Enzyme for Auxin Production Discovered

Applied Genetics News,  Feb, 2001  

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Researchers led by Joane Chory of the Salk Institute for Biological Studies (San Diego, CA) have identified a flavin monooxygenase-like (FMO) enzyme involved in the production of auxin, a plant growth hormone that influences many aspects of plant growth, including cell division and flowering.

The role of the FMO-like enzymes was discovered when the scientists created a mutant form of the plant Arabidopsis that had growth characteristics indicative of auxin overproduction. Arabidopsis, a relative of the mustard plant, is a model organism often used in plant biology research.

"We were randomly inserting into the Arabidopsis genome DNA sequences, called enhancer sequences, that promote gene activity," says Chory. "Specifically, we were looking for mutants in the light-responsive pathway."

One mutant produced during their experiments showed signs of overgrowth that is characteristic of auxin overproduction. "This mutant had long hypocotyls (primary stems) and increased apical dominance with down-curling leaves," says Chory. Apical dominance is the inhibition of lateral branching characteristically induced by auxin. The mutant was named "yucca" due to its resemblance to a yucca plant.

Analysis of the yucca-mutant plants revealed that they did, indeed, show increased auxin levels, and that these high-auxin levels caused the distinctive growth characteristics of the yucca mutant. Also, the scientists were able to repress yucca's distinctive growth characteristics by using genetic techniques that specifically reduced auxin levels in the mutant plant. Finally, the researchers overexpressed the yucca gene in tobacco plants. These experiments dramatically altered the tobacco plant, creating tobacco plants that resembled the yucca-induced changes in Arabidopsis.

The yucca gene resembled FMO genes found in mammals. Searches of the genome database of Arabidopsis revealed that the plant possessed two families of FMO-like genes. Such redundancy, says Chory, finally explains why past efforts to produce knockout auxin mutants failed.

Studies of this FMO-like enzyme in yucca revealed that the enzyme most likely catalyzes the oxygenation of the compound tryptamine. "That finding made us go back and look at the proposed pathways for auxin biosynthesis, none of which have been sorted out in the plant. And since the tryptamine that this enzyme acts on comes from the tryptophan-dependent pathway for auxin biosynthesis, this shows that in yucca, auxin biosynthesis proceeds via a tryptophan- dependent pathway," Chory concludes.

However, the picture of auxin biosynthesis is complicated by the fact that other scientists have produced mutant plants that cannot produce tryptophan, yet they can still produce auxin. "Thus, there is an alternative route, but at least now we can begin to propose a pathway for the tryptophan-dependent part of auxin biosynthesis," says Chory.

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