Molecular Distinction between Alternative Oxidase from Monocots and Dicots

Abstract

The alternative oxidase (Aox) is encoded in two discrete gene subfamilies in higher plants. Aox1 is most widely known for its induction by stress stimuli in many tissues and is present in both monocot and eudicot plant species. Aox2, on the other hand, is usually constitutive or developmentally expressed in eudicot species but is absent from the genomes of all monocot species examined to date. This molecular distinction suggests a divergence of Aox across plant families and may even have implications for the role of this enzyme in different plant species. The attention of plant biologists was first drawn to this respiratory oxidase through its pivotal role in the thermogenic climacteric that occurs during the fertilization of flowers from aroid monocot species. The first Aox antibody and gene sequence came from work on the thermogenic spadix of Sauromatum guttatum (Aox1;Elthon et al., 1989; Rhoads and McIntosh, 1991). Subsequent approaches including cDNA library screening, PCR with degenerate primers designed from conserved regions, and complementation methods have identified cDNAs encoding Aox1-type sequences from a wide variety of non-thermogenic monocot and dicot plants, together with fungi and protista. Initial reports indicated a single nuclear gene in many species (Rhoads and McIntosh, 1991; Kumar and Soll, 1992). It has subsequently been shown that a small gene family exists in soybean (Glycine max), tobacco (Nicotiana tabacum), rice (Oryza sativa), Arabidopsis, and mango (Mangifera indica; Whelan et al., 1996; Ito et al., 1997; Saisho et al., 1997; Considine et al., 2001).