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Prokaryotic and eukaryotic traits support the biological role of the chloroplast outer envelope.
Barth, MA, Soll, J, Akbaş, Ş
Biochimica et biophysica acta. Molecular cell research. 2022;(5):119224
Abstract
The plastid outer envelope (OE) is a mixture of components inherited from their prokaryotic ancestor like galactolipids, carotenoids and porin type ion channels supplemented with eukaryotic inventions to make the endosymbiotic process successful as well as to control plastid biogenesis and differentiation. In this review we wanted to highlight the importance of the OE proteins and its evolutionary origin. For a long time, the OE was thought to be a diffusion barrier only, but with the recent discoveries of all kinds of different proteins in the OE it has been shown that the OE can modulate various functions within the cell. The phenotypic changes show that channels like the outer envelope proteins OEP40, OEP16 or JASSY have a pronounced ion selectivity that cannot be replaced by other ion channels present in the OE. Eukaryotic additions, like the GTPase receptors Toc33 and Toc159 or the ubiquitin proteasome system for chloroplast protein quality control, round up the profile of the OE.
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2.
Recent advances in plant thermomemory.
Nishad, A, Nandi, AK
Plant cell reports. 2021;(1):19-27
Abstract
This review summarizes the process of thermal acquired tolerance in plants and the knowledge gap compared to systemic acquired resistance that a plant shows after pathogen inoculation. Plants are continuously challenged by several biotic stresses such as pests and pathogens, or abiotic stresses like high light, UV radiation, drought, salt, and very high or low temperature. Interestingly, for most stresses, prior exposure makes plants more tolerant during the subsequent exposures, which is often referred to as acclimatization. Research of the last two decades reveals that the memory of most of the stresses is associated with epigenetic changes. Heat stress causes damage to membrane proteins, denaturation and inactivation of various enzymes, and accumulation of reactive oxygen species leading to cell injury and death. Plants are equipped with thermosensors that can recognize certain specific changes and activate protection machinery. Phytochrome and calcium signaling play critical roles in sensing sudden changes in temperature and activate cascades of signaling, leading to the production of heat shock proteins (HSPs) that keep protein-unfolding under control. Heat shock factors (HSFs) are the transcription factors that read the activation of thermosensors and induce the expression of HSPs. Epigenetic modifications of HSFs are likely to be the key component of thermal acquired tolerance (TAT). Despite the advances in understanding the process of thermomemory generation, it is not known whether plants are equipped with systemic activation thermal protection, as happens in the form of systemic acquired resistance (SAR) upon pathogen infection. This review describes the recent advances in the understanding of thermomemory development in plants and the knowledge gap in comparison with SAR.
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3.
What is the Role of Lipid Membrane-embedded Quinones in Mitochondria and Chloroplasts? Chemiosmotic Q-cycle versus Murburn Reaction Perspective.
Manoj, KM, Gideon, DA, Parashar, A
Cell biochemistry and biophysics. 2021;(1):3-10
Abstract
Quinones are found in the lipid membranes of prokaryotes like E. coli and cyanobacteria, and are also abundant in eukaryotic mitochondria and chloroplasts. They are intricately involved in the reaction mechanism of redox phosphorylations. In the Mitchellian chemiosmotic school of thought, membrane-lodged quinones are perceived as highly mobile conveyors of two-electron equivalents from the first leg of Electron Transport Chain (ETC) to the 'second pit-stop' of Cytochrome bc1 or b6f complex (CBC), where they undergo a regenerative 'Q-cycle'. In Manoj's murburn mechanism, the membrane-lodged quinones are perceived as relatively slow-moving one- or two- electron donors/acceptors, enabling charge separation and the CBC resets a one-electron paradigm via 'turbo logic'. Herein, we compare various purviews of the two mechanistic schools with respect to: constraints in mobility, protons' availability, binding of quinones with proteins, structural features of the protein complexes, energetics of reaction, overall reaction logic, etc. From various perspectives, the murburn mechanism appeals as a viable alternative explanation well-rooted in thermodynamics/kinetics and one which lends adequate structure-function correlations for the roles of quinones, lipid membrane and associated proteins.
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4.
Progress in Research on the Mechanisms Underlying Chloroplast-Involved Heat Tolerance in Plants.
Zeng, C, Jia, T, Gu, T, Su, J, Hu, X
Genes. 2021;(9)
Abstract
Global warming is a serious challenge plant production has to face. Heat stress not only affects plant growth and development but also reduces crop yield and quality. Studying the response mechanisms of plants to heat stress will help humans use these mechanisms to improve the heat tolerance of plants, thereby reducing the harm of global warming to plant production. Research on plant heat tolerance has gradually become a hotspot in plant molecular biology research in recent years. In view of the special role of chloroplasts in the response to heat stress in plants, this review is focusing on three perspectives related to chloroplasts and their function in the response of heat stress in plants: the role of chloroplasts in sensing high temperatures, the transmission of heat signals, and the improvement of heat tolerance in plants. We also present our views on the future direction of research on chloroplast related heat tolerance in plants.
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5.
Starch granule initiation in Arabidopsis thaliana chloroplasts.
Mérida, A, Fettke, J
The Plant journal : for cell and molecular biology. 2021;(3):688-697
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Abstract
The initiation of starch granule formation and the mechanism controlling the number of granules per plastid have been some of the most elusive aspects of starch metabolism. This review covers the advances made in the study of these processes. The analyses presented herein depict a scenario in which starch synthase isoform 4 (SS4) provides the elongating activity necessary for the initiation of starch granule formation. However, this protein does not act alone; other polypeptides are required for the initiation of an appropriate number of starch granules per chloroplast. The functions of this group of polypeptides include providing suitable substrates (maltooligosaccharides) to SS4, the localization of the starch initiation machinery to the thylakoid membranes, and facilitating the correct folding of SS4. The number of starch granules per chloroplast is tightly regulated and depends on the developmental stage of the leaves and their metabolic status. Plastidial phosphorylase (PHS1) and other enzymes play an essential role in this process since they are necessary for the synthesis of the substrates used by the initiation machinery. The mechanism of starch granule formation initiation in Arabidopsis seems to be generalizable to other plants and also to the synthesis of long-term storage starch. The latter, however, shows specific features due to the presence of more isoforms, the absence of constantly recurring starch synthesis and degradation, and the metabolic characteristics of the storage sink organs.
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6.
Low Light/Darkness as Stressors of Multifactor-Induced Senescence in Rice Plants.
Gad, AG, Habiba, , Zheng, X, Miao, Y
International journal of molecular sciences. 2021;(8)
Abstract
Leaf senescence, as an integral part of the final development stage for plants, primarily remobilizes nutrients from the sources to the sinks in response to different stressors. The premature senescence of leaves is a critical challenge that causes significant economic losses in terms of crop yields. Although low light causes losses of up to 50% and affects rice yield and quality, its regulatory mechanisms remain poorly elucidated. Darkness-mediated premature leaf senescence is a well-studied stressor. It initiates the expression of senescence-associated genes (SAGs), which have been implicated in chlorophyll breakdown and degradation. The molecular and biochemical regulatory mechanisms of premature leaf senescence show significant levels of redundant biomass in complex pathways. Thus, clarifying the regulatory mechanisms of low-light/dark-induced senescence may be conducive to developing strategies for rice crop improvement. This review describes the recent molecular regulatory mechanisms associated with low-light response and dark-induced senescence (DIS), and their effects on plastid signaling and photosynthesis-mediated processes, chloroplast and protein degradation, as well as hormonal and transcriptional regulation in rice.
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7.
Mesophyll conductance: the leaf corridors for photosynthesis.
Gago, J, Daloso, DM, Carriquí, M, Nadal, M, Morales, M, Araújo, WL, Nunes-Nesi, A, Flexas, J
Biochemical Society transactions. 2020;(2):429-439
Abstract
Besides stomata, the photosynthetic CO2 pathway also involves the transport of CO2 from the sub-stomatal air spaces inside to the carboxylation sites in the chloroplast stroma, where Rubisco is located. This pathway is far to be a simple and direct way, formed by series of consecutive barriers that the CO2 should cross to be finally assimilated in photosynthesis, known as the mesophyll conductance (gm). Therefore, the gm reflects the pathway through different air, water and biophysical barriers within the leaf tissues and cell structures. Currently, it is known that gm can impose the same level of limitation (or even higher depending of the conditions) to photosynthesis than the wider known stomata or biochemistry. In this mini-review, we are focused on each of the gm determinants to summarize the current knowledge on the mechanisms driving gm from anatomical to metabolic and biochemical perspectives. Special attention deserve the latest studies demonstrating the importance of the molecular mechanisms driving anatomical traits as cell wall and the chloroplast surface exposed to the mesophyll airspaces (Sc/S) that significantly constrain gm. However, even considering these recent discoveries, still is poorly understood the mechanisms about signaling pathways linking the environment a/biotic stressors with gm responses. Thus, considering the main role of gm as a major driver of the CO2 availability at the carboxylation sites, future studies into these aspects will help us to understand photosynthesis responses in a global change framework.
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8.
Regulation of Iron Homeostasis and Use in Chloroplasts.
Kroh, GE, Pilon, M
International journal of molecular sciences. 2020;(9)
Abstract
Iron (Fe) is essential for life because of its role in protein cofactors. Photosynthesis, in particular photosynthetic electron transport, has a very high demand for Fe cofactors. Fe is commonly limiting in the environment, and therefore photosynthetic organisms must acclimate to Fe availability and avoid stress associated with Fe deficiency. In plants, adjustment of metabolism, of Fe utilization, and gene expression, is especially important in the chloroplasts during Fe limitation. In this review, we discuss Fe use, Fe transport, and mechanisms of acclimation to Fe limitation in photosynthetic lineages with a focus on the photosynthetic electron transport chain. We compare Fe homeostasis in Cyanobacteria, the evolutionary ancestors of chloroplasts, with Fe homeostasis in green algae and in land plants in order to provide a deeper understanding of how chloroplasts and photosynthesis may cope with Fe limitation.
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9.
Chloroplast avoidance movement: a novel paradigm of ROS signalling.
Majumdar, A, Kar, RK
Photosynthesis research. 2020;(1):109-121
Abstract
The damaging effects of supra-optimal irradiance on plants, often turning to be lethal, may be circumvented by chloroplast avoidance movement which realigns chloroplasts to the anticlinal surfaces of cells (parallel to the incident light), essentially minimizing photon absorption. In angiosperms and many other groups of plants, chloroplast avoidance movement has been identified to be a strong blue light (BL)-dependent process being mediated by actin filaments wherein phototropins are identified as the photoreceptor involved. Studies through the last few decades have identified key molecular mechanisms involving Chloroplast Unusual Positioning 1 (CHUP1) protein and specific chloroplast-actin (cp-actin) filaments. However, the signal transduction pathway from strong BL absorption down to directional re-localization of chloroplasts by actin filaments is complex and ambiguous. Being the immediate cellular products of high irradiance absorption and having properties of remodelling actin as well as phototropin, reactive oxygen species (ROS) deemed to be more able and prompt than any other signalling agent in mediating chloroplast avoidance movement. Although ROS are presently being identified as fundamental component for regulating different plant processes ranging from growth, development and immunity, its role in avoidance movement have hardly been explored in depth. However, few recent reports have demonstrated the direct stimulatory involvement of ROS, especially H2O2, in chloroplast avoidance movement with Ca2+ playing a pivotal role. With this perspective, the present review discusses the mechanisms of ROS-mediated chloroplast avoidance movement involving ROS-Ca2+-actin communication system and NADPH oxidase (NOX)-plasma membrane (PM) H+-ATPase positive feed-forward loop. A possible working model is proposed.
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10.
Regulatory thiol oxidation in chloroplast metabolism, oxidative stress response and environmental signaling in plants.
Vogelsang, L, Dietz, KJ
The Biochemical journal. 2020;(10):1865-1878
Abstract
The antagonism between thiol oxidation and reduction enables efficient control of protein function and is used as central mechanism in cellular regulation. The best-studied mechanism is the dithiol-disulfide transition in the Calvin Benson Cycle in photosynthesis, including mixed disulfide formation by glutathionylation. The adjustment of the proper thiol redox state is a fundamental property of all cellular compartments. The glutathione redox potential of the cytosol, stroma, matrix and nucleoplasm usually ranges between -300 and -320 mV. Thiol reduction proceeds by short electron transfer cascades consisting of redox input elements and redox transmitters such as thioredoxins. Thiol oxidation ultimately is linked to reactive oxygen species (ROS) and reactive nitrogen species (RNS). Enhanced ROS production under stress shifts the redox network to more positive redox potentials. ROS do not react randomly but primarily with few specific redox sensors in the cell. The most commonly encountered reaction within the redox regulatory network however is the disulfide swapping. The thiol oxidation dynamics also involves transnitrosylation. This review compiles present knowledge on this network and its central role in sensing environmental cues with focus on chloroplast metabolism.