Identification and characterization of GABA, proline and quaternary ammonium compound transporters from Arabidopsis thaliana.

Arabidopsis thaliana grows efficiently on GABA as the sole nitrogen source, thereby providing evidence for the existence of GABA transporters in plants. Heterologous complementation of a GABA uptake-deficient yeast mutant identified two previously known plant amino acid transporters, AAP3 and ProT2, as GABA transporters with Michaelis constants of 12.9 +/- 1.7 and 1.7 +/- 0.3 mM at pH 4, respectively. The simultaneous transport of [1-14C]GABA and [2,3-3H]proline by ProT2 as a function of pH, provided evidence that the zwitterionic state of GABA is an important parameter in substrate recognition. ProT2-mediated [1-14C]GABA transport was inhibited by proline and quaternary ammonium compounds.

LeProT1, a transporter for proline, glycine betaine, and gamma-amino butyric acid in tomato pollen.

During maturation, pollen undergoes a period of dehydration accompanied by the accumulation of compatible solutes. Solute import across the pollen plasma membrane, which occurs via proteinaceous transporters, is required to support pollen development and also for subsequent germination and pollen tube growth. Analysis of the free amino acid composition of various tissues in tomato revealed that the proline content in flowers was 60 times higher than in any other organ analyzed. Within the floral organs, proline was confined predominantly to pollen, where it represented >70% of total free amino acids. Uptake experiments demonstrated that mature as well as germinated pollen rapidly take up proline. To identify proline transporters in tomato pollen, we isolated genes homologous to Arabidopsis proline transporters. LeProT1 was specifically expressed both in mature and germinating pollen, as demonstrated by RNA in situ hybridization. Expression in a yeast mutant demonstrated that LeProT1 transports proline and gamma-amino butyric acid with low affinity and glycine betaine with high affinity. Direct uptake and competition studies demonstrate that LeProT1 constitutes a general transporter for compatible solutes.

Fungal elicitors induce a transient release of active oxygen species from cultured spruce cells that is dependent on Ca(2+) and protein-kinase activity.

Cell-wall components from the ectomycorrhizal fungi Amanita muscaria and Hebeloma crustuliniforme and from the spruce pathogen Heterobasidion annosum elicited a transient release of active oxygen species from cultured spruce cells (Picea abies (L.) Karst.). Since the detection of active oxygen was suppressed by catalase, H2O2 was assumed to be the prevailing O2 species. On the other hand, superoxide dismutase enhanced the concentration of detectable H2O2 indicating that the superoxide anion was formed before dismutating to H2O2. The elicitors induced the formation of active oxygen in a dose-dependent manner. Interestingly, elicitors from mycorrhizal fungi had a lower H2O2-inducing activity than equal amounts of cell-wall preparations from the pathogen H. annosum. In Ca(2+)-depleted medium the production of active oxygen by elicitor-treated spruce cells was suppressed. Additionally, the ionophore A 23187 induced active oxygen formation in a medium with Ca(2+) but not in a Ca(2+)-depleted medium. Furthermore, the protein-kinase inhibitor staurosporine inhibited the oxidative burst. At a concentration of 34 nM the effect was diminished to 50%. From these results it is suggested that the release of active oxygen species from cultured spruce cells triggered by cell-wall-derived fungal elicitors depends on external Ca(2+) and a protein-kinase activity. In these respects the effect shows similarities with the well-studied respiratory burst of mammalian neutrophils.