Stachyose and mannitol transport in olive (Olea europaea L.)

Abstract

Source leaves of Olea europea L. (olive) were photosynthetically labelled with 14CO2 and then transferred to 20 mM EDTA solutions to allow exudation of phloem sap. Label in phloem sap was recovered predominantly as stachyose (50–60% of the total label) while only small amounts of label were recovered in the sugar alcohol mannitol (less than 5%). In contrast, in leaf tissues stachyose accounted for only 5% of the total leaf label while mannitol accounted for 30%. Vacuum-infiltration of exuding leaves with 1 mM p-chloromercuriphenylsulfonic acid had no effect on subsequent exudation or on the distribution of label in stachyose or mannitol in either leaf tissues or phloem exudates. In contrast, a similar treatment inhibited the uptake of exogenously supplied [14C]stachyose and [14C]sucrose into leaf discs. Following short-term pulse-chase experiments with 14CO2 (15s pulse, 5 s-20 min chase), sucrose, mannitol and galactinol were rapidly labelled within the first 2 min following the pulse. Stachyose and raffinose, on the other hand, were not appreciably labelled until 10 min after the pulse. Taken together, the data indicate that phloem loading of raffinose oligosaccharides or of mannitol may not require an apoplastic step. Additionally, it appears that there may be a spatial separation of the synthesis of these sugars within the leaf, with mannitol synthesis occurring within the photosynthetic mesophyll tissues and raffinose oligosaccharide synthesis occurring closer to, and probably within, the minor veins.