Plant geometry secret revealed

Researchers from the University of Leeds have discovered how plants set the angles of their branches.

The mechanism is vital to elucidating the shape of plants, and explaining how some have a broader form than others.

Dr Stefan Kepinski, senior lecturer in the University of Leeds’ Faculty of Biological Sciences and lead author of the paper published in Current Biology said: “We began working on this after a train commute into Leeds. Looking out of the window, I was struck by the fact that the way we recognise tree and other plant species from a distance is largely informed by the angle at which their branches grow.”

The angle of root and shoot branches is not usually set relative to the main root or stem from which they grow, but relative to gravity. So if a plant is placed on its side, these branches will begin a phase of bending growth, known as gravitropism that orientates them back toward their original angle of growth relative to gravity.

But many of the angles in branch and root architectures are at an angle to gravity, rather than being completely upright. The conundrum for researchers was how plants were able to set, relative to gravity, the particular non-vertical angle of growth for their branches that determines their architecture.

The team found that another growth component – the ‘anti-gravitropic offset’ counteracts the normal gravitropic growth in these lateral branches.

Branches that are growing close to the vertical have a weak anti-gravitropic offset, while in branches that are growing out at shallow angles away from the vertical the anti-gravitropic offset is relatively strong.

The researchers proved the presence of the offset by using a clinostat, which slowly rotates a plant growing on its side, thereby withdrawing a stable gravity reference. It enabled the team to monitor the anti-gravitropic offset mechanism working unopposed by a coordinated gravitropic response.

Under these conditions, they observed 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.

Kepinski said: “You can compare it to the way a tank or paddle steamer is steered. If you want to go one direction, you speed up the track or paddle on the other side. If you want to straighten up, you balance the speeds - or in our case the ‘speed’ of growth on either side of the branch. In a given non-vertical branch, the anti-gravitropic offset is constant, while gravity responsive growth increases in magnitude according to how far the branch is away from the vertical, generating a robust system for maintaining a whole array of branch angles.”