The vaccine could increase yields of major crops such as wheat and barley by up to 30% by activating plant defense mechanisms to knock out diseases before they take hold, in the same way a shot in the arm can protect humans from flu.

But unlike other forms of genetically modified (GM) food, rejected by many consumers around the world, the Australian technique does not change plants by inserting a foreign gene. It simply silences an existing gene.

``From that point of view it's perhaps more acceptable to consumers,'' scientist Peter Waterhouse said on Wednesday. ``It's very exciting. It's enabling technology, you can do all sorts of different things with it.''

The vaccine could be used to knock out plant genes to produce non-browning bananas, caffeine-free coffee and many other changes in plants that do not involve altering their protein structure, said Waterhouse, a scientist with the government's Commonwealth Scientific and Industrial Research Organization (CSIRO).

The technique can also silence unwanted genes that produce allergens in nuts or pollen.

TECHNOLOGY READY TO ROLL

The technology, recently proven in barley plants raised in greenhouses and about to be used in trial crops, involves inserting a small, incomplete piece of virus RNA into plant DNA.

DNA, or deoxyribonucleic acid, is the material from which genes--which transmit hereditary characteristics--are made. RNA, or ribonucleic acid, helps cells make proteins, and is also the hereditary material in some viruses.

The plant recognizes the virus RNA as foreign and activates its defense mechanism, degrading the invader before it multiplies.

This results in immunity to the virus, which scientists say can be passed down through plant generations.

When the breakthrough will be brought to market is complicated by cross-patent negotiations with multinational corporations active in the GM business, as well as by consumer reaction. But technologically the process is ready to roll.

``From a technical point of view there's nothing stopping us getting plants that would be ready to be commercialized,'' Waterhouse told Reuters.

The technology is likely to be commercialized first in a proven process which switches off unwanted genes in cottonseed, producing ``healthier'' cooking oil, he said.This is three to five years away from commercialization, research team leader Allan Green told Reuters on Wednesday.

The CSIRO team has also already developed potatoes resistant to potato leaf roll virus using the technique, with plants tested in laboratories, glasshouses and in field trials.

Waterhouse said the vaccination technology would be worth millions, if not billions, of dollars in license fees and increased productivity.

HUGE SAVINGS SEEN

Savings to Australia's big wheat crop alone, worth around A billion a year on world markets (about billion US), would be huge.

Multinationals would have to buy licenses from the CSIRO if they wanted to use the technology, which was in the process of being patented, he said. Hundreds of international laboratories had already requested technical details.

``It's a technology everyone wants to use.''

CSIRO's biggest breakthrough with the new technique has come with successful trials to knock out the major world-wide crop disease barley yellow dwarf virus (BYDV), which affects Australia's wheat, barley and oats crops.