Reproductive organ identity in Arabidopsis is controlled by the B, C and SEPALLATA classes of floral homeotic genes. We have identified a recessive mutation in a novel gene, HUA ENHANCER2 , which, when combined with mutations in two weak class C genes, HUA1 and HUA2 , leads to the production of third whorl sepal-petal-stamens and fourth whorl sepal-carpels. Quadruple mutant analysis and in situ localization of A, B, C and SEPALLATA floral homeotic RNAs suggest that HUA ENHANCER2 is required for the maintenance of B and C gene expression in the reproductive whorls. In addition to its role in floral homeotic gene expression, HUA ENHANCER2 is required for normal spacing and number of perianth organ primordia. We show that HUA ENHANCER2 encodes a putative DExH-box RNA helicase that is expressed in specific patterns in the inflorescence meristem and developing flowers. As a possible ortholog of the yeast exosome-associated protein, Dob1p (Mtr4p), HUA ENHANCER2 may affect floral organ spacing and identity through the regulation of protein synthesis or mRNA degradation. Therefore, our studies on HUA ENHANCER2 not only demonstrate that B and C gene expression is established and maintained separately, but also implicate the existence of post-transcriptional mechanisms in the maintenance of B and C gene expression.