Importance of the alternative pathway of respiration for avoidance of ROS production and for optimisation of photosynthesis in Chlamydomonas

The physiological function of the alternative pathway of respiration has been investigated by analysing two RNAi C.reinhardtii lines deprived of alternative oxidase protein (AOX1). Compared to wild-type, AOX1- lines exhibited modified growth curves and reduced maximal cell density. These differences were more pronounced at high irradiance and in nitrate-containing medium (TAP NO3) rather than in ammonium-containing medium (TAP NH4). Although the alternative pathway was inactive, respiration was not significantly altered in transgenics. Light-saturation curves of O2-evolution were only slightly modified. However, non-photochemical quenching of fluorescence (NPQ) was strongly reduced. Further analysis showed that AOX1- transgenics present a reduced ability to promote the change in energy distribution between photosystems, known as state transition. This effect, which explains low NPQ in the light, was most pronounced in high-light cells cultivated in TAP NO3 medium. Moreover, AOX1- transgenics exhibited higher levels of intracellular peroxides, which suggests that inhibition of state transition might result from higher ROS production. In support of this hypothesis, addition of millimolar-range concentrations of H2O2 to wild-type inhibited the state transition promoted by the reduction of the plastoquinone pool in darkness.