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

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 fru it, was developed from an interspecific cross with Lycopersicon hirsutum. A comparison of in vitro activities of enzymes of the sucrose-to-starch meta bolic pathway indicated that only the activity of ADPglucose pyrophosphoryl ase was significantly increased in the high starch line, compared to the Ly copersicon esculentum cultivar, M-82. Polymerase chain reaction (PCR) marke rs 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 i t carries the LS1 allele from L. hirsutum (LS1(H)). In segregating populati ons from the cross between the high starch line and M-82 the LS1H allele wa s correlated with increases in ADPglucose pyrophosphorylase activity and st arch level in the immature fruit and soluble solids content in the mature f ruit. 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 a cid duplication of Glu-Lys-Lys near the N-terminal, and three additional am ino acid changes. The four genes were mapped to different chromosomal segme nts, using Lycopersicon pennellii introgression lines, which is further evi dence that these are distinct ADPglucose pyrophosphorylase genes in tomato. These results suggest that ADPglucose pyrophosphorylase activity is limiti ng to starch synthesis in developing fruits; furthermore; there exists natu ral 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 describe d QTL for fruit Brix levels. (C) 2000 Elsevier Science Ireland Ltd. All rig hts reserved.