Phloem Structure and Function

Cronshaw, James

doi:10.1146/annurev.pp.32.060181.002341

Cited by 95 publications

(45 citation statements)

References 101 publications

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“…P-proteins have been identified in all dicotyledonous plants studied thus far and also in some monocotyledonous plants, with palms and grasses as the main exceptions (3).…”

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confidence: 99%

“…The synthesis of P-proteins begins in immature, nucleated sieve elements (5), resulting in electron-dense proteinaceous structures. Their ultrastructural characteristics have been described variously as granular, fibrillar, or tubular, even within the same cell (6)(7)(8)(9)(10)(11)(12)(13)(14), and today these structures are thought to represent the various different stages of P-protein differentiation (3). In young sieve elements, subunits accumulate within the cytoplasm, forming large P-protein bodies (9).…”

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confidence: 99%

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Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

Ernst

Jekat

Zielonka

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

100

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The sieve element occlusion (SEO) gene family originally was delimited to genes encoding structural components of forisomes, which are specialized crystalloid phloem proteins found solely in the Fabaceae. More recently, SEO genes discovered in various nonFabaceae plants were proposed to encode the common phloem proteins (P-proteins) that plug sieve plates after wounding. We carried out a comprehensive characterization of two tobacco (Nicotiana tabacum) SEO genes (NtSEO). Reporter genes controlled by the NtSEO promoters were expressed specifically in immature sieve elements, and GFP-SEO fusion proteins formed parietal agglomerates in intact sieve elements as well as sieve plate plugs after wounding. NtSEO proteins with and without fluorescent protein tags formed agglomerates similar in structure to native Pprotein bodies when transiently coexpressed in Nicotiana benthamiana, and the analysis of these protein complexes by electron microscopy revealed ultrastructural features resembling those of native P-proteins. NtSEO-RNA interference lines were essentially devoid of P-protein structures and lost photoassimilates more rapidly after injury than control plants, thus confirming the role of P-proteins in sieve tube sealing. We therefore provide direct evidence that SEO genes in tobacco encode P-protein subunits that affect translocation. We also found that peptides recently identified in fascicular phloem P-protein plugs from squash (Cucurbita maxima) represent cucurbit members of the SEO family. Our results therefore suggest a common evolutionary origin for P-proteins found in the sieve elements of all dicotyledonous plants and demonstrate the exceptional status of extrafascicular P-proteins in cucurbits.photoassimilate transport | wound response | exudation | phloem protein 1

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“…P-proteins have been identified in all dicotyledonous plants studied thus far and also in some monocotyledonous plants, with palms and grasses as the main exceptions (3).…”

mentioning

confidence: 99%

mentioning

confidence: 99%

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

Ernst

Jekat

Zielonka

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

100

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“…Because mature sieve elements lack both a nucleus and ribosomes (4), they are incapable of protein synthesis. Clearly, the ongoing presence of proteins in the translocation stream requires their continual replacement by movement from adjacent nucleate cells.…”

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confidence: 99%

Turnover of Soluble Proteins in the Wheat Sieve Tube

Fisher¹,

Wu²,

Ku³

1992

Plant Physiol.

238

177

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Although the enucleate conducting cells of the phloem are incapable of protein synthesis, phloem exudates characteristically contain low concentrations of soluble proteins. The role of these proteins and their movement into and out of the sieve tubes poses important questions for phloem physiology and for cell-to-cell protein movement via plasmodesmata. Because mature sieve elements lack both a nucleus and ribosomes (4), they are incapable of protein synthesis. Clearly, the ongoing presence of proteins in the translocation stream requires their continual replacement by movement from adjacent nucleate cells. Companion cells are the most likely origin of such proteins and characteristically possess an active-appearing cytoplasm, including abundant ribosomes. Recently, Nakamura et al. (16) (17) showed that even 'structural' P-proteins are involved in rapid turnover and presented microautoradiographic evidence for their synthesis in companion cells.The occurrence of protein turnover in sieve tubes raises a number of intriguing and physiologically significant questions. Movement of proteins into and out of the sieve tube presumably occurs via plasmodesmata, which, except for pathological conditions, appear to provide passageways too small for intercellular protein movement (21). Although their functions are unknown, the proteins presumably have some role in source-sink and/or sieve tube-companion cell relations. As Raven (20) recently emphasized, the striking longevity of sieve elements as enucleate cells poses ongoing maintenance problems that almost certainly require intercellular protein transport. Finally, the synthesis and movement of phloem proteins in healthy plants may provide insight into the replication and movement of phloem-limited viruses and Mycoplasma-like organisms.The following experiments were undertaken to investigate some of the overall characteristics of soluble sieve tube proteins, especially their number and variability along the transport pathway, and their pattems of synthesis, transport, and turnover. MATERIALS AND METHODS Plant MaterialWheat plants (Triticum aestivum L. cv SUN 9E) were grown in a growth chamber as described previously (9). Experiments were performed with plants in the middle portion of the grain-filling stage (approximately 15-25 d after anthesis).

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“…Transition in tall fescue leaves, and specifically in tall fescue cells, may occur regardless of the carbon equilibrium of the leaf blades. Vascular bundle maturity may be a more determinant factor of the transition (Cronshaw 1981;Giaquinta 1983;Turgeon 1989;Riesmeier et al 1992).…”

Section: Resultsmentioning

confidence: 99%

Grass leaf blade development: Modification of the source

Brégard¹,

Allard

1999

Can. J. Plant Sci.

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. Grass leaf blade development: Modification of the source. Can. J. Plant Sci. 79: 361-363. Developing leaf blades of tall fescue (Festuca arundinacea Schreb.) were subjected to three treatments that modify their source capacity: (1) emerged tips cut daily, (2) grown in darkness or (3) grown at the CO 2 compensation point. Treated and control leaves developed similarly but treated leaves senesced rapidly at the leaf tip. Reduction of carbon availability in developing tall fescue leaf blades seems not to penalize normal blade development but does reduce the lifespan of mature leaves.Key words: Source, leaf development, tall fescue Brégard, A. et Allard, G. 1999. Développement de limbes de graminées: modification de la source. Can. J. Plant Sci. 79: 361-363. Des limbes en développement de fétuque élevée (Festuca arundinacea Schreb.) ont été soumis à trois traitements afin de modifier leur capacité source: (1) extrémités émergées des feuilles coupées quotidiennement, (2) maintenues à l'obscurité ou (3) développées au point de compensation en CO 2 . Les développements des feuilles traitées et témoins étaient similaires mais l'extrémité des feuilles traitées se nécrosaient rapidement. La disponibilité réduite en carbone dans les limbes n'affecte pas leur développement mais réduit la durée de vie des feuilles matures. Mots clés: Source, développement foliaire, fétuque élevéeIn developing grass leaves, the tip of the blade is structurally mature and photosynthetically active by the time it emerges from the whorl formed by the sheath of older leaves. Hence photoassimilates produced by the leaf tip could be exported towards the growing leaf base which, in turn, also imports photoassimilates from mature leaves (Anderson and Dale 1983). Anderson and Dale (1983) determined that by the time a barley leaf is half expanded it can provide about half of its own photoassimilate requirement. Similarly, Ryle and Powell (1972) determined that in Lolium leaves, 60 to 80% of photoassimilates produced in the emerged leaf tip are used in the basal growing zone when the blade is one-third to one-half expanded.The balance between imported and produced photoassimilates in a developing leaf blade is continuously modified by the increasing photosynthetic capacity of the expanding leaf tip. Eventually, a positive carbon balance is reached, i.e. more carbon is fixed by the leaf than is required for its growth and respiration. On average, when a leaf from a dicotyledonous species reaches 50% of its full development, the pattern of importation changes and the sink-to-source transition occurs (Turgeon 1989). In tall fescue (Festuca arundinacea Schreb.) leaf blades, a monocotyledon, the sink-to-source transition occurs when the blade reaches 80% of its final length (Brégard and Allard 1999); this also corresponds to the onset of sheath elongation and the development of the next leaf.Leaf blade elongation and development could therefore be related to the carbon autotrophy of the blade and as such, the period of the sink-to-source transition could b...

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Phloem Structure and Function

Cited by 95 publications

References 101 publications

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

Turnover of Soluble Proteins in the Wheat Sieve Tube

Grass leaf blade development: Modification of the source

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