Potato granule-bound starch synthase promoter-controlled GUS expression: regulation of expression after transient and stable transformation

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Abstract
Chimaeric genes of promoter sequences from the potato gene encoding granule-bound starch synthase (GBSS) and the β-glucuronidase (GUS) reporter gene were used to study GBSS expression and regulation. Analysis of stable transformants revealed that a GBSS promoter sequence of 0.4 kb was sufficient to result in tissue-dependent GUS expression: levels in stably transformed microtubers exceeded levels in corresponding leaves by orders of magnitude. GBSS-GUS constructs could be transiently expressed in leaf protoplasts from wild-type and amylose-free potato lines, etuberosumSolanum brevidens, Nicotiana tabacum andArabidopsis thaliana. Transient expression levels in potato leaf protoplasts were clearly lower than in corresponding suspension cell protoplasts. This lower expression in leaf protoplasts could not be elevated by increasing DNA concentrations during transfection. Light incubation of electroporated suspension cell protoplasts reduced transient GBSS-GUS expression, whereas incubation of transfected protoplasts in media with different sucrose concentrations did not affect transient expression levels. However, electroporated protoplasts, isolated from suspensions, which had been grown on media with increasing amounts of sucrose showed a sucrose concentration-dependent transient expression profile. This indicates that studying GBSS regulation by transient expression experiments needs pre-treatment of the protoplast source. Sequence data of the GBSS promoter were compared to those of two other potato alleles.