The Rebeiz Foundation for Basic Research (RFFBR) Paper Award for 2008 has been awarded to Professor. Alison Smith and her colleagues, Michael Moulin, Alex C. McCormac, and Mattew J. Terry, for their paper entitled "Tetrapyrrole profiling in Arabidopsis seedlings reveals that retrograde plastid nuclear signaling is not due to Mg-protoporphyrin IX accumulation”. The paper appeared in Proceedings of the .National Academy of Sciences, USA 105:15178-15183(2008) and was edited by Diter von Wettstein. Essentially it refutes the long standing hypothesis that Mg-protoporphyrin IX accumulation is responsible for retrograde plastid-nuclear signaling
Chloroplast biogenesis involves careful coordination of both plastid and nuclear gene expression, which is achieved in part by retrograde signaling from the chloroplast to the nucleus. This can be demonstrated by the fact that the herbicide, Norflurazon (NF), which causes bleaching of chloroplasts, prevents the light induction of photosynthesis-related genes in the nucleus. It has been proposed that the chlorophyll biosynthetic intermediate, Mg-protoporphyrin IX, acts as the signaling molecule in this pathway and accumulates in the chloroplasts and cytosol of the cell after NF treatment. In this paper, it is demonstrated that this model is too simplistic. A sensitive liquid chromatography-mass spectrometry (LC/MS) method was developed to measure tetrapyrrole intermediates, and it was shown that no Mg-protoporphyrin IX, nor indeed any other chlorophyll-biosynthetic intermediate, can be detected in NF-treated plants under conditions in which nuclear gene expression is repressed. Conversely when endogenous Mg-protoporphyrin IX levels were artificially increased by supplementation with the tetrapyrrole precursor, 5-aminolevulinic acid, the expression of nuclear-encoded photosynthetic genes was induced instead of being repressed. It was also demonstrate that NF-treatment leads to a strong down-regulation of tetrapyrrole biosynthesis genes, in the absence of an accumulation of tetrapyrrole intermediates. Finally, no correlation was observed between nuclear-gene expression and any chlorophyll biosynthetic intermediates over a range of growth conditions and treatments. It was proposed that a possible perturbation of tetrapyrrole biosynthesis might lead to localized production of reactive oxygen species (ROS) or to an altered redox state of the plastid, which could mediate retrograde signaling.
In selecting this paper for the thirdRFFBR Paper Award, the Board of directors of the Rebeiz Foundation felt that the described work will greatly benefit other chloroplast researchers, and enhance our knowledge of the chlorophyll biosynthetic pathway and its regulation.