Isolation and characterization of the Arabidopsis organ fusion gene HOTHEAD.

The outer epidermal plant cell wall and cuticle play an important role in regulating both abiotic and biotic interactions between the plant and its environment. In addition to acting as a protective barrier that limits water loss, the effects of detrimental irradiation and invasion by pathogens, the epidermis also offers an interface that is inert to interactions between organs and ensures proper separation and expansion of organs at the growing points of the plant. Here, we describe the molecular cloning and characterization of HOTHEAD (HTH), a gene required to limit cellular interactions between contacting epidermal cells during floral development. HTH is a member of a small gene family in Arabidopsis and encodes an enzyme related to a group of FAD-containing oxidoreductases that have been described in several other species. Characterization of 11 independently derived mutant alleles suggests that key amino acids are shared between these related groups of enzymes and identify a cluster of other functionally important residues that are highly conserved only within the Arabidopsis gene family. Our findings add this new type of enzyme to a growing list of enzymes that have been shown to be involved in regulating post-genital organ fusion. Expression analysis of the HTH gene shows that it is expressed in all tissues tested, including roots, and is not epidermis-specific. Furthermore, the sequence data unequivocally show that none of the alleles isolated are epigenetic alleles as suggested by genetic behavior previously observed at this locus.