Compartmental cross-talk in the regulation of light harvesting complex transcription under short-term light and temperature stress in Chlamydomonas reinhardtii

Short-term light stress in Chlamydomonas leads to transient changes in light harvesting complex (LHC) transcription. This requires cross-talk between the plastid and nucleus to coordinate chloroplast function with nuclear transcription. None of the components of this signalling pathway have been identified although several sensor candidates have been proposed. To examine the regulation of nuclear photosynthetic gene expression, we constructed an LHC:: Arylsulphatase (ARS) reporter system in Chlamydomonas reinhardtii allowing us to examine short-term regulatory changes on a fine scale. We modulated plastoquinone (PQ) redox state via photosynthetic inhibitors, changes in light and (or) temperature and found no evidence that either the PQ pool or Q(A) redox state were directly involved in short-term retrograde signalling. Shifts in light level and (or) temperature indicated that LHC transcriptional activity is tightly coordinated to photosynthetic production. Transient switching between photoautotrophic and mixotrophic growth, plus the use of mitochondrial inhibitors indicated that nuclear photosynthetic gene expression is coupled to mitochondrial activity. These short-term effects on LHC transcription demonstrate an interdependence of photosynthetic production and mitochondrial activity, suggesting Chlamydomonas is able to respond to environmental changes by monitoring metabolite pools between the chloroplast and mitochondria and not in the chloroplast directly.