Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots

Abstract

The upper side of the angiosperm leaf is specialized for efficient capture of sunlight whereas the lower side is specialized for gas exchange. In Arabidopsis, the establishment of polarity in the leaf probably requires the generation 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 Arabidopsis PHABULOSA and PHAVOLUTA genes in the perception of radial positional information in the leaf primordium. Dominant phabulosa (phb) and phavoluta (phv) mutations cause a dramatic transformation of abaxial leaf fates into adaxial leaf fates. They do so by altering the predicted sterol/lipid-binding domains of ATHB14 and ATHB9, proteins of previously unknown function that also contain DNA-binding motifs. This change probably renders the protein constitutively active, implicating this domain as a central regulator of protein function and the PHB and PHV proteins as receptors for an adaxializing signal.

Of Blades and Branches: Understanding and Expanding the Arabidopsis Ad/Abaxial Regulatory Network through Target Gene Identification

“…Plants carrying mutations that cause adaxialization show upward leaf curling, indicating that the bottom half of the leaf grows more than the top half (McConnell et al 2001;Ochando et al 2006;T. Huang et al, unpubl.).…”

“…In the embryo, the formation of distinct adaxial/abaxial domains of the cotyledons (or seed leaves) is detectable in the globular embryo before cotyledon outgrowth occurs (McConnell et al 2001). Roughly equal rates of growth of the ad-and abaxial cotyledon domains cause the embryo Schematic of cross section through a typical leaf.…”

“…2A,B), whereas the abaxial cotyledon domains are continuous with the outer cortical and epidermal layers of the hypocotyl. The criterion for calling these domains is based on domains of gene expression -for instance, the adaxial domain accumulates HD-ZIPIII mRNA (McConnell et al 2001; Fig. 2C,D).…”

Embryogenesis: Pattern Formation from a Single Cell

“…Although single mutants have no abnormal phenotype, double and triple mutants cannot properly generate a SAM (McConnell et al, 2001;Emery et al, 2003;Prigge et al, 2005). The class III HD-ZIP expression domain is restricted by microRNA, namely the miR165/6 family (Williams et al, 2005).…”

“…However, the antagonistic, gain-offunction genotypes caused by ectopic expression of FIL or YAB3 in which the SAM stops functioning support an important role for the YAB genes in promoting abaxial fate (Siegfried et al, 1999). A current model describing this network has the class III HD-ZIP genes defi ning the central region of the embryo, as well as the adaxial side of the cotyledons, while the KAN and YAB genes defi ne the periphery of the embryo and the abaxial side of the cotyledon primordia as shown in fi gure 15H (Kerstetter et al, 2001;McConnell et al, 2001). In summary, a complex network of interdependent processes regulates patterning in the apical domain.…”