PINning down the connections: transcription factors and hormones in leaf morphogenesis.

The past year has seen major advances in delineating the regulatory circuits that control leaf morphogenesis. Regulation of auxin transporters is pivotal in generating auxin gradients that define the sites of leaf inception. In addition, auxin acts via the ARGOS protein to regulate leaf size. HD-ZIPIII transcription factors controlled by micro-RNAs are required to specify adaxial identity in the leaf and vasculature. Elaboration of final leaf shape also depends on the regulated arrest of cell division, and the TCP and JAGGED transcription factors are key regulators of this process. Furthermore, the juxtaposition of abaxial and adaxial identities regulates leaflet placement in dissected leaf species.