A symplasmic flow of sucrose contributes to phloem loading in Ricinus cotyledons |
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Authors: | Gabriele Orlich Markus Hofbrückl Alexander Schulz |
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Institution: | Lehrstuhl Pflanzenphysiologie, Universit?t Bayreuth, D-95440 Bayreuth, Germany, DE Botanisches Institut LS II, Christian-Albrechts-Universit?t, D-24098 Kiel, Germany, DE
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Abstract: | External sucrose, supplied by the endosperm in vivo, is the physiological source of sucrose for Ricinus communis L. seedlings. It is taken up by the cotyledons and exported via the sieve tubes to the growing hypocotyl and root. Two parallel
pathways of external sucrose to the sieve tubes, directly via the apoplasm and indirectly after transit through the mesophyll,
have already been established (G. Orlich and E. Komor, 1992). In this study, we analysed whether a symplasmic flow of sucrose
contributes to phloem loading. Uptake of external sucrose into the mesophyll and into the sieve tubes, and export of total
sucrose were measured with intact and exuding seedlings in the presence of p-chloromercuribenzenesulfonic acid (PCMBS). Sucrose uptake into the mesophyll and into the sieve tubes was inhibited by 80–90%.
Consequently, export of total sucrose slowed down. However, after the addition of PCMBS, sucrose was transiently exported
in such a high amount that could not be accounted for by the residual uptake activity nor by the amount of sucrose confined
to the sieve element-companion cell complex (seccc). From the results, we conclude that most of the sucrose exported transiently
had moved to the sieve tubes from a symplasmic domain larger than the seccc, comprising at least all the cells of the bundle
including the bundle sheath. We suggest that the symplasmic flow of sucrose observed is a mass flow driven by a turgor pressure.
As a structural prerequisite for a symplasmic flow, plasmodesmata interconnect all the cells from the bundle sheath to the
sieve tubes and also occur between the bundle sheath and the mesophyll. The phloem loading pathway of Ricinus cotyledons can thus be classified as a combination of three different routes.
Received: 17 October 1997 / Accepted: 9 March 1998 |
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Keywords: | : Mass flow Phloem loading Ricinus Sucrose transport (apoplasmic symplasmic) |
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