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.
Highlights
Expression of nuclear-encoded plastid proteins and import of those proteins into plastids are indispensable for plastid biogenesis
This observation suggests that plastid protein import and nuclear gene expression are tightly coordinated to regulate the flow of nuclear-encoded plastid proteins in response to the functional and developmental states of the plastids
Plastid protein import and the expression of nuclearencoded plastid proteins are both integral parts of plastid biogenesis
Summary
Expression of nuclear-encoded plastid proteins and import of those proteins into plastids are indispensable for plastid biogenesis. Because plastids need a distinct set of proteins in specific amounts dictated by their functional and metabolic states, it is critical for plastids to give feedback information to the nucleus, so that nuclear gene expression can be adjusted appropriately This fine-tuning mechanism is important for determining which nuclear-encoded proteins are imported, and it regulates the differentiation of. When an artificial substrate of atToc159, preSSU-GFP, was expressed in ppi under the control of a constitutive promoter, the expression of preSSU-GFP was no longer down-regulated and ppi accumulated a significant amount of the unprocessed protein in the cytosol (Smith et al, 2004) These observations suggest that plastid protein import and nuclear gene expression are tightly coordinated to regulate the flow of nuclear-encoded plastid proteins in response to the functional and developmental states of the plastids. The molecular mechanism that coordinates protein import and nuclear gene expression remains elusive
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