PSI Photoinhibition and Changing CO2 Levels Initiate Retrograde Signals to Modify Nuclear Gene Expression

Photosystem I (PSI) is a critical component of the photosynthetic machinery in plants. Under conditions of environmental stress, PSI becomes photoinhibited, leading to a redox imbalance in the chloroplast. PSI photoinhibition is caused by an increase in electron pressure within PSI, which damages the iron-sulfur clusters. In this study, we investigated the susceptibility of PSI to photoinhibition in plants at different concentrations of CO2, followed by global gene expression analyses of the differentially treated plants. PSI photoinhibition was induced using a specific illumination protocol that inhibited PSI with minimal effects on PSII. Unexpectedly, the varying CO2 levels combined with the PSI-PI treatment neither increased nor decreased the likelihood of PSI photodamage. All PSI photoinhibition treatments, independent of CO2 levels, upregulated genes generally involved in plant responses to excess iron and downregulated genes involved in iron deficiency. PSI photoinhibition also induced genes encoding photosynthetic proteins that act as electron acceptors from PSI. We propose that PSI photoinhibition causes a release of iron from damaged iron-sulfur clusters, which initiates a retrograde signal from the chloroplast to the nucleus to modify gene expression. In addition, the deprivation of CO2 from the air initiated a signal that induced flavonoid biosynthesis genes, probably via jasmonate production.

Nicotiana benthamiana Kunitz peptidase inhibitor-like protein involved in chloroplast-to-nucleus regulatory pathway in plant-virus interaction

Plant viruses use a variety of strategies to infect their host. During infection, viruses cause symptoms of varying severity, which are often associated with altered leaf pigmentation due to structural and functional damage to chloroplasts that are affected by viral proteins. Here we demonstrate that Nicotiana benthamiana Kunitz peptidase inhibitor-like protein (KPILP) gene is induced in response to potato virus X (PVX) infection. Using reverse genetic approach, we have demonstrated that KPILP downregulates expression of LHCB1 and LHCB2 genes of antenna light-harvesting complex proteins, HEMA1 gene encoding glutamyl-tRNA reductase, which participates in tetrapyrrole biosynthesis, and RBCS1A gene encoding RuBisCO small subunit isoform involved in the antiviral immune response. Thus, KPILP is a regulator of chloroplast retrograde signaling system during developing PVX infection. Moreover, KPILP was demonstrated to affect carbon partitioning: reduced glucose levels during PVX infection were associated with KPILP upregulation. Another KPILP function is associated with plasmodesmata permeability control. Its ability to stimulate intercellular transport of reporter 2xGFP molecules indicates that KPILP is a positive plasmodesmata regulator. Moreover, natural KPILP glycosylation is indispensable for manifestation of this function. During PVX infection KPILP increased expression leads to the reduction of plasmodesmata callose deposition. These results could indicate that KPILP affects plasmodesmata permeability via callose-dependent mechanism. Thus, virus entering a cell and starting reproduction triggers KPILP expression, which leads to downregulation of nuclear-encoded chloroplast genes associated with retrograde signaling, reduction in photoassimilates accumulation and increase in intercellular transport, creating favorable conditions for reproduction and spread of viral infection.

Transcriptomic and Metabolomic Response to High Light in the Charophyte Alga Klebsormidium nitens

The characterization of the molecular mechanisms, such as high light irradiance resistance, that allowed plant terrestralization is a cornerstone in evolutionary studies since the conquest of land by plants played a pivotal role in life evolution on Earth. Viridiplantae or the green lineage is divided into two clades, Chlorophyta and Streptophyta, that in turn splits into Embryophyta or land plants and Charophyta. Charophyta are used in evolutionary studies on plant terrestralization since they are generally accepted as the extant algal species most closely related to current land plants. In this study, we have chosen the facultative terrestrial early charophyte alga Klebsormidium nitens to perform an integrative transcriptomic and metabolomic analysis under high light in order to unveil key mechanisms involved in the early steps of plants terrestralization. We found a fast chloroplast retrograde signaling possibly mediated by reactive oxygen species and the inositol polyphosphate 1-phosphatase (SAL1) and 3'-phosphoadenosine-5'-phosphate (PAP) pathways inducing gene expression and accumulation of specific metabolites. Systems used by both Chlorophyta and Embryophyta were activated such as the xanthophyll cycle with an accumulation of zeaxanthin and protein folding and repair mechanisms constituted by NADPH-dependent thioredoxin reductases, thioredoxin-disulfide reductases, and peroxiredoxins. Similarly, cyclic electron flow, specifically the pathway dependent on proton gradient regulation 5, was strongly activated under high light. We detected a simultaneous co-activation of the non-photochemical quenching mechanisms based on LHC-like stress related (LHCSR) protein and the photosystem II subunit S that are specific to Chlorophyta and Embryophyta, respectively. Exclusive Embryophyta systems for the synthesis, sensing, and response to the phytohormone auxin were also activated under high light in K. nitens leading to an increase in auxin content with the concomitant accumulation of amino acids such as tryptophan, histidine, and phenylalanine.

CIA2 and CIA2-LIKE are required for optimal photosynthesis and stress responses in Arabidopsis thaliana

Chloroplast-to-nucleus retrograde signaling is essential for cell function, acclimation to fluctuating environmental conditions, plant growth and development. The vast majority of chloroplast proteins are nuclear-encoded, and must be imported into the organelle after synthesis in the cytoplasm. This import is essential for the development of fully functional chloroplasts. On the other hand, functional chloroplasts act as sensors of environmental changes and can trigger acclimatory responses that influence nuclear gene expression. Signaling via mobile transcription factors (TFs) has been recently recognized as a way of communication between organelles and the nucleus. In this study, we performed a targeted reverse genetic screen to identify dual-localized TFs involved in chloroplast retrograde signaling during stress responses. We found that CHLOROPLAST IMPORT APPARATUS 2 (CIA2) has a functional plastid transit peptide, and can be located both in chloroplasts and the nucleus. Further, we found that CIA2, along with its homolog CIA2-like (CIL) are involved in the regulation of Arabidopsis responses to UV-AB, high light and heat shock. Finally, our results suggest that both CIA2 and CIL are crucial for chloroplast translation. Our results contribute to a deeper understanding of signaling events in the chloroplast-nucleus cross-talk.

Targeted Control of Chloroplast Quality to Improve Plant Acclimation: From Protein Import to Degradation

The chloroplast is an important energy-producing organelle acting as an environmental sensor for the plant cell. The normal turnover of the entire damaged chloroplast and its specific components is required for efficient photosynthesis and other metabolic reactions under stress conditions. Nuclear-encoded proteins must be imported into the chloroplast through different membrane transport complexes, and the orderly protein import plays an important role in plant adaptive regulation. Under adverse environmental conditions, the damaged chloroplast or its specific components need to be degraded efficiently to ensure normal cell function. In this review, we discuss the molecular mechanism of protein import and degradation in the chloroplast. Specifically, quality control of chloroplast from protein import to degradation and associated regulatory pathways are discussed to better understand how plants adapt to environmental stress by fine-tuning chloroplast homeostasis, which will benefit breeding approaches to improve crop yield.

Mg chelatase in chlorophyll synthesis and retrograde signaling in Chlamydomonas reinhardtii: CHLI2 cannot substitute for CHLI1

The oligomeric Mg chelatase (MgCh), consisting of the subunits CHLH, CHLI, and CHLD, is located at the central site of chlorophyll synthesis, but is also thought to have an additional function in regulatory feedback control of the tetrapyrrole biosynthesis pathway and in chloroplast retrograde signaling. In Arabidopsis thaliana and Chlamydomonas reinhardtii, two genes have been proposed to encode the CHLI subunit of MgCh. While the role of CHLI1 in A. thaliana MgCh has been substantially elucidated, different reports provide inconsistent results with regard to the function of CHLI2 in Mg chelation and retrograde signaling. In the present report, the possible functions of both isoforms were analyzed in C. reinhardtii Knockout of the CHLI1 gene resulted in complete loss of MgCh activity, absence of chlorophyll, acute light sensitivity, and, as a consequence, down-regulation of tetrapyrrole biosynthesis and photosynthesis-associated nuclear genes. These observations indicate a phenotypical resemblance of chli1 to the chlh and chld C. reinhardtii mutants previously reported. The key role of CHLI1 for MgCh reaction in comparison with the second isoform was confirmed by the rescue of chli1 with genomic CHLI1 Because CHLI2 in C. reinhardtii shows lower expression than CHLI1, strains overexpressing CHLI2 were produced in the chli1 background. However, no complementation of the chli1 phenotype was observed. Silencing of CHLI2 in the wild-type background did not result in any changes in the accumulation of tetrapyrrole intermediates or of chlorophyll. The results suggest that, unlike in A. thaliana, changes in CHLI2 content observed in the present studies do not affect formation and activity of MgCh in C. reinhardtii.