Characterization of two members of the Arabidopsis thaliana gene family, At beta fruct3 and At beta fruct4, coding for vacuolar invertases.

We have isolated and characterized two Arabidopsis thaliana cDNAs and their cognate genes, At beta fruct3 and At beta fruct4, encoding vacuolar forms of invertase. Our sequencing results showed that the gene At beta fruct3 is located downstream of the 3-ketoacyl-acyl carrier protein synthase III gene (AtKasIII). At beta fruct3 and 4 are functional and organized into seven exons and six introns with an identical organization. The At beta fruct3 and At beta fruct4 genes encode, respectively, polypeptides of 648 and 664 residues that contain all the characteristic hallmarks of vacuolar invertases. A. thaliana is the first plant of which both cell-wall (At beta fruct1 and At beta fruct2) and vacuolar (At beta fruct3 and At beta fruct4) genes are characterized. The same number of exons and introns is seen in the genes At beta fruct1, At beta fruct3 and At beta fruct4 as well as in all other invertase genes described to date. However, the position of the third intron is different in At beta fruct3 and At beta fruct4. At beta fruct2 shows a different organization. A neighbour-joining distance tree shows that the A. thaliana vacuolar invertases described here are, as expected, more closely related to vacuolar invertases from other plant species (e.g., carrot) than to the A. thaliana cell-wall invertases. The evolution of plant invertase genes from a common ancestral gene is discussed. Our results demonstrate that in A. thaliana, at least two genes encoding vacuolar invertases are expressed during the development of the plant. Southern blot hybridization experiments suggest the presence of one copy of, respectively, At beta fruct3 and At beta fruct4 per haploid genome, and Northern blot analysis demonstrates that vacuolar invertase genes are highly expressed in stems, roots, flowers and at very low levels in mature leaves.