Vein Patterning by Tissue-Specific Auxin Transport
Unlike in animals, in plants vein patterning does not rely on direct cell-cell interaction and cell migration; instead, it depends on the transport of the plant signal auxin, which in turn depends on the activity of the PIN-FORMED1 (PIN1) auxin transporter. The current hypotheses of vein patterning by auxin transport propose that in the epidermis of the developing leaf PIN1-mediated auxin transport converges to peaks of auxin level. From those convergence points of epidermal PIN1 polarity, auxin would be transported in the inner tissues where it would give rise to major veins. Here we tested predictions of this hypothesis and found them unsupported: epidermal PIN1 expression is neither required nor sufficient for auxin-transport-dependent vein patterning, whereas inner-tissue PIN1 expression turns out to be both required and sufficient for auxin-transport-dependent vein patterning. Our results refute all vein patterning hypotheses based on auxin transport from the epidermis and suggest alternatives for future tests.