An apple gene highly expressed in fruit.

Abstract

Although apple is an important horticultural crop, its molecular biology is not well understood. Only a few apple genes have thus far been isolated, and these include genes encoding ACC synthase (Dong et al., 1991a) and ACC oxidase (Dong et al., 1991b; Ross et al., 1992) enzymes in the ethylene biosynthetic pathway (Yang and Hoffman, 1984). The identification of other genes controlling important fruit characteristics, and a better understanding of their regulation, would enhance prospects for improving this crop using genetransfer technology . We report here a cDNA clone (AP1) that encodes a message highly expressed in apple fruit (Malus domestica Borkh. cv Golden Delicious). The clone was isolated from a cDNA library constructed using poly(A)+ RNA from cortical tissue of ripe apple fruit (Ross et al., 1992). The nucleic acid sequence of AP1 is 705 bp and contains an open reading frame encoding a 119-amino acid polypeptide (Table I). This deduced polypeptide is hydrophilic, with 32 charged residues out of 119. Southern analysis indicates that the gene coding for APl has a low copy number. Northern analysis has shown that APl expression, although highest in fruit tissue, was also detectable in apple leaf, stem, petiole, seed, and root tissue (our unpublished data). In fruit, APl mRNA was detected in a range of developmental stages, from ovaries of flowers at full bloom to ripe fruit. The AP1 nucleic acid and deduced polypeptide sequence are 66 and 83% identical, respectively, to those of XSAR5, a cDNA clone coding for a strawberry gene that is repressed by exogenously applied auxin (Reddy and Poovaiah, 1990).