How Sugars Might Coordinate Chloroplast and Nuclear Gene Expression during Acclimation to High Light Intensities

The concept of retrograde control of nuclear gene expression assumes the generation of signals inside the chloroplasts, which are either released from or sensed inside of the organelle. In both cases, downstream signaling path- ways lead eventually to a differential regulation of nuclear gene expression and the production of proteins required in the chloroplast. This concept appears reasonable as the majority of the over 3000 predicted plastidial proteins are encoded by nuclear genes. Hence, the nucleus needs information on the status of the chloroplasts, such as during acclimation responses, which trigger massive changes in the protein composition of the thylakoid membrane and in the stroma. Here, we propose an additional control mechanism of nuclear- and plastome-encoded photosynthesis genes, taking advantage of pathways involved in sugar- or hormonal signaling. Sugars are major end products of photosynthesis and their con- tents respond very sensitively to changes in light intensities. Based on recent findings, we ask the question as to whether the carbohydrate status outside the chloroplast can be directly sensed within the chloroplast stroma. Sugars might syn- chronize the responsiveness of both genomes and thereby help to coordinate the expression of piastome- and nuclear- encoded photosynthesis genes in concert with other, more specific retrograde signals.     

Induction of the AOX1D Isoform of Alternative Oxidase in A. thaliana T-DNA Insertion Lines Lacking Isoform AOX1A Is Insufficient to Optimize Photosynthesis when Treated with Antimycin A

Plant respiration is characterized by two pathways for electron transfer to O2, namely the cytochrome pathway （CP） that is linked to ATP production, and the alternative pathway （AP）, where electrons from ubiquinol are directly transferred to O2 via an alternative oxidase （AOX） without concomitant ATP production. This latter pathway is well suited to dispose of excess electrons in the light, leading to optimized photosynthetic performance. We have characterized T- DNA-insertion mutant lines of Arabidopsis thaliana that do not express the major isoform, AOXIA. In standard growth conditions, these plants did not show any phenotype, but restriction of electron flow through CP by antimycin A, which induces AOXIA expression in the wild-type, led to an increased expression of AOXID in leaves of the aoxla-knockout mutant. Despite the increased presence of the AOX1D isoform in the mutant, antimycin A caused inhibition of photosyn- thesis, increased ROS, and ultimately resulted in amplified membrane leakage and necrosis when compared to the wild- type, which was only marginally affected by the inhibitor. It thus appears that AOX1 D was unable to fully compensate for the loss of AOXIA when electron flow via the CP is restricted. A combination of inhibition studies, coupled to metabolite profiling and targeted expression analysis of the P-protein of glycine decarboxylase complex （GDC）, suggests that the aoxla mutants attempt to increase their capacity for photorespiration. However, given their deficiency, it is intriguing that increase in expression neither of AOX1D nor of GDC could fully compensate for the lack of AOXIA to optimize pho- tosynthesis when treated with antimycin A. We suggest that the aoxla mutants can further be used to substantiate the current models concerning the influence of mitochondrial redox on photosynthetic performance and gene expression.