Tese de doutoramento, Biologia (Biologia do Desenvolvimento), Universidade de Lisboa, Faculdade de Ciências, 2015Upon hydration in the female gametophyte, pollen grains germinate and develop a cytoplasmic extension designated as the pollen tube. This cell is dramatically polarized, grows exclusively at its apex (apical growth) and responds to specific female cues by changing growth axis in order to direct it towards the entrance of the ovule. When germinated in vitro, pollen tubes display oscillations in growth, which is underlied by ion dynamics, namely ion fluxes across the membrane and cytosolic ionic gradients. By means of imaging techniques and by Ion-Specific Vibrating Probes, Zonia et al., (2002) identified and characterized chloride (Cl-) fluxes in growing pollen tubes from Lilium longiflorum and Nicotiana tabacum, showing an massive oscillatory efflux of Cl- at the tip and influxes all over the shank of the tube, starting approximately at 15 μm from the tip. The authors suggested that the Cl- fluxes are essential to pollen tube growth due to its role on the maintenance of the cellular volume, presumably by water flux control. The molecular nature and mechanisms of the pollen tube anion transporters are still mostly unknown. This thesis aims at characterizing the anionic currents present in the pollen grain protoplasts of two species, Lilium longiflorum (Lily) and Arabidopsis thaliana, and to characterize the putative transporters underlying these currents in Arabidopis. Lily pollen was chosen because (1) it’s one of the most studied species because of the favourable features of its pollen for Cell Biology studies and (2) it allows for a comparison between the same type of currents in two very different species. During the course of this project single channel events were detected by patch-clamp, and the existence of anionic currents across channels in the plasma membrane of un-germinated grains was demonstrated. In addition, by germinating Lily pollen we were able to find anionic currents similar to those measured in hydrated Lily protoplasts. We speculate that many of the channels present in the plasma membrane of the grain protoplasts may suffer a spatial redistribution to specific regions of the membrane during pollen tube germination and growth or that they undergo recycling. Arabidopsis was chosen due to its genetic tractability, namely the established resources for reverse-genetics approaches that facilitate the identification of candidate channels and transporters responsible for Cl- transport determined by patch-clamp. A full search into the available transcriptomic datasets for pollen grain (PG) and tube (PT) resulted in the selection of four candidate proteins that, due to their known electrophysiological profiles, could account for the observed currents and fluxes previously measured, respectively. Four transporter candidates were chosen: AtCCC (The Cation Chloride Cotransporter), SLAH3 (SLow Anion channel Homolog 3), ALMT12 (Aluminum-activated Malate Transporter 12) and At1g73020 (here designated by AtTMEM16, Arabidopsis thaliana TransMEMbrane 16). Mutant lines were generated and a electrophysiological characterization study and localization assays were performed. By patch clamp we could measure current alterations on hydrated pollen grain protoplasts of mutant for these genes. Pollen protoplasts (sporoplats) of the single gene mutants slah3-/-, almt12-/- and atmem16-/-, and the double mutant slah3-/-;almt12-/- present significant specific differences in currents than those measured in the wild-type (wt) group, namely: (1) a higher initial current (IInitial) in slah3-/- and attmem16-/- protoplasts; (2) almt12-/- had reduced IInitial and final currents (IFinal) for the negative membrane potentials (Vm) tested; (3) increased rundown currents (ILostRD) for the negative Vm in all three single mutants, (4) higher loss of ILostRd in slah3-/- and attmem16-/- protoplasts for positive Vm; (5) different sensitivities to 5-Nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) were measured for all three single mutants and (6) different sensitivities and conductive states to Vm variations in the three single mutants. In addition, the currents measured from the double mutant line almt12-/-; slah3-/-, presented key differences when compared to wt, and to the respective single mutants, namely: a significant decrease in IInitial, IFinal and ILostRd, a higher rundown for the negative Vm, and a higher inhibition of ILostRd in relation to the slah3-/- single mutant. In the mutant context, the observed differences in the current amplitudes throughout the experimental protocol strongly suggest that the regulation of Cl- transport is very complex and involves several regulatory loops and different types of compensation, either by modulation/activation of the remaining channels or by transcriptional activation of other transporters. By overexpressing Green Fluorescent Protein (GFP) fusion chimeras of SLAH3, ALMT12 and AtTMEM16 channels and AtCCC under the control of the LAT52 pollen specific promoter we observe that AtCCC localizes to the plasma membrane (PM) in the shank of the PT, but not in the tip; AtTMEM16 localizes to the plasma membrane of the shank, and to enlogated membranous structures, presumably endoplasmatic reticulum, vesicle accumulation, or stretched young vacuoles; SLAH3 and ALMT12 are present in small circular organelles. We were able to successfully establish the localization of AtCCC at the shank of the PT and allocate SLAH3 to the tip. These results are suggestive that SLAH3 and AtCCC transporters could account for the fluxes measured by Zonia et al., (2002) in the growing PT. Vibrating Probe analysis in the 4 selected transporter mutants, further sustains the previous statement: the anionic fluxes collected from the tip of germinating PTs of slah3-/- and atccc-/- was significantly lower than those measured from the wt and almt12-/- and atmem16-/- mutants. We further expressed the candidate genes on mammalian cell lines, in order to characterize them electrophysiologically and to establish their molecular identity. ALMT12 expressed in COS-7 was technically challenging, but we were able to measure statistically relevant inward currents, resembling the electrophysiological profile of that described in Meyer et al., (2010). Finally, we show a direct regulation of the elicited currents in hydrated PGs by gamma-Aminobutyric acid (GABA). A motif that contains a binding site to GABA is shared between GABAA receptors and the ALMT family (Ramesh et al., 2015). When added to wt, 80-500 μM GABA inhibit significantly the overall currents, but this effect is completely abrogated in almt12-/-.sporoplasts. In addition, when GABA was added to two ALMT12 transfected COS-7 mammalian cell it induced single channel flickering events. These results, strongly suggest that GABA directly binds to ALMT12 channels, and establishes ALMT channels as prominent candidates for GABA binding and consequent biological implications in plants. The physiological implications of the present findings are discussed, in the context of pollen tube growth regulation.Fundação para a Ciência e a Tecnologia (FCT
