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.
Auxin is believed to work by appearing on the surface of leaves and triggering certain cells to transport it out of the leaf through the stem. In a self-reinforcing process, these transporting cells become able to move more auxin and stimulate neighboring cells to do the same.
Eventually the surface cells differentiate into veins, creating a network rather like tributaries flowing down to a river.
But the problem with this mechanism, Jagla pointed out, is that a river network doesn't form closed loops.
With the cell-collapse hypothesis, "veins can form closed paths without any limitation," Jagla said.
One thing his model does not explain is how plant species can have different patterns of veins at the largest scale. For instance, in some species, secondary veins branch off the either side of the main vein in an alternating pattern, while others branch off symmetrically. Jagla proposes that genetic factors driven by auxin may determine this big-picture pattern, while the smaller vein patterns form via cell collapse.
"We are convinced that the full explanation involves both auxin and elastic stress," he said.
How such a two-phase mechanism would come about is difficult to imagine, said Pavel Dimitrov of Yale University in New Haven, Conn. He published a model that explained closed loops in leaf vein networks using an auxin-based mechanism that relies on how auxin concentrations change across the leaf over time. He sees no need for another explanation.
Jagla hopes his model will encourage biologists to seek evidence for this hypothesis at the molecular and cellular level within real leaves to help settle the question.
"The jury is still out on leaf venation, and it will be out for a long time," Kramer said.
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