ADPglucose pyrophosphorylase activity and starch accumulation in immature tomato fruit: the effect of a Lycopersicon hirsutum-derived introgression encoding for the large subunit

Abstract

A breeding line of tomato with an increased level of starch in the immature fruit, and a concomitantly increased soluble sugar level in the mature fruit, was developed from an interspecific cross with Lycopersicon hirsutum. A comparison of in vitro activities of enzymes of the sucrose-to-starch metabolic pathway indicated that only the activity of ADPglucose pyrophosphorylase was significantly increased in the high starch line, compared to the Lycopersicon esculentum cultivar, M-82. Polymerase chain reaction (PCR) markers were developed to distinguish between the L. esculentum and L. hirsutum alleles for each of the four known genes of ADPglucose pyrophosphorylase in tomato (LS1, LS2, LS3, SS). Analysis of the high starch line showed that it carries the LS1 allele from L. hirsutum (LS1 H). In segregating populations from the cross between the high starch line and M-82 the LS1 H allele was correlated with increases in ADPglucose pyrophosphorylase activity and starch level in the immature fruit and soluble solids content in the mature fruit. The LS1 H allele was cloned and sequenced and a comparison with the consensus sequence of the L. esculentum LS1 E allele shows a three amino acid duplication of Glu-Lys-Lys near the N-terminal, and three additional amino acid changes. The four genes were mapped to different chromosomal segments, using Lycopersicon pennellii introgression lines, which is further evidence that these are distinct ADPglucose pyrophosphorylase genes in tomato. These results suggest that ADPglucose pyrophosphorylase activity is limiting to starch synthesis in developing fruits; furthermore, there exists natural genetic variability within the Lycopersicon genus that may be utilized to increase enzyme activity and starch accumulation in the cultivated fruit. Furthermore, we proposes a possible gene function for a hitherto described QTL for fruit Brix levels.