Plant guard cell anion channel SLAC1 regulates stomatal closure

Plants are rooted to their growth place; therefore it is important that they react adequately to changes in environmental conditions. Stomatal pores, which are formed of a pair of guard cells in leaf epidermis, regulate plant gas-exchange. Importantly, guard cells protect the plant from desiccation in drought conditions by reducing the aperture of the stomatal pore. They serve also as the first barrier against the major air pollutant ozone, but the behaviour of guard cells during ozone exposure has not been sufficiently addressed. Aperture of the stomatal pore is regulated by the influx and efflux of osmotically active ions via ion channels and transporters across the guard cell membrane, however the molecular identity of guard cell plasma membrane anion channel has remained unknown. In the frame of this study, guard cell behaviour during ozone exposure was studied using the newly constructed Arabidopsis whole-rosette gas-exchange system. Ozone induced a Rapid Transient Decrease (RTD) in stomatal conductance within 10 min from the start of exposure, which was followed by a recovery in the conductance within the next 40 min. The decrease in stomatal conductance was dependent on the applied ozone concentration. Three minutes of ozone exposure was sufficient to induce RTD and further ozone application during the closure-recovery process had no effect on RTD, demonstrating that the whole process is programmed within the first three minutes. To address the molecular components responsible for RTD, the ozone response was measured in 59 different Arabidopsis mutants involved in guard cell signalling. Four of the tested mutants slac1 (originally rcd3), ost1, abi1-1 and abi2-1 lacked RTD completely. As the ozone sensitive mutant slac1 lacked RTD, the next aim of this study was to identify and characterize SLAC1. SLAC1 was shown to be a central regulator in response to all major factors regulating guard cell aperture: CO2, light/darkness transitions, ozone, relative air humidity, ABA, NO, H2O2, and extracellular Ca2+. It encodes the first guard cell plasma membrane slow type anion channel to be identified at the molecular level. Interestingly, the rapid type anion conductance was intact in slac1 mutant plants. For activation, SLAC1 needs to be phosphorylated. Protein kinase OST1 was shown to phosphorylate several amino acids in the N-terminal tail of SLAC1, Ser120 was one of its main targets, which led to SLAC1 activation. The lack of RTD in type 2C protein phosphatase mutants abi1-1 and abi2-1, suggests that these proteins have a regulatory role in ozoneinduced activation of the slow type anion channel.

Metrology of gaseous air pollutants

The need for mutual recognition of accurate measurement results made by competent laboratories has been very widely accepted at the international level e.g., at the World Trade Organization. A partial solution to the problem was made by the International Committee for Weights and Measures (CIPM) in setting up the Mutual Recognition Arrangement (CIPM MRA), which was signed by National Metrology Institutes (NMI) around the world. The core idea of the CIPM MRA is to have global arrangements for the mutual acceptance of the calibration certificates of National Metrology Institutes. The CIPM MRA covers all the fields of science and technology for which NMIs have their national standards. The infrastructure for the metrology of the gaseous compounds carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), sulphur dioxide (SO2) and ozone (O3) has been constructed at the national level at the Finnish Meteorological Institute (FMI). The calibration laboratory at the FMI was constructed for providing calibration services for air quality measurements and to fulfil the requirements of a metrology laboratory. The laboratory successfully participated, with good results, in the first comparison project, which was aimed at defining the state of the art in the preparation and analysis of the gas standards used by European metrology institutes and calibration laboratories in the field of air quality. To confirm the competence of the laboratory, the international external surveillance study was conducted at the laboratory. Based on the evidence, the Centre for Metrology and Accreditation (MIKES) designated the calibration laboratory at the Finnish Meteorological Institute (FMI) as a National Standard Laboratory in the field of air quality. With this designation, the MIKES-FMI Standards Laboratory became a member of CIPM MRA, and Finland was brought into the internationally-accepted forum in the field of gas metrology. The concept of ‘once measured - everywhere accepted’ is the leading theme of the CIPM MRA. The calibration service of the MIKES-FMI Standards Laboratory realizes the SI traceability system for the gas components, and is constructed to enable it to meet the requirements of the European air quality directives. In addition, all the relevant uncertainty sources that influence the measurement results have been evaluated, and the uncertainty budgets for the measurement results have been created.