Plasmodesmata: Dynamic Regulation and Role in Macromolecular Cell-To-Cell Signaling

Plasmodesmata (PD) are dynamic channels in the cell wall that connect adjacent cells via cytoplasmic bridges and allow the cell-to-cell trafficking of molecules. Trafficking is likely to occur via the cytoplasmic annulus located between the plasma membrane lining of the channel and the centrally located desmotubule, a cylindrical strand of modified endoplasmic reticulum (ER). PD facilitate the transport of photo-assimilates, ions and growth regulators from cell to cell. When they originate during cytokinesis, they are called primary PD. PD formed later on during development are termed secondary PD as they originate through pre-existing walls in response to changing needs during plant development.
Changes observed in PD during development: During development, primary PD are transformed into more complex structures as the leaf matures and undergoes the sink-to-source transition. The structural transformation of PD is paralleled by changes in their SEL. Whereas PD in young and suger-importing leaves are characterized by a rather large SEL allowing macromolecules of sizes up to 50 kDa to spread between cells, the SEL of PD in mature, sugar-exprting, leaves is much lower, permitting the passage of small molecules only. Despite this down-regulation in overall SEL, PD maintain a dynamic nature allowing the pore to selectively dilate in order to permit the trafficking of specific RNA and protein macromolecules, including transcription factors, silencing signals and viruses.In a review article published in the December. 2002 issue of Current Opinion in Plant Biology (vol. 5:543-552), Manfred Heinlein at the Friedrich Miescher Institute for Biomedical Research, Basel, updates the latest development of PD research with a special emphasis on the mechanisms of PD-mediated intercellular transport of macromolecules.Systemic transport of mRNA signals: The author cites two recent reports in which cell-to-cell and systemic transport of specific protein and mRNA signals have been shown to play an important role in developmental regulation. The results of a grafting experiment with tomato involving a wild type as scion and a mutant, Mouse ears (Me) as rootstock, revealed a change in the leaf morphology of the scion. One explanation is that rootstock-encoded transcript of the dominant Me may have reached the meristamatic zone of the scion via PD, altering the leaf morphology in the scion. The results of the grafting experiment, reported by M. Kim et al. (Science vol. 293: 287-89), support the view that cell-to-cell and long-distance transport of regulatory mRNAs have an impact on modifying the leaf morphology of the wild-type scion. However, the other possibility that the morphological alterations in the scion may have originated by the trafficking of a graft-transmissible signal other than the Me transcript cannot be ruled out.

Movement of a gene product through PD can alter the cell fate: The second report by Nakajima (Nature: 413:307-11), presented evidence that the protein SHORTROOT (SHR) moves from cells in the stele into neighboring cells to determine endodermal cell fate. Movement of this protein thus has a pivotal role during root patterning in Arabidopsis It has been shown that the SHORT ROOT (SHR) protein is required for the formation of the endodermal cell layer in the root and for induction of the longitudinal division of the cortex/endodermis initial daughter cells. Although SHR protein accumulates in the endodermis, its transcript is restricted to the stele, suggesting the intercellular movement of the protein. Expression of SHR from the endodermis-specific promoter of one of its target genes (SCARECROW) leads to the formation of multiple endodermal cell layers and thus confirms the non-autonomous role of SHR in endodermal cell specification.

Comparison of intercellular movement of GFP::KN1 protein vs ER-targeted GFP: The author describes the results of investigation made by Kim et al. (PNAS vol. 99:4103-08) on the intercellular transport of GFP::KN1 fusion protein in the leaf and shoot meristem of Arabidopsis. Both veins and the surrounding tissues show green fluorescence, when GFP::KN1 fusion protein is expressed in the veins of the leaf, compared to the expression of ER-targeted GFP, which stays in the veins. Similarly, when expressed in the L1 layer of the apical meristem GFP::KN1 spread into adjacent layers whereas non-fused GFP did not. The above results confirm the ability of KN1 to move between cells and indicate that this ability is also exerted in meristem tissues in which the protein usually acts.

Recent studies by Kim et al (Development 129:1261-72) have shown that reduction of size exclusion limit (SEL) of PD (downregulation) below 10 kDa was correlated with the torpedo stage of embryo development in Arabidopsis thaliana, suggesting the importance of intercellular signaling pathways during embryogenesis. The group monitored the various stages of embryo development by using tracer molecules such as HPTS (8-hydroxypyrene 1,3,6 trisulfonic acid) and 10 kDa F-dextran. They observed that flowering induction and transient isolation of shoot apex take place simultaneously, suggesting a relationship between floral commitment and symplastic movement.

Targeting and manipulation of PD by macromolecules: The author describes the results of interesting experiments reported by Crawford and Zambryski (Plant Physiol. 125:1802-12). Crawford and Zambryski showed the comparative transport rate between PD-targeted GFP i.e., GFP fused to a viral MP (movement protein) and PD-non-targeted GFP. Whereas the transmission of non-targeted GFP through PD was dependent upon the age of the leaf and growth conditions, transmission of targeted GFP was not influenced by these factors. It was shown that while in young leaves the movement of P30:GFP was 1.5 times greater compared to its GFP:GFP counterpart, in mature leaves the movement increases 26-fold. These results are consistent with the ability of MP to increase the pore size of plasmodesmata despite their overall downregulation during plant development.
Callose deposition in the regulation of PD SELs: A comparison between the dissected and nondissected epidermal root cells in Arabidopsis thaliana revealed that the dissected cells had a lower size exclusion limit (SEL), compared to their counterparts. In Tradescantia staminal hair cells, the same results were noted, namely, undissected staminal hair cells had a higher SEL. Reduction in the pore size upon dissection is associated with callose deposition. The inference was made (cf. because the down-regulation of PD of dissected material was blocked by the treatment with 2-deoxy-D-glucose, a callose synthesis inhibitor. Further confirmation of the role of callose comes from the results of a set of experiments reported by Bucher et al (Plant J., 28:361-69). Their results indicate that virus and MP transmission rate is reduced in plants expressing lower levels of -1,3-glucanase, a callose degrading enzyme.

Association of PD with cytoskeletal components: A whole set of cytoskeletal components appears to be involved in PD-mediated trafficking, in PD structureand in the control of the PD size exclusion limit (SEL). In addition, there is a potential link between the cytoskeleton and virus movement through PD mediated by 70-kD heat shock proteins (Hsp70). Results of recent experiments BY WHOM? uggest that the Hsp70 homolog encoded by Beet yellows clostervirus (BYV) is a microtubule-assocuiated protein that functions as a viral movement protein and is involved in virion movement through PD.

Conclusion: The dynamic and pivotal role of PD in plant development has only recently been realized, although PD were discovered more than a century ago. The use of a number of molecular tools has made it possible to determine that protein and RNA macromolecules can function as messengers for cell-to-cell and long-distance signaling. Results of recent studies have demonstrated that PD take part in the control of systemic defense responses. The author suggests that the future research in PD should be directed towards the elucidation of PD structure and towards studies to unravel a role of RNA gating and RNA trafficking in yet unknown signaling mechanisms in the induction of flowering and the induction of systemic acquired resistance. The author expects more precise and elaborate structural information about PD, which would emerge as a result of applying improved genetic and biochemical approaches.

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