Alder (Alnus glutinosa) and more than 200 angiosperms that encompass 24 genera are collectively called actinorhizal plants. These plants form a symbiotic relationship with the nitrogen-fixing actinomycete Frankia strain HFPArI3. The plants provide the bacteria with carbon sources in exchange for fixed nitrogen, but this metabolite exchange in actinorhizal nodules has not been well defined. We isolated an alder cDNA from a nodule cDNA library by differential screening with nodule versus root cDNA and found that it encoded a transporter of the PTR (peptide transporter) family, AgDCAT1. AgDCAT1 mRNA was detected only in the nodules and not in other plant organs. Immunolocalization analysis showed that AgDCAT1 protein is localized at the symbiotic interface. The AgDCAT1 substrate was determined by its heterologous expression in two systems. Xenopus laevis oocytes injected with AgDCAT1 cRNA showed an outward current when perfused with malate or succinate, and AgDCAT1 was able to complement a dicarboxylate uptake-deficient Escherichia coli mutant. Using the E. coli system, AgDCAT1 was shown to be a dicarboxylate transporter with a K m of 70 m for malate. It also transported succinate, fumarate, and oxaloacetate. To our knowledge, AgDCAT1 is the first dicarboxylate transporter to be isolated from the nodules of symbiotic plants, and we suggest that it may supply the intracellular bacteria with dicarboxylates as carbon sources.Some plants and microorganisms engage in reciprocal symbiosis for the purpose of exchanging nutrients. For example, in nitrogen-fixing nodules, the intracellular bacteria supply the host plant with combined nitrogen and are in turn provided with carbon sources (Mylona et al., 1995). There are two types of nodule symbioses between nitrogen-fixing soil bacteria and higher plants, namely, the symbiosis between legumes and rhizobia, and the actinorhizal symbiosis between actinomycetes of the genus Frankia and a diverse group of angiosperms collectively called actinorhizal plants. In both cases, nutrient exchange between the host and its microsymbiont is controlled by the plant plasma membrane-derived interface enclosing the microsymbiont. In most legume symbioses, where the bacteria are taken up into the plant cells in a complete endocytotic process (Verma, 1992), this interface is the peribacteroid membrane (PBM). In primitive legume symbioses (de Faria et al., 1987) and the actinorhizal symbioses (Mylona et al., 1995), the interface is reported to be the invaginated and incompletely enclosed plasma membrane of the infected cell.The nutrient exchange between the symbiotic partners requires transporters of the carbon sources and trace elements that flow from the plant to the microsymbiont along with the transporters of the products of bacterial nitrogen fixation that flow from the microsymbiont to the plant (Pawlowski and Bisseling, 1996). Soybean (Glycine max) nodules, which represent legume symbioses, show evidence of the physiological or biochemical activities of transporters of ammonium, which i...
Light-induced leaflet movement of Samanea saman depends on the regulation of membrane transporters in motor cells. Blue light (BL) stimulates leaflet opening by inducing K ϩ release from the flexor motor cells. To elucidate the mechanism of K ϩ -efflux (K D )-channel regulation by light, flexor motor cell protoplasts were patch-clamped in a cell-attached configuration during varying illumination. Depolarization elicited outward currents through single open K D channels. Changes in cell membrane potential (E M ) were estimated by applying voltage ramps and tracking the change of the apparent reversal potential of K D -channel current. BL shifted E M in a positive direction (i.e. depolarized the cell) by about 10 mV. Subsequent red light pulse followed by darkness shifted E M oppositely (i.e. hyperpolarized the cell). The BL-induced shifts of E M were not observed in cells pretreated with a hydrogen-pump inhibitor, suggesting a contribution by hydrogen-pump to the shift. BL also increased K D -channel activity in a voltage-independent manner as reflected in the increase of the mean net steady-state patch conductance at a depolarization of 40 mV relative to the apparent reversal potential (G @40 ). G @40 increased by approximately 12 pS without a change of the single-channel conductance, possibly by increasing the probability of channel opening. Subsequent red-light and darkness reversed the change in G @40 . Thus, K ϩ efflux, a determining factor for the cell-volume decrease of flexor cells, is regulated by BL in a dual manner via membrane potential and by an independent signaling pathway.
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