulation is important in breaking seed dormancy, and stimulating seed germination and protection from pathogens. However, excessive ROS accumulation can be detrimental. Therefore, knowledge of the mechanisms by which ROS influence seed physiology will provide insights that may not only allow the development of seed quality markers but also help us understand how dormancy can be broken in several recalcitrant species.Conclusions Reactive oxygen species have a dual role in seed physiology. Understanding the relative importance of beneficial and detrimental effects of ROS provides great scope for the improvement and maintenance of seed vigour and quality, factors that may ultimately increase crop yields.     

Analysis of dynamic protein carbonylation in rice embryo during germination through AP‐SWATH

Seed germination is an important aspect of the plant life cycle, during which, reactive oxygen species (ROS) accumulate. The accumulation of ROS results in an increase in protein oxidation of which carbonylation is the most canonical one. However, there is insufficient information concerning protein oxidation, especially carbonylation and its contribution to seed germination. In this study, biotin hydrazide labeled chromatography combined with sequential window acquisition of all theoretical fragment ion spectra (SWATH) method was used to analyze the dynamic pattern of protein carbonylation in rice embryos during germination. A total of 1872 unique proteins were quantified, among which 288 carbonylated peptides corresponding to 144 proteins were determined based on the filtering through mass shifts of modified amino acids. In addition, 66 carbonylated proteins were further analyzed based on their carbonylation intensity in four stages of germination. These identified carbonylated proteins were mainly involved in maintaining the levels of ROS, abscisic acid and seed reserves. Remarkably, a peroxiredoxin was found with 23 unique carbonylated peptides, and the expression of which was consistent with its increased activity. This study describes the dynamic pattern of carbonylated proteins during seed germination, and may help to further understand the biochemical mechanisms on this process.     

Reactive oxygen species and seed germination

Reactive oxygen species (ROS) are continuously produced by the metabolically active cells of seeds, and apparently play important roles in biological processes such as germination and dormancy. Germination and ROS accumulation appear to be linked, and seed germination success may be closely associated with internal ROS contents and the activities of ROS-scavenging systems. Although ROS were long considered hazardous molecules, their functions as cell signaling compounds are now well established and widely studied in plants. In seeds, ROS have important roles in endosperm weakening, the mobilization of seed reserves, protection against pathogens, and programmed cell death. ROS may also function as messengers or transmitters of environmental cues during seed germination. Little is currently known, however, about ROS biochemistry or their functions or the signaling pathways during these processes, which are to be considered in the present review.     

种子老化的生理生化与分子机理研究进展

种子作为植物遗传资源的有效保存体以及重要的种质创新原料,其老化或者劣变将直接导致发芽率、活力、生活力降低,抑制种胚正常发育以及幼苗生长,由此造成植物生产水平及其品质大幅下降。这也将进一步涉及因种质资源匮乏、土壤种子库系统功能紊乱所引发的全球生物多样性减小、草地退化和荒漠化加剧等生态危机问题。对种子老化生理生化特性和分子机理等研究进行了综述。总结了近年来关于种子老化涉及的理化反应包括保护酶活性的改变、核酸以及蛋白质的分解、内源激素的消长、质膜完整性降低等相关研究;并从蛋白代谢、核酸代谢、种子含水量以及基因重组等多角度总结和阐述了与老化机理有关的最新研究观点,以期为种子老化、种子活力修复和种子寿命延长等机理研究提供基础理论参考。目前对种子老化的研究多集中于传统的生理生化过程和内外影响因子相对独立变化的片段性研究,缺乏系统综合的多层面体系研究。种子作为生命体,随着探讨生命衰老机理的生物技术日新月异,通过蛋白组学、酶学、基因工程技术、转录组测序等新技术的应用,必将对未来种子老化机理机制的揭示有突破性推进作用。     

Genetic differences in seed longevity of various Arabidopsis mutants

Seeds gradually lose their viability during dry storage. The damage that occurs at the biochemical level can alter the seed physiological status and is affected by the storage conditions of the seeds. Although these environmental conditions controlling loss of viability have been investigated frequently, little information is available on the genetics of seed longevity. Using Arabidopsis mutants in defined developmental or biochemical pathways such as those affected in seed coat composition, seed dormancy, hormone function and control of oxidative stress, we tried to gain insight into the genes and mechanisms controlling viability of stored seeds. Mutations like abscisic acid insensitive3 ( abi3 ) as well as abscisic acid deficient1 ( aba1 ) show reduced longevity, which may be partially related to the seed dormancy phenotype of these mutants. Mutants with seed coat alterations, especially aberrant tests shape ( ats ), showed a stronger reduction in germination percentage after storage, indicating the importance of a 'functional' seed coat for seed longevity. A specific emphasis was placed on mutants affected in dealing with Reactive Oxygen Species (ROS). Because several pathways are involved in protection against ROS and because gene redundancy is a common feature in Arabidopsis , 'double' mutants were generated. These 'double' mutants and the corresponding single mutants were subjected to a controlled deterioration test (CDT) and a germination assay on hydrogen peroxide (H₂O₂) after prolonged storage at two relative humidities. CDT and germination on H₂O₂ affected all genotypes, although it appears that other effects like genetic background are more important than the deficiencies in the ROS scavenging pathway. Explanations for this limited effect of mutations affecting ROS scavenging are discussed.     

植物细胞活性氧种类、代谢及其信号转导

越来越明显的证据表明,植物体十分活跃的产生着活性氧并将之作为信号分子、进而控制着诸如细胞程序性死亡、非生物胁迫响应、病原体防御和系统信号等生命过程,而不仅是传统意义上的活性氧是有氧代谢的附产物。日益增多的证据显示,由脱落酸、水杨酸、茉莉酸与乙烯以及活性氧所调节的激素信号途径,在生物和非生物胁迫信号的“交谈”中起重要作用。活性氧最初被认为是动物吞噬细胞在宿主防御反应时所释放的附产物,现在的研究清楚的表明,活性氧在动物和植物细胞信号途径中均起作用。活性氧可以诱导细胞程序性死亡或坏死、可以诱导或抑制许多基因的表达,也可以激活上述级联信号。近来生物化学与遗传学研究证实过氧化氢是介导植物生物胁迫与非生物胁迫的信号分子,过氧化氢的合成与作用似乎与一氧化氮有关系。过氧化氢所调节的下游信号包括钙“动员”、蛋白磷酸化和基因表达等。     

Oxidative metabolism-related changes during germination of mono maple (<Emphasis Type="Italic">Acer mono</Emphasis> Maxim.) seeds under seasonal frozen soil

The influence of seasonal frozen soil and buried depth on germination of mono maple (Acer mono Maxim.) seeds was studied in field conditions in winter in a sub-alpine region. Mono maple seeds almost lost their ability to germinate in non-freezing soil, while seasonal frozen soil treatments facilitated the germination accompanied with a progressive accumulation of reactive oxygen species (ROS). The result indicates that ROS may act as a positive signal for seed dormancy. However, exceeding accumulation of ROS led to decrease in germination rate. We suggest that the shift from a signaling to a deleterious role may be related to the accumulation of these ROS above a threshold level that leads to various cellular alterations and damage. The enhanced activities of antioxidant enzymes appear to be more closely related to freezing tolerance, because of their ability to scavenge ROS to avoid deleterious events. Seasonal frozen soil was beneficial in accelerating the germination of mono maple seeds. However, a slight increase of freezing temperature may have also facilitated the germination of mono maple seeds by enhanced activities of antioxidant enzymes. Hence, moderate winter warming may be beneficial to mono maple regeneration due to the improvement of seed germination, but the disappearance of seasonal frozen soil may lead to germination failure of the mono maple seeds.     

Allantoin Alleviates Seed Germination Thermoinhibition in <i>Arabidopsis</i>

Allantoin as the metabolite of purine catabolism can store and remobilize nitrogen for plant growth and development. However, emerging evidence suggests it also contributes to plant tolerance to stress response through altering abscisic acid (ABA) and reducing reactive oxygen species (ROS) level. 1-CYS PEROXIREDOXIN (PER1) is a seed-specific antioxidant that enhances seed longevity through scavenging ROS over-accumulation. High temperature (HT) suppresses seed germination and induces seed secondary dormancy, called as seed germination thermoinhibition. However, the mechanism that allantoin and PER1 regulate seed germination thermoinhibition remains unknown. In this study, we reported that allantoin treatment enhances seed germination under HT stress. Consistently, the aln mutants displayed higher seed germination, as well as more accumulation of endogenous allantoin, than that of wild-type control. Further biochemical and genetic analyses showed that allantoin reduces ABA content under HT, and allantoin targets PER1 to efficiently scavenge HT-induced ROS accumulation, meanwhile, the function of allantoin requires PER1 during seed gemination thermotolerance. Collectively, our finding proposes a novel function of allantoin in enhancing seed germination tolerance to HT, and uncovers the underlying mechanism by which allantoin regulates seed germination through altering ABA metabolism and PER1-mediated ROS level under HT stress.     

Nitric oxide enhances desiccation tolerance of recalcitrant Antiaris toxicaria seeds via protein S-nitrosylation and carbonylation

The viability of recalcitrant seeds is lost following stress from either drying or freezing. Reactive oxygen species (ROS) resulting from uncontrolled metabolic activity are likely responsible for seed sensitivity to drying. Nitric oxide (NO) and the ascorbate-glutathione cycle can be used for the detoxification of ROS, but their roles in the seed response to desiccation remain poorly understood. Here, we report that desiccation induces rapid accumulation of H(2)O(2), which blocks recalcitrant Antiaris toxicaria seed germination; however, pretreatment with NO increases the activity of antioxidant ascorbate-glutathione pathway enzymes and metabolites, diminishes H(2)O(2) production and assuages the inhibitory effects of desiccation on seed germination. Desiccation increases the protein carbonylation levels and reduces protein S-nitrosylation of these antioxidant enzymes; these effects can be reversed with NO treatment. Antioxidant protein S-nitrosylation levels can be further increased by the application of S-nitrosoglutathione reductase inhibitors, which further enhances NO-induced seed germination rates after desiccation and reduces desiccation-induced H(2)O(2) accumulation. These findings suggest that NO reinforces recalcitrant seed desiccation tolerance by regulating antioxidant enzyme activities to stabilize H(2)O(2) accumulation at an appropriate concentration. During this process, protein carbonylation and S-nitrosylation patterns are used as a specific molecular switch to control antioxidant enzyme activities.     

Ascorbic acid and reactive oxygen species are involved in the inhibition of seed germination by abscisic acid in rice seeds

The antagonism between abscisic acid (ABA) and gibberellin (GA) plays a key role in controlling seed germination, but the mechanism of antagonism during this process is not known. The possible links among ABA, reactive oxygen species (ROS), ascorbic acid (ASC), and GA during rice seed germination were investigated. Unlike in non-seed tissues where ROS production is increased by ABA, ABA reduced ROS production in imbibed rice seeds, especially in the embryo region. Such reduced ROS also led to an inhibition of ASC production. GA accumulation was also suppressed by a reduced ROS and ASC level, which was indicated by the inhibited expression of GA biosynthesis genes, amylase genes, and enzyme activity. Application of exogenous ASC can partially rescue seed germination from ABA treatment. Production of ASC, which acts as a substrate in GA biosynthesis, was significantly inhibited by lycorine which thus suppressed the accumulation of GA. Consequently, expression of GA biosynthesis genes was suppressed by the low levels of ROS and ASC in ABA-treated seeds. It can be concluded that ABA regulates seed germination in multiple dimensions. ROS and ASC are involved in its inhibition of GA biosynthesis.     

Proanthocyanidins accelerate the germination of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) seeds

Proanthocyanidins (PAs) are the end products of the flavonoid biosynthetic pathway in many seeds, but their biological function is rarely unknown during seed germination. In the present study, we observed that PAs pretreatment accelerated cucumber seeds germination with maximum efficiency at 0.15% by measuring germination percentage and radical length. Using inhibitors of abscisic acid (ABA), gibberellins (GA) and alternative oxidase (AOX) and H₂O₂ scavenger pretreatment and gene expression analysis, we found that the accelerated effect of 0.15% PAs on seed germination was due to the decreased ABA biogenesis and enhanced GA production. ROS are induced by PAs pretreatment. Then, the enhanced ROS contributed to GA and ethylene accumulation and ABA decrease in seeds. Moreover, the improvement of GA was involved in the further induction of antioxidant enzymes activities. Therefore, our findings uncover a novel role of PAs in seed germination and clarify the relationships between ROS, ABA, GA and ethylene during seed germination.     

Proteomic profiling and redox status alteration of recalcitrant tea (<Emphasis Type="Italic">Camellia sinensis</Emphasis>) seed in response to desiccation

Tea seed is believed to be recalcitrant based on its sensitivity to chilling or drying stress. Reactive oxygen species (ROS) and alterations in cytosolic redox status have been implicated in intolerance to desiccation by recalcitrant seed, but there is little information available regarding how ROS are regulated in seeds susceptible to drying stress. We investigated changes in protein expression and activity in tea embryo in response to desiccation using physiological and proteomic methods. Results showed that desiccation treatment dramatically induced the accumulation of H₂O₂ in tea embryos, accompanied by increased activities of antioxidant enzymes like ascorbate peroxidase (APX) and superoxide dismutase (SOD). Proteomic analyses also demonstrated that 23 proteins associated with defense response, metabolism and redox status were up-regulated following desiccation. Increase in antioxidants, ascorbic acid (AsA) and catalase (CAT) (H₂O₂ scavengers) partially assuaged desiccation damage to tea seed, resulting in improved germination rates. Higher accumulation of H₂O₂ aggravated desiccation damage to seeds leading to lower germination activity. We propose that desiccation causes an over-accumulation of ROS that are not efficiently scavenged by increased levels of antioxidant enzymes. High levels of ROS alter the redox status and are detrimental to seed viability. Reducing ROS to appropriate concentrations is an efficient way to reduce desiccation damage and improve germination rates of recalcitrant seeds.     

PI3K调控水稻种子萌发的机理研究

PI3K在植物的正常生长和胁迫应答过程中发挥着很重要的功能.在本文中,我们证实了在种子萌发过程中,PI3K促进了种子的萌发.通过RT-PCR的分析,我们发现,PI3K在整个过程中,伴随着吸胀时间的延长,表达量逐渐上升.而PI3K抑制剂的处理,证实了萌发过程中,PI3K促萌发的效应.DCF检测ROS的产生发现,在萌发的过程中,PBK促进了ROS的生成.PI3K产物PI3P亚细胞定位的分析发现,PI3P主要定位于膜结构区域.总之,本文借助于光学分子成像技术发现PI3K在种子萌发过程中的重要作用,提出PI3K通过调控ROS生成促进种子萌发的重要机理.     

Plant peroxiredoxins: alternative hydroperoxide scavenging enzymes

The role of plant peroxiredoxins in the detoxification systems is discussed in relation with the existence of many isoforms of this protein in distinct plant compartments. Phylogenetic analyses indicate that plant peroxiredoxins can be divided into four classes. Two of these classes correspond to chloroplastic enzymes. All isoforms contain at least one conserved catalytic cysteine. The enzymes belonging to the 1-Cys Prx class seem to be seed restricted and to play a role of detoxification during the germination process. At least one putative cytosolic isoform can use both thioredoxin and glutaredoxin as an electron donor, but the chloroplastic isoforms characterized depend on reduced thioredoxin. Mutagenesis and plant transformation studies support the proposal that the chloroplastic peroxiredoxins play an important role in combating the ROS species generated at the level of the chloroplastic electron transfer chain. This revised version was published online in June 2006 with corrections to the Cover Date.