Tracks for cellulose

Flow makes vein L eaf veins, just like roots and shoots, use fl ows of the plant hormone auxin to drive their patterning, say Enrico Scarpella (University of Alberta, Canada), Thomas Berleth (University of Toronto, Canada), and colleagues. Models for leaf vein patterning have been based on either auxin fl ows or reaction–diffusion systems. The fl ow models had a hard time explaining how loops would arise. But “all the modeling so far has been based on the fi nal pattern,” says Berleth. “Now fl ow can be reconciled with closed networks, although other types of cellular interactions may also play a role.” His team used the auxin effl ux protein AtPIN1 as an early marker of vein formation. Expression was found at a series of “convergence points” at the leaf margin, from where it led down paths that foreshadowed fi rst the leaf’s main central vein and then other major veins connected to the main vein. Auxin is presumed to fl ow from leaf margin to the interior, consistent with AtPIN1’s polarization to the parts of the cells closest to the main vein. Loops could also form, however, because certain cells had bipolar AtPIN1, allowing auxin fl ow to the main vein via two alternate routes. Auxin turns on AtPIN1 expression, thus refi ning and reinforcing the pattern. Interfering with auxin fl ow disturbs vein formation and spacing. Vein placement seems to depend on the positions of convergence points, but those may be defi ned both by auxin’s positive feedback on its own fl ow and on undefi ned genetic circuits.