Master hormone controls plant growth

Master hormone controls plant growth

A single hormone co-ordinates how a plant grows in response to the environment, researchers have found.

Plant molecular biologist Dr Phil Brewer, of the University of Queensland, and colleagues, report their findings about a chemical called strigolactone, this week in the Proceedings of the National Academy of Sciences.

"It's a very key hormone," says Brewer.

Three years ago, Brewer and colleagues published research in the journal Nature that showed strigolactone determines whether a plant grows long and skinny or broad and bushy.

When nutrient levels or light levels are low, strigolactone levels rise, suppressing the development of buds into branches, so the plant grows tall and skinny.

This enables the plant to reach more light and also maximises the amount of energy that goes into reproduction. Energy is therefore focussed on producing flowers and seeds, rather than vegetative growth.

When there is a lot of light and nutrients around, strigolactone levels fall, encouraging branching and making a plant that is broad and bushy and able to make the most of abundant resources.

Now, the researchers have found this is not all that strigolactone does.

"Initially we thought the hormone strigolactone was all about branching," says Brewer. "But now we're finding the hormone is involved in quite a lot of other things."

Stem thickening

Brewer and colleages have found that when strigolactone levels are high, not only does this stop buds from turning into branches, but it also thickens up the main stem.

This makes sure that a plant growing tall to reach the light, also has the structural strength to do so.

"We now think that this is a hormone that co-ordinates a response for the whole plant," says Brewer. "It's not just about the branching, it's also about other parts of the plant. It's about optimising its growth."

He says that for many years, scientists thought the thickening of stems was controlled by a chemical called auxin, but these latest findings challenge this.

"This is a breakthrough for us because it shows that auxin works through strigolactones to do this job," says Brewer. "It's a big change in the dogma of the field."

Brewer says he and collaborators are also finding strigolactone influences other parts of the plant too.

When nutrient levels are low, strigolactone levels rise and this stimulates production of root hairs and beneficial mycorrhizal fungi, which both help increase uptake of nutrients.

On the downside, some parasitic weeds have hijacked this system, says Brewer.

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