Leaf Veins 'Crack' Under Stress

April 11, 2008 -- Find a leaf and take a close look. Before inspecting, you might have assumed that the leaf's vein patterns fork, like tree branches splitting into twigs as they get further from the trunk, but peer carefully and you'll see that the veins make innumerable closed loops.

The pattern looks more like what you see in cracked mud or paint, where irregular mosaic patterns form on the surface as it shrinks relative to the layer beneath.

Now, researchers propose that something similar can explain the formation of the closed-loop vein pattern in leaves. But instead of the top layer shrinking, Eduardo Jagla and colleagues of the Centro Atómico Bariloche in Bariloche, Argentina, describe a faster-growing middle layer stuck between slower-growing outer layers of the leaf.

According to their model, the pressure of the extra material bulging in the middle causes some cells to collapse, creating a "crack" pattern of squished, cigar-shaped cells that differentiate into veins.

The team used a computer simulation to predict the pattern of collapsed cells that would result from the three-layer model and compared the vein widths, lengths and angles resulting from the simulation with measurements of real leaves. It was a close match.

"They have produced patterns that quantitatively look like real veins," said developmental biologist Eric Kramer of Bard College at Simon's Rock in Great Barrington, Mass., who was not involved in the study, published Thursday in PLoS Computational Biology.

Researchers continue to debate the mechanism behind leaf formation. The plant hormone auxin clearly plays a role, since plants with mutations in the auxin transport system have distorted vein patterns. However, Jagla and others suggest auxin may not be enough to explain the loop patterns.