Auxin polar transport flanking incipient primordium initiates leaf adaxial-abaxial polarity patterning

The leaves of most higher plants are polar along their adaxial-abaxial axis, and the development of the adaxial domain （upper side） and the abaxial domain （lower side） makes the leaf a highly efficient photosynthetic organ. It has been proposed that a hypothetical signal transported from the shoot apical meristem （SAM） to the incipient leaf primordium, or conversely, the plant hormone auxin transported from the leaf primordium to the SAM, initiates leaf adaxial-abaxial patterning. This hypothetical signal has been referred to as the Sussex signal, because the research of fan Sussex published in 1951 was the first to imply its existence. Recent results, however, have shown that auxin polar transport flanking the incipient leaf primordium, but not the Sussex signal, is the key to initiate leaf polarity. Here, we review the new findings and integrate them with other recently published results in the field of leaf development, mainly focusing on the early steps of leaf polarity establishment.