Hexose metabolism in discs excised from developing potato (Solanum tuberosum L.) tubers

Abstract

This work aimed at establishing whether labelled precursors supplied to discs excised from developing potato (Solanum tuberosum L.) tubers can be metabolised prior to uptake. Discs were excised from developing tubers and incubated with 25 mM 2-(N-morpholino)-ethanesulfonic acid (pH 6.5) and 300 mM mannitol. Analyses of the bathing medium at the end of the incubation time revealed the presence of sucrose, hexoses and metabolic intermediates such as hexose phosphate (hexose-P), uridine 5′-diphosphate (UDP) and uridine 5′-diphosphoglucose (UDPGlc). When [U-¹⁴C]glucose or [U-¹⁴C]fructose were included in the bathing medium, labelled sucrose, hexoses, hexose-P and UDPGlc were detected in the medium itself at the end of the incubation time. Metabolism of [U-¹⁴C]fructose in the medium was much more extensive than that of [U-¹⁴C]glucose. Analysis of intracellular label distribution in discs incubated with a medium containing [U-¹⁴C]fructose and active or inactive acid invertase demonstrated uptake of labelled sucrose synthesised in the medium. When the discs were incubated with UDP[U-¹⁴C]glucose radioactivity was detected in sucrose and hexose-P in the medium at the end of incubation. The conclusion is drawn that metabolic intermediates, sucrose and hexoses are readily interconverted in the medium of incubation of tuber discs. When the bathing medium contained unlabelled fructose (10 mM) the conversion of [U-¹⁴C]fructose and UDP[U-¹⁴C]glucose into sucrose was markedly stimulated. The inclusion of unlabelled glucose in the medium had no effect on metabolism of precursors. These results are interpreted as evidence that sucrose synthesis in the bathing medium was catalysed by sucrose synthase. Assays of fructokinase, glucokinase and sucrose synthase activities in the bathing medium showed that release of enzymes from the tuber tissue into the medium occurred during incubation. However, the finding that these activities were substantially higher when assays were carried out in media containing discs indicates that the enzyme responsible for the metabolism of labelled precursors in the bathing medium were strongly associated with the tuber tissue. Repeated washings of the discs prior to incubation failed to prevent conversion of [U-¹⁴C]fructose in the medium. When the discs were incubated with the anionic dye lucifer yellow, fluorescence was localised in a layer of collapsed cells at the disc surface and (more intensely) in companion cells throughout the tissue. This suggests a possible involvement of phloem tissue in the metabolic events observed.