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  • Author or Editor: Ray Ming x
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The vegetative forms of male (XY), female (XX), and hermaphrodite (XYh) papaya (Carica papaya L.) plants are phenotypically identical. However, the flower and inflorescence morphology of each sex type is unique. Gynodioecious varieties SunUp, SunUp Diminutive mutant, and dioecious AU9 were used to test the response of papaya to gibberellic acid (GA3). Exogenous applications of GA3 on female and hermaphrodite flowers of papaya did not yield any sex reversal phenotype but caused a significant increase in peduncle elongation and inflorescence branch number in all treated plants. An increase in flower number was seen in females but not hermaphrodites or males. There was an increase in plant height for all treated plants except SunUp Diminutive mutant, suggesting that the mechanism causing the dwarf phenotype is independent of gibberellins. Gibberellin metabolism genes were identified in the papaya genome, none of which mapped to the sex-determining region of either the male- or hermaphrodite-specific region of papaya Y or Yh chromosome. We hypothesize that a transacting regulatory element that enhances gibberellin biosynthesis plays a role in the extreme length of the male papaya peduncle.

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An invertase gene was isolated and its mRNA activity and protein levels were determined during papaya (Carica papaya L.) fruit development. A complete invertase cDNA (AF420223) and a partial sucrose synthase cDNA (AF420224) were isolated from papaya fruit cDNA libraries. The invertase cDNA encoded a predicted polypeptide of 582 residues (MW 65,537 Da), and was 68% and 45% identical with carrot apoplastic and vacuolar invertases, respectively. Key amino acids indicative of an apoplastic invertase were conserved. A full-length gene corresponding to the putative apoplastic invertase cDNA was isolated and was organized into seven exons and six introns. Exon 2 (9 bp long) encoded part of a highly conserved region (NDPNG/A). Invertase mRNA and activity levels increased during fruit maturation and sugar accumulation just before ripening. In contrast, sucrose synthase mRNA levels were high during early fruit growth and low during the fruit sugar accumulation stage. A 73-kDa cell wall extractable protein that cross-reacted with carrot apoplastic invertase antisera substantially increased during papaya fruit maturation and declined in full ripe fruit. The increase in invertase protein levels occurred 2 to 4 weeks before maturity and was markedly higher than the overall increase in enzyme activity at this stage. Subsequently, the increase in enzyme activity was higher than the increase in protein levels between 2 weeks before maturity and fully ripe. The results suggested that mRNA level and invertase activity were related to maturity. The data suggested that the invertase was apoplastic, and that post-translational control of enzyme activity occurred, in which a significant accumulation of invertase occurred before the peak of enzymes activity.

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