Biology subjects

Bag, P.

Publications and source records attributed to Bag, P..

9 recordsLinked to original sources

bioRxiv2026

Neofunctionalised plastid translocon regulates chloroplast ageing in Arabidopsis

Protein import across the chloroplast outer membrane is mediated by the TOC complex and regulated by GTP hydrolysis at two receptor GTPases, large and small. The former are the {beta}-barrel-containing Toc159 family of proteins (Sun and Jarvis 2023; H. Li and Chiu 2010; Shi and Theg 2013a; Kessler and Schnell 2009). The smallest member of this family in higher plants is Toc90, which has long been regarded as functionally unimportant due to the absence of obvious phenotypes in early mutant studies (Kubis et al. 2004; Infanger et al. 2011a). It was hypothesised that Toc90 is a relic of an ancestral Toc159 protein from early green lineage organisms (Hofmann and Theg 2003; Reumann et al. 2005). Here, we challenge this view by combining genetic, biochemical, phylogenetic, and in silico structural analyses to show that Toc90 is not an ancient remnant, but rather a neofunctionalised derivative of Toc159, and likely the most recently evolved member of the family. We show that Toc90 physically associates with Toc33 as a part of a photosynthetic TOC complex. Furthermore, we identify Toc90 as a regulator of chloroplast ageing. Our findings redefine Toc90 as an active and specialised component of the chloroplast protein import machinery, playing a pivotal role in senescence, especially in reducing photosynthetic protein abundance. Together, these results highlight Toc90 as a promising target for manipulating chloroplast longevity and plant senescence, with potential to improve crop productivity and yield.

plant biology↗

bioRxiv2025

Spillover is the dominant non-photochemical quenching mechanism in angiosperms

Non-photochemical quenching (NPQ) is an important photoprotective process in plants, but all molecular details of the process(es) involved are yet not understood. We have used advanced spectroscopic techniques (including simultaneous time- and spectrally-resolved room-temperature chlorophyll fluorescence analysis and spectro-kinetic deconvolution) to analyse the processes in Arabidopsis, hybrid aspen and Scots pine plants. We used four well-characterized Arabidopsis lines (npq1, npq2, npq4 and L17) affected in NPQ, together with hybrid aspen lines with corresponding modifications that we generated. The data are described best by a model for NPQ induction with up to five fluorescence components representing distinct biochemical entities. A dominant fluorescing species at the end of NPQ induction was identified as functionally detached and quenched LHCII but most importantly we believe that one represents a "PSII-PSI (Photosystem II-Photosystem I) complex" where direct energy transfer between PSII and PSI (spillover) take place. This provides strong quenching in all three plant species. We suggest a new integrated model for NPQ in higher plants where spillover is a major element and suggest roles for PsbS and zeaxanthin. Moreover, we discuss the link between NPQ and thylakoid rearrangements as thylakoid destacking facilitates direct contact between PSII and PSI; a prerequisite for spillover.

plant biology↗

bioRxiv2023

Distinctive mechanism of LHCSR3 expression and function under osmotic stress in Chlamydomonas reinhardtii

Light-harvesting complex stress-related protein 3 (LHCSR3) expression is observed in various protoxidizing conditions like high light and nutrient starvation. LHCSR3 expression is essential for energy-dependent quenching (qE), whereas its role under nutrient starvation is elusive. It is also unclear how nutrient starvation can induce LHCSR3 expression under subsaturating light intensities. To study the role of LHCSR3 under nutrient starvation, the C. reinhardtii cells are grown under osmotic stress that would prevent water uptake; therefore same holds true for soluble nutrients in the medium. In this work, we have shown that LHCSR3 expression can occur under osmotic stress and subsaturating light intensities, whereas it does not elicit qE. Further examination of thylakoid membrane architecture from wild-type and npq4 mutant grown under nutrient starvation revealed that LHCSR3 expression affects the interaction between the PSII core with its peripheral LHCII antenna and possibly can prevent excitation energy transfer. Thylakoid lumen acidification is essential for the expression and function of LHCSR3. Under saturating light intensities, this is achieved by the increased rate of photosynthetic electron flow coupled with proton translocation into the thylakoid lumen. Whereas, under nutrient starvation, the reports of LHCSR3 expression also showed reduced photosynthetic electron flow. Therefore, an alternative mechanism should exist for developing the proton gradient. We observed the downregulation of chloroplast (cp) ATP synthase activity and its abundance under osmotic stress, suggesting the role of (cp) ATP synthase in thylakoid lumen acidification under reduced photosynthetic electron flow. This observation is supported by the expression of LHCSR3 in (cp) ATP-synthase mutant atpF upon exposure to moderate light intensity. This study proposes that the mechanism of LHCSR3 expression and its functionality can vary with the type of photooxidizing stress.

plant biology↗

bioRxiv2022

O2 photoreduction at acceptor side of Photosystem I provide photoprotection to conifer thylakoids in early spring

Green organisms evolve O2 via photosynthesis and consume by respiration. Net O2 consumption only becomes dominant when photosynthesis is suppressed at night. Here, we show that green thylakoid membranes of Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies) needles demonstrate strong O2 consumption even in the presence of light when extremely low temperatures coincide with high solar irradiation during early spring. This phenomenon deviates from the general finding that photosynthetic organisms evolve O2 upon illumination. By using different electron transport chain inhibitors, we showed that O2 consumption occurred around photosystem (PS) I and correlated with higher abundance of flavodiiron (Flv) A protein in ES thylakoid membranes. Furthermore, by measuring P700 absorption changes, we separated different alternative electron flow pathways and demonstrated that electron scavenging from the acceptor-side of PSI via O2 photoreduction is a major alternative pathway in ES. This photoprotection mechanism in vascular plants indicates that conifers have developed an adaptative evolution trajectory for growing in harsh environments.

plant biology↗

bioRxiv2022

Timing Is Everything In Autumn - Coordination Of Senescence Onset By A Transcriptional Program In Response To Environmental And Phytohormone Signals

European aspen (Populus tremula L.) undergoes a coordinated senescence program during autumn; however, it is not known what exactly triggers it. To identify the cellular program leading to senescence, we utilized natural variation among Swedish aspen genotypes in a common garden to study senescence timing and the underlying changes in leaf phytohormone and transcriptome profiles. Apart from the patterns of major transcriptional cascade that was similar between the genotypes and closely associated with cytokinin and auxin metabolite levels and gradually decreasing air temperature during autumn, we detected patterns that consistently preceded or coincided with senescence onset in individual genotypes. Another cascade seemed to respond to short-term changes in weather conditions that re-wired the transcriptional network; the up-regulation of genes related to ethylene and abiotic stress, programmed cell death and translation occurred first in the early-senescing genotypes and later in the late one. Network analyses displayed a connection between the two cascades, metabolic stress and immunity responses mediated by salicylic acid (SA)-signalling pathway that was repressed along with SA levels at senescence onset. We propose that autumn senescence in aspen trees is affected by environmental variation that evokes stress and the timing is fine-tuned by their stress tolerance mechanism. One sentence summarySalicylic acid signalling pathway fine-tunes the timing of senescence onset under challenging environmental conditions in autumn.

plant biology↗

bioRxiv2021

Nitrate fertilization may delay autumn leaf senescence, while amino acid treatments do not

Fertilization with nitrogen (N)-rich compounds leads to increased growth, but may compromise phenology and winter survival of trees in boreal regions. During autumn, N is remobilized from senescing leaves and stored in other parts of the tree to be used in the next growing season. However, the mechanism behind the N fertilization effect on winter survival is not well understood and it is unclear how N levels or forms modulate autumn senescence. We performed fertilization experiments and showed that treating Populus saplings with high or low levels of inorganic nitrogen resulted in a delay in senescence. In addition, by using precise delivery of solutes into the xylem stream of Populus trees in their natural environment, we found that delay of autumn senescence was dependent on the form of N administered: inorganic N (NO3-1) delayed senescence but amino acids (Arg, Glu, Gln, and Leu) did not. Metabolite profiling of leaves showed that the levels of tricarboxylic acids (TCA), arginine catabolites (ammonium, ornithine), glycine, glycine-serine ratio and overall carbon-to-nitrogen (C/N) ratio were affected differently by the way of applying NO3-1 and Arg treatments. In addition, the onset of senescence did not coincide with soluble sugar accumulation in any of the treatments. Taken together, metabolomic rearrangement under different N forms or experimental setups could modulate senescence process, but not initiation and progression in Populus. We propose that the different regulation of C and N status through direct molecular signaling of NO3-1 could account for the contrasting effects of NO3-1 and Arg on senescence. One sentence summaryNitrate, administered by precision fertilization through injection into the trunk, may delay autumn senescence and change metabolism in Populus leaves, while the same amount of amino acids does not have the same effect.

plant biology↗

bioRxiv2021

An atlas of the Norway spruce needle seasonal transcriptome

Boreal conifers possess a tremendous ability to survive and remain evergreen during harsh winter conditions and resume growth during summer. This is enabled by coordinated regulation of major cellular functions at the level of gene expression, metabolism, and physiology. Here we present a comprehensive characterization of the annual changes in the global transcriptome of Norway spruce needles as a resource to understand needle development and acclimation processes throughout the year. In young, growing needles (May 15 - June 30), cell walls, organelles etc. were formed, and this developmental program heavily influenced the transcriptome, explained by over represented Gene Ontology (GO) categories. Later changes in gene expression were smaller but four phases were recognized: summer (July-August), autumn (September-October), winter (November-February) and spring (March-April), where over represented GO categories demonstrated how the needles acclimated to the various seasons. Changes in the seasonal global transcriptome profile were accompanied by differential expression of members of the major transcription factor families. We present a tentative model of how cellular activities are regulated over the year in needles of Norway spruce, which demonstrates the value of mining this dataset, accessible in ConGenIE together with advanced visualization tools. Significance statementThe development of Norway spruce needles and their annual cycle of biochemical activities (photosynthesis in the summer, adaptation and survival of the harsh boreal winter) is not well understood. We use deep RNA sequencing to profile the transcriptome over the season, and show how the dataset could be used to give information about "what needles do" over the year, and are here making the dataset available for the scientific community. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=190 SRC="FIGDIR/small/452085v1_ufig1.gif" ALT="Figure 1"> View larger version (54K): org.highwire.dtl.DTLVardef@1225307org.highwire.dtl.DTLVardef@791821org.highwire.dtl.DTLVardef@1f22dforg.highwire.dtl.DTLVardef@2a8270_HPS_FORMAT_FIGEXP M_FIG C_FIG

plant biology↗

bioRxiv2021

GIGANTEA gene expression influence leaf senescence in Populus in two different ways

GIGANTEA (GI) genes have a central role in plant development and influence several processes such as light signaling, circadian rhythm and abiotic stress tolerance. Hybrid aspen T89 (Populus tremula x tremuloides) trees with low GI expression through RNAi have a severely compromised growth. In order to study the effect of reduced GI expression on leaf traits with special emphasis on leaf senescence, we grafted GI-RNAi scions onto wild type (WT) rootstocks and managed to restore scions growth. The RNAi line had distorted leaf shape and reduced photosynthesis, probably caused by modulating phloem or stomatal function, increased starch accumulation, higher carbon-to-nitrogen (C/N) ratio and a reduced capacity to withstand moderate light stress. GI-RNAi also induced senescence under long day (LD) and moderate light conditions. Furthermore, the GI-RNAi lines were affected in their capacity to respond to "autumn environmental cues" inducing senescence, a type of leaf senescence with characteristics different from senescence induced directly by stress under LD conditions. Whereas Overexpression of GI delayed senescence. The two different effects on leaf senescence were not affected by the expression of FT (Flowering locus T), were "local" - they followed the genotype of the branch independent on the position in the tree - and trees with modified gene expression grown in the field were affected in a similar way as under controlled conditions. Taken together, GI plays a central role to sense the environmental changes during autumn and determine the appropriate timing for leaf senescence in Populus. One sentence summaryLeaf senescence is a complex process that is not well understood, but this paper shows that changing the expression of one gene could influence leaf senescence in Populus trees in two separate ways.

plant biology↗

bioRxiv2020

Direct energy transfer from photosystem II to photosystem I is the major regulator of winter sustainability of Scots pine

Evergreen conifers in boreal forests can survive extremely cold (freezing) temperatures during the long dark winter and fully recover during the summer. A phenomenon called sustained quenching putatively provides photoprotection and enables their survival, but its precise molecular and physiological mechanisms are not understood. To unveil them, we have analyzed the seasonal adaptation of the photosynthetic machinery of Scots pine (Pinus sylvestris) trees by monitoring multi-year changes in weather, chlorophyll fluorescence, chloroplast ultrastructure, and changes in pigment-protein composition. Recorded Photosystem II and Photosystem I performance parameters indicate that highly dynamic structural and functional seasonal rearrangements of the photosynthetic apparatus occur. Although several mechanisms might contribute to sustained quenching of winter/early spring pine needles, time-resolved fluorescence analysis shows that extreme down-regulation of photosystem II activity along with direct energy transfer from photosystem II to photosystem I plays a major role. This mechanism is enabled by extensive thylakoid destacking allowing for mixing of PSII with PSI complexes. These two linked phenomena play crucial roles in winter acclimation and protection. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=125 SRC="FIGDIR/small/144170v1_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@b9d757org.highwire.dtl.DTLVardef@e8ba28org.highwire.dtl.DTLVardef@115c5b2org.highwire.dtl.DTLVardef@6634f5_HPS_FORMAT_FIGEXP M_FIG C_FIG

plant biology↗