Scientists have discovered why some plants grow tall and upright and others spread out wide, in a breakthrough that could revolutionise agriculture, potentially boosting crop production worldwide.
The way plants respond to gravity and light has been long understood, but scientists at the University of Leeds have identified an anti-gravity mechanism in branches which acts as a counterbalancing force. The anti-gravitropic offset mechanism, or AGO, is stronger in some species and varieties than others.
An easy example of how different species vary in their "architecture" is to compare the tall poplar, where branches are more upright and close to the trunk, and the wide, spreading nature of an oak, where branches grow at right angles to the centre.
The same principles apply in herbaceous plants and, crucially, vegetable crops. All plants have a hormone, auxin, which responds to gravity. Auxin also governs the magnitude of the AGO. Where plants grow upright, the AGO is weaker, while those with branches spreading outwards have stronger AGO.
The mechanism is active in both shoots and roots, and scientists believe the magnitude could be increased or reduced in different crops to make them more suitable to certain soil and environmental conditions, boosting yields. In poor soils where nutrients are close to the surface but sparse further down, one improvement could be to force the roots of cereal crops to spread laterally, rather than grow downwards.
Similarly, manipulating the leaves of brassicas to be more spreading, meaning they can absorb more sunlight across the plant surface, would boost production. Varieties of oilseed rape with more upright branches are known to be better for crop yields. Altering the mechanism in crops could be achieved either through conventional breeding, or through biotechnological alternatives such as genetic modification (although the latter would be more controversial). Changing the magnitude of AGO would remove the need for unsustainable, expensive fertilisers.
The discovery at Leeds is described in the paper "Auxin Controls Gravitropic Setpoint Angle in Higher Plant Lateral Branches", published in Current Biology. Stefan Kepinski, a senior lecturer in the university's Faculty of Biological Sciences and lead author of the paper, 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.
"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."
In the lab, the scientists took an Arabidopsis thaliana (thale cress), as well as pea, bean and rice plants, turned them on their side and rotated them very slowly, which removed gravity as a factor. The AGO was identified because some species showed more bending of branches, revealing an additional factor at play.
Dr Kepinski added: "The angle of growth of branches is an exceptionally important adaptation because it determines the plant's capacity to capture resource above and below ground. Depending on what sort of soil a plant is in, it might be beneficial to be foraging for nutrients in the top soil or to be going deeper.
"Similarly, in the shoot, a plant might gain an advantage from having more steeply pitched branches to avoid shading from neighbouring plants. Until now, nobody really knew how non-vertical growth angles, referenced to gravity like this, were set and maintained."
Dr Kepinski said the benefits could be profound: "The predominant crop improvement strategy in developed countries has been to generate crop varieties that perform well on soils with high fertiliser inputs. This is undesirable because fertilisers are expensive to produce, have a significant carbon footprint and, in some cases, run-off from farmland can adversely affect natural habitats."
Suruchi Roychoudhry, who was part of the Leeds team, added: "Being able to develop crops with optimised root growth-angle characteristics which can take up nutrients in the soil more effectively would contribute to more cost-effective, environmentally-friendly and sustainable agriculture. In identifying the mechanisms controlling branch growth angle, our work offers new opportunities that are relevant to both conventional plant breeding and other biotechnological approaches to the improvement of crop performance across a range of soil and growth conditions throughout the world."
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