How do grass leaves get their shape?

Every time a human cuts grass with a lawnmower or an animal eats it, the leaves always grow back remarkably fast, and this is due to their shape. However, the way in which the shape of a grass leaf arises has not been fully understood by scientists.

A collaborative project led by researchers at the John Innes Centre has been investigating exactly how the shape of grass leaves arise, applying the concept to other crops such as wheat, rice, and maize.

Grass is a form of monocot, a type of flowering plant that has leaves which encircle the stem at their base and have parallel veins throughout. The other key type of flowering plant is the eudicot (such as brassicas, legumes, shrubs, and trees), and these are structured in the typical manner of stalks, petioles, and net-like veins

Furthermore, the tube-like structure that forms the base of the leaf, known as the sheath, allows grasses to increase in height whilst protecting themselves from lawnmowers and animals by keeping its base tip very close to the ground.

The researchers used cutting-edge computational modeling and developmental genetic techniques in order to get to the bottom of the mystery. 

“By formulating and testing different models for its evolution and development, we’ve shown that current theories are likely incorrect and that a discarded idea proposed in the 19^th century is much nearer the mark,” explained study co-author Professor Enrico Coen.

It was initially proposed by 19th-century botanists that the sheath of grass was similar to the petiole of eudicot leaves. However, this was disputed in the 20^th century, as plant anatomists discovered that petioles had parallel veins, assuming that the majority of the grass leaf (excluding the tip of the stem) was derived from the petiole.

With new analysis tools at their disposal, the collaborative research team revisited the controversial debate, modeling different hypotheses for the growth of grass leaves and comparing the predictions with experimental results. The researchers concluded that the original 19th-century idea of sheath-petiole similarity was significantly stronger than the 20th-century reassessment. 

Ultimately, the findings demonstrate how small modulations of growth rules based on common genetic activity patterns can generate a wide range of different leaf shapes across many crops.

The study is published in the journal Science.

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By Calum Vaughan, Earth.com Staff Writer