BLUE LIGHT-DEPENDENT MONOVALENT ANION UPTAKE

Blue light is one of the most important environmental signals regulati ng monovalent anion transport in plant cells. In the unicellular fresh water chlorophyte Monoraphidium braunii, blue light is essential for t he activation of HCO3-, NO3-, NO2-, and Cl- transport systems. These a nions are taken up when blue light is present but the uptake ceases wh en this radiation is suppressed, indicating that blue light is a switc h signal for the monovalent anion transport system(s) of this alga. Si milar results have been obtained in other green algae and higher plant s. The action spectra for the uptake of NO3- and Cl- in M. braunii are very similar and resemble the absorption spectra of flavins or a comb ination of flavins and pterins. It is proposed that both anions share the same transport system(s). The uptake of monovalent anions consists of a cotransport with H+, thus producing alkalinization of the extern al medium. The time between the onset of blue light and the beginning of alkalinization can be as short as 2 s, Taken together, the results suggest that the photoreceptor mediating the blue light activation of monovalent anion uptake in this green alga is a plasma membrane-bound flavoprotein.