Bulletin

Issue: July/August 2007

News

Top of the Crop - La Trobe University scientists have designed a genetic switch to boost plant production

By Tim Thwaites

Imagine a breeding system that within one generation could provide farmers with crop plants which are up to 30 per cent more productive and show greater resistance to disease.

It already exists. And we’ve known about it for at least a century. It’s called hybrid vigour or out-breeding and results from crossing two distinctly different genetic lines of a species. The most vigorous plants arise in the first generation from the initial cross. But there’s a catch which limits its use – which a research team led by Professor Roger Parish and Dr Song Li of the Department of Botany has now overcome.

In flowering plants female ovules are fertilised by male pollen. But most flowering plants, including most crop plants, carry both anthers, in which the male pollen is produced as well as a carpel containing female ovules. And that means they can self pollinate or inbreed – not good for hybrid vigour.

To produce seed guaranteed to be vigorous, you need somehow to ensure that all the pollen which fertilises the ovules comes from a plant of a different genetic line. And the only way of doing that is to make sure the seed-producing plants are totally male sterile.

In some species this can be done manually, simply by removing the anthers. Seed companies in the US, for instance, pay school children to tear the pollen producing tassels off flowering maize. Genetic mutations which prevent normal pollen development can also be used. They form the basis of several systems that have been developed, but all are either inefficient at producing the male sterile plants, or tend to break down under certain conditions.

Professor Parish’s research team believes it has found the answer. In a paper recently published in The Plant Biotechnology Journal the La Trobe researchers provide details of a genetic system they have developed, tested and patented worldwide which can guarantee total male sterility in seed producing plants of almost any species. It can also be reversed easily in the first generation hybrids to ensure normally fertile seed and plants. The research group is involved in discussions with universities, agricultural research organisations and seed companies in Australia, the US, New Zealand and several other countries over further research and practical ways of applying their new reversible male sterility system.

‘It’s not only good for producing seeds with hybrid vigour,’ says Professor Parish. ‘Male sterility can also ensure containment of genetically engineered plants, such as special flowers. And because there is no seed set in male sterile plants, they keep on putting energy into making more leaves. Hence, productivity is significantly increased, important for forage crops and relevant to increasing the biomass required for bioethanol production.’

The work to develop the new system has taken about eight years, and was funded by the Grains Research and Development Council.

‘We needed to find genes that were essential for pollen development and only pollen development, so that when you turn them off, the only thing affected is the pollen. And we needed to have 100 per cent certainty there was no fertile pollen whatsoever.’

The group began its search among genes coding for the master switch proteins which control development in plants. They used the genetic workhorse of the botanical world, Arabidopsis thaliana, the thale cress. The group found six genes which play a role in controlling pollen production, but finally focused on one which seemed to display the qualities they were seeking.

‘When you knocked it out, the only thing that happened to the plant was that it became male sterile. The gene didn’t seem to affect growth or anything else.’

So the researchers investigated more closely what happened when they knocked out the activity of the gene, known as AtMYB103. They were able to switch it off permanently by inserting a piece of foreign DNA. Then they observed the impact of doing this under the electron microscope. The effect was restricted to a specialised tissue called the tapetum which surrounds the chamber in which the pollen develops and provides nutrients and cell wall components. Without AtMYB103, says Professor Parish, the tapetum dies and breaks down early, resulting in malformed and sterile pollen.

But switching off the gene by inserting DNA is of no value for the production of hybrid seed because it makes the resulting plants irreversibly male sterile, and thereby reduces their capacity to reproduce and pass on the benefits of hybrid vigour beyond the first generation.

So the research group tried a couple of other ways of curtailing the activity of the gene. The AtMYB103 gene actually codes for a protein which binds to a short length of the genetic material, called a promoter. This stimulates the production of compounds which keep the tapetum healthy and functioning. What worked best in disrupting the activity of the promoter was fusing a small, carefully crafted piece of DNA onto the end of the AtMYB103 gene. This in turn added a tiny extra piece onto the protein. With this addition the AtMYB103 protein now represses rather than activates the promoters of the genes it regulates.

The scheme works perfectly, says Professor Parish. It is 100 per cent effective, and produces male sterile plants. But what’s even better is that it can be reversed. You simply add copies of the original gene without the extra piece of DNA to the plants providing the pollen in the hybrid cross. The relatively large amounts of the AtMYB103 protein they produce then successfully competes with and displaces the repressor form of the protein in the hybrid plants. Viable pollen is again produced.

‘We now have proof of concept. The whole technology is there. It just has to be applied to the various crops,’ Professor Parish says. He is convinced it can be up and running, producing hybrid seeds within a couple of years.

‘And we found the same gene is present in rice, wheat, barley, and the Brassicas, such as canola, broccoli and cabbages. It seems to be in all plants. Our future research will focus on broadening the system to other crops.

‘We are also examining how the AtMYB103 gene is turned on and the means by which it prevents programmed cell death of the tapetum.’