Abstract
Transgenic tobacco (Nicotiana tabacum) was studied to localize the activity of phloem loading during development and to establish whether the endoplasmic reticulum (ER) of the companion cell (CC) and the sieve element (SE) reticulum is continuous by using a SUC2 promoter-green fluorescent protein (GFP) construct targeted to the CC-ER. Expression of GFP marked the collection phloem in source leaves and cotyledons as expected, but also the transport phloem in stems, petioles, midveins of sink leaves, nonphotosynthetic flower parts, roots, and newly germinated seedlings, suggesting that sucrose retrieval along the pathway is an integral component of phloem function. GFP fluorescence was limited to CCs where it was visualized as a well-developed ER network in close proximity to the plasma membrane. ER coupling between CC and SEs was tested in wild-type tobacco using an ER-specific fluorochrome and fluorescence redistribution after photobleaching (FRAP), and showed that the ER is continuous via pore-plasmodesma units. ER coupling between CC and SE was quantified by determining the mobile fraction and half-life of fluorescence redistribution and compared with that of other cell types. In all tissues, fluorescence recovered slowly when it was rate limited by plasmodesmata, contrasting with fast intracellular FRAP. FRAP was unaffected by treatment with cytochalasin D. The highest degree of ER coupling was measured between CC and SE. Intimate ER coupling is consistent with a possible role for ER in membrane protein and signal exchange between CC and SE. However, a complete lack of GFP transfer between CC and SE indicated that the intraluminal pore-plasmodesma contact has a size exclusion limit below 27 kD.
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Claims 10
Claim · model-written summary
GFP marked collection phloem in source leaves and cotyledons.
- Taxon
- tobacco
Nicotiana tabacumt0 - Organ / stage
- source leaves and cotyledonse0
- Gene
- GFPe0
- Outcome
- marked the collection phloeme0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Expression of GFP marked the collection phloem in source leaves and cotyledons as expected”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
GFP also marked transport phloem across several sink and nonphotosynthetic organs.
- Taxon
- tobaccoNicotiana tabacumt0
- Gene
- GFPe0
- Outcome
- transport phloem in stems, petioles, midveins of sink leaves, nonphotosynthetic flower parts, roots, and newly germinated seedlingse0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Expression of GFP marked the collection phloem in source leaves and cotyledons as expected, but also the transport phloem in stems, petioles, midveins of sink leaves, nonphotosynthetic flower parts, roots, and newly germinated seedlings”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
The transport phloem signal suggests sucrose retrieval is integral to phloem function.
- Taxon
- tobaccoNicotiana tabacumt0
- Outcome
- sucrose retrieval along the pathwaye0
- Qualifier
- integral component of phloem functione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“suggesting that sucrose retrieval along the pathway is an integral component of phloem function.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
GFP fluorescence was confined to companion cells, showing a developed ER network near the plasma membrane.
- Taxon
- tobaccoNicotiana tabacumt0
- Organ / stage
- CCse0
- Gene
- GFPe0
- Outcome
- well-developed ER network in close proximity to the plasma membranee0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“GFP fluorescence was limited to CCs where it was visualized as a well-developed ER network in close proximity to the plasma membrane.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
Companion and sieve element ER were continuous through pore plasmodesma units.
- Taxon
- tobaccoNicotiana tabacumt0
- Outcome
- ER is continuous via pore-plasmodesma unitse0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“showed that the ER is continuous via pore-plasmodesma units.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
Fluorescence recovered slowly when limited by plasmodesmata, versus fast intracellular recovery.
- Taxon
- tobaccoNicotiana tabacumt0
- Outcome
- fluorescence recoverede0
- Direction
- slowlye0
- Comparator
- fast intracellular FRAPe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“In all tissues, fluorescence recovered slowly when it was rate limited by plasmodesmata, contrasting with fast intracellular FRAP.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
Cytochalasin D did not affect FRAP.
- Taxon
- tobaccoNicotiana tabacumt0
- Treatment
- treatment with cytochalasin De0
- Outcome
- FRAPe0
- Direction
- unaffectede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“FRAP was unaffected by treatment with cytochalasin D.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
ER coupling was highest between companion and sieve elements.
- Taxon
- tobaccoNicotiana tabacumt0
- Organ / stage
- CC and SEe0
- Outcome
- degree of ER couplinge0
- Direction
- higheste0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The highest degree of ER coupling was measured between CC and SE.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
ER coupling may support membrane protein and signal exchange between companion and sieve elements.
- Taxon
- tobaccoNicotiana tabacumt0
- Outcome
- membrane protein and signal exchange between CC and SEe0
- Qualifier
- Intimate ER couplinge0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Intimate ER coupling is consistent with a possible role for ER in membrane protein and signal exchange between CC and SE.”
t0“Transgenic tobacco (Nicotiana tabacum)”
Claim · model-written summary
No GFP transferred between companion and sieve elements, implying size exclusion below 27 kD.
- Taxon
- tobaccoNicotiana tabacumt0
- Gene
- GFPe0
- Outcome
- size exclusion limit below 27 kDe0
- Value
- 27 kDe0
- Qualifier
- complete lack of GFP transfer between CC and SEe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a complete lack of GFP transfer between CC and SE indicated that the intraluminal pore-plasmodesma contact has a size exclusion limit below 27 kD.”
t0“Transgenic tobacco (Nicotiana tabacum)”