Transformation of the collateral vascular bundles into amphivasal vascularbundles in an arabidopsis mutant

Arabidopsis inflorescence stems develop a vascular pattern similar to that found in most dicots. The arrangement of vascular tissues within the bundle is collateral, and vascular bundles in the stele are arranged in a ring. A lthough auxin has been shown to be an inducer of vascular differentiation, little is known about the molecular mechanisms controlling vascular pattern formation. By screening ethyl methanesufonate-mutagenized populations of A rabidopsis, we have isolated an avb1 (amphivasal vascular bundle) mutant wi th a novel vascular pattern. Unlike the collateral vascular bundles seen in the wild-type stems, the vascular bundles in the avb1 stems were similar t o amphivasal bundles, i.e. the xylem completely surrounded the phloem. Furt hermore, branching vascular bundles in the avb1 stems abnormally penetrated into the pith, which resulted in a disruption in the ring-like arrangement of vascular bundles in the stele. The avb1 mutation did not affect leaf ve nation pattern and root vascular organization. Auxin polar transport assay indicated that the avb1 mutation did not disrupt the auxin polar transport activity in inflorescence stems. The avb1 mutation also exhibited pleiotrop ic phenotypes, including curled stems and extra cauline branches. Genetic a nalysis indicated that the avb1 mutation was monogenic and partially domina nt. The avb1 locus was mapped to a region between markers mi69 and ASB2, wh ich is covered by a yeast artificial chromosome clone, CIC9E2, on chromosom e 5. Isolation of the avb1 mutant provides a novel means to study the evolu tionary mechanisms controlling the arrangement of vascular tissues within t he bundle, as well as the mechanisms controlling the arrangement of vascula r bundles in the stele.