Abstract
The transporters responsible for sugar uptake into non-photosynthetic sink tissues in plants, such as roots and flowers, have not been fully identified and analyzed. Plants encode around 100 putative sugar transporters within the major facilitator superfamily, yet only a few have been studied. Here we report the analysis of a sugar alcohol permease homolog (AtPLT5, At3g18830) from Arabidopsis. A wide range of sugars including hexoses, pentoses, tetroses, a sugar acid, and sugar alcohols but not disaccharides induced inward currents in oocytes expressing AtPLT5. AtPLT5 expression also resulted in 14C-labeled substrate uptake in oocytes, indicating that AtPLT5 encodes an ion-coupled uptake transporter. K(0.5) values for glucose and sorbitol were highly dependent on external pH. Expression of AtPLT5 was found primarily in sink tissues: in the elongation zone of roots, in the inflorescence stem, and several floral structures, especially in the floral abscission zone. Expression was induced by mechanical wounding and insect feeding. Analysis of transport properties and expression in Arabidopsis indicate that AtPLT5 functions to transport a wide range of sugars into specific sink tissues in the plant.
Highlights
Most plant cells are connected by plasmodesmata, forming multicellular “symplasmic domains” [1] that allow exchange of small metabolites between cells
The Arabidopsis sugar alcohol permease homolog AtPLT5 was expressed in Xenopus oocytes and analyzed by voltage clamping
When AgMAT1 from celery was expressed in yeast, glucose was found to inhibit labeled mannitol uptake [20] indicating that glucose could compete for transport
Summary
Most plant cells are connected by plasmodesmata, forming multicellular “symplasmic domains” [1] that allow exchange of small metabolites between cells. Symplasmic domains of sink cells, for example, in fruit [2], require metabolite uptake transporters to support growth and development (for review, see Ref. 3). AtPLT5 (At3g18830) is a member of a third group of 6 genes that is most closely related to the celery mannitol permease AgMAT1 [14], and the sorbitol permeases PcSOT1 and PcSOT2 from cherry fruit [15] and PmPLT1 and PmPLT2 from Plantago [16]. Whereas celery and Plantago are known to transport mannitol and sorbitol, respectively, in the phloem and sour cherry fruit accumulate sorbitol during ripening, Arabidopsis does not translocate or accumulate either sugar alcohol in significant quantities. It is likely that AtPLT5 functions as a nonspecific sugar uptake transporter in developing sinks and in cells surrounding wounded tissue
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