Recent Advances in vitro Fertilization of Gramineae  

Zhong an Wang
School of Life Science, Xia Men University, Xiamen, 361005,P.R. China
Author    Correspondence author
Molecular Plant Breeding, 2011, Vol. 2, No. 17   doi: 10.5376/mpb.2011.02.0017
Received: 08 Oct., 2011    Accepted: 07 Nov., 2011    Published: 16 Feb., 2012
© 2011 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:

Wang., 2011, Recent Advances on in vitro Fertilization of Gamineae, Molecular Plant Breeding, Vol.2, No.17 119-122 (doi: 10.5376/mpb.2011.02.0017)

Abstract

Gramineae is one of the largest families of monocot plants, it contains most important staple cereal crops in the world, such as wheat, rice, maize, sorghum, barley, oats, and millet. Fertilization studies of gramineae have been considered very difficult, especially in vitro fertilization. The so-called in vitro fertilization refers to the separation of sperm cells and egg cells, fusion of both cells and regeneration of zygote. The technologies of in vitro fertilization have been widely used in the filed of gramineae, which include transferring the gene into egg cell and zygote, regeneration of plants, in vitro development from fusion products between gametes of alien species, and so on. Some questions of the gamety and embryo were discussed in this review.

Keywords
Gramineae; in vitro fertilization; Sperm cell; Egg cell; Zygote

Gramineae is one of the largest families with the widest distribution in the world, including the world's most important crops ,such as rice, corn, wheat, barley, sorghum, sugar cane, and so on. There is a important practical significance to learn of the mechanism of in vitro fertilization of gramineae because we can further improve the yield and quality of the gramineae. To date, the first successful case of in vitro fertilization is Kranz et al (1991) making the fusion of corn sperm cells and egg cells by power fusion, and resulting in some plants. Since then, there has been ongoing studies on the gramineae family in vitro fertilization from other researchers, who have made some prog- resses. In this article, we will briefly introduce recent advances in in vitro fertilization of gramineae.

1 Isolation of sperm cells
Isolation and purification of sperm cells arethe bases of physiological and biochemical in vitro fertilization study. They are also the premise of physiology, biochemistry and molecular biology on the sperm and egg identification process in higher plants. There are two types of Angiosperm pollens: bicellular pollen type, a generative cell and a vegetative cell in a mature pollen grain. tricellular pollen type, two sperm cells and a vegetative cell in a mature pollen grain.The pollen tubes of bicellular pollen type grains would be firstly induced in vitro cultivation to isolate sperm cells in the pollen tube, after the generative cell of pollen tube divides to form two sperm cells, The mature pollens of gramineae of a tricellular pollen cell could be used directly to isolate sperm cells. There are 2 main ways to separate sperm cells from tricellular pollen cells: the impact of infiltration and grinding. In 1973, Cass soaked pollens of barley in hypotonic solution of 10% BK sucrose solution, and obtained some sperm cells successfully (Cass, 1973). The impact of infiltration method is relatively simple, and it has high ratio of efficiency separation. To date the method was adopted in the sperm cells isolation of the graminea, such as corn (Dupuis et al., 1987; Yang and Zhou, 1989), rice (Guo et al., 1999), and so on, but different sperm cells masters are required and different optimum concentrations of separation are observed depending on the specific plants. There is little application of grinding on the isolation of the sperm cells of gramineae.

2 Isolation of egg cell
Egg cells are located in the embryo sac, and the embryo sac is born in the ovule, which made it more difficult to isolate the egg cells. We can isolate embryo sac, and then isolate the egg sac, or we can also isolate egg cell directly from the ovule. At present, we obtained the egg cell from gramineae mainly by enzyme or anatomical dissection. Enzymatic dissection methods which began in the 1980s have been relied on for the isolation of egg cell from maize (Kranz and Lorz, 1993), wheat (Kovacs et al., 1995), Rice (Zhao et al., 2000). Corn is by far the only case that achieved in vitro fertilization using isolated egg cell obtained from enzyme dissection method. Another method, micro-dissection was developed in the 1990s, which is performed universally for the gramineae in particular. At present, micro-dissection has been applied successfully to isolate the egg cell of barley (Holm et al., 1994), wheat (Holm et al., 1994; Kovacs et al., 1995), maize (Kovacs et al., 1995), Rice (Zhao et al., 2000). Zhao et al (2000) further attempted to compare the effects of the ovule from the vertical profile, crossprofile extrusion and separation of rice on egg and found cross-sectional ovule very easy to maintain egg cell in situ, to identify and to operate. 

It is generally believed that dissection enzyme should be used more in the plants (such as tobacco) with thin beads heart, and easily separated ovules, while Microdissection applies to plants (such as corn) with bulky ovules and small numbers of ovules. The two methods can complement each other, for example, in the separation of rice and corn, the ovule of the short-term digestion of the egg will help improve the efficiency of separation, but if the enzyme is not thoroughly cleaned or the over-all follow-up on the homozygous which would be a negative impact on culture. 

3 Egg, zygote and sperm in vitro culture, sperm cells and egg cell fusion
The embryo sac cells which have been in vitro culture include egg cell ,help cells, central cell, zygote, and artificial insemination central cells. The most eyecatching ones are egg cell and zygote in vitro culture. Kranz et al (1995) first reported that the egg cell of corn starts splitting and developing small cell mass in high concentration of 2,4-D, but it can not further grow. Subsequently, Zhao et al (2000) inducted the first split in the cultivation of egg cells. Zygote can be divided into homozygous and artificial zygote. Homozygous zygote was easier to get than artificial zygote, so homozygous zygote has been separated in many species for studies. Homozygous of the four natural crops, barley (Holm et al., 1994), wheat (Holm et al., 1994; Kumlehn et al., 1998; Pónya et al., 1999), maize (Leduc et al., 1996) and Rice (Han et al., 1998; Zhang et al., 1999) in vitro culture have been able to regenerate plants. Zhang et al (1999) found the homozygous development of rice is not in accordance with normal embryonic development mode in the cultivation of rice’s homozygous, but generated plants after by the bipolar embryoid or the formation of adventitious root. Zhao et al (2002) used embryonic cell lines of rice as suspension cell husbandries, obtained the desired result in inducing the growth of zygote by micro-feeding room culture, the rice zygote successfully splits into multi-cell mass. Artificial zygote is induced in vitro by a combination of methods. The methods by which the egg and sperm cells were induced to form artificial zygote are as following: electric fusion, calcium-induced fusion, and PEG-induced fusion. Zygote obtained from corn after sperm-egg fusion, formed the plant in vitro culture through electric fusion (Kranz et al., 1991). Faure et al (1994) put the egg and sperm cells of the corn into 5 mmol/L cacl2, and found that the sperm cells and the egg cell fused in a very short time. By increasing the concentration to 0.05 mol/L cacl2, the frequency of corn sperm-egg fusion had also increased, but the zygote formed multi-cell mass and no plant was obtained after the fusion. Kovacs et al (1995) found slightly inequal split in the cultivation artificial homozygous of wheat. Such successful studies in vitro culture will form a quality test system for genetic transformation and improvement of crops.

4 Molecular biology research
The use of in vitro fertilization system in the process of gene expression is a new field. The two major technological bases of study have matured, they are, access to low-volume manual egg cells and zygote, and the method of building a micro-cell cDNA library. RT_PCR application technology, 128 egg cells by the separation of corn and 104 artificial zygote in vitro fertilization after 18 hours of construction of cDNA library, and a number of specific expression in the egg or the fertilization induced expression gene were isolated (Leduc et al., 1996). Gou et al (2001) first built a CDNA library of rice sperm cells, and isolated the expressional gene from the sperm cells. Bai et al (2002) first isolated the full-length gene RSG6f from rice sperm cells. RT-PCR results show that the gene is expressed in the roots, leaves, the pollen cells and mature pollen, pollination ovary and sperm cell with particularly high volume in the sperm cells, thus it is a differentially expressed genes. Miao et al (2003) used cloning which has been expressed in the sperm cells as a probe to screen rice sperm cell CDNA library, and got a two full-length CDNA cloning. They then successfully constructed expression vector PRGFP1 for Agrobacterium-mediated genetic transformation of rice. In addition to building a library of cDNA to study gene expression, the researchers also tried to introduce foreign gene into the zygote experimentby microinjection of two genes (GUS gene, the gene regulation of anthocyanin) were separated from the corn import Zygote, the train after a short period of time, prove to be imported in the homozygous gene expression in the moment (Leduc et al., 1996). 

5 Conclusion
After the model Corn fertilization succeeded, the researchers mainly aimed to create more systems of in vitro operation. At present, as an important food crop, rice has a better foundation for embryo culture. The culture of renewable natural homozygous plants has also been successful. The large number of sperm cells of rice can be isolated, but separation of egg cells in rice quantitatively restricts the result of in vitro fertilized. In addition to the kinds of in vitro fertilization, the distant vitro fertilization, the use of an egg, a zygote with genetically engineered receptosr also enjoyed great vitality. Whether natural or artificial zygote, there are very high frequency splits in vitro, which have incomparable advantages with a somatic general and other cell systems. 

The in vitro fertilization has two major advantages which are the possibility of performing it live and with control. we can study the process of signal transduction in vivo of fertilization and embryo which will not be as effective if studied by physiology and biophysics methods, Additionally, we can apply biological methods to study the process of gene expression, find out the various aspects of the development of functional genes, and then apply transgenic technology to change the process of fertilization and embryo development. If we find the mechanism that activates the egg-related genes, there is the hope that we can use the gene to create operating new plant type without fertilization and embryogenesis. Its significance is self-evident. 

Authors' Contributions
ZAW performed the experiment, wrote and modified the manuscript. The author has read and approved the final manuscript.

Acknowledgements
I thanked two anonymous reviewers for their strict criticism on this paper.

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