Phloem loading. A reevaluation of the relationship between plasmodesmatal frequencies and loading strategies.
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The incidence of plasmodesmata in the minor vein phloem of leaves varies widely between species. On this basis, two pathways of phloem loading have been proposed: symplastic where frequencies are high, and apoplastic where they are low. However, putative symplastic-loading species fall into at least two categories. In one, the plants translocate raffinose-family oligosaccharides (RFOs). In the other, the primary sugar in the phloem sap is sucrose (Suc). While a thermodynamically feasible mechanism of symplastic loading has been postulated for species that transport RFOs, no such mechanism is known for Suc transporters. We used p-chloromercuribenzenesulfonic acid inhibition of apoplastic loading to distinguish between the two pathways in three species that have abundant minor vein plasmodesmata and are therefore putative symplastic loaders. Clethra barbinervis and Liquidambar styraciflua transport Suc, while Catalpa speciosa transports RFOs. The results indicate that, contrary to the hypothesis that all species with abundant minor vein plasmodesmata load symplastically, C. barbinervis and L. styraciflua load from the apoplast. C. speciosa, being an RFO transporter, loads from the symplast, as expected. Data from these three species, and from the literature, also indicate that plants with abundant plasmodesmata in the minor vein phloem have abundant plasmodesmata between mesophyll cells. Thus, plasmodesmatal frequencies in the minor veins may be a reflection of overall frequencies in the lamina and may have limited relevance to phloem loading. We suggest that symplastic loading is restricted to plants that translocate oligosaccharides larger than Suc, such as RFOs, and that other plants, no matter how many plasmodesmata they have in the minor vein phloem, load via the apoplast. (+info)
Glandulocalyx upatoiensis, a fossil flower of Ericales (Actinidiaceae/Clethraceae) from the Late Cretaceous (Santonian) of Georgia, USA.
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Triterpene saponins from Clethra barbinervis and their hyaluronidase inhibitory activities.
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An extract of Clethra barbinervis with an inhibitory effect on hyaluronidase activity was fractionated guided by the results of an assay. From the active fractions, seven new triterpene saponins (1-4, 6-8) and a new lignan glycoside (14) were isolated together with 14 known compounds (5, 9-13, 15-22). Some of the saponins (2, 3, 9) were revealed as hyaluronidase inhibitors similar to epicatechin (17). (+info)
Structures of ryobunins A-C from the leaves of Clethra barbinervis.
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From a MeOH extract of the leaves of Clethra barbinervis Sieb. et Zucc., ryobunins A-C, three new triterpene glucosides, i.e. one ursane, one seco-ursane and one oleanane-type glucoside, along with four known compounds were isolated. Their structures were elucidated based on chemical and spectral evidence. (+info)