ABSTRACT
In kiwifruit, much of the softening process occurs prior to the respiratory climacteric and production of ethylene. This fruit therefore represents an excellent model system for dissecting the process of softening in the absence of endogenous ethylene production. We have characterized the expression of three polygalacturonase (PG) cDNA clones (CkPGA, B and C) isolated from fruit of Actinidia chinensis. Expression of CkPGA and B was detected by northern analysis only in fruit producing endogenous ethylene, and by RT-PCR in other tissues including flower buds, petals at anthesis, and senescent petals. CkPGA promoter fragments of 1296, 860 and 467 bp fused to the beta-glucuronidase (uidA) reporter gene directed fruit-specific gene expression during the climacteric in transgenic tomato. CkPGC gene expression was observed in softening fruit, and reached maximum levels (50-fold higher than for CkPGA and B) as fruit passed through the climacteric. However, expression of this gene was also readily detected during fruit development and in fruit harvested prior to the onset of softening. Using RT-PCR, expression of CkPGC was also detected at low levels in root tips and in senescent petals. These results suggest that PG expression is required not only during periods of cell wall degeneration, but also during periods of cell wall turnover and expansion.
Subject(s)
Fruit/genetics , Polygalacturonase/genetics , Amino Acid Sequence , Base Sequence , Blotting, Northern , DNA, Complementary/chemistry , DNA, Complementary/genetics , DNA, Complementary/isolation & purification , DNA, Plant/genetics , Ethylenes/biosynthesis , Fruit/enzymology , Fruit/growth & development , Gene Dosage , Gene Expression Regulation, Developmental , Gene Expression Regulation, Enzymologic , Gene Expression Regulation, Plant , Glucuronidase/genetics , Glucuronidase/metabolism , Solanum lycopersicum/genetics , Molecular Sequence Data , Plants, Genetically Modified/genetics , Promoter Regions, Genetic/genetics , RNA, Plant/genetics , RNA, Plant/metabolism , Recombinant Fusion Proteins/genetics , Recombinant Fusion Proteins/metabolism , Sequence Analysis, DNA , Sequence Homology, Amino Acid , Sequence Homology, Nucleic Acid , Tissue DistributionABSTRACT
Levels of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and polygalacturonase (PG) mRNAs were characterized during ripening of Royal Gala, Braeburn and Granny Smith apples. Both ACC-oxidase and PG mRNAs were up-regulated in ripening fruit of all three cultivars. Expression in Royal Gala was detected earlier than in Braeburn and Granny Smith, relative to internal ethylene concentration. Genomic clones corresponding to the ACC-oxidase and PG mRNAs expressed in ripe apple fruit were isolated and ca. 2 kb of each promoter was sequenced. The start point of transcription in each gene was mapped by primer extension, and sequences homologous to elements in other ethylene-responsive or PG promoters were identified. The fruit specificity of the apple ACC-oxidase and PG promoters was investigated in transgenic tomato plants using a nested set of promoter fragments fused to the beta-glucuronidase (gusA) reporter gene. For the ACC-oxidase gene, 450 bp of 5' promoter sequence was sufficient to drive GUS expression, although this expression was not specific to ripening fruit. Larger fragments of 1966 and 1159 bp showed both fruit and ripening specificity. For the PG gene, promoter fragments of 1460 and 532 bp conferred ripening-specific expression in transgenic tomato fruit. However GUS expression was down-regulated by 2356 bp of promoter, suggesting the presence of a negative regulatory element between positions -1460 and -2356.