Plant tissue generally grows at the tips of plants, so how can grass grow back when you mow your lawn? In grasses, the base of the leaf forms a tube-like structure called the sheath. The sheath allows the plant to increase in height while keeping its growing tip close to the ground, protecting it from lawnmowers’ blades or herbivores’ incisors.
The secret to its remarkable regenerative powers lies in part in the shape of its leaves, but how that shape arises has been a topic of longstanding debate. The debate is relevant to our staple crops wheat, rice and maize, because they are members of the grass family with the same type of leaf.
Flowering plants can be categorised into monocots and eudicots. Monocots, which include the grass family, have leaves that encircle the stem at their base and have parallel veins throughout. Eudicots, which include brassicas, legumes and most common garden shrubs and trees, have leaves that are held away from the stem by stalks, termed petioles, and typically have broad laminas with net-like veins.
In the 19th Century, botanists proposed that the grass sheath was equivalent to the petiole of eudicot leaves. But this view was challenged in the 20th century, when plant anatomists noted that petioles have parallel veins, similar to the grass leaf, and concluded that the entire grass leaf (except for a tiny region at its tip) was derived from the petiole.
Richardson and colleagues combined developmental genetics and computational modelling to examine both types of leaf. In their paper, the team write: “These modeled hypotheses make different assumptions and predictions. The petiole-leaf hypothesis assumes additional proximal-distal domains and is therefore less parsimonious. The petiole-leaf hypothesis also predicts that petiole mainly derives from the middle of the early primordium…, whereas the petiole-sheath hypothesis predicts that petiole derives from the primordium base…. Cell tracking shows that petiole derives from proximal primordium cells with high proximodistal growth rates, supporting the petiole-sheath prediction…”
One of the corresponding authors, Professor Enrico Coen said in a press release: “The grass leaf has been a conundrum. 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 the 19th century is much nearer the mark.”
The grass study shows how simple modulations of growth rules, based on a common pattern of gene activities, can generate a remarkable diversity of different leaf shapes, without which our gardens and dining tables would be much poorer.
Richardson, A.E., Cheng, J., Johnston, R., Kennaway, R., Conlon, B.R., Rebocho, A.B., Kong, H., Scanlon, M.J., Hake, S., Coen, E., 2021. Evolution of the grass leaf by primordium extension and petiole-lamina remodeling. Science. https://doi.org/10.1126/science.abf9407