Slow grow.
Flowers without the growth hormone auxin (bottom) struggle to grow normal female parts (top).

Yunde Zhao, UCSD

Bud on Tap

Ever since Charles Darwin and his son Francis noticed that plants with severed tips don't grow towards light, scientists have known that plants somehow control their own growth. The mechanism, discovered in the 1920s, turned out to be controlled by hormones called auxins. Now, a study identifies a class of genes that determines exactly where and when these hormones are released. The new insights could help breeders more accurately control the size and character of plants.

The newly identified mechanism involves a family of 11 genes called YUCCA, or YUC for short. In 2001, researchers showed that plants that overexpressed YUC genes were taller than normal plants, much like plants that overproduce auxins. But that still didn't tell researchers exactly what the YUC genes did.

For that, plant biologist Yunde Zhao of the University of California, San Diego, and his colleagues developed a way to visualize the expression of YUC genes. The team genetically modified an Arabidopsis plant so that its tissues turned blue if the YUC genes were active. The researchers found that the coloration appeared wherever the plant was growing--in floral buds and young leaves, for example--but was barely seen in mature plants.

Furthermore, by disabling some of the YUC genes, the team showed that they play a direct role in making auxins. Various combinations of knockouts caused defects in reproductive organs, floral patterning, and the vascular system--the kinds of defects caused by a lack of auxins. To seal the deal, the researchers modified the plants to express auxin genes; with the hormone present, successive generations of the plants began growing normally. The team reports its results in the current issue of Genes and Development.

Zhao believes the research will have far reaching applications in both science and industry. If researchers can use a plant's genes to manipulate its growth, he says, "we can engineer plants with stronger vascular tissue and can produce more biomass" without having to rely on adding external auxins. And because auxins also control seed formation, manipulating YUC genes may be a good way to produce new kinds of seedless fruits.

The study is an important step in understanding the mechanisms involved in plant growth, says biologist Ottoline Leyser of the University of York, U.K.

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