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 avbl 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 avbl 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 pattern-ing, the avbl 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 fascia-tion of inflorescence. Ectopic overexpression of the avbl mutant cDNA not only phenocopied most of the avbl 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 sil-iques. Taken together, these results suggest that the avbl gain-of-function mutation of the IFL1/REV gene alters the positional information that determines vascular patterning and organ polarity.