Centre for Biological Sciences, University of Southampton Southampton, UK ; Institute for Life Sciences, University of Southampton Southampton, UK.
Front Plant Sci. 2013 Feb 13;4:14. doi: 10.3389/fpls.2013.00014. eCollection 2013.
Chloroplast biogenesis involves the co-ordinated expression of the chloroplast and nuclear genomes, requiring information to be sent from the developing chloroplasts to the nucleus. This is achieved through retrograde signaling pathways and can be demonstrated experimentally using the photobleaching herbicide, norflurazon, which in seedlings results in chloroplast damage and the reduced expression of many photosynthesis-related, nuclear genes. Genetic analysis of this pathway points to a major role for tetrapyrrole synthesis in retrograde signaling, as well as a strong interaction with light signaling pathways. Currently, the best model to explain the genetic data is that a specific heme pool generated by flux through ferrochelatase-1 functions as a positive signal to promote the expression of genes required for chloroplast development. We propose that this heme-related signal is the primary positive signal during chloroplast biogenesis, and that treatments and mutations affecting chloroplast transcription, RNA editing, translation, or protein import all impact on the synthesis and/or processing of this signal. A positive signal is consistent with the need to provide information on chloroplast status at all times. We further propose that GUN1 normally serves to restrict the production of the heme signal. In addition to a positive signal re-enforcing chloroplast development under normal conditions, aberrant chloroplast development may produce a negative signal due to accumulation of unbound chlorophyll biosynthesis intermediates, such as Mg-porphyrins. Under these conditions a rapid shut-down of tetrapyrrole synthesis is required. We propose that accumulation of these intermediates results in a rapid light-dependent inhibition of nuclear gene expression that is most likely mediated via singlet oxygen generated by photo-excitation of Mg-porphyrins. Thus, the tetrapyrrole pathway may provide both positive and inhibitory signals to control expression of nuclear genes.
叶绿体生物发生涉及叶绿体和核基因组的协调表达，需要将信息从发育中的叶绿体发送到细胞核。这是通过逆行信号通路来实现的，可以通过使用光漂白除草剂 norflurazon 进行实验来证明，norflurazon 在幼苗中导致叶绿体损伤和许多与光合作用相关的核基因表达减少。该途径的遗传分析表明四吡咯合成在逆行信号中起主要作用，并且与光信号途径强烈相互作用。目前，解释遗传数据的最佳模型是，通过铁螯合酶-1 通量产生的特定血红素池作为促进叶绿体发育所需基因表达的正信号。我们提出，这种血红素相关信号是叶绿体生物发生过程中的主要正信号，并且影响叶绿体转录、RNA 编辑、翻译或蛋白质导入的处理和突变都会影响该信号的合成和/或加工。正信号与随时提供有关叶绿体状态的信息的需求一致。我们进一步提出，GUN1 通常用于限制血红素信号的产生。除了在正常条件下增强叶绿体发育的正信号外，由于未结合的叶绿素生物合成中间产物（如 Mg-卟啉）的积累，异常的叶绿体发育可能会产生负信号。在这些条件下，需要迅速停止四吡咯合成。我们提出，这些中间产物的积累导致核基因表达的快速光依赖性抑制，这很可能是通过 Mg-卟啉的光激发产生的单线态氧介导的。因此，四吡咯途径可能为核基因表达的控制提供正信号和抑制信号。
