Plant's genetic 'master switch' subverted.

Plant's genetic 'master switch' subverted

Minutes after a water-starved plant begins wilting, concentrations of a natural hormone soar to 10 times their previous levels. The dramatic rise in the hormone, known as abscisic acid or ABA, triggers a cascade of chemical changes that helps the plant resist drought and disease.

Investigators have also found that ABA influences a plant's early maturation, preventing vulnerable seeds from germinating too rapidly if drought conditions prevail. But until recently, biologists knew little about how the hormone worked on the molecular level.

Researchers have now identified a genetic sequence that appears to act as a "master switch" turning on several genes in response to ABA. The work suggests that by placing copies of the switch near plant genes not normally activated by the hormone, scientists might create plants in which ABA triggers the production of a wide variety of protective proteins. In addition, studies of the switch may provide clues to how other plant hormones function on the molecular level.

Molecular biologist Ralph Quatrano began the work by soaking wheat embryos (the source of wheat germ) in a solution of ABA. He found that the drenched embryos responded by accelerating production of two normal proteins -- a storage protein known as triticin and a compound known as Em, which may play a role in drought resistance. Quatrano, then at Oregon State University in Corvallis, documented that ABA had two main effects on the genes encoding these proteins. The hormone increased the transcription of DNA into messenger RNA (mRNA) -- a key step in protein production -- and it stabilized the normally fragile mRNA produced. ABA's dual role may explain its ability to activate several genes in both mature plants and embryos, Quantrano says.

Working at the University of North Carolina at Chapel Hill with William Marcotte and Mark Guiltinan, Quantrano went on to identify and examine a genetic sequence within the gene encoding the Em protein in wheat embryos. When this sequence, known as the Em promoter, senses high levels of ABA, it activates a chain of commands that switches on the Em gene to produce the Em protein, he says.

Recently, Quatrano and his colleagues isolated a 76-base-pair fragment from the sequence, which has a total of 1,800 base pairs. That fragment contains the active Em switching site common to both wheat and rice plants, he reported this week in Richmond, Va., at the annual meeting of the American Institute of Biological Sciences.

The team also discovered several tantalizing hints that genetic engineering might someday enhance ABA's influence on plant development. Quantrano and his co-workers developed a raid in vitro assay, which showed that the Em promoter, in the presence of ABA, could activate a bacterial gene placed near it -- genetic material not normally controlled by the hormone. Moreover, in searching through mRNA derived from wheat embryo, they identified several DNA segments coding for proteins that appear to interact with the 76-base, ABA-sensitive fragment of the Em promoter.

Quatrano suggests that genetic engineers might insert the Em promoter near several plant genes to bring them under the control of naturally occurring or even artificially produced ABA.

Marcotte adds that the group's results complement other researchers' findings that similar master switches influence the response to other hormones -- including ethylene, which promotes ripening, and gibberellins, which affect growth. But because ABA can occur in high concentrations in plant embryos and can protect both immature and adult plants from sudden environmental changes, he says, its switch may have special significance for strengthening crops.