Mark Tester

Genetically Modified (GM) Plants

What's it all about ?

Genetic modification (GM) is the heritable alteration of the genetic make-up of an organism. As a natural process, genetic modification (GM) is as old as genes themselves, and has been utilised by human beings since the beginnings of agriculture. Recently, the term has come to apply specifically to newly developed DNA technologies, where the genome of an organism is modified using artificial techniques.

These rely on the ability

to cut DNA precisely, isolate desired fragments and insert them

into a single cell of another organism. From this transformed cell

a new multicellular organism can be regenerated. There is a wide

range of applications of the new GM technology, from employing micro-organisms

to synthesise recombinant human insulin or growth hormone, to making

crops resistant to pest and diseases. However, it has also attracted

much opposition. It has been criticised for being unnatural, for

posing an unassessable risk to the environment and to human health,

and for providing an instrument for the manipulation of human genetic

make-up that might invite serious abuse.

What is genetic modification?

Genetic modification (GM) is the modification of genetic make-up

such that the modification is passed on to the organism's descendants.

Strictly, it is a general term that covers many processes - some

of these are new, some have been occurring since life began, and

some have been strategically used for 10,000 years, since agriculture

began at the end of the last ice age. However, the term 'genetic

modification' has more recently come to be used for the process

of 'genetic engineering', where newly developed processes of molecular

biotechnology are employed to insert relatively few genes into an

organism's genome. Other terms that have been used to describe this

technology include 'recombinant DNA technology' and 'genetic manipulation'.

In this article, GM using the new technologies is distinguished

from GM using traditional techniques by referring to the former

as 'new GM'.

Methods of GM (Genetic modification) :

There are at least three traditional methods of genetic modification:

a) Selecting for variability within existing populations arising

from genetic recombinations resulting from sexual reproduction.

It is noteworthy that most modern crops have been so altered using

this technique that it is difficult to identify their wild progenitors.

Variability in dog breeds provides another example of the results

of this type of GM.

b) Crossing closely related species. For example, modern bread wheat

has arisen from two sequential crossings of, in total, three species.

c) Isolating mutants. For example, herbicide resistant oilseed rape

has been developed from plants that appeared spontaneously in Canadian

fields.

In addition to these traditional approaches, there is new GM, involving

the modification of specific genes in single cells using recently

developed biotechnologies. Essentially, this process involves:

1) The identification and isolation of a linear polymer of nucleotides

that will either direct the synthesis of a protein with desired

characteristics (a gene), or alter the level of protein synthesis

directed by another gene. The process of isolation of specific nucleotide

sequences requires in particular the action of bacterial enzymes

that cut nucleotide polymers in precise locations identified by

their nucleotide sequence. These are called restriction enzymes,

the first of which was isolated in 1970. (More recently, another

technique for the rapid in vitro replication of DNA sequences, termed

the polymerase chain reaction or PCR, has become an increasingly

central tool to aid the isolation of desired DNA fragments.)

2) The movement of the desired gene(s) into another organism. Genes

are usually moved into other organisms by exploiting natural pathogens

whose mode of infection involves the injection into the host of

genetic material. Herbert Boyer and Stanley Cohen performed the

first successful gene transfer in 1973.

3) This gene transfer occurs into a single cell. When genetically

modifying plants or animals, an entire organism must then be regenerated

from this single cell. Thus, transfer into single-celled embryos

is technically desirable, as these are programmed for growth into

a multicellular organism. Plants, however, have the useful trait

of totipotency, whereby cells from the adult plant have the ability

to regenerate into new adults - thus a range of cell types can be

used for gene manipulations. The recent cloning of mammals from

adult cells, most famously 'Dolly' the sheep, opens the door for

this technology being transferred to mammals.

Traditional techniques for gene modification limited modifications

to those occurring between closely related organisms. New GM can

be used for similar types of gene modifications, but it also enables

the transfer of genes between any two organisms. Thus, although

new GM enables the addition to an organism's genome of just one

gene, with a specific trait (unlike traditional breeding programmes,

where thousands of genes are transferred at a time), the one gene

could come from any organism, or even be created de novo in the

laboratory. Overall, new GM tends to bring in fewer genes, but potentially

from 'further away', than old GM.

Applications of new GM

The first major product of new GM was developed in 1982, for the

production of human insulin by bacteria for the treatment of diabetes.

In 1990, the first GM food product, an enzyme employed in cheese

making, was approved for use in the USA. In 1994 the first food

product was sold commercially, the so-called FlavrSavr tomato,

that had reduced activity of a gene essential for ripening. The

development of GM animals with disrupted gene function is providing

numerous insights into the molecular basis of disease, and there

is the distinct possibility of modifying pigs to provide organs

for human transplants.

Recent commercial applications of new GM include the introduction

of herbicide tolerance into crops such as soya bean and oilseed

rape, and the ability to synthesise insecticidal proteins in cotton

and maize. Many other applications of new GM are being developed,

including conferral of the ability to make antibodies in fruits

and the ability to decontaminate polluted land by degrading organic

pollution. New GM also provides opportunities to alter the composition

of food to increase its nutritive value, such as increasing the

mineral and vitamin content of grain (e.g. 'golden rice'). Increases

in food production are also possible, by improving overall plant

qualities (e.g. dwarfing rice) and by increasing tolerance to biotic

stresses (pests and diseases) and abiotic stresses (e.g. low temperature,

drought or salinity).

Problems of application

GM has been subject to various types of objection. It has been pronounced

unnatural, held to pose an unjustifiable risk to the environment

and to human health, and felt to bring us unacceptably close to

being able to manipulate human genetic make-up. Only the last of

these is specific to GM, and then not to new GM - concern about

eugenic policies antedates modern biotechnology.

The charge of being unnatural has been levelled at a host of targets.

Those who bring it in this case still have to explain what is specifically

unnatural about new GM that it does not share with many long accepted

procedures.

More serious is the contention that new GM threatens unintended,

undesirable and perhaps also unforeseeable environmental and medical

consequences. It brings the risk of the escape of organisms, or

at least their genes, into wild populations. For example, the spread

of insecticidal proteins into wild plants could confer a competitive

advantage on those plants, disrupting semi-natural systems. Likewise,

effects on insect populations could be significant. Although having

herbicide-resistant crops will decrease herbicide use, it will increase

the effectiveness of applications, reducing weed densities and thus

continuing a decline of wildlife that has been going on since agriculture

began.

Exposure of human populations to large amounts of novel proteins

that have never previously been in the human food chain could cause

unpredictable problems. In particular, allergenicity could cause

problems that would be difficult to detect, as symptoms can take

a long time to develop.

Issues involving new GM and animals raise a further range of ethical

questions. They also suggest the potential for the transfer of GM

technology to humans and its use as an instrument for manipulation

of human genetics - giving rise to the fear of its use for objectionable

ends, or of damage to human life as an unforeseen consequence of

well-meaning actions. The thought here may be that there are limits

to the powers with which human beings are good enough, or clever

enough, to be trusted.

These issues, however, concern only particular applications of

new GM, not the technology per se. Since it has so many applications,

generalisations about the whole technology are difficult. Most features

of most applications of new GM are not profoundly different from

the processes that have been performed on plants and animals for

the past 10,000 years, so objections to new GM need to be carefully

formulated to address its unique features.

Problems of public acceptance

Recent concerns with genetically modified (GM) plants have not

arisen in a significant way with GM bacteria and GM yeast, despite

the widespread commercial application of new GM techniques to these

organisms for many years. Diabetics have used insulin from a synthetic,

human-like gene inserted into bacteria and yeast, for well over

two decades, with no loud public protest. Another example of widespread

application of new GM is vegetarian cheese - people have been consuming

genetically engineered rennet protein for many years.

However, current traits conferred on crops reduce application of

insecticides and herbicides, and although benefiting producers offer

no evident benefits to consumers. For those who experience no benefit

but sense a possible risk, the natural reaction is one of rejection.

Furthermore, allowing the patenting of the technological processes

of GM (in contrast to plant variety rights that protect the output

of traditional breeding) places new GM in a different position to

traditional GM, a situation related to the fact that this second

'green revolution' is privately funded, in contrast to the publicly

funded first Green Revolution. This comes in the context of a general

distrust of science, fed by the historically recent misuse of science

(e.g. for the development of bio-weapons) and mistakes by scientists

(e.g. BSE).

- September 2005

About the Author

Mark Tester runs a research group at the Australian Centre for Plant Functional Genomics, Adelaide.