Molecular distinction between alternative oxidase from monocots and dicots.

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 nonthermogenic 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). Studies have revealed differential expression of the soybean genes in response to developmental cues and environmental perturbations (Finnegan et al., 1997; Millar and Day, 1997; McCabe et al., 1998; Tanudji et al., 1999). In soybean, Aox1 expression has only been documented after respiratory inhibition of cell cultures by antimycin A (Finnegan et al., 1998; Tanudji et al., 1999). The two soybean Aox2-type isozymes, Aox2 and Aox3 (now renamed Aox2a and Aox2b), are more prevalent, although differentially regulated (Finnegan et al., 1997; McCabe et al., 1998). Most studies to date have focused on the relationship of Aox1-type gene expression to stress adaptation (Vanlerberghe and McIntosh, 1997, and refs. therein). We present results from our own experimental studies and a review of available genomic and expressed sequence tag (EST) data, showing that whereas Aox2 is found in many eudicots, including soybean, mango, tobacco, tomato (Lycopersicon esculentum), and Arabidopsis, it is absent in all monocots examined to date. The fact that the Aox2 family represents the major form expressed in eudicots studied to date, where it shows tissue and developmental stage specificity but does not respond to stress, has implications for the role and function of Aox proteins in different plant species.