Alteration of the shoot radial pattern in Arabidopsis thaliana by a gain-of-function allele of the class III HD-Zip gene INCURVATA4.

Class III HD-Zip (HD-Zip III) family genes play key roles in a number of fundamental developmental programs in Arabidopsis thaliana, such as embryo patterning, meristem initiation and homeostasis, lateral organ polarity and vascular development. Semidominant gain-of-function alleles of the HD-Zip III genes PHABULOSA (PHB), PHAVOLUTA (PHV) and REVOLUTA (REV) disrupt the negative regulation of these genes by a mechanism of microRNA interference. We provide evidence that the gain-of-function icu4-1 allele of INCURVATA4, a gene encoding the HD-Zip III transcription factor ATHB15/CORONA (CNA), stimulates the production of vascular tissues, supporting a role for ICU4 in promoting vascular development. Occasionally, homozygous mutants for this allele show a reduced number of thick shoot vascular bundles, although normal collateral polarity remains unchanged. Genetic analysis of icu4-1 and phb-1D, a gain-of-function allele of the related PHB gene, revealed antagonism in lateral organ polarity between both mutations and a synergistic interaction in shoots, with transformation of the polarized collateral bundles into a radialized amphivasal pattern. These results indicate that the precise regulation of HD-Zip III genes confers positional information which is required to establish the number and pattern of vascular bundles in the stem. In addition, we present results that suggest an interaction between ICU4 function and auxin signaling.