Are sucrosyl-oligosaccharides synthesized in mesophyll protoplasts of mature leaves of Cucumis melo?

Abstract

Biosynthesis of sucrosyl-oligosaccharides (raffinose, stachyose) was traced in source leaves of Cucumis melo after (14)C-photoassimilation. The main carbon compound exported was (14)C-labeled stachyose. No oligosaccharide synthesis was detected in young, importing leaves. Mesophyll protoplasts, isolated from mature leaves which had previously photosynthesized (14)CO2, did not contain (14)C-oligosaccharides but contained [(14)C]-sucrose and (14)C-hexoses. Isolated minor-vein-enriched fractions from the same leaves, however, showed nearly 30% of the (14)C of the neutral fraction to be in oligosaccharides. Isolated, viable mesophyll protoplasts incubated with NaH(14)CO3 also failed to incorporate radioactivity into oligosaccharides, although sucrose and galactinol synthesis was unimpaired. Galactinolsynthase activity in leaf extracts and in mesophyll protoplasts was 16.8 μmol·h(-1)·mg(-1) protein and 13.8 μmol·h(-1)·mg(-1) protein, respectively. Galactosyltransferase (EC 2.4.1.67), which synthesizes stachyose from raffinose and galactinol, had an activity of 50 nmol·h(-1)·mg(-1) protein in leaf extracts and was also present in the minor-vein-enriched fraction, but could not be detected in mesophyll protoplast lysates. The results indicate that mesophyll cells may not be the site of stachyose synthesis although precursor compounds like sucrose and galactinol are synthesized there.