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。     

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。     

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。     

不同成熟度花生胚萌发时子叶中贮藏蛋白质的降解

花生(Arachis hypogaea L.)汕油71果针入土20d(20 DAP)的种子剥去种皮后,10％的胚可以萌发,至40 DAP发芽率达98％。不同发育时期的花生胚萌发 10d后子叶盐溶蛋白质和花生球蛋白降解表明,20和32 DAP胚萌发后,子叶中这些蛋白质只有部分降解。随着胚成熟度增加,子叶中降解这些蛋白质的能力不断提高。20～40 DAP胚萌发4d时,子叶的BAPAase和GHE活性较低。50～80DAP胚萌发 4d,子叶中上述两种酶均显示较高的活性。     

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。     

种子的成熟脱水与耐脱水性

成熟脱水是大多数植物种子从发育走向成熟必须经历的过程,它使种子的生理代谢从发育转向萌发,耐脱水性是种子在发育过程中获得的一种综合特性。大多数种子在成熟脱水前即已经历从不耐脱水至耐脱水的转变过程。种子耐脱水性与种子类型、耐脱水蛋白、ＡＢＡ的调控及某些非还原性寡糖和抗氧化系统的存在有关。     

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。     

酶促和非酶促抗氧化系统在玉米胚脱水耐性获得中的作用

以发育中的玉米胚为材料,研究了玉米胚脱水耐性的发育变化及其与抗氧化系统之间的关系。结果表明,授粉后18d的胚获得萌发能力,但不耐脱水;授粉后36d的胚开始获得耐脱水能力,并随着发育逐渐增加。随着发育,胚的超氧化物歧化酶（SOD）、抗坏血酸过氧化物酶（APX）、谷胱甘肽还原酶（GR）和脱氢抗坏血酸还原酶（DHAR）的活性逐渐降低,过氧化氢酶（CAT）活性逐渐增加。授粉后16～22d的玉米胚中检测不到抗坏血酸,24d后胚中抗坏血酸的含量显著增加;还原性谷胱甘肽含量在整个发育过程中逐渐增加。脱水胚的SOD、APX和DHAR的活性比对照（未脱水）胚低,而GR和CAT活性在发育早期比对照胚低,在发育中、后期高于对照胚。脱水胚的抗坏血酸和还原性谷胱甘肽含量明显低于对照胚。胚中丙二醛的含量随着发育逐渐下降,脱水胚的丙二醛含量显著高于对照。这些结果说明CAT活性和谷胱甘肽含量的增加以及脂质过氧化产物丙二醛含量的下降与玉米胚脱水耐性的获得密切相关。     

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。     

白木香种子脱水耐性的发育变化及贮藏特性

研究白木香种子发育进程中种子性状、萌发能力和脱水耐性的变化,以及不同光温条件对种子萌发的影响和种子的贮藏特性。结果表明：白木香种子在花后78d获得最大干重,进入生理成熟期,此时萌发率接近最大值;胚在花后57～85d,脱水耐性逐渐增强,并在花后85d获得最大脱水耐性。种子萌发的适宜温度范围为25℃-35℃,光照对种子萌发有一定的抑制作用。新鲜自木香种子（含水量27．45％）在4℃低温条件下贮藏1个月后萌发率仅为30％左右,而含水量7．38％的干燥种子在4℃低温条件下贮藏120d,萌发率仍有53．33％,因此,4℃低温和适度脱水有利于种子短期贮藏。白木香种子能忍耐一定程度的脱水,但干燥至含水量7．50％以下时种子会发生损伤,因此推测白木香种子是一种中间性种子。     

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.     

花生种子耐脱水力的获得与热稳定蛋白的关系

花生（ＡｒａｃｈｉｓｈｙｐｏｇａｅａＬ．）种子的耐脱水能力在果针入土后４５ｄ以后的胚胎发育期逐渐增加,与一组９～１５．５ｋＤ低分子量热稳定蛋白的丰富表达有关。缓慢干燥可以诱导不耐脱水的果针入土后２５ｄ及３５ｄ花生胚获得耐脱水能力并同时诱导胚轴表达这组热稳定蛋白。成熟脱水促进花生胚耐脱水能力的获得,并增加了花生球蛋白的热稳定性。     

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。     

Fourier Transform Infrared Microspectroscopy Detects Changes in Protein Secondary Structure Associated with Desiccation Tolerance in Developing Maize Embryos

Isolated immature maize (Zea mays L.) embryos have been shown to acquire tolerance to rapid drying between 22 and 25 d after pollination (DAP) and to slow drying from 18 DAP onward. To investigate adaptations in protein profile in association with the acquisition of desiccation tolerance in isolated, immature maize embryos, we applied in situ Fourier transform infrared microspectroscopy. In fresh, viable, 20- and 25-DAP embryo axes, the shapes of the different amide-I bands were identical, and this was maintained after flash drying. On rapid drying, the 20-DAP axes had a reduced relative proportion of α-helical protein structure and lost viability. Rapidly dried 25-DAP embryos germinated (74%) and had a protein profile similar to the fresh control axes. On slow drying, the α-helical contribution in both the 20- and 25-DAP embryo axes increased compared with that in the fresh control axes, and survival of desiccation was high. The protein profile in dry, mature axes resembled that after slow drying of the immature axes. Rapid drying resulted in an almost complete loss of membrane integrity in the 20-DAP embryo axes and much less so in the 25-DAP axes. After slow drying, low plasma membrane permeability ensued in both the 20- and 25-DAP axes. We conclude that slow drying of excised, immature embryos leads to an increased proportion of α-helical protein structures in their axes, which coincides with additional tolerance of desiccation stress.     

Onset of desiccation tolerance during development of the barley embryo

We have investigated events which take place in the developing barley (Hordeum vulgare L.) embryo during its acquisition of desiccation tolerance. Excised embryos are capable of precocious germination as early as 8 d after pollination (DAP). At this age, however, they are not capable of resisting a desiccation treatment which induces a loss of 96–98% of their initial water content. At 16 DAP the embryos germinate despite the drastic drying treatment. The pattern of in-vivo and in-vitro proteins synthesized by the developing embryos from 12 DAP (desiccation-intolerant) and 16 DAP (desiccation-tolerant) were compared. A set of 25–30 proteins was identified which is denovo synthesized or enhanced during the developmental period leading to desiccation tolerance. Abscisic acid (ABA; 100 M) applied in vitro for 5 d to 12-DAP embryos induces desiccation tolerance and represses a subset of polypeptides preferentially associated with 16-DAP embryos. During in vitro culture of barley embryos ABA stimulates the appearance of a set of proteins and prevents the precocious germination allowing embryogenesis to continue in vitro. It also suppresses a set of germination-related proteins which appear 4 h after the incubation of the dissected embryo on a germination medium without ABA. Almost all mRNAs remain functional for translation when isolated embryos are dried at the desiccation-intolerant and tolerant stages of embryo development.Abbreviations ABA abscisic acid - DAP days after pollination - GM germination medium - poly(A)RNA polyadenylated RNA - SDS sodium dodecyl sulfate     

Significance of the zygotic seed coat on quiescence and desiccation tolerance of Medicago sativa L. somatic embryos

Summary The influence of the zygotic seed coat on precocious germination and desiccation tolerance of somatic embryos has been studied using alfalfa (Medicago sativa L.). When cultured in contact with somatic embryos, seed coats at certain developmental stages inhibited precocious germination and induced desiccation tolerance in the somatic embryos. Germination of somatic embryos was inhibited by seed coats at the age of 16–26 days after pollination (DAP) and desiccation tolerance was induced after 20–26 DAP. Both phenomena were related to the synthesis of abscisic acid in the seed coat. The absence of a quiescent phase and desiccation tolerance in alfalfa somatic embryos may be related to the lack of developmental control by the seed coat.Abbreviations ABA Abscisic acid - DAP Days after pollination     

Developmental changes in the germinability, desiccation tolerance, hardseededness, and longevity of individual seeds of Trifolium ambiguum

Background and Aims

Using two parental clones of outcrossing Trifolium ambiguum as a potential model system, we examined how during seed development the maternal parent, number of seeds per pod, seed position within the pod, and pod position within the inflorescence influenced individual seed fresh weight, dry weight, water content, germinability, desiccation tolerance, hardseededness, and subsequent longevity of individual seeds.

Methods

Near simultaneous, manual reciprocal crosses were carried out between clonal lines for two experiments. Infructescences were harvested at intervals during seed development. Each individual seed was weighed and then used to determine dry weight or one of the physiological behaviour traits.

Key Results

Whilst population mass maturity was reached at 33–36 days after pollination (DAP), seed-to-seed variation in maximum seed dry weight, when it was achieved, and when maturation drying commenced, was considerable. Individual seeds acquired germinability between 14 and 44 DAP, desiccation tolerance between 30 and 40 DAP, and the capability to become hardseeded between 30 and 47 DAP. The time for viability to fall to 50 % (p₅₀) at 60 % relative humidity and 45 °C increased between 36 and 56 DAP, when the seed coats of most individuals had become dark orange, but declined thereafter. Individual seed f. wt at harvest did not correlate with air-dry storage survival period. Analysing survival data for cohorts of seeds reduced the standard deviation of the normal distribution of seed deaths in time, but no sub-population showed complete uniformity of survival period.

Conclusions

Variation in individual seed behaviours within a developing population is inherent and inevitable. In this outbreeder, there is significant variation in seed longevity which appears dependent on embryo genotype with little effect of maternal genotype or architectural factors.