Carbohydrate Balance and Accumulation during Development of Near-isogenic Tomato Lines Differing in the AGPase-L1 Allele

Abstract

Soluble sugar accumulation is a major determinant of tomato (Solanum lycopersicum) fruit quality. One strategy of increasing sugar levels in the mature fruit is via the increase of the transient starch pool in the immature fruit, which is subsequently degraded to contribute to its soluble sugar levels. ADP-glucose pyrophosphorylase [AGPase (E.C. 2.7.7.27)] is a limiting enzyme in starch synthesis and we therefore developed introgression lines of cultivated tomato harboring the wild species (Solanum habrochaites) allele for the regulatory large subunit (L1H ) of this heterotetrameric enzyme. Comparison of numerous near-isogenic lines of tomato segregating for the L1 allele, during multiple seasons, showed that the wild species allele led to an increase in fruit total soluble solids concentration (TSS) without a concomitant decrease in fruit size. Rather, in practically all lines studied, fruit size increased together with TSS, leading to an even larger increase in TSS × yield. A comparative developmental study of fruit carbohydrates, starch, and sugars between genotypes showed that the wild species allele led to increases in fruit size, carbohydrate concentration, and carbohydrate content of the whole fruit unit. This was related to a large increase in the transient starch reservoir that, upon degradation, accounted for the subsequent increase in soluble sugars. These results are evidence that modifying fruit sink carbohydrate metabolism via a single rate-limiting enzymatic step can increase the net import of photoassimilate into the fruit.