Researchers offer clues to how leaf patterns are formed

“For years people have been trying to understand this beautiful formation,” said Dr. Enrico Scarpella, from the U of A's Department of Biological Sciences. “We were able to connect the mechanism responsible for the initiation of the veins in the leaf with that of formation of the shoot and root. With our piece of the puzzle added, it indeed seems the same mechanism is responsible for all these events.”

What Scarpella and his research team–Dr. Thomas Berleth's group from the University of Toronto and Dr. Jiri Friml from the University of Tuebingen–discovered has interested scientists around the world. For several years it has been known that a hormone called auxin stimulated the formation of the veins. “It was believed that auxin would behave like man–build the streets on which man himself would travel,” said Scarpella. “However, the theory argued that in each individual vein auxin could only run one way at any given time, making them sort of alternate one-way streets.”

By labeling the protein that transports the hormone auxin with a fluorescent tag, he could then shine a light on the leaves and watch how auxin was being transported during vein formation. Thanks to this approach, the team identified cells within individual veins that transport the hormone auxin in two opposite directions. He also showed for the first time that the epidermis of the leaf is very important in the transport of this hormone and in the formation of the veins.

One of the objections to the idea that veins might act as a channel to transport auxin was that there were mutant leaves that produced dotted, rather than continuous veins for auxin to run through. But the research team showed that the leaves with the dotted veins were a mature version and that at an earlier stage, the veins were continuous and did act as transporters. “We didn't have the technology to see those early stages before and now we do,” he said. “We now know that the veins are the backbone of the leaf and are somehow responsible for the final shape of the plant.”

But one of the biggest discoveries, perhaps the one with the most evolutionary implications, is that plants use the same mechanism to regulate vein formation in the leaf and branch formation on the main trunk and on the main root. The finding that the leaf is like a two-dimensional model of a tree may change the way plant scientists work, says Scarpella. “If each leaf can make more than 100 veins, you can see the process over and over compared to the formation of branches in a big, three-dimensional slowly growing tree or the difficulties in studying root branching in their natural environment, which is the dirt,” he said. “Our findings will contribute to the way we will manipulate plant development to our advantage. Once we know all the players in the game we will be able to say, we want more leaves on this, more branches on this one or fewer flowers on this plant.”