Scientists Reveal Secrets of Plant Architecture: The Angles of Trees

Plants rely on their ability to absorb light efficiently, gathering it in their leaves in order to create energy to grow and thrive. You'd think that an angle at which a plant sets its branches and leaves would be crucial for this adaptation. Yet some trees have vertical branches while others have much flatter ones. Now, scientists have discovered what determines the architecture of a plant, revealing a little bit more about how they function.

The lead author, Stefan Kepinski, first became interested in the shape of plants during a train commute into Leeds. As he glanced out the window, he noticed how all of the different trees and branches that flashed by the train had different structures. But why, he wondered, did they have all of these different shapes in the first place?

"These characteristic angles are all around us and the same thing is happening underground; different varieties within species often have very distinct root-system architectures that are determined mainly by the growth angle of lateral roots," said Kepinski in a news release.

In order to find out exactly what was responsible for these different shapes, Kepinski and his team examined the mechanisms for root and shoot growth. The angle of root and shoot branches is not usually set relative to the main root or stem from which they grow. Instead, it's set relative to gravity. Yet while scientists knew that gravity played a role, they weren't sure how it set the architecture of branches and roots to be at a particular non-vertical angle of growth, known as the "gravitropic set-point angle."

The scientists ended up using a clinostat in order to examine plant growth. This tool slowly rotates a plant growing on its side, which withdraws a stable gravity reference and enabled the researchers to monitor the anti-gravitropic offset mechanism working unopposed by a coordinated gravitropic response. In the end, the scientists found that shoot and root branches displayed an outward bending growth, away from the main root and shoot that would normally be masked by the interaction with gravity-sensitive growth.

"We have found that another growth component--the 'anti-gravitropic offset'--counteracts the normal gravitropic growth in these lateral branches," said Kapinski in a news release. "This offset mechanism sustains growth on the other side of a branch from the gravity-sensitive growth and prevents the branch from being moved beyond a set angle to the vertical. It turns out that this countervailing growth is also driven by auxin, the same hormone that causes gravity responsive growth on the lower side of the branch."

The findings are important for understanding plants and for breeding and biotechnological approaches to crop improvement. The architecture of plants play a huge role in how well a plant functions, which means that researchers could potential alter the shape of a species in order to optimize its performance.

The findings are published in the journal Current Biology.