Biotechnology - the gene revolution's next frontier
epigenetics
For the first time scientists are
exploring epigenetically controlled
genes in wheat, barley and canola
pre-breeding programs.

From its inception, molecular biology has rested on a central theory: that the traits inherited by an organism are encoded and transmitted by the DNA sequence of genes.

As a molecular biologist, Dr Liz Dennis of CSIRO Plant Industry in no way disputes the theory, having used it with her colleagues to discover a gene that switches plants between vegetative growth and flowering. Nonetheless, she has seen first-hand that even the most entrenched genetic assumptions occasionally go haywire due to an entirely different mechanism that overrides conventional genetics.

This extra player has been termed 'epigenetics' - the ability of something other than the inherited DNA sequence to determine the organism and its progeny.

"Some 20 years ago, a number of top plant research journals wouldn't publish anything to do with epigenetics," Dr Dennis says. "But now it is clear that there is another level of control over and above the gene sequence. It has to do with how DNA is packaged and modified."

This 'packaging' is required because DNA molecules are extremely long, exceeding the length of a cell many times over. To get around this problem, DNA is coiled around tiny reel-like structures - called histones - and then the coils are twisted into ever-tighter bundles.

"If the DNA is wound up tightly enough, the proteins required to express genes cannot gain access and the genes remain inactive," Dr Dennis says. That implies that changes in DNA packaging could, potentially, determine whether a gene is switched on or off.

Around the world, a cadre of geneticists - Dr Dennis among them - quietly realised that anything capable of systematically controlling the way DNA is packaged amounts to a whole new kind of control over genes and an organism's traits.

For many years scientists questioned whether such a control system existed. In the past decade, as scientists started to identify the underlying molecular mechanisms, there has been a growing recognition of epigenetics and its over-arching influences.

"Epigenetics works by either chemically modifying the DNA or the histone proteins in ways that can switch gene expression on or off," Dr Dennis says. "The modifications primarily involve adding or removing acetyl or methyl groups."

The effect on organisms can trump conventional expectations. Dr Dennis relates the example of a gene involved in the formation of stamens and the superman mutation of that gene which results in an over-abundance of the flower's male parts. The same effect can be produced by an epigenetic modification on the normal, non-mutated gene.

"Epigenetics has its own, seemingly erratic, logic and utility," Dr Dennis says. "Environmental factors, pathogens or changes in growth rate and nutrition can trigger some genes to epigenetically switch on or off. It is now known that in plants, epigenetics is a factor during the major developmental transitions, such as flowering and seed setting."

Overall, the early findings of this new field of research create a tantalising possibility. It is the idea that epigenetics provides an otherwise conserved and rigidly regulated set of genes with a dose of interactivity and responsiveness to their environment: "You can think of it as a higher-order level of control in which the genetic is dominant only if there is no epigenetic override on the DNA."

This is significant because scientists can now trace a pathway by which the environment is able to influence gene expression and plant traits. That opens up the possibility of developing crop varieties with better-adapted responses to detrimental environmental events, such as frost.

Dr Dennis adds that the overrides are routinely replicated during cell divisions in any one organism but are not necessarily passed on to the next generation - rather, the progeny inherit the ability to make their own epigenetic modifications.

However, because DNA sequencing techniques are unable to detect epigenetic modifications, data gathered by genome projects is missing an important layer of information that is liable to prove important to plant breeders. In response, initiatives dubbed 'epigenomics' are starting up around the world to map epigenetic modifications across genomes.

After 20 years of epigenetic research by Dr Dennis, Australia has a strong record. "We didn't participate in genome sequencing projects very much, but there is an opportunity now for Australia to play a lead role in the emerging field of epigenomics," she says.


Editor's Note: First published in the September - October 2007 issue of GRDC's Ground Cover (Issue 70). For permission to reproduce this article please contact GRDC.