Coordination of Plastid Protein Import and Nuclear Gene Expression by Plastid-to-Nucleus Retrograde Signaling

Abstract

Expression of nuclear-encoded plastid proteins and import of those proteins into plastids are indispensable for plastid biogenesis. One possible cellular mechanism that coordinates these two essential processes is retrograde signaling from plastids to the nucleus. However, the molecular details of how this signaling occurs remain elusive. Using the plastid protein import2 mutant of Arabidopsis (Arabidopsis thaliana), which lacks the atToc159 protein import receptor, we demonstrate that the expression of photosynthesis-related nuclear genes is tightly coordinated with their import into plastids. Down-regulation of photosynthesis-related nuclear genes is also observed in mutants lacking other components of the plastid protein import apparatus. Genetic studies indicate that the coordination of plastid protein import and nuclear gene expression is independent of proposed plastid signaling pathways such as the accumulation of Mg-protoporphyrin IX and the activity of ABA INSENSITIVE4 (ABI4). Instead, it may involve GUN1 and the transcription factor AtGLK. The expression level of AtGLK1 is tightly correlated with the expression of photosynthesis-related nuclear genes in mutants defective in plastid protein import. Furthermore, the activity of GUN1 appears to down-regulate the expression of AtGLK1 when plastids are dysfunctional. Based on these data, we suggest that defects in plastid protein import generate a signal that represses photosynthesis-related nuclear genes through repression of AtGLK1 expression but not through activation of ABI4.Plastids are a diverse group of organelles that perform essential metabolic and signaling functions within all plant cells. It is generally believed that plastids originated from a unicellular photosynthetic bacterium that was taken up by a eukaryotic host cell (Dyall et al., 2004). During evolution, most of the genes encoded by the bacterial ancestor have been transferred to the host nuclear genome; for example, the plastid genome of Arabidopsis (Arabidopsis thaliana) encodes fewer than 100 open reading frames (Martin et al., 1998). Consequently, plastid biogenesis is dependent on the import of nuclear-encoded plastid proteins (Keegstra and Cline, 1999;Soll and Schleiff, 2004;Kessler and Schnell, 2006;Inaba and Schnell, 2008;Jarvis, 2008), the genes for which must be expressed at an appropriate level. For example, many of the photosynthesis-related nuclear genes that are required for chloroplast biogenesis are induced via photoreceptors, such as phytochrome, in response to light quality and quantity (Terzaghi and Cashmore, 1995), so that the photosynthesis-related proteins will be available for import into the developing chloroplasts. Other types of plastids, according to their specific metabolic functions, need other sets of nuclear-encoded proteins. Therefore, the expression of specific sets of nuclear genes and the import of their translation products are indispensable for plastid differentiation.After they have been imported into plastids, nuclear-encoded plastid proteins combi...

Plastid Proteome Assembly without Toc159: Photosynthetic Protein Import and Accumulation of N-Acetylated Plastid Precursor Proteins

“…Of these genes, 1998 (8.2%) were significantly upregulated in the wild type and 2268 (9.3%) in ppi2 ( Figure 3A; see Supplemental Data Set 4 online). Our transcriptomics data confirmed the trend that photosynthetic proteins are expressed at lower levels in the import mutant as previously reported in published microarray, SAGE and RT-PCR data (Kubis et al, 2003;Kakizaki et al, 2009;Lee et al, 2009b). Notably, 84% of the downregulated genes reported in the SAGE data set were also found downregulated under our experimental conditions (see Supplemental Data Set 4 online).…”

“…Notably, 84% of the downregulated genes reported in the SAGE data set were also found downregulated under our experimental conditions (see Supplemental Data Set 4 online). Similarly, we could confirm the reported upregulation of several heat shock-related proteins (Kakizaki et al, 2009). Of the 1046 nuclear-encoded genes coding for plastid proteins for which we had array data, 381 (36.4%) had significantly reduced RNA levels in ppi2, with a significant overrepresentation of genes encoding proteins for photosynthetic functions ( Figure 3B; see Supplemental Data Set 4 online).…”

“…This suggests that these nonphotosynthetic proteins are integrated in a signaling circuit that is different from the signaling circuit that controls the expression of photosynthetic proteins. Together, our data support the view that the accumulation of nuclear-encoded plastid precursor proteins triggers an unimported precursor protein response (Kakizaki et al, 2009;Lee et al, 2009b). This signaling response could act through additional unknown signaling pathways that do not depend on the functional state of the plastids.…”

“…The assembly and maintenance of the chloroplast proteome are highly regulated processes and involve not only the organellar import machinery but also multiple anterograde and retrograde signaling events that affect the transcription of nuclear-and chloroplast-encoded genes (Mochizuki et al, 2001(Mochizuki et al, , 2008Strand et al, 2003;Larkin et al, 2003;Koussevitzky et al, 2007;Moulin et al, 2008;Kakizaki et al, 2009). Similar transcript patterns observed under different environmental conditions or genetic modifications identified key regulators that affect entire sets of plastid proteins and biochemical networks (Richly et al, 2003;Biehl et al, 2005).…”

Sumoylation of transcription factor MYB30 by the small ubiquitin-like modifier E3 ligase SIZ1 mediates abscisic acid response in Arabidopsis thaliana

“…To determine if MYB30 functions in previously identified ABA signaling pathways, we crossed myb30-2 to abi1-3, abi2-2, abi3, abi4, and abi5-8 (all in Col-0) (37)(38)(39)(40)(41). Double-mutants of abi1-3 myb30-2 and abi2-2 myb30-2 exhibited enhanced ABA sensitivity relative to their single mutants, suggesting that MYB30 functions in parallel pathways with ABI1 or ABI2 during ABA signaling (Fig.…”