成熟脱水对种子发育和萌发的作用

成熟脱水是正常性种子发育的末端事件。种子在成熟时胚的脱水耐性增加;当种子萌发时胚变得不耐脱水。当种子获得脱水耐性时,糖、蛋白质和抗氧化防御系统等保护性物质积累;当脱水耐性丧失时,这些物质被降解。成熟脱水是种子从发育过程向萌发过程转变的“开关”,它降低发育的蛋白质和mRNA的合成,终止发育事件和促进萌发事件。顽拗性种子不经历成熟脱水的发育阶段,对脱水高度敏感。     

与种子耐脱水性有关的基础物质研究进展

耐脱水的获得和维持与种子的类型有关,正常型种子耐脱水,而顽拗形种子对脱水高度敏感。正常型种子的脱水耐性随发育过程而变化,种子成熟时胚的脱水耐性增强,而萌发时胚变为不耐脱水。当种子获得脱水耐性时,糖、LEA蛋白质和抗氧化防御系统等保护性物质积累。但脱水耐性是一种复杂的数量的特性,任何一种单一的机制都不能充分地解释脱水耐性,各种保护性物质协同调节脱水耐性。本文综述了近几年来关于种子耐脱水性与保护性物质相关性的研究进展。     

与种子耐脱水性有关的基础物质研究进展

耐脱水的获得和维持与种子的类型有关,正常型种子耐脱水,而顽拗形种子对脱水高度敏感,正常型种子的脱水耐性随发良[过程而变化,种子成熟时胚的脱水耐性增强,其萌发时胚变为不耐脱水,当种子获得脱水耐性时,糖,LEA蛋白质和抗氧化防御系统等保护性物质积累,但脱水耐性是一种复杂的数量的特性,任何一种单一的机制都不能 充分地解释脱水耐性,各种保护性物质协同调节脱水耐性,本文综述了近几年来关于种子耐脱水性与保护性物质相关性的研究进展。     

小芸木胚发育过程中脱水耐性的变化

对热带植物小芸木的种子和胚在整个发育阶段的形态学特征、含水量、萌发率和电导率进行了研究。结果显示:(1)小芸木种子在发育过程中形态学特征、电导率、萌发率和胚的脱水耐性有明显的变化;(2)在55~80 d种子鲜重和干重逐渐增加,随后又稍微降低,整个发育过程未经历明显的成熟脱水阶段;种子萌发率从55~85 d逐渐达到最大,随后又稍微降低;(3)胚的脱水耐性从55~90 d逐渐增加,于85~90 d达到最大,在95~103 d时又有所下降。表明小芸木胚的最大脱水耐性的获得时间与种子干物质积累达到最大的时间一致。     

种子脱水耐性及其与种子类型和发育阶段的相关性

种子脱水耐性是种子发育过程中获得的综合特性,是判断种子贮藏持性的一个重要依据。当种子获得脱水耐性时,生理,形态和结构会发生相应的变化,包括糖,蛋白质,脂类和抗氧化系统等保护性物质的合成,各种保护性物质不是单独作用的,而是协同调节种子的脱水耐性。不同的植物种子,其脱水耐性不同,并且随着种子的发育而变化。关于种子脱水耐性的获得,主要有2种观点,一种认为是数量性状,另一种认为是突变性状。种子库收集种子保存时,适时采集和适度脱水才能有效地延长种子的贮藏寿命。     

玉米胚发育过程中脱水耐性的变化

对离体玉米胚脱水耐性的变化以及不同脱水速率对其脱水耐性的影响进行了研究。授粉后16d的玉米胚能耐轻微脱水,含水量从1.45降低到0.28gH2Og-1DW时胚的萌发率为100%,但含水量低于0.1gH2Og-1DW时胚死亡。胚的脱水耐性随着发育逐渐加强,表现为电解质渗漏速率逐渐降低,萌发率和幼苗干重逐渐增加。授粉后20d胚内超氧化物歧化酶(SOD)和抗坏血酸过氧化物酶(APX)活性较高,过氧化氢酶(CAT)活性较低;授粉后24d,这些酶的活性与授粉后20d的正好相反。脂质过氧化产物丙二醛(MDA)在种子发育过程中呈下降趋势。不同脱水速率明显地影响胚的脱水耐性:在慢速脱水到含水量0.1~0.18gH2Og-1DW时,胚的萌发率和幼苗干重比快速脱水高,电解质渗漏速率比快速脱水低;在快速脱水条件下胚中的SOD、APX活性和MDA含量也比慢速脱水高;CAT活性的变化不明显。     

假槟榔种子脱水耐性的发育变化

对假槟榔（Archontophoenix alexandrae）种子和胚发育过程中脱水耐性的变化、不同脱水速率对脱水耐性的影响及种子的萌发和贮藏特性进行了研究。种子含水量在花后55～70d逐步降低,随后不再变化,并保持在较高水平（37％）;花后90d的种子获得最大干重。花后60d后种子获得萌发能力,花后70d达到最大值。在交替光照下（14h光照,10h黑暗,12μnmol m^-2s^-1）,种子在15℃～40℃下均能萌发,其萌发的适宜温度范围为30℃～35℃;但光照对种子的萌发有较大的抑制作用。种子和胚在花后55～90d,脱水耐性逐渐增强;花后90d种子和胚的脱水耐性最强,此时种子和胚的半致死含水量分别为0．18g／g和0．3g／g。脱水至相同含水量,快速脱水的种子的存活率明显高于慢速脱水。无论是否进行脱水处理,-18℃下贮藏1个月后,种子均丧失萌发能力;在4℃,10℃和15℃下,适度脱水能延长种子的贮藏寿命。假槟榔种子不耐脱水,不适合在低温、低含水量条件下长期贮藏,属顽拗性种子。     

顽拗性黄皮胚脱水过程中核酸、蛋白质代谢的变化

黄皮种子发育晚期,胚内核酸、蛋白质合成能力增强,而花生胚的核酸、蛋白质合成能力在发育晚期则呈下降趋势。黄皮胚的发育在达到生理成熟后维持着活跃的生理代谢并转入萌发状态;而花生胚的代谢活性逐步降低并转入生理静止状态。脱水处理引起生理成熟期黄皮胚核酸、蛋白质合成能力急剧下降,核酸水解酶活性增强。不同程度脱水的黄皮胚吸胀２４ｈ,核酸、蛋白质的合成能力随脱水程度的加深而降低;生物大分子代谢能力的变化是顽拗性     

萌发玉米种子脱水耐性与胚根细胞超微结构的变化

玉米种子脱水试验表明,25℃下萌发24h种子脱水耐性开始丧失,丧失50％和100％的时间分别为33h、58h。萌发过程中随着吸胀时间增加,玉米种子脱水耐性逐步丧失。显微观察显示,种子吸胀过程中,胚根细胞的贮藏物质逐步减少,线粒体等细胞器的分化程度则不断提高,尤其是脂类物质的分解程度与脱水耐性变化的关系似乎更明显。     

沙芥种子发育过程中的脱水耐性

以发育过程中经脱水和未脱水处理的沙芥种子为试验材料,测定了其含水量、萌发率和抗氧化酶系统,探讨了沙芥种子脱水耐性与抗氧化系统之间的关系。结果表明：在20～60DAF,沙芥种子含水量逐渐下降,干重逐渐增加;60DAF种子具有萌发能力,萌发率为24％;且脱水可促进沙芥种子的萌发,人工脱水至含水量为12％和5％时,萌发率分别为56％和44％,自然脱水至含水量为12％和5％时,萌发率分别为52％和60％;发育过程中沙芥种子SOD活性逐渐降低,而CAT、POD、LOX活性以及MDA含量均呈上升趋势;在脱水过程中,随着种子含水量的下降SOD活性逐渐降低,CAT和LOX活性逐渐升高,而POD活性呈先降低后升高的变化趋势。脱水后,种子中MDA含量均高于CK。60DAF的沙芥种子已获得脱水耐性。     

种子顽拗性的形成机理及其保存技术

根据种子的脱水行为将种子分为正常性种子、顽拗性种子和中间性种子。顽拗性种子在发育的末期不经历成熟脱水．脱落时有相对高的含水量,并且对低温和脱水干燥非常敏感。在自然界,顽拗性种子存在一个连续群,即低度、中度和高度顽拗性种子,其差异在于对脱水伤害的敏感程度。影响种子顽拗性的因素,既有种子本身的生理生化物质基础,也有种子在母株上发育过程中所经受的外界环境的影响。目前,对种子脱水耐性的分子机制及其保存技术研究得较多。本文综述了有关顽拗性种子研究的近期进展。     

Maturation proteins associated with desiccation tolerance in soybean

A set of proteins that accumulates late in embryogenesis (Lea proteins) has been hypothesized to have a role in protecting the mature seed against desiccation damage. A possible correlation between their presence and the desiccation tolerant state in soybean seeds (Glycine max L. Chippewa) was tested. Proteins that showed the same temporal pattern of expression as that reported for Lea proteins were identified in the axes of soybean. They were distinct from the known storage proteins and were resistant to heat coagulation. The level of these “maturation” proteins was closely correlated with desiccation tolerance both in the naturally developing and in the germinating seed: increasing at 44 days after flowering, when desiccation tolerance was achieved, and decreasing after 18 hours of imbibition, when desiccation tolerance was lost. During imbibition, 100 micromolar abscisic acid or Polyethylene glycol-6000 (−0.6 megapascals) delayed disappearance of the maturation proteins, loss of desiccation tolerance, and germination. During maturation, desiccation tolerance was prematurely induced when excised seeds were dried slowly but not when seeds were held for an equivalent time at high relative humidity. In contrast, maturation proteins were induced under both conditions. We conclude that maturation proteins may contribute to desiccation tolerance of soybean seeds, though they may not be sufficient to induce tolerance by themselves.     

Effects of Drying Rates on the Desiccation Tolerance of Citrus maxima ‘Feizhouyou’ Seeds

The effects of drying rates on the desiccation tolerance of Citrus maxima ‘Feizhouyou’ seeds at different developmental stages were studied in this paper. For seeds harvested at 130 days after anthesis (DAA), 190 DAA, 245 DAA and 275 DAA, slow dried seeds had higher desiccation tolerance than those rapid dried, with difference at significant level (P < 005). However, such improvement was little for seeds harvested at 155 DAA and 220 DAA, indicating that effect of drying rate on desiccation tolerance depends on seed developmental stages. These results accorded with previous reports on orthodox soybean seeds and maize embrys. It was suggested that the effects of drying rate on desiccation tolerance of intermediate Citrus maxima ‘Feizhouyou’ seeds mainly resulted from expression and accumulation of some desiccation related proteins induced by slow drying. On the required genetic basis, desiccation tolerance in seeds can be induced only at suitable seed developmental stages.     

The role of sugar, vitrification and membrane phase transition in seed desiccation tolerance

In a search for the mechanism of desiccation tolerance, a comparison was made between orthodox (desiccation-tolerant) soybean ( Glycine max [L.] Merrill) and recalcitrant (desiccation-intolerant) red oak ( Quercus rubra L.) seeds. During the maturation of soybean seeds, desiccation tolerance of seed axes is correlated with increases in sucrose, raffinose and stachyose. In cotyledons of mature oak seeds, sucrose levels are equal to those in mature soybeans, but oligosaccharides are absent. By using the thermally stimulated current method, we observed the glassy state in dry soybean seeds during maturation. Oak cotyledons showed the same phase diagram for the glass transition as did mature soybeans. By using X-ray diffraction, we found the maturation of soybeans to be associated with an increased ability of membranes to retain the liquid crystalline phase upon drying, whereas the mature oak cotyledonary tissue existed in the gel phase under similar dry conditions. These findings lead to the conclusion that the glassy state is not sufficient for desiccation tolerance, whereas the ability of membranes to retain the liquid crystalline phase does correlate with desiccation tolerance. An important role for soluble sugars in desiccation tolerance is confirmed, as well as their relevance to membrane phase changes. However, the presence of soluble sugars does not adequately explain the nature of desiccation tolerance in these seeds.     

Temporal profiling of the heat-stable proteome during late maturation of Medicago truncatula seeds identifies a restricted subset of late embryogenesis abundant proteins associated with longevity

Developing seeds accumulate late embryogenesis abundant (LEA) proteins, a family of intrinsically disordered and hydrophilic proteins that confer cellular protection upon stress. Many different LEA proteins exist in seeds, but their relative contribution to seed desiccation tolerance or longevity (duration of survival) is not yet investigated. To address this, a reference map of LEA proteins was established by proteomics on a hydrophilic protein fraction from mature Medicago truncatula seeds and identified 35 polypeptides encoded by 16 LEA genes. Spatial and temporal expression profiles of the LEA polypeptides were obtained during the long maturation phase during which desiccation tolerance and longevity are sequentially acquired until pod abscission and final maturation drying occurs. Five LEA polypeptides, representing 6% of the total LEA intensity, accumulated upon acquisition of desiccation tolerance. The gradual 30-fold increase in longevity correlated with the accumulation of four LEA polypeptides, representing 35% of LEA in mature seeds, and with two chaperone-related polypeptides. The majority of LEA polypeptides increased around pod abscission during final maturation drying. The differential accumulation profiles of the LEA polypeptides suggest different roles in seed physiology, with a small subset of LEA and other proteins with chaperone-like functions correlating with desiccation tolerance and longevity.     

Preliminary Study on Development,Germination and Desiccation Tolerance of Jatropha curcas (Euphorbiaceae)

The developmental changes in morphology and germinability of Jatropha curcas seeds, effects of temperature and light on seed germination , and changes in desiccation tolerance of mature seeds were studied in this paper. The results indicated that J. curcas seeds reached a physiological maturation at 58 days after anthesis, and that seed germination percentage reached a peak at physiological maturation, and then decreased. The optimal germination temperature was 25 to 30℃. J. curcas seeds were insensitive to dehydration at physiological maturation . There was not a notable effect of light on seed germination. Therefore, J. curcas seed was a non-photoblastic and orthodox seeds .     

Proteomics of desiccation tolerance during development and germination of maize embryos

Maize seeds were used to identify the key embryo proteins involved in desiccation tolerance during development and germination. Immature maize embryos (28N) during development and mature embryos imbibed for 72 h (72HN) are desiccation sensitive. Mature maize embryos (52N) during development are desiccation tolerant. Thiobarbituric acid reactive substance and hydrogen peroxide contents decreased and increased with acquisition and loss of desiccation tolerance, respectively. A total of 111 protein spots changed significantly (1.5 fold increase/decrease) in desiccation-tolerant and -sensitive embryos before (28N, 52N and 72HN) and after (28D, 52D and 72HD) dehydration. Nine pre-dominantly proteins, 17.4 kDa Class I heat shock protein 3, late embryogenesis abundant protein EMB564, outer membrane protein, globulin 2, TPA:putative cystatin, NBS-LRR resistance-like protein RGC456, stress responsive protein, major allergen Bet v 1.01C and proteasome subunit alpha type 1, accumulated during embryo maturation, decreased during germination and increased in desiccation-tolerant embryos during desiccation. Two proteins, Rhd6-like 2 and low-molecular-weight heat shock protein precursor, showed the inverse pattern. We infer that these eleven proteins are involved in seed desiccation tolerance. We conclude that desiccation-tolerant embryos make more economical use of their resources to accumulate protective molecules and antioxidant systems to deal with maturation drying and desiccation treatment.