Differential expression of sucrose transporter and polyol transporter genes during maturation of common plantain companion cells.

The cDNAs of two sorbitol transporters, common plantain (Plantago major) polyol transporter (PLT) 1 and 2 (PmPLT1 and PmPLT2), were isolated from a vascular bundle-specific cDNA library from common plantain, a dicot plant transporting Suc plus sorbitol in its phloem. Here, we describe the kinetic characterization of these sorbitol transporters by functional expression in Brewer's yeast (Saccharomyces cerevisiae) and in Xenopus sp. oocytes and for the first time the localization of plant PLTs in specific cell types of the vascular tissue. In the yeast system, both proteins were shown to be uncoupler sensitive and could be characterized as low-affinity and low-specificity polyol symporters. The Km value for the physiological substrate sorbitol is 12 mm for PmPLT1 and even higher for PmPLT2, which showed an almost linear increase in sorbitol transport rates up to 20 mm. These data were confirmed in the Xenopus sp. system, where PmPLT1 was analyzed in detail and characterized as a H+ symporter. Using peptide-specific polyclonal antisera against PmPLT1 or PmPLT2 and simultaneous labeling with the monoclonal antiserum 1A2 raised against the companion cell-specific PmSUC2 Suc transporter, both PLTs were localized to companion cells of the phloem in common plantain source leaves. These analyses revealed two different types of companion cells in the common plantain phloem: younger cells expressing PmSUC2 at higher levels and older cells expressing lower levels of PmSUC2 plus both PLT genes. The putative role of these low-affinity transporters in phloem loading is discussed.