art%3A10.1134%2FS1022795412110130

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EXPRESSION OF HETEROLOGOUS GENES IN PLANT SYSTEMS 1073

Virus structure

Replicase

Transport protein

Envelope protein

Gene substitution

Replicase

Target gene

Envelope protein

Gene insertion

Replicase

Transport protein

Target gene

Envelope protein

Substitution of a gene region

Replicase

Transport protein

TG

Envelope protein

Fig. 4. A diagram of transient expression of target genes and sequences with the use of viruses. TG, target gene.

technology on an industrilal scale and a low viral vector

capacity; i.e., integration and expression of largesize

gene sequences are limited.

The key advantages of transient expression of heterologous

genes in plants are a high level of expression

of target genes; posttranslational modifications of

plant proteins; the yield of a target protein can reach

80% relative to the total protein. Despite the abovementioned

merits, this method has several drawbacks.

The protein products of some genes are toxic for plants

(e.g,. hepatitis B surface antigen); a problem is the

expression of oligomultimeric peptides (such as IgG

antibodies), which requires manipulations with viral

vectors for the expression of two and more polypeptides

in equimolar quantities.

CURRENT AREAS OF RESEARCH

ON THE EXPRESSION OF HETEROLOGOUS

GENES IN PLANTS AND PROSPECTS

OF DEVELOPMENT

Modeling of Plant Metabolism Through Expression

of One or Several Heterologous Genes

At present, heterologous expression of one gene is

predominantly used to improve the characteristics of

agricultural plants. However, a useful trait or a useful

physiological function in plants is, as a rule, a consequence

of the realization of a complex metabolic pathway

involving the products of several genes. In connection

with this, one of the areas in plant genetic engineering is

targeted plant metabolism modeling [21].

In order to directionally modify the metabolic

pathway, data on metabolomics, transcriptomics, and

proteomics are necessary for elucidating the key stages

of metabolism and possible mechanisms of its regulation.

However, plant metabolism is as yet poorly

understood and complete information about enzymes

and genes encoding them is known only for individual

biosynthetic pathways in a number of crops.

There are two basic approaches to modify the metabolic

pathway in plants: (1) alteration of the metabolic

pathway in a desired direction; (2) introduction

of a new biosynthetic activity from another organism

[22]. For instance, the biosynthesis of a particular

product can be stimulated by regulating the rate-limiting

stage of the process via alteraton of the level of an

enzyme catalyzing this reaction. The key problem is

associated with the knowledge of this stage and the

enzyme catalyzing it. It is also possible to inhibit the

activity of an enzyme responsible for the delivery of

the substrate or, finally, to manipulate the regulatory

factors.

As compared to directional selection, genetic engineering

permits a desirable trait to be imparted to a

plant much more rapidly due to modifications of

metabolism of certain compounds.

We shall present several examples of successful

modeling of metabolic pathways in plants as a result of

transfer of one or several genes.

One of such examples is modeling of metabolism of

steroid compounds using heterologous expression of

one gene (CYP11A1 cDNA of cytochrome P450scc

from the bovine adrenal cortex) in transgenic tobacco

plants [23]. Cytochrome P450scc is known to catalyze

the rate-limiting step of biosynthesis of mammalian,

as well as human, steroid hormones. The recent literature

data suggest structural and functional conservation

of the steroidogenic and steroid regulatory systems

in plants and animals. On this theoretical basis

animal CYP11A1 cDNA was chosen for being transferred

and expressed in transgenic tobacco plants. The

analysis of the obtained transgenic plants showed that

the protein product of animal CYP11A1 cDNA is compatible

with the electron-transport proteins of the

plants, it displays a specific functional activity and is

able to integrate into the steroidogenic system of

plants. The expression of this heterologous gene

caused an alteration in the metabolism of steroid compounds

and the formation of metabolites that are not

specific for tobacco plants, such as pregnenolone and

progesterone.

There are many other examples of successful modeling

of biosynthesis of secondary metabolites in

plants: an increase in the level of phenolic antioxidants

in potato tubers simultaneously associated with

enhanced expression of the StMtf1 M gene and silencing

of the F3'5'h gene [24]; alteration of the caffeine

level in coffee plants [25]; modeling of tropane alkaloid

biosynthesis in various plant species via the transfer

of one or two genes [26].

Modeling of metabolic pathways in plants via

expression of one or several genes is primarily aimed at

creating plants that produce biologically active substances

or to improve the nutritional quality of agricultural

crops (biofortification).

RUSSIAN JOURNAL OF GENETICS Vol. 48 No. 11 2012