Distinct Physiological Roles of Fructokinase Isozymes Revealed by Gene-Specific Suppression of Frk1 and Frk2Expression in Tomato

Abstract

There are two divergent fructokinase isozymes, Frk1 and Frk2 in tomato (Lycopersicon esculentum Mill.) plants. To investigate the physiological functions of each isozyme, the expression of each fructokinase mRNA was independently suppressed in transgenic tomato plants, and the respective phenotypes were evaluated. Suppression of Frk1 expression resulted in delayed flowering at the first inflorescence. Suppression of Frk2 did not effect flowering time but resulted in growth inhibition of stems and roots, reduction of flower and fruit number, and reduction of seed number per fruit. Localization of Frk1 and Frk2 mRNA accumulation by in situ hybridization in wild-type tomato fruit tissue indicated that Frk2 is expressed specifically in early tomato seed development. Fruit hexose and starch content were not effected by the suppression of either Frk gene alone. The results collectively indicate that flowering time is specifically promoted by Frk1 and that Frk2 plays specific roles in contributing to stem and root growth and to seed development. Because Frk1 and Frk2 gene expression was suppressed individually in transgenic plants, other significant metabolic roles of fructokinases may not have been observed if Frk1 and Frk2 play, at least partially, redundant metabolic roles.Suc translocated from source leaves to sink tissue is first metabolized by Suc synthase (SS) and/or invertase (INV) to form a pool of hexose. Fru is subsequently phosphorylated to Fru-6-phosphate by fructokinase (EC 2.7.1.4) and used as the substrate for respiration and biosynthesis of starch and the other complex carbohydrates. Fructokinase has been characterized from various plant tissues such as pea (Pisum sativum) seeds (Copeland et al., 1978), avocado (Persea americana) fruit (Copeland and Tanner, 1988), maize (Zea mays) kernels (Doehlert, 1990), potato (Solanum tuberosum) tubers (Gardner et al., 1992; Renz and Stitt, 1993), taproots of sugar beet (Beta vulgaris; Chaubron et al., 1995), barley (Hordeum vulgare) leaves (Baysdorfer et al., 1989), spinach (Spinacia oleracea) leaves (Schnarrenberger, 1990), rice (Oryza sativa) embryo (Guglielminetti et al., 2000), and tomato (Lycopersicon esculentum) fruit (Martinez-Barajas and Randall, 1996). The purified fructokinases have been characterized by a generally high affinity for Fru and ATP and, in some cases, two to three fructokinase isoforms have been identified.It has been proposed that fructokinase may regulate starch synthesis coordinately with SS in sink tissue such as potato tubers and tomato fruit. Potato tubers accumulate starch throughout development (Ross et al., 1994), whereas in tomato fruit, starch is transiently accumulated in young fruit and then degraded to a negligible level in mature fruit (Schaffer and Petreikov, 1997a). In both sink tissues, the activities of fructokinase and SS parallel starch content (Ross et al., 1994; Appeldoorn et al., 1997; Schaffer and Petreikov, 1997a). In addition, the localization of mRNA of both enzymes is closely associated with starch-accumulat...

Fructose sensitivity is suppressed in Arabidopsis by the transcription factor ANAC089 lacking the membrane-bound domain

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