The Phloem Sieve Element: A River Runs through It.

Sjölund, Richard D.

doi:10.1105/tpc.9.7.1137

Cited by 189 publications

(118 citation statements)

References 72 publications

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“…If mature phloem elements are damaged, massive callose synthesis occurs on the sieve plate, resulting in occlusion of the pores and retention of phloem contents behind the callose plugs (Evert and Derr, 1964;Hao et al, 2008;Mullendore et al, 2010). Indeed, because callose is formed very rapidly upon damage to the phloem, its existence in sieve plate pores in undamaged plants has been disputed in the past: some authors have argued that its presence is an artifact of sectioning and preparation techniques (Evert and Derr, 1964;Walsh and Melaragno, 1976;Spanner, 1978;Sjö lund, 1997; for summary, see van Bel, 2003). However, callose linings have been observed in sieve plate pores from a wide range of plant species and organs and with a range of techniques designed to minimize the possibility of callose synthesis during tissue preparation (Ehlers et al, 2000;van Bel et al, 2002).…”

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“…However it is derived, the mature pore usually has a callose lining (Bouck and Cronshaw, 1965;Deshpande, 1974Deshpande, , 1975Thorsch and Esau 1988;Eleftheriou, 1990; for summary, see Sjö lund, 1997). If mature phloem elements are damaged, massive callose synthesis occurs on the sieve plate, resulting in occlusion of the pores and retention of phloem contents behind the callose plugs (Evert and Derr, 1964;Hao et al, 2008;Mullendore et al, 2010).…”

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Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis

et al. 2010

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One isoform of callose synthase, Glucan Synthase-Like7 (GSL7), is tightly coexpressed with two isoforms of sucrose synthase (SUS5 and SUS6) known to be confined to phloem sieve elements in Arabidopsis (Arabidopsis thaliana). Investigation of the phenotype of gsl7 mutants of Arabidopsis revealed that the sieve plate pores of stems and roots lack the callose lining seen in wild-type plants. Callose synthesis in other tissues of the plant appears to be unaffected. Although gsl7 plants show only minor phenotypic alterations during vegetative growth, flowering stems are reduced in height and all floral parts are smaller than those of wild-type plants. Several lines of evidence suggest that the reduced growth of the inflorescence is a result of carbohydrate starvation. Levels of sucrose, hexoses, and starch are lower in the terminal bud clusters of gsl7 than in those of wild-type plants. Transcript levels of "starvation" genes expressed in response to low sugars are elevated in the terminal bud clusters of gsl7 plants, at the end of the night, and during an extended night. Pulse-chase experiments with 14 CO 2 show that transport of assimilate in the flowering stem is much slower in gsl7 mutants than in wild-type plants. We suggest that the callose lining of sieve plate pores is essential for normal phloem transport because it confers favorable flow characteristics on the pores.

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Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis

et al. 2010

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“…In response to injury in SEs of intact broad bean plants, spindle-like protein bodies (forisomes) disperse, and parietal proteins detach from the plasma membrane and SE reticulum (3), plugging the sieve plates (4), whereas callose deposition constricts the pores in the sieve plates from the apoplast side (7,8). Protein meshworks inside SEs that collapse as a consequence of wounding are described for some dicotyledonous plants (6,9) and water-soluble proteins, which become insoluble after wounding, thereby plugging the sieve plate, are postulated for Poaceae (10). Different combinations of these mechanisms can be observed in various plant families (11) with exception of forisomes that are restricted to the Fabaceae (12,13).…”

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Molecular sabotage of plant defense by aphid saliva

Will

Tjallingii

Thönnessen

et al. 2007

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

455

413

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Aphids, which constitute one of the most important groups of agricultural pests, ingest nutrients from sieve tubes, the photoassimilate transport conduits in plants. Aphids are able to successfully puncture sieve tubes with their piercing mouthparts (stylets) and ingest phloem sap without eliciting the sieve tubes' normal occlusion response to injury. Occlusion mechanisms are calciumtriggered and may be prevented by chemical constituents in aphid saliva injected into sieve tubes before and during feeding. We recorded aphid feeding behavior with the electrical penetration graph (EPG) technique and then experimentally induced sieve tube plugging. Initiation of sieve tube occlusion caused a change in aphid behavior from phloem sap ingestion to secretion of watery saliva. Direct proof of ''unplugging'' properties of aphid saliva was provided by the effect of aphid saliva on forisomes. Forisomes are proteinaceous inclusions in sieve tubes of legumes that show calcium-regulated changes in conformation between a contracted state (below calcium threshold) that does not occlude the sieve tubes and a dispersed state (above calcium threshold) that occludes the sieve tubes. We demonstrated in vitro that aphid saliva induces dispersed forisomes to revert back to the nonplugging contracted state. Labeling Western-blotted saliva proteins with 45 Ca 2؉ or ruthenium red inferred the presence of calcium-binding domains. These results demonstrate that aphid saliva has the ability to prevent sieve tube plugging by molecular interactions between salivary proteins and calcium. This provides aphids with access to a continuous flow of phloem sap and is a critical adaptation instrumental in the evolutionary success of aphids.calcium-binding ͉ plant-aphid interaction ͉ plugging ͉ saliva proteins ͉ sieve element

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“…Sieve elements are connected to each other through converted plasmodesmata, forming sieve pores that facilitate translocation (4). The synthesis of P-proteins begins in immature, nucleated sieve elements (5), resulting in electron-dense proteinaceous structures.…”

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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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The Phloem Sieve Element: A River Runs through It.

Cited by 189 publications

References 72 publications

Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis

Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis

Molecular sabotage of plant defense by aphid saliva

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

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