Changes in activities of acid invertase isoforms affect sugar accumulation and composition during ripening of tomato fruit

The observation that the wild, red fruited tomato Lycopersicon pimpinellifolium has higher soluble solids than the cultivated variety L. esculentum may be related to differential controls on fruit carbohydrate metabolism. We therefore investigated the role of acid invertase (AI) in determining fruit sugar composition in these two species. At all stages of fruit development, the majority of AI was localized in the vacuole with about 10&percent; of the total activity in the extra-cellular space. Four forms of soluble AI activity were identified. Isoform 1 persisted throughout development. Isoform 2 showed a strong relationship to total fruit sugar content in both species, suggesting that this isoform is the tiv1 gene product located in the vacuole, and that it plays a major role in determining fruit soluble solids. Isoform 3 was very high in L. pimpinellfolium but not in L. esculentum. We suggest that AI has a major role in determining fruit sugar accumulation in L. pimpinellifolium, but not in L. esculentum. Pulse-chase in vitro unloading experiments with [3h]-glucose showed that a substantial amount of [3h]-sucrose was present in the apoplast, suggesting an exchange of sugars between extra- and intra-cellular spaces. Hence ``futile cycles'' of sucrose synthesis and degradation involving apoplastic AI can occur during fruit development.