In this article we will discuss about:- 1. Gene Expression in Vegetative Organs 2. Gene Expression in Reproductive Organs 3. Gene Expression in Seeds 4. Gene Expression in Fruits.
Successful studies on gene expression in plants are undoubtedly due to the contribution of early botanical monograph in which cytological structure and function of organs were defined. With the advent of various reporter gene system and transformation efficiency of plants, cells can be defined at the molecular level by the gene that they express.
Cell specific gene expression studies have enlightened the functions of each cell types and have greater impact on agriculture biotechnology. The differential expressions of plant genes in various organs have been well documented. With the help of reporter gene trapping system, tissue and cell specific gene expressions in plants have been well defined.
It is possible to monitor gene expression within organs of single plant due to repetition of organogenesis over its life span. Plant cells undergo differentiation in response to external signals which have significant impact on gene expression. The literature on tissue-specific gene expression in plants is comprehensive and incorporates studies of large multi-gene families in many species.
Gene Expression in Vegetative Organs:
Leaf-Specific Gene Expression:
Leaves contains innumerable gene products of which many are primarily involved in photosynthesis and produced in a leaf specific manner, regulated by light or photosynthetic metabolism. To understand the significance of leaf specific gene expression at the cellular level, it is essential to highlight the specialized functions of different leaf cell-types in C3 and C4 plants.
In C4 plants like maize and sugarcane, CO2 is first fixed into a C4 acid in M (mesophyll) cells. Further decarboxylation and refixation steps occur in BS (sheath) Bundle cells where the O2-sensitive ribulose-1, 5-bisphosphate carboxylase (RUBISCO) enzyme is sequestered. The cell specific distribution of C4 metabolic enzymes in BS and M cells is regulated by gene expression.
Studies have shown that genes for RUBISCO and NADP malic enzymes are expressed specifically in bundle sheath (BS) cells in light grown C4 plants. In contrast, other C4 genes like phosphoenol carboxylase and NADP-malate dehydrogenase are expressed specifically in mesophyll cells.
Light had greater role in cell specific expression of C4 genes. In dark grown plants, RUBISCO gene expression occurs in M cells. Upon illumination, RUBISCO gene expression is changed to BS cell. At the same time, light induces the expression of many other C4 genes in M cells indicating the role of light in positive and negative control of cell specific gene expression.
In C3 plants also cell specific expression takes place predominantly, For example, small sub unit of RUBISCO (rbcS) are regulated by light and are expressed in leaf specific manner. The rbcs gene expression takes place in leaf mesophyll cells, guard cells of the leaf epidermis, and cells of mid rib. The rbcS gene expression is clearly under the control of light.
Stem Specific Gene Expression:
Many plant genes are expressed abundantly in stems. The genes which are expressed specifically in stems are mainly encoding cell wall proteins and other metabolic enzymes of the plant vasculture. The main cell wall proteins in plants are hydroxyproline rich glycoproteins (HRGPs), the glyco-rich proteins (GRPS) and proline or hydroxyproline rich proteins (PRPS).
Presence of repeating basic amino acid motif is one of the common features of these proteins. Hydroxyproline-rich glycoproteins are main structural components of the plant cell wall and are identified to play a defensive role against pathogen. Similarly, the cell specific expression of genes encoding the glycine rich proteins of petunia has been examined.
These are confirmed as cell wall proteins and associated with protoxylem cells in hypocotyls, ovaries, and seed coat. Several gene encoding enzymes are expressed in a vascular-specific manner. Examples are the genes encoding S-adenosylmethionine synthetase and glutamine synthetase, involved in biosynthesis of polyamines and ethylene.
Root Specific Gene Expression:
The primary functions of root are the mechanical support and nutrient absorption. Several genes have been shown to express specifically in root organ. Cell specific gene expression in roots has been examined for the gene encoding tobacco cell wall protein, HRGPnt3. The promoter of this gene facilitates the expression in the endodermis.
In legumes, root -specific genes play a role in plant—Rhizobium interactions. Root hair cells express certain genes in response to Rhizobium attachment. Transgenic experiments have shown that lectins are synthesized as host determinants in roots. High level expression of plant transcription factors that are active predominantly in root has been identified.
Biotechnological Applications of Gene Expression in Vegetative Organs:
The study of the gene expression in vegetative organs has many applications in agriculture biotechnology. For example, study of cell specific expression of glutamine synthetase (GS) genes has an application in the genetic engineering of herbicide resistant plants. It may be essential to express herbicide resistant forms of GS in both mesophyll and phloem cells to confer herbicide resistant.
Plant promoter that direct vascular specific GRP gene expression can be used to express proteins that protect against phloem or xylem borne pathogen. Similarly, leaf and root specific expression of genes encoding pharmaceutically important proteins have high significance.
Gene Expression in Reproductive Organs:
One of the most complex organs in plant is the flower that consists of several types of tissues. The complex process of flower development and plant reproductive biology has been dissected at the molecular level.
Genes Controlling Fertilization:
The self-incompatibility (SI) in several plants is due to genetically controlled mechanism, whereby the pistil rejects pollen derived from the same plant. The SI phenotype is determined by the genetic constitution of the pollen grain (gamete) in gametophytic plants, and by the diploid genotype of the parent (sporophytic) in sporophytic plants.
The morphological difference between these two SI systems is the region of the pistil in which pollen tube is arrested. SI pollen germinates, but pollen tube growth is arrested in the style. In some other sporophytic plants, e.g., SI pollen fails to germinate. SI is controlled by locus known as S-locus and genes encoding S-locus specific glycoproteins are expressed in stigma throughout the pathway of pollen growth in the style.
Stamen and Pistil Specific Expression:
Both stamen and pistil specific gene expression have been studied. The stamen specific genes are involved in pollen development and their lack of expression results in male sterility. The pistil specific gene is expressed in stigma, style, or ovaries at certain stages of development. For example, one pistil specific gene is expressed exclusively in their stylar transmitting tissue just prior to fertilization. This suggests do that this gene product may play a role in regulating pollen tube growth.
Petal Specific Expression:
The genes which are expressed specifically in petals are structural and regulatory genes involved in pigment production. Most of the flower pigments are flavonoids synthesized via phenylpropanoid and flavonoid biosynthetic pathways. The best known genes encoding enzymes are phenylalanine ammonia lyase (PAL), chalcone synthase (CH), chalcone isomerase (CHI) etc. are produced in petals.
Certain members of these gene families are also expressed in vegetative organs for the production of lignins and plant defense compounds. Another petal specific expression of the gene is enolpyruvylshikimate 3-phosphate synthase (EPSPs), an enzyme involved in biosynthesis of phenylalanine feed into phenylpropanoid pathways.
Biotechnological Applications of Floral Specific Gene Expression:
The significance of floral specific expression studies facilitated the production of male sterile plants for the production of hybrid seeds. Male sterile flowers can be produced by using anther specific promoter to direct the expression of a ribonuclease. Further, floral expression studies have applications in flower color manipulations. Even flower color can be inhibited by anitisence RNA technique by antisense CHS mRNA production.
Gene Expression in Seeds:
The expression of seed specific genes is restricted to embryo or endosperm tissues and their expression pattern have been studied at molecular level in wide number of species.
Embryo and Endosperm Specific Gene Expression:
Many seed storage protein genes like globulin-storage protein genes are specifically expressed in embryonic tissues of the dicot seed. Other seed storage protein genes and their expression also restricted to embryo including β conglycinin, glycinin, legumines, vicilins, helianthinin, and β-phaseolin.
Transgenic experimental evidence has shown that legume seed storage genes are expressed in transgenic tobacco. This indicated that DNA sequence and transacting factors which controls the expression of above seed storage protein genes are conserved between these distantly related plants.
Several defence genes like proteinase inhibitors and lectins are also expressed in response to wounding in embryos. Phytohemaglutenin (PHA) is a lectin seed storage protein in seeds of soyabean and others, and their expression is controlled by 63bp repeat core sequence in the promoters of PHA genes.
Multi-gene family like zeins seed storage proteins is specific to endosperm tissues. Tobacco is the model system for the study of endosperm genes from cereals. This has been proved when zein promoter was fused to GUS reporter gene and expressed efficiently in transgenic tobacco endosperm.
Wheat storage proteins are divided into glutenins and gliadins that together form the gluten complex responsible for the bread making properties of wheat. These storage proteins are expressed in endosperm-specific manner. Glutenin gene was cloned from wheat and after being introduced into tobacco, expressed correctly in endosperm tissue.
Some of the genes are also expressed in aleurone layer. One of the best studied genes expressed in aleurone layer is alpha amylase gene. The enzyme amylase specifically expressed in aleurone, break down the endosperm tissue to provide nourishment for the developing seedling.
Biotechnological Applications of Seed Specific Gene Expression:
Most of the cereals storage proteins are of considerable economic importance. Modification of protein quality for better nutrition is significant at the back drop of studies on seed specific gene expression. Since cereal grains are poor in lysine, and legumes have a lacuna in methionine amino acid, it is essential to modify genes for desirable amino acids in their storage proteins. In addition, it is possible to produce fine bread making quality of protein and protection of crop and seed against insects and pests by understanding gene expression profiles of seed storage protein genes.
Gene Expression in Fruits:
Ripening of fruit is a highly complex process controlled by ethylene hormone. Several genes expressed in response to ethylene during ripening have been studied and indeed has significant agriculture implications. Although several genes encoding fruit specific mRNA have been cloned, functions of many of these mRNAs encoded products are unknown.
A member of this group that is well characterized is polygalacturonase (PG) gene. It is thought to play a role in fruit softening. Ethylene plays an important role in fruit ripening and a gene encoding the rate limiting enzyme of ethylene biosynthesis like ACC synthase and ACC oxidase have been successfully cloned.
Biotechnological Applications of Fruit Specific Gene Expression:
In an attempt to modify fruit ripening and texture, fruit specific genes are used as molecular tools using antisense technology for extending shelf life. Using antisense mRNA for PG in transgenic plants, an attempt has been made to reduce PG enzyme and increasing the shelf life of tomato fruits. In addition, anti-sense technology has also been used to reduce ethylene synthesis by producing antisense mRNA in transgenic plants.