Changes In Light Quality Research Articles

During State 1 to State 2 Transition in Arabidopsis thaliana, the Photosystem II Supercomplex Gets Phosphorylated but Does Not Disassemble

Plants are exposed to continuous changes in light quality and quantity that challenge the performance of the photosynthetic apparatus and have evolved a series of mechanisms to face this challenge. In this work, we have studied state transitions, the process that redistributes the excitation pressure between photosystems I and II (PSI/PSII) by the reversible association of LHCII, the major antenna complex of higher plants, with either one of them upon phosphorylation/dephosphorylation. By combining biochemical analysis and electron microscopy, we have studied the effect of state transitions on the composition and organization of photosystem II in Arabidopsis thaliana. Two LHCII trimers (called trimers M and S) are part of the PSII supercomplex, whereas up to two more are loosely associated with PSII in state 1 in higher plants (called "extra" trimers). Here, we show that the LHCII from the extra pool migrates to PSI in state 2, thus leaving the PSII supercomplex and the semicrystalline PSII arrays intact. In state 2, not only is the mobile LHCII phosphorylated, but also the LHCII in the PSII supercomplexes. This demonstrates that PSII phosphorylation is not sufficient for disconnecting LHCII trimers S and M from PSII and for their migration to PSI.

Role of CBFs as Integrators of Chloroplast Redox, Phytochrome and Plant Hormone Signaling during Cold Acclimation

Cold acclimation of winter cereals and other winter hardy species is a prerequisite to increase subsequent freezing tolerance. Low temperatures upregulate the expression of C-repeat/dehydration-responsive element binding transcription factors (CBF/DREB1) which in turn induce the expression of COLD-REGULATED (COR) genes. We summarize evidence which indicates that the integration of these interactions is responsible for the dwarf phenotype and enhanced photosynthetic performance associated with cold-acclimated and CBF-overexpressing plants. Plants overexpressing CBFs but grown at warm temperatures mimic the cold-tolerant, dwarf, compact phenotype; increased photosynthetic performance; and biomass accumulation typically associated with cold-acclimated plants. In this review, we propose a model whereby the cold acclimation signal is perceived by plants through an integration of low temperature and changes in light intensity, as well as changes in light quality. Such integration leads to the activation of the CBF-regulon and subsequent upregulation of COR gene and GA 2-oxidase (GA2ox) expression which results in a dwarf phenotype coupled with increased freezing tolerance and enhanced photosynthetic performance. We conclude that, due to their photoautotrophic nature, plants do not rely on a single low temperature sensor, but integrate changes in light intensity, light quality, and membrane viscosity in order to establish the cold-acclimated state. CBFs appear to act as master regulators of these interconnecting sensing/signaling pathways.

Ectopic expression of Arabidopsis phytochrome B in Troyer citrange affects photosynthesis and plant morphology

Phytochromes are photoreceptors that regulate signal transduction of metabolic pathways and plant development in response to changes in environmental light quality and levels. Studies have shown that mutant plants deficient in phytochromes and transgenic plants overexpressing phytochrome genes exhibit altered canopy growth characteristics. Troyer citrange was transformed with a binary vector system of Agrobacterium tumefaciens carrying the neomycin phosphotransferase II (nptII) and Arabidopsis thaliana phytochrome B (PHYB) gene. Three transgenic lines were used to investigate the influence of overexpression of phytochrome B on fruit tree plant development through morphological and physiological analyses in addition to gene expression analyses. All the regenerated transgenic plants showed altered physiological and architectural canopy development compared with wild-type plants. In particular PHYB1 transgenic clone was further characterized under particular growing conditions and molecular analysis was used to study the effect of the transgene on gene expression during photosynthesis. The expression of chloroplast and nuclear genes that are directly involved in the light and dark phases of photosynthesis was found to be particularly enhanced in the transgenic plant.

The receptor-like kinase ERECTA contributes to the shade-avoidance syndrome in a background-dependent manner.

Plants growing at high densities perceive a decrease in the red to far-red (R/FR) ratio of incoming light. These changes in light quality trigger a suite of responses collectively known as the shade-avoidance syndrome (SAS) including hypocotyl and stem elongation, inhibition of branching and acceleration of flowering. Quantitative trait loci (QTLs) were mapped for hypocotyl length to end-of-day far-red (EOD), a simulated shade-avoidance response, in recombinant inbred line (RIL) populations of Arabidopsis thaliana seedlings, derived from Landsberg erecta (Ler) and three accessions (Columbia, Col; Nossen, No-0; and Cape Verde Islands, Cvi-0). Five loci were identified as being responsible for the EOD response, with a positive contribution of Ler alleles on the phenotype independently of the RIL population. Quantitative complementation analysis and transgenic lines showed that PHYB is the candidate gene for EODRATIO5 in the Ler × Cvi-0 RIL population, but not for two co-localized QTLs, EODRATIO1 and EODRATIO2 mapped in the Ler × No-0 and Ler × Col RIL populations, respectively. The ERECTA gene was also implicated in the SAS in a background-dependent manner. For hypocotyl length EOD response, a positive contribution of erecta alleles was found in Col and Van-0, but not in Ler, Cvi-0, Hir-1 or Ws. Furthermore, pleiotropic effects of ERECTA in the EOD response were also detected for petiole and lamina elongation, hyponastic growth, and flowering time. The results show that the analysis of multiple mapping populations leads to a better understanding of the SAS genetic architecture. Moreover, the background- and trait-dependent contribution of ERECTA in the SAS suggest that its function in shaded natural environments may be relevant for some populations in different phases of plant development. It is proposed that ERECTA is involved in canalization processes buffering the genetic variation of the SAS against environmental light fluctuations.

<i>Pinus taeda</i> cDNA Microarray as a Tool for Candidate Gene Identification for Local Red/Far-Red Light Adaptive Response in <i>Pinus sylvestris</i>

Light quality response is a vital environmental cue regulating plant development. Conifers, like angiosperms, respond to the changes in light quality including the level of red (R) and far-red (FR) light, which follows a latitudinal cline. R and FR wavelengths form a significant component of the entire plant life cycle, including the initial developmental stages such as seed germination, cotyledon expansion and hypocotyl elongation. With an aim to identify differentially expressed candidate genes, which would provide a clue regarding genes involved in the local adaptive response in Scots pine (Pinus sylvestris) with reference to red/far-red light; we performed a global expression analysis of Scots pine hypocotyls grown under two light treatments, continuous R (cR) and continuous FR (cFR) light; using Pinus taeda cDNA microarrays on bulked hypocotyl tissues from different individuals, which represented different genotypes. This experiment was performed with the seeds collected from northern part of Sweden (Ylinen, 68?N). Interestingly, gene expression pattern with reference to cryptochrome1, a blue light photoreceptor, was relatively high under cFR as compared to cR light treatment. Additionally, the microarray data analysis also revealed expression of 405 genes which was enhanced under cR light treatment; while the expression of 239 genes was enhanced under the cFR light treatment. Differentially expressed genes were re-annotated using Blast2GO tool. These results indicated that cR light acts as promoting factor whereas cFR antagonises the effect in most of the processes like C/N metabolism, photosynthesis and cell wall metabolism which is in accordance with former findings in Arabidopsis. We propose cryptochrome1 as a strong candidate gene to study the adaptive cline response under R and FR light in Scots pine as it shows a differential expression under the two light conditions.

A phosphorylation map of the photosystem II supercomplex C2S2M2.

PS II PHOSPHORYLATION It was 36 years ago when John Bennett first discovered chloroplast protein phosphorylation (Bennett, 1977). The most conspicuous chloroplast phosphoprotein was the apoprotein of the Light Harvesting Complex II (LHC II) of photosystem II (PS II). Other phosphoproteins included the reaction center protein CP43 and the quinone-binding proteins D1 and D2 of PS II. These earlier studies also identified the PsbH protein, the minor subunit of PS II, as another chloroplast phosphoprotein (Allen, 1992). Nearly all of the first-known chloroplast phosphoproteins belong to PS II, the water-oxidizing photosystem of oxygenic photosynthesis. With more than 25 protein subunits and 350 kDa in size, the PS II core monomer is a massive structure (Umena et al., 2011). This enormous size is in addition to the peripheral light harvesting complexes. The major peripheral antenna of PS II is LHC II, which is a trimer. Some, so-called minor, LHC are monomeric and act as linkers in connecting the trimeric LHC II to the core. The core is composed of the reaction center proteins D1 and D2 and an internal antenna made up of CP43 and CP47. The PS II core with its peripheral antenna forms supercomplexes, which mainly differ in the number of LHC II bound to the core (Caffarri et al., 2009). The core itself is dimeric (C2) to which the LHC II could strongly (S), or moderately (M), or loosely (L) bind. The monomeric linker associated with the Sform is CP26, and for the M-form are CP29 and CP24. A dimeric core (C2) with 2 S and 2 M LHC II (C2S2M2) is the most abundant supercomplex in plant photosynthetic membranes (Kouril et al., 2012). PS II phosphorylation is light dependent. It turned out, however, light acts through the redox state of the plastoquinone (PQ) pool (Allen et al., 1981; Bennett, 1991). PS II phosphorylation can be divided into the core phosphorylation (D1, D2, CP43, and PsbH) and the peripheral antenna phosphorylation (LHC II). In plants, the protein kinases that phosphorylate the core and the LHC II have been identified as Stn8 and Stn7, respectively, (Bellafiore et al., 2005; Bonardi et al., 2005). PS II phosphorylation is only observed in green algae and plants, and not in cyanobacteria and red algae where the light harvesting antenna is phycobilisomebased (Pursiheimo et al., 1998). Core phosphorylation is responsive to light intensity with increasing levels of phosphorylation observed in high light (Elich et al., 1992; Tikkanen and Aro, 2012). On the contrary, LHC II phosphorylation was restricted to low light condition specific to PS II (Rintamaki et al., 2000). At higher light intensities, the activity of the Stn7 is inhibited by the reduced stromal electron carrier thioredoxin (Rintamaki et al., 2000; Puthiyaveetil, 2011). It was also noted that only the L-form of the LHC II trimer is phosphorylated, while the LHC II isoforms comprising the Sand M-trimers are not phosphorylated or do not contain phosphorylation sites (Galka et al., 2012). LHC II phosphorylation is the basis of state transitions, an acclimatory response to changes in light quality (Bonaventura and Myers, 1969; Murata, 1969). The precise function of PS II core phosphorylation, however, is still unclear. PS II undergoes damage in light, which results in photoinhibition—light-induced loss of photosynthetic activity (Aro et al., 1993; Long et al., 1994). In plants and green algae, PS II is mainly found in the stacked granal regions of the thylakoid membrane where it forms almost immobile supercomplexes (Kirchhoff et al., 2008; Mullineaux, 2008). The centrally located reaction center protein D1 is the main target of photodamage in PS II (Kyle et al., 1984). To maintain photosynthetic efficiency in high light, chloroplasts have evolved a robust repair process known as the PS II repair cycle, wherein the damaged D1 protein is degraded and replaced with a newly synthesized copy (Melis, 1999; Nixon et al., 2010). The PS II repair cycle operates through many constraints (Kirchhoff, 2013b), chief among them are how the repair machinery located in the unstacked regions of the thylakoid membrane accesses the damaged photosystems in the stacked, crowded granal membranes and how the damaged photosystems are mobilized in the rather “immobile” grana. The proposed functions of PS II phosphorylation include “marking” the damaged photosystems for degradation (Aro et al., 1993), increasing the mobility of photosystems (Goral et al., 2010; Herbstova et al., 2012), disassembly of PS II (Tikkanen et al., 2008; Fristedt and Vener, 2011), decreasing the granal diameter (Herbstova et al., 2012; Kirchhoff, 2013a), and for maintaining electron flow in high light (Harrison and Allen, 1991). Some of these proposals are not mutually exclusive. Functions such as increased mobility and disassembly are especially supported but how precisely phosphorylation produces these effects are unclear.

Microtubule Dependent Anomalous Diffusion of Chloroplasts in Moss

Organelle motility via molecular motors plays an important role in eukaryotic cell functioning. In the model plant, Physcomitrella patens, reorganization of the chloroplasts to adapt to changes in light intensity and quality is driven by the actin or microtubule (MT) cytoskeleton. In this work, we investigate the motility of chloroplasts, in the absence of the actin cytoskeleton. To characterize this motility, we analyze the mean squared displacement (MSD) of chloroplasts in moss cells at steady state and constant illumination, and show that they are actively transported via the MT cytoskeleton. Our results show that while the apparent diffusion coefficient is dependent on the intensity of blue light, the super-diffusive nature of the movement determined by the MSD exponents is not. In order to develop a mechanistic understanding of this process, we developed a coarse-grained model of chloroplast motility. The model incorporates various MT network topologies, consistent with experimental data, and uses point like cargo to mimic chloroplasts. Our simulations show a strong dependence of MSD exponents to the MT network topology, while the apparent diffusion coefficient is mainly influenced by the MT density and the presence of passive cross-linkers.

Protein kinases and phosphatases involved in the acclimation of the photosynthetic apparatus to a changing light environment

Photosynthetic organisms are subjected to frequent changes in light quality and quantity and need to respond accordingly. These acclimatory processes are mediated to a large extent through thylakoid protein phosphorylation. Recently, two major thylakoid protein kinases have been identified and characterized. The Stt7/STN7 kinase is mainly involved in the phosphorylation of the LHCII antenna proteins and is required for state transitions. It is firmly associated with the cytochrome b(6)f complex, and its activity is regulated by the redox state of the plastoquinone pool. The other kinase, Stl1/STN8, is responsible for the phosphorylation of the PSII core proteins. Using a reverse genetics approach, we have recently identified the chloroplast PPH1/TAP38 and PBPC protein phosphatases, which counteract the activity of STN7 and STN8 kinases, respectively. They belong to the PP2C-type phosphatase family and are conserved in land plants and algae. The picture that emerges from these studies is that of a complex regulatory network of chloroplast protein kinases and phosphatases that is involved in light acclimation, in maintenance of the plastoquinone redox poise under fluctuating light and in the adjustment to metabolic needs.

Seed germination of Pinus koraiensis in response to light regimes caused by shading and seed positions

Pinus koraiensis Siebold & Zucc. (Korean pine), the dominant tree species in the mixed broadleaved Korean pine forests (regional climax), is severely restricted by its regeneration failure. To determine the effects of light regimes on P. koraiensis regeneration, the seed germination process was examined in shade houses and forest stands (before and after leaf expansion) with various light levels created by shading and seed positions. Despite the large size of P. koraiensis seeds (500-600 mg), both light intensity and quality significantly affected the germination percentage in both shade houses and forests. Substantial changes in light intensity and quality led the majority of seeds (80%) to germinate in leafless forests and shade houses, while only a minority (≤ 20%) germinated after leaf expansion in the forests. Moreover, seed germination in shade houses and leafless forests exhibited similar patterns; they consistently reached a 70% shading degree, which was optimal for the seed germination of P. koraiensis on topsoil. Seed positioning significantly affected germination for each shading degree, especially when litter and soil coverings drastically inhibited germination. In conclusion, (1) when seeds were not stressed by temperature and moisture, light irradiance played a critical role in the seed germination of P. koraiensis; (2) seed positioning, in relation to alterations in light intensity and quality, affected the germination of P. koraiensis; (3) a shade house experiment using neutral cloth provided an applicable and controllable way to monitor the P. koraiensis seed germination in early spring before leaf expansion. The light requirement for the germination of P. koraiensis played a key role in the regeneration of P. koraiensis throughout the temperate secondary forests.

Light acclimation, retrograde signalling, cell death and immune defences in plants

This review confronts the classical view of plant immune defence and light acclimation with recently published data. Earlier findings have linked plant immune defences to nucleotide-binding site leucine-rich repeat (NBS-LRR)-dependent recognition of pathogen effectors and to the role of plasma membrane-localized NADPH-dependent oxidoreductase (AtRbohD), reactive oxygen species (ROS) and salicylic acid (SA). However, recent results suggest that plant immune defence also depends on the absorption of excessive light energy and photorespiration. Rapid changes in light intensity and quality often cause the absorption of energy, which is in excess of that required for photosynthesis. Such excessive light energy is considered to be a factor triggering photoinhibition and disturbance in ROS/hormonal homeostasis, which leads to cell death in foliar tissues. We highlight here the tight crosstalk between ROS- and SA-dependent pathways leading to light acclimation, and defence responses leading to pathogen resistance. We also show that LESION SIMULATING DISEASE 1 (LSD1) regulates and integrates these processes. Moreover, we discuss the role of plastid-nucleus signal transduction, photorespiration, photoelectrochemical signalling and 'light memory' in the regulation of acclimation and immune defence responses. All of these results suggest that plants have evolved a genetic system that simultaneously regulates systemic acquired resistance (SAR), cell death and systemic acquired acclimation (SAA).

Steady-State Phosphorylation of Light-Harvesting Complex II Proteins Preserves Photosystem I under Fluctuating White Light

According to the "state transitions" theory, the light-harvesting complex II (LHCII) phosphorylation in plant chloroplasts is essential to adjust the relative absorption cross section of photosystem II (PSII) and PSI upon changes in light quality. The role of LHCII phosphorylation upon changes in light intensity is less thoroughly investigated, particularly when changes in light intensity are too fast to allow the phosphorylation/dephosphorylation processes to occur. Here, we demonstrate that the Arabidopsis (Arabidopsis thaliana) stn7 (for state transition7) mutant, devoid of the STN7 kinase and LHCII phosphorylation, shows a growth penalty only under fluctuating white light due to a low amount of PSI. Under constant growth light conditions, stn7 acquires chloroplast redox homeostasis by increasing the relative amount of PSI centers. Thus, in plant chloroplasts, the steady-state LHCII phosphorylation plays a major role in preserving PSI upon rapid fluctuations in white light intensity. Such protection of PSI results from LHCII phosphorylation-dependent equal distribution of excitation energy to both PSII and PSI from the shared LHCII antenna and occurs in cooperation with nonphotochemical quenching and the proton gradient regulation5-dependent control of electron flow, which are likewise strictly regulated by white light intensity. LHCII phosphorylation is concluded to function both as a stabilizer (in time scales of seconds to minutes) and a dynamic regulator (in time scales from tens of minutes to hours and days) of redox homeostasis in chloroplasts, subject to modifications by both environmental and metabolic cues. Exceeding the capacity of LHCII phosphorylation/dephosphorylation to balance the distribution of excitation energy between PSII and PSI results in readjustment of photosystem stoichiometry.

Initialism as a mechanism of weed interference: can a crop plant be blinded?

Initialism is a new word proposed to indicate the "shade-avoidance syndrome". Plants detect the presence of neighbor plants very early in the growing season through changes in light quality. They modify the allocation of photosynthesis products privileging shoot growth over the roots. One of the hypotheses of the authors is that, when weed management is timely scheduled, a "blind" crop could be more productive because it would avoid an imbalance on the shoot:root ratio (S:R). Two strategies were developed to test this hypothesis: a) to use the classical Yoda's Law to screen several crops for insensitivity to S:R imbalance; b) to evaluate several growth regulators to control the plant responses to crowding. Experimental results confirm that both strategies can yield insensitive plants. The possibilities of the use of this knowledge are discussed.

When Zostera marina is intermixed with Ulva, its photosynthesis is reduced by increased pH and lower light, but not by changes in light quality

To evaluate the effects of Ulva on the photosynthetic capacities of the temperate seagrass Zostera marina, we compared it in the laboratory under normal light and light filtered through layers of Ulva intestinalis, and repeated the experiments with the addition of pH-induced changes in carbon speciation and availability. One thallus of Ulva reduced photosynthetically available irradiance to underlying seagrass by about 50% and shifted the quality of remaining light towards the green part of the spectrum (approximately 520–570nm). There was no significant difference in photosynthetic performance between Z. marina under normal light and under Ulva-filtered light when adjusted to the same irradiance as for the control plants, indicating that the green spectrum transmitted through Ulva layers was being absorbed by the seagrass and was as efficient in driving photosynthesis as was the normal light. On the other hand, algae-generated pH shifts which could extend up to pH 10 had significant negative effects on photosynthesis of the seagrass, reducing the electron transport rates with up to 75%.

Blue light regulated shade avoidance

Most plants grow in dense vegetation with the risk of being out-competed by neighboring plants. These neighbors can be detected not only through the depletion in light quantity that they cause, but also through the change in light quality, which plants perceive using specific photoreceptors. Both the reduction of the red:far-red ratio and the depletion of blue light are signals that induce a set of phenotypic traits, such as shoot elongation and leaf hyponasty, which increase the likelihood of light capture in dense plant stands. This set of phenotypic responses are part of the so called shade avoidance syndrome (SAS). This addendum discusses recent findings on the regulation of the SAS of Arabidopsis thaliana upon blue light depletion. Keller et al. and Keuskamp et al. show that the low blue light attenuation induced shade avoidance response of seedling and rosette-stage A. thaliana plants differ in their hormonal regulation. These studies also show there is a regulatory overlap with the R:FR-regulated SAS.

Phylogenetic composition of Prochlorococcus and Synechococcus in cold eddies of the South China Sea

Two mesoscale cyclonic eddies (CE I and CE II) occurred along the Vietnam coast in the South China Sea during summer 2007; CE I was in the decaying stage and CE II was in the intensifying stage. CE II was also influenced by the Mekong River plume. The upwelling effect of the 2 eddies on the phylogenetic diversity of Prochlorococcus and Synechococcus was investi- gated by clone library construction based on sequences of the internal transcribed spacer (ITS) region, with station SEATS in the oligotrophic basin as a comparison. Phylogenetic analysis revealed the presence of sequences affiliated with 3 Prochlorococcus clades (HL I, HL II & LL IV) and 7 Synechococcus clades (II, III, VII, X, XV, XVI, and sub-cluster 5.2) from the various water bodies, with HL II Prochlorococcus and clade II Synechococcus being the dominant phylotypes. Multivariate analysis demonstrated that spatial distribution of the 2 genera was mostly explained by temperature, but also affected by salinity and chlorophyll a concentration. Upwelling of the nutrient-rich subsurface water at the center of the cold eddy resulted in a significant increase in the abundance of Synechococcus in the young and intensifying CE II, whereas Prochlorococcus dominated in the old and decaying CE I and the surrounding oligotrophic waters. It is intriguing that Synechococcus clades III, XV and XVI, which are known to contain chromatically adapting members, were most numerous in eddy and river plume influenced waters, where light quality changes might be expected, compared to the oligotrophic basin waters. The disparity in cyanobacterial phylogenetic composition between the 2 eddies clearly reflected the differing hydrographic dynamics and developmental stages of these eddy systems.

Coarse-Grained Model of Cooperative Chloroplast Transport in Moss

Organelle motility is essential for the functioning of the eukaryotic cell. Actively modifying intracellular structures allows cells to change and adapt to different conditions. One of these cellular structures is the microtubule cytoskeleton, which is comprised of polarized filaments that function as tracks to transport cargo via molecular motors. Recent studies have revealed the importance of cooperative transport in living cells. In the moss Physcomitrella patens, reorganization of the chloroplasts is critical to adapt to changes in light quality and intensity. In this work, we performed detailed analysis of the transport of chloroplasts in protonemal moss cells, and showed that their transport is facilitated by molecular motors and the microtubule cytoskeleton. Our findings were recapitulated using coarse-grained modeling of this cargo transport over different microtubule network topologies. Our simulations include a detailed description of motor and microtubule dynamics, in the presence of thermal fluctuations. Our modeling sheds light into the molecular mechanisms at play in the photo-relocation response of chloroplasts in moss.

Light Quantity Affects the Regulation of Cell Shape in Fremyella diplosiphon

In some cyanobacteria, the color or prevalent wavelengths of ambient light can impact the protein or pigment composition of the light-harvesting complexes. In some cases, light color or quality impacts cellular morphology. The significance of changes in pigmentation is associated strongly with optimizing light absorption for photosynthesis, whereas the significance of changes in light quality-dependent cellular morphology is less well understood. In natural aquatic environments, light quality and intensity change simultaneously at varying depths of the water column. Thus, we hypothesize that changes in morphology that also have been attributed to differences in the prevalent wavelengths of available light may largely be associated with changes in light intensity. Fremyella diplosiphon shows highly reproducible light-dependent changes in pigmentation and morphology. Under red light (RL), F. diplosiphon cells are blue-green in color, due to the accumulation of high levels of phycocyanin, a RL-absorbing pigment in the light-harvesting complexes or phycobilisomes (PBSs), and the shape of cells are short and rounded. Conversely, under green light (GL), F. diplosiphon cells are red in color due to accumulation of GL-absorbing phycoerythrin in PBSs, and are longer and brick-shaped. GL is enriched at lower depths in the water column, where overall levels of light also are reduced, i.e., to 10% or less of the intensity found at the water surface. We hypothesize that longer cells under low light intensities at increasing depths in the water column, which are generally also enriched in green wavelengths, are associated with greater levels of total photosynthetic pigments in the thylakoid membranes. To test this hypothesis, we grew F. diplosiphon under increasing intensities of GL and observed whether the length of cells diminished due to reduced pressure to maintain larger cells and the associated increased photosynthetic membrane capacity under high light intensity, independent of whether it is light of green wavelengths.

Modulation of Phototropic Responsiveness in Arabidopsis through Ubiquitination of Phototropin 1 by the CUL3-Ring E3 Ubiquitin Ligase CRL3NPH3

Plant phototropism is an adaptive response to changes in light direction, quantity, and quality that results in optimization of photosynthetic light harvesting, as well as water and nutrient acquisition. Though several components of the phototropic signal response pathway have been identified in recent years, including the blue light (BL) receptors phototropin1 (phot1) and phot2, much remains unknown. Here, we show that the phot1-interacting protein NONPHOTOTROPIC HYPOCOTYL3 (NPH3) functions as a substrate adapter in a CULLIN3-based E3 ubiquitin ligase, CRL3(NPH3). Under low-intensity BL, CRL3(NPH3) mediates the mono/multiubiquitination of phot1, likely marking it for clathrin-dependent internalization from the plasma membrane. In high-intensity BL, phot1 is both mono/multi- and polyubiquitinated by CRL3(NPH3), with the latter event targeting phot1 for 26S proteasome-mediated degradation. Polyubiquitination and subsequent degradation of phot1 under high-intensity BL likely represent means of receptor desensitization, while mono/multiubiquitination-stimulated internalization of phot1 may be coupled to BL-induced relocalization of hormone (auxin) transporters.

Interaction of Brassinosteroids with Light Quality and Plant Hormones in Regulating Shoot Growth of Young Sunflower and Arabidopsis Seedlings

Sunflower hypocotyls elongate as light quality changes from the normal red to far-red (R/FR) ratio of sunlight to a lower R/FR ratio. This low R/FR ratio-induced elongation significantly increases endogenous concentrations of indole-3-acetic acid (IAA) and also of three gibberellins (GAs): GA20, GA1, and GA8. Of these, it is likely GA1 that drives low R/FR-induced growth. Brassinosteroids are also involved in shoot growth. Here we tested three R/FR ratios: high, normal, and low. Significant hypocotyl elongation occurred with this stepwise reduction in R/FR ratio, but endogenous castasterone concentrations in the hypocotyls remained unchanged. Brassinolide was also applied to the seedlings and significantly increased hypocotyl growth, though one that was uniform across all three R/FR ratios. Applied brassinolide increased hypocotyl elongation while significantly reducing (usually) levels of IAA, GA20, and GA8, but not that of GA1, which remained constant. Given the above, we conclude that endogenous castasterone does not mediate the hypocotyl growth that is induced by enriching FR light, relative to R light. Similarly, we conclude that the hypocotyl growth that is induced by applied brassinolide does not result from an interaction of brassinolide with changes in light quality. The ability of applied brassinolide to influence IAA, GA20, and GA8 content, yet have no significant effect on GA1, is hard to explain. One speculative hypothesis, though, could involve the brassinolide-induced reductions that occurred for endogenous IAA, given IAA’s known ability to differentially influence the expression levels of GA20ox, GA3ox, and GA2ox, key genes in GA biosynthesis.

Comparative phosphoproteome profiling reveals a function of the STN8 kinase in fine-tuning of cyclic electron flow (CEF)

Important aspects of photosynthetic electron transport efficiency in chloroplasts are controlled by protein phosphorylation. Two thylakoid-associated kinases, STN7 and STN8, have distinct roles in short- and long-term photosynthetic acclimation to changes in light quality and quantity. Although some substrates of STN7 and STN8 are known, the complexity of this regulatory kinase system implies that currently unknown substrates connect photosynthetic performance with the regulation of metabolic and regulatory functions. We performed an unbiased phosphoproteome-wide screen with Arabidopsis WT and stn8 mutant plants to identify unique STN8 targets. The phosphorylation status of STN7 was not affected in stn8, indicating that kinases other than STN8 phosphorylate STN7 under standard growth conditions. Among several putative STN8 substrates, PGRL1-A is of particular importance because of its possible role in the modulation of cyclic electron transfer. The STN8 phosphorylation site on PGRL1-A is absent in both monocotyledonous plants and algae. In dicots, spectroscopic measurements with Arabidopsis WT, stn7, stn8, and stn7/stn8 double-mutant plants indicate a STN8-mediated slowing down of the transition from cyclic to linear electron flow at the onset of illumination. This finding suggests a possible link between protein phosphorylation by STN8 and fine-tuning of cyclic electron flow during this critical step of photosynthesis, when the carbon assimilation is not commensurate to the electron flow capacity of the chloroplast.