Desiccation tolerance is necessary for the completion of the normal life cycle, which is the adaptive strategy for the survival of the seed in storage or environmental stress, and to ensure the better reproduction of the species. In early 1973, Roberts will be made by seed desiccation tolerance were divided into normal and recalcitrant 2 [1]. The different types of seeds, which have a great difference in their resistance to dehydration, will directly affect the storage and preservation of germplasm resources. Therefore, the research on the desiccation tolerance of seeds is of great significance both in theory and in practice. From the last century 80's, the research on the desiccation tolerance of plant seeds has become one of the hot topics in plant physiology. Since 1997, there are more than one hundred papers and articles published in China and abroad, and the related research papers are increasing year by year. Only in 2012, the relevant papers are more than 260 articles (of Science Web), the species of Oryza (sativa Arabidopsis), mays Zea (thaliana), [2-5] Medicago (truncatula), and other 40 species.
Throughout on seed desiccation - tolerance research found that most of the domestic researches focus on recalcitrant seeds and species studied with Sanqi (Panax notoginseng) [6], chestnut [7-8], yellow [9-15], longan and litchi [16], Archontophoenix alexandrae [17], green glauca (18), the loquat [19], tea tree [20],. These studies mainly aimed at the changes of desiccation tolerance during seed development, soluble protein, soluble sugar, antioxidant system, dehydration sensitivity and cell membrane permeability, cell ultrastructure changes, lipid peroxidation, endogenous hormones and the differences in the process of dehydration. In addition, there are part of the study focused on the normal seed development in the process of desiccation tolerance changes, relates to the species with Arabidopsis [21], corn [22], peanut [23], cassia Mao [24] and so on, mainly study the normal seed desiccation tolerance formation and soluble sugar, endogenous hormone, protein. In recent years, the research on the change of desiccation tolerance during normal seed priming, such as [25], [26], Ca2+, and [27], has been studied. Recently some scholars to different sources of cattle ear Maple seeds as experimental material, through germination test to study seed desiccation tolerance, pointed out that the UI Maple seed size and germination rate was negatively related to [28]. In foreign countries, the research on the desiccation tolerance mainly concentrated in recalcitrant seeds of Quercus robur [29-32] etc., on rice, wheat and other normal seed development in desiccation tolerance during changes in [33], seed development in the process of desiccation tolerance and free radical [34], antioxidants, carbohydrate, enzyme activity of relationship also made some research. In the genus Acer found recalcitrant species as well as normal, normal Norway maple (Acer platanoides L.) and recalcitrant Eurasian maple (Acer pseudoplatanus L.) seed differences in mature sugar metabolism and desiccation tolerance of the [35,36]. In recent years, the changes of protein synthesis and [37], [38], ABA, [39], and in the process of seed drying and dehydration.
And so on. This paper summarized the progress in the research on 4 aspects, such as protein and desiccation tolerance, desiccation tolerance of seeds, desiccation tolerance and antioxidant system, and the regulation of seed desiccation tolerance and seed shedding.
1, the seed desiccation tolerance and protein
In the literature, the proteins associated with desiccation tolerance were generally divided into 2 categories: small heat shock protein (HSPs) and late embryonic stage (LEA). The small heat shock protein occurred and accumulated in the late stage of seed embryos, and it was found in the dry seeds of [40].
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During seed drying, sHSP gene expression occurred in the end in the embryo cells, indicating that during dehydration and dehydration state, the cells within the small heat shock protein Baiqi protective effect [41]. The interaction of small heat shock proteins and other proteins as a molecular chaperone also suggests that it can counteract the effects of inappropriate proteins and proteins. Thus, in the dry state, this type of protein usually plays a protective role in [42]. Late stage embryos were found in the mature embryos of cotton (hirsutum Gossypium) for the first time in 1981. LEA protein is a kind of special mRNA transcription and related proteins in the late stage of seed embryos, mRNA and protein synthesis. The LEA gene is considered to be a serious loss of water loss and protection, which has been confirmed in the mature of the normal seed and recovery plant dehydration test [44]. Partial protection of the LEA protein has been predicted, including the role of DNA in protecting the [45], the role of stabilizing the cytoskeletal filaments and acting as a molecular chaperone. There are experiments showed that: LEA protein can act as synergistic sugar (such as trehalose) role, prevent the aggregation of [46] protein during dehydration. In recent years there are test showed that the seeds are not tolerant to drying process and LEA protein synthesis is not entirely consistent, such as in soybean hypocotyl obtained before desiccation tolerance existing dehydration pigment synthesis [47]. From this we can know, the function of LEA protein may be very complex, and it is not related to the desiccation tolerance of seeds. Resistance to dehydration ability and not only by the LEA protein expression or accumulation generated, recalcitrant seeds is not entirely due to the lack of lea gene and LEA protein that has the characteristics of desiccation sensitivity. Therefore, there are still other factors in the dehydration process of desiccation tolerance of seeds play an important protective role.
In the first view, the expression of gene expression in the control of desiccation tolerance in the leaves of the plant and the leaf is different. But in recent years, there are great similarities between them, especially in the SSP (seed storage protein) gene and lea gene control. The researchers found that the gene promoter cis element and the interaction of transcription factors can be replaced in the form of a single leaf plant and a twin. This suggests that the desiccation tolerance gene is protected (Figure 1). This diagram illustrates the major genes associated with the mature process of the seed maturation and desiccation tolerance of Arabidopsis thaliana, and also shows that the active oxygen and antioxidant system are potential signals for dehydration tolerance. It can be inferred that the late stage of desiccation tolerance may occur in the late stage of seed maturation. Figure 1
Figure 1. The early stage of the embryo formation, mature stage, ABA accumulation stage
Illustrates the main control genes in mature process, including desiccation tolerance obtained (embryo morphology occur using gray ellipses to represent; between the major genes controlling network is represented by black; solid lines represent control process; dotted lines representing different stages regulation; downstream activation of gene with a center of the dotted line represents the; question mark is used to represent the unknown path; cotyledons LEC; cotyledon phenotypes Fus3; ABI3 said not sensitive to abscisic acid; on behalf of glutathione GSH; cat stands for catalase; GR on behalf of glutathione reductase; SSPs on behalf of seed storage proteins).
2, seed dehydration tolerance and sugar
Sugar metabolism plays a key role in the protection of cell in the process of seed dehydration. Studies have shown that: in the process of development, the seed of the dehydration and tolerance of the accumulation of a large amount of soluble sugar [48], it is worth noting that the non reducing sugar, sugar, sugar and the seed of the desiccation tolerance. The accumulation and disappearance of oligosaccharides on the membrane, which is relatively low or dry, has a very important effect on the accumulation and disappearance of [49], whereas sucrose has little effect on the seed desiccation tolerance. On the contrary, the adverse effects caused by the dehydration of trehalose with the seeds of two sugars are relatively more. Trehalose is rarely found in the normal seeds of the mature, but trehalose has been found in the seeds of Arabidopsis.
Aircraft such as the d.510, fait et al speculated, the role of trehalose in Arabidopsis seed dehydration process may is to maintain a consistent representation of storage and housekeeping proteins. But the high concentration of sucrose in all the normal seeds of the water, it is now generally considered that sucrose replaced trehalose. In general, the desiccation tolerance of seeds is related to the concentration of sucrose and the concentration of oligosaccharides, and the ratio of oligosaccharides and sucrose, the higher the ratio, the longer the storage time of seed storage. During different stages of development, there was a great difference in desiccation tolerance of seeds, which had a great relationship with the changes of sugar content and concentration in different stages of embryo development. [51].
Seeds played in sugar dehydration or drying effect is mainly reflected in 3 aspects:
1) soluble oligosaccharides can replace the water. When the water loss of seed cells, oligosaccharide hydroxyl can instead of water play a role, mainly distributed to the cell membrane and related proteins and other macromolecules, including trehalose, a disaccharide can replace the water molecules, is the role of the principle and film very protein polar region combination acts as a membrane pole protein of linings, avoid the bilayer lipid molecules stacked one over the other, to further stabilize the cell membrane structure [52-53];
2) soluble oligosaccharides were able to stabilize the protein. The results showed that: when the seeds were subjected to water stress, the hydroxyl groups of the two sugars and proteins can be combined to form water in the form of hydrogen bond, which can prevent the change of protein conformation in the process of water stress;
3) soluble oligosaccharides were easy to form cytoplasm glass, and there was evidence that the vitrification of [54-55] in the normal and other plant tissues could promote the vitrification of cell protoplasts. The presence of oligosaccharides could enhance the glass state temperature (Tg) of cell protoplasts, and effectively inhibit the crystallization of sucrose during water stress.
When the seeds were relatively low and soluble sugar concentration was high, the glass may restrict the movement of the molecules, or destroy the movement of the free radical [53]. As a result, the intracellular vitrification is a form of damage caused by dehydration to the cell in the dry condition of normal seeds, which is compatible with the stability of the cell environment.
3, seed dehydration tolerance and antioxidant system
In general, when the seeds are in normal water, the generation and removal of environmental free radicals is a dynamic balance. When the seeds are subjected to water stress, the electron transport in the stem cells is hindered by the loss of water. When the concentration exceeds a certain limit, it can cause damage to the body. It can also cause the damage of cell membrane. At the same time, there are a series of antioxidant system in the seed, they can remove excess reactive oxygen species in the cell, maintain the active oxygen metabolism balance, protect the cell membrane from free radical damage.
Come reviews the recalcitrant seeds in the dehydration process with the uncoordinated metabolism and produce a large number of free radicals is the main damage factor [57]. Recalcitrant seeds with antioxidant mechanism [58-59]. But in the dehydrated state. This defense mechanism gradually becomes invalid, and free radicals was becoming more and more accumulation. This is [60] the main causes of desiccation sensitivity. Free radical damage should be the primary factor in the occurrence of dehydration sensitivity, and the protective effects of oligosaccharides and heat shock proteins are the premise of [61]. As far as the link between the three to be further studied.
4, seed dehydration tolerance and shedding acid
For most of the seeds, the early and mid levels of maturity are largely controlled by the function of ABA. It was concluded that the acid was synthesized in the first mature tissues, but not in the late stage of seed embryo and endosperm in the development of [62]. It is generally believed that the higher the content of ABA is the main factor that the seed remains dormant or dormant, and the formation of the seed development and seed vigor by decreasing the content of [63]. A good report on the dehydration tolerance of lea gene transcription regulation of [64-66] gene. As a protective agent, the expression of ABA can be used as a protective agent, and the expression of many genes in the water stress requires an increase in the content of endogenous ABA. Studies show that many lea genes are activated by ABA, and LEA protein expression is also consistent with the changes of [66]. Therefore, the loss of the dehydration tolerance of the acid to the seed of the seed has an important role in regulation.
5, summary and Outlook
Research on seed desiccation tolerance is mainly from normal and recalcitrant seed development in protein and carbohydrate storage substance changes and plant hormone on growth and development regulation and seed metabolic changes, especially for studies reporting more recalcitrant seed desiccation tolerance. The mechanism of seed desiccation tolerance is regulated by multiple mechanisms, which is accumulated in the process of seed development. Some progress has been made in the preservation, induction of recalcitrant seed desiccation tolerance. But these results reveal that there are many outstanding questions to be answered in the process of dehydration tolerance. The main results are as follows: 2. 3. How to find and explain the phenomenon of [67], how to find and explain a process and the other process, and to explain the mechanism and the effect of seed desiccation tolerance.