A Second Cell Wall Acid Invertase Gene in Arabidopsis thaliana

Invertases, or P-fructofuranosidases, hydrolyze SUC into Fru and Glc. Immobilized acid invertases in plants have been described as localized in the cell wall (Eschrich, 1980; Gogarten, 1986; Sturm and Chrispeels, 1990). Soluble acid invertases were ascribed to the vacuole (Leigh et al., 1979). At least in dicotyledonous plants these organelle-specific isoforms are encoded by separate genes (Sturm and Chrispeels, 1990; Unger et al., 1992). In maize a single mutation causes miniature seeds (Miller and Chourey, 1992) and these mutant seeds do not contain insoluble or soluble acid invertase. A characteristic of the known cell wall invertases in dicotyledonous plants is their acid pH optimum and their high pI (Eschrich, 1980; Gogarten, 1986; Hedley et al., 1993). The latter probably facilitates immobilization of the enzyme in the cell wall by ionic interactions; the former was interpreted to represent an acidcontrolled switch for photosynthate partitioning (Eschrich, 1980). We report here the nucleotide sequence of a gene encoding an acid invertase in Arabidopsis thaliana (Table I). The predicted pI of the encoded protein and the high sequence similarity to the carrot cell wall invertase (Table I), as well as our phylogenetic analysis (Fig. l), indicate that this gene encodes a cell wall isoform. Recently another cell wall acid invertase from A. thaliana (isoform 1) was reported (Schwebel-Dugue et al., 1994). The deduced amino acid sequences of the two Arabidopsis acid invertases are only 54% identical; however, both invertases have a high predicted pI (isoform 1,9.5; isoform 2,9.7), and both are more similar to the carrot cell wall invertase (isoform 1, 54.3%; isoform 2, 66.5% identity) than to the soluble intracellular acid invertase from carrot (isoform 1,46.4%; isoform 2,43% identity) (see legend to Fig. 1 for accession numbers; see Unger et al. [19921, for the characterization of the soluble invertase from carrots). To determine whether these two cell wall isoforms are restricted to Arabidopsis or whether they have a wider phylogenetic distribution, we reconstructed the evolutionary history of severa1 cell wall invertases whose sequences had been submitted to GenBank (Fig. 1). As expected from