Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots

The upper side of the angiosperm leaf is specialized for efficient capture of sunlight whereas the lower side is specialized for gas exchange. In Arab idopsis, the establishment of polarity in the leaf probably requires the ge neration and perception of positional information along the radial (adaxial versus abaxial or central versus peripheral) dimension of the plant. This is because the future upper (adaxial) side of the leaf develops from cells closer to the centre of the shoot, whereas the future under (abaxial) side develops from cells located more peripherally. Here we implicate the Arabid opsis PHABULOSA and PHAVOLUTA genes in the perception of radial positional information in the leaf primordium. Dominant phabulosa (phb)(1) and phavolu ta (phv) mutations cause a dramatic transformation of abaxial leaf fates in to adaxial leaf fates. They do so by altering the predicted sterol/lipid-bi nding domains of ATHB14 and ATHB9, proteins of previously unknown function that also contain DNA-binding motifs. This change probably renders the prot ein constitutively active, implicating this domain as a central regulator o f protein function and the PHB and PHV proteins as receptors for an adaxial izing signal.