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Cellular protein homeostasis (proteostasis) is maintained through the balance between de novo synthesis and proteolysis. The unfolded/misfolded protein response (UPR) that is triggered by stressed endoplasmic reticulum (ER) also plays an important role in proteostasis in both plants and animals. Although ER-triggered UPR has been extensively studied in plants, the molecular mechanisms underlying mitochondrial and chloroplastic UPRs are largely uncharacterized despite the fact that these organelles are sites of production of harmful reactive oxygen species (ROS), which damage proteins. In this study, we demonstrate that chloroplasts of the Arabidopsis yellow leaf variegation 2 (var2) mutant, which lacks the metalloprotease FtsH2, accumulate damaged chloroplast proteins and trigger a UPR-like response, namely the accumulation of a suite of chloroplast proteins involved in protein quality control (PQC). These PQC proteins include heat-shock proteins, chaperones, proteases, and ROS detoxifiers. Given that FtsH2 functions primarily in photosystem II proteostasis, the accumulation of PQC-related proteins may balance the FtsH2 deficiency. Moreover, the apparent up-regulation of the cognate transcripts indicates that the accumulation of PQC-related proteins in var2 is probably mediated by retrograde signaling, indicating the occurrence of a UPR-like response in var2. [ABSTRACT FROM AUTHOR]

Seedlings of Arabidopsis have been exposed to norflurazon. Depending on the developmental stage at which seedlings were first exposed to the inhibitor, enhanced production of reactive oxygen species occurred and, among others, 1O2-mediated and EXECUTER-dependent retrograde signaling was induced.Chloroplast development depends on the synthesis and import of a large number of nuclear-encoded proteins. The synthesis of some of these proteins is affected by the functional state of the plastid via a process known as retrograde signaling. Retrograde plastid-to-nucleus signaling has been often characterized in seedlings of Arabidopsis thaliana exposed to norflurazon (NF), an inhibitor of carotenoid biosynthesis. Results of this work suggested that, throughout seedling development, a factor is released from the plastid to the cytoplasm that indicates a perturbation of plastid homeostasis and represses nuclear genes required for normal chloroplast development. The identity of this factor is still under debate. Reactive oxygen species (ROS) were among the candidates discussed as possible retrograde signals in NF-treated plants. In the present work, this proposed role of ROS has been analyzed. In seedlings grown from the very beginning in the presence of NF, ROS-dependent signaling was not detectable, whereas, in seedlings first exposed to NF after light-dependent chloroplast formation had been completed, enhanced ROS production occurred and, among others, 1O2-mediated and EXECUTER-dependent retrograde signaling was induced. Hence, depending on the developmental stage at which plants are exposed to NF, different retrograde signaling pathways may be activated, some of which are also active in non-treated plants under light stress. [ABSTRACT FROM AUTHOR]

Enhanced levels of singlet oxygen (1O2) in chloroplasts trigger programmed cell death. The impact of 1O2 production in chloroplasts was monitored first in the conditional fluorescent (flu) mutant of Arabidopsis thaliana that accumulates 1O2 upon a dark/light shift. The onset of 1O2 production is rapidly followed by a loss of chloroplast integrity that precedes the rupture of the central vacuole and the final collapse of the cell. Inactivation of the two plastid proteins EXECUTER (EX1) and EX2 in the flu mutant abrogates these responses, indicating that disintegration of chloroplasts is due to EX-dependent signaling rather than 1O2 directly. In flu seedlings, 1O2 -mediated cell death signaling operates as a default pathway that results in seedlings committing suicide. By contrast, EX-dependent signaling in the wild type induces the formation of microlesions without decreasing the viability of seedlings. 1O2 -mediated and EX-dependent loss of plastid integrity and cell death in these plants occurs only in cells containing fully developed chloroplasts. Our findings support an as yet unreported signaling role of 1O2 in the wild type exposed to mild light stress that invokes photoinhibition of photosystem II without causing photooxidative damage of the plant. [ABSTRACT FROM AUTHOR]

The NADPH-dependent protochlorophyllide (Pchlide) oxidoreductase (POR) is unique because it is a photoenzyme that requires light for its catalytic activity and uses Pchlide itself as a photoreceptor. In Arabidopsis, there are three structurally related PORs, denoted PORA, PORB, and PORC. The import of one of them, PORA, into plastids of cotyledons is substrate dependent. This substrate dependence is demonstrated in intact seedlings of wild-type Arabidopsis and two mutants, xantha2, which is devoid of Pchlide, and flu, which upon redarkening rapidly accumulates Pchlide. In true leaves, PORA up, take does not require the presence of Pchlide. The organ specificity of the substrate-dependent import of PORA reveals a means of controlling plastid protein translocation that is closely associated with a key step in plant development, the light-dependent transformation of cotyledons from a storage organ to a photosynthetically active leaf. [ABSTRACT FROM AUTHOR]