Radial patterning of Arabidopsis shoots by class III HD-ZIP and KANADI genes

BACKGROUND: Shoots of all land plants have a radial pattern that can be considered to have an adaxial (central)-abaxial (peripheral) polarity. In Arabidopsis, gain-of-function alleles of PHAVOLUTA and PHABULOSA, members of the class III HD-ZIP gene family, result in adaxialization of lateral organs. Conversely, loss-of-function alleles of the KANADI genes cause an adaxialization of lateral organs. Thus, the class III HD-ZIP and KANADI genes comprise a genetic system that patterns abaxial-adaxial polarity in lateral organs produced from the apical meristem. RESULTS: We show that gain-of-function alleles of REVOLUTA, another member of the class III HD-ZIP gene family, are characterized by adaxialized lateral organs and alterations in the radial patterning of vascular bundles in the stem. The gain-of-function phenotype can be obtained by changing only the REVOLUTA mRNA sequence and without changing the protein sequence; this finding indicates that this phenotype is likely mediated through an interference with microRNA binding. Loss of KANADI activity results in similar alterations in vascular patterning as compared to REVOLUTA gain-of-function alleles. Simultaneous loss-of-function of PHABULOSA, PHAVOLUTA, and REVOLUTA abaxializes cotyledons, abolishes the formation of the primary apical meristem, and in severe cases, eliminates bilateral symmetry; these phenotypes implicate these three genes in radial patterning of both embryonic and postembryonic growth. CONCLUSIONS: Based on complementary vascular and leaf phenotypes of class III HD-ZIP and KANADI mutants, we propose that a common genetic program dependent upon miRNAs governs adaxial-abaxial patterning of leaves and radial patterning of stems in the angiosperm shoot. This finding implies that a common patterning mechanism is shared between apical and vascular meristems.     

Amphivasal vascular bundle 1, a gain-of-function mutation of the IFL1/REV gene, is associated with alterations in the polarity of leaves, stems and carpels

In Arabidopsis stems, the vascular bundles in the stele are arranged in a ring-like pattern and the vascular tissues in each bundle are organized in a collateral pattern. We have shown previously that the semidominant amphivasal vascular bundle 1 (avb1) mutation transforms the collateral vascular bundles into amphivasal bundles and disrupts the ring-like arrangement of vascular bundles in the stele. In this study, we show that the avb1 mutation occurred in the putative microRNA 165 target sequence in the IFL1/REV gene and caused an amino acid substitution in the putative sterol/lipid-binding START domain. We present direct evidence that the wild-type IFL1/REV mRNA was cleaved within the microRNA 165 target sequence and the avb1 mutation resulted in an inhibition of cleavage and a higher level accumulation of full-length mRNA, suggesting a role of microRNA 165 in the regulation of IFL1/REV gene expression. In addition to an alteration in vascular patterning, the avb1 mutation also caused dramatic changes in fiber cell wall thickening and organ polarity, including aberrant formation and proliferation of cauline leaves and branches, production of trumpet-shaped leaves with reversed adaxial-abaxial identity, ectopic growth of carpel-like structures on the outer surface of carpels, and fasciation of inflorescence. Ectopic overexpression of the avb1 mutant cDNA not only phenocopied most of the avb1 mutant phenotypes but also led to additional novel phenotypes such as formation of leaves with extremely narrow blades and ectopic production of branches in the axil of siliques. Taken together, these results suggest that the avb1 gain-of-function mutation of the IFL1/REV gene alters the positional information that determines vascular patterning and organ polarity.     

Establishment of polarity in lateral organs of plants

BACKGROUND: Asymmetric development of plant lateral organs initiates by partitioning of organ primordia into distinct domains along their adaxial/abaxial axis. A recent model proposes that a meristem-born signal, acting in a concentration-dependent manner, differentially activates PHABULOSA-like genes, which in turn suppress abaxial-promoting factors. As yet, no abaxial factors have been identified that when compromised give rise to adaxialized organs. RESULTS: Single mutants in either of the closely related genes KANADI1 (KAN1) or KANADI2 (KAN2) have little or no effect on plant morphology. However, in kan1 kan2 double mutant plants, there is a replacement of abaxial cell types by adaxial ones in most lateral organs. The alterations in polarity establishment are associated with expansion in the expression domain of the PHB-like genes and reduction in the expression of the previously described abaxial-promoting YABBY genes. Ectopic expression of either of the KANADI genes throughout leaf primordia results in dramatic transformation of adaxial cell types into abaxial ones, failure of lateral blade expansion, and vascular tissue formation. CONCLUSION: The phenotypes of KANADI loss- and gain-of-function alleles suggest that fine regulation of these genes is at the core of polarity establishment. As such, they are likely to be targets of the PHB-mediated meristem-born signaling that patterns lateral organ primordia. PHB-like genes and the abaxial-promoting KANADI and YABBY genes appear to be expressed throughout primordia anlagen before becoming confined to their corresponding domains as primordia arise. This suggests that the establishment of polarity in plant lateral organs occurs via mutual repression interactions between ab/ad factors after primordium emergence, consistent with the results of classical dissection experiments.