Chloroplast Accumulation Response Enhances Leaf Photosynthesis and Plant Biomass Production

Hyperspectral imaging for chloroplast movement detection

We employed hyperspectral imaging to detect chloroplast positioning and assess its influence on common vegetation indices. In low blue light, chloroplasts move to cell walls perpendicular to the direction of the incident light. In high blue light, chloroplasts exhibit the avoidance response, moving to cell walls parallel to the light direction. Irradiation with high light resulted in significant changes in leaf reflectance and the shape of the reflectance spectrum. Using mutants with disrupted chloroplast movements, we found that blue light-induced changes in the reflectance spectrum are mostly due to chloroplast relocations. We trained machine learning methods in the classification of leaves according to the chloroplast positioning, based on the reflectance spectra. The convolutional network showed low levels of misclassification of leaves irradiated with high light even when different species were used for training and testing, suggesting that reflectance spectra may be used to detect chloroplast avoidance in heterogeneous vegetation. We also examined the correlation between chloroplast positioning and values of indices of normalized-difference type for various combinations of wavelengths and identified an index sensitive to chloroplast positioning. We found that values of some of the vegetation indices, including those sensitive to the carotenoid levels, may be altered due to chloroplast rearrangements.

Phototropin switches between cis- and trans-autophosphorylation in light-induced chloroplast relocation in Marchantia polymorpha

In the accumulation response, chloroplasts move toward weak blue light (BL) to maximize photosynthetic efficiency; in the avoidance response, they move away from strong BL to reduce photodamage. The BL receptor kinase phototropin (phot) mediates these chloroplast relocation responses, and the chloroplast relocation response requires phot kinase activity. Upon receiving BL, phot undergoes autophosphorylation; however, the molecular mechanisms that regulate chloroplast relocation through phot autophosphorylation remain unclear. In this study, we conducted biochemical experiments using phot in the liverwort Marchantia polymorpha and revealed that phot employs cis-autophosphorylation under weak BL and both cis- and trans-autophosphorylation under strong BL. Inhibiting trans-autophosphorylation reduced phot autophosphorylation and suppressed the avoidance response, but not the accumulation response. These findings suggest that phot employs two modes of autophosphorylation to alternate between the accumulation and avoidance responses in plants.

Comparative transcriptome and metabolomic analysis reveal key genes and mechanisms responsible for the dark-green leaf color of a strawberry mutant

Photosynthesis is a source of energy for various types of plant life activities and is essential for plant growth and development. Consequently, the study of photosynthetic mechanisms has been a hot spot. Leaf color mutants has always been ideal materials for exploring the mechanisms of chlorophyll metabolism and photosynthesis. In this study, we identified a leaf color mutant of 'Benihoppe' strawberry in the field, which exhibited a darker green leaf color compared with the wild type. The content of total chlorophyll and carotenoid in the mutant leaves was elevated by 7.44-20.23% and 8.9-21.92%, respectively, compared with that of the wild type. Additionally, net photosynthetic rate in the mutant increased by 20.13%. Further transcriptome analysis showed that significant upregulation of genes such as GLK1, PPR, and MORF9 in the mutant leaves, which promoted chloroplast development. The expression levels of UROD, PPOC, PORA, CHLG, and CPOX were significantly upregulated during chlorophyll synthesis, while the expression levels of HCAR and CYP89A9 were significantly downregulated during chlorophyll degradation, thus leading to the accumulation of chlorophyll in mutant leaves. The upregulation of gene expression levels such as PetM, AtpD, PGK, and RPI4 during photosynthesis promoted multiple stages of light and dark reaction, thereby enhancing the photosynthetic capacity of the mutant. And the changes in metabolites such as monogalactosyl monoacylglycerol (MGMG), glucuronosyldiacylglycerol (GlcADG), raffinose, etc. also indicate that the mutant has metabolic differences in chloroplast composition and photosynthesis compared to 'Benihoppe'. The above results not only deepen our understanding of the mechanism behind the dark-green leaf color in strawberry mutants but also provide potential genetic resources for cultivating strawberry varieties with enhanced photosynthetic capacity.

Light-dependent chloroplast relocation in wild strawberry (Fragaria vesca)

Chloroplast photorelocation is a vital organellar response that optimizes photosynthesis in plants amid fluctuating environmental conditions. Chloroplasts exhibit an accumulation response, in which they move toward weak light to enhance photoreception, and an avoidance response, in which they move away from strong light to avoid photodamage. Although chloroplast photorelocation has been extensively studied in model plants such as Arabidopsis thaliana, little is known about this process in the economically important crop strawberry. Here, we investigated chloroplast photorelocation in leaf mesophyll cells of wild strawberry (Fragaria vesca), a diploid relative of commercially cultivated octoploid strawberry (F. × ananassa). Microscopy observation revealed that the periclinal area of leaf mesophyll cells in F. vesca is considerably smaller than that of A. thaliana. Given this small cell size, we investigated chloroplast photorelocation in F. vesca by measuring light transmittance in leaves. Weak blue light induced the accumulation response, whereas strong blue light induced the avoidance response. Unexpectedly, strong red light also induced the accumulation response in F. vesca. These findings shed light on chloroplast photorelocation as an intracellular response, laying the foundation for enhancing photosynthesis and productivity in Fragaria.

Biomass Allocation in Gentianella turkestanorum is Driven by Environmental Factors and Functional Traits

Exploring the elevation distribution characteristics, biomass allocation strategies, and the effects of elevation, soil factors, and functional traits on the biomass of Gentianella turkestanorum (Gand.) Holub is of great significance for the production, development, utilization, and protection of the medicinal material resources. In this study, we investigated the biomass and functional traits of the root, stem, leaf, and flower of G. turkestanorum, analyzing their elevation distribution patterns, allometric growth trajectories, and their correlations. The results showed that the biomass of different organs of G. turkestanorum decreases with increasing elevation, and the belowground biomass/aboveground biomass increases with elevation. The flower biomass accounts for 59.24% of the total biomass, which was significantly higher than that of other organs. G. turkestanorum biomass follows the optimal allocation theory, adopting a 'pioneering' growth strategy at low elevations and a 'conservative' strategy at high elevations. Chlorophyll content and leaf thickness of G. turkestanorum were positively correlated with elevation, but leaf dry matter content and the number of flowers were negatively correlated with elevation. Compared to functional traits, elevation and soil factors have a stronger explanatory power regarding the biomass of G. turkestanorum. Elevation, soil moisture content, pH, available phosphorus, total nitrogen, and ammonium nitrogen significantly affect the biomass of G. turkestanorum, with only pH showing a positive correlation with biomass. Among these factors, elevation, soil moisture content, and pH significantly impact the accurate prediction of G. turkestanorum biomass. The number of flowers, crown width, root length, root diameter, and leaf dry matter content all have a significantly positive correlation with the biomass of G. turkestanorum, with the number of flowers and root diameter making significant contributions to the accurate prediction of biomass. Elevation can directly affect the biomass of G. turkestanorum and can also indirectly affect it through other pathways, with the direct effect being greater than the indirect effect.

A Kinesin-Like Protein, KAC, is Required for Light-Induced and Actin-Based Chloroplast Movement in Marchantia polymorpha

Chloroplasts accumulate on the cell surface under weak light conditions to efficiently capture light but avoid strong light to minimize photodamage. The blue light receptor phototropin regulates the chloroplast movement in various plant species. In Arabidopsis thaliana, phototropin mediates the light-induced chloroplast movement and positioning via specialized actin filaments on the chloroplasts, chloroplast-actin filaments. KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT (KAC) and CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1) are pivotal for actin-based chloroplast movement and positioning in land plants. However, the mechanisms by which KAC and CHUP1 regulate chloroplast movement and positioning remain unclear. In this study, we characterized KAC and CHUP1 orthologs in the liverwort Marchantia polymorpha, MpKAC and MpCHUP1, respectively. Their knockout mutants, Mpkacko and Mpchup1ko, impaired the light-induced chloroplast movement. Although Mpchup1ko showed mild chloroplast aggregation, Mpkacko displayed severe chloroplast aggregation, suggesting the greater contribution of MpKAC to the chloroplast anchorage to the plasma membrane. Analysis of the subcellular localization of the functional MpKAC-Citrine indicated that MpKAC-Citrine formed a punctate structure on the plasma membrane. Structure-function analysis of MpKAC revealed that the deletion of the conserved C-terminal domain abrogates its targeting to the plasma membrane and its function. The deletion of the N-terminal motor domain retains the plasma membrane targeting but abrogates the formation of punctate structure and shows a severe defect in the light-induced chloroplast movement. Our findings suggest that the formation of the punctate structure on the plasma membrane of MpKAC is essential for chloroplast movement.

CHUP1 restricts chloroplast movement and effector-triggered immunity in epidermal cells

Chloroplast Unusual Positioning 1 (CHUP1) plays an important role in the chloroplast avoidance and accumulation responses in mesophyll cells. In epidermal cells, prior research showed silencing CHUP1-induced chloroplast stromules and amplified effector-triggered immunity (ETI); however, the underlying mechanisms remain largely unknown. CHUP1 has a dual function in anchoring chloroplasts and recruiting chloroplast-associated actin (cp-actin) filaments for blue light-induced movement. To determine which function is critical for ETI, we developed an approach to quantify chloroplast anchoring and movement in epidermal cells. Our data show that silencing NbCHUP1 in Nicotiana benthamiana plants increased epidermal chloroplast de-anchoring and basal movement but did not fully disrupt blue light-induced chloroplast movement. Silencing NbCHUP1 auto-activated epidermal chloroplast defense (ECD) responses including stromule formation, perinuclear chloroplast clustering, the epidermal chloroplast response (ECR), and the chloroplast reactive oxygen species (ROS), hydrogen peroxide (H2O2). These findings show chloroplast anchoring restricts a multifaceted ECD response. Our results also show that the accumulated chloroplastic H2O2 in NbCHUP1-silenced plants was not required for the increased basal epidermal chloroplast movement but was essential for increased stromules and enhanced ETI. This finding indicates that chloroplast de-anchoring and H2O2 play separate but essential roles during ETI.

Starch metabolism in guard cells: At the intersection of environmental stimuli and stomatal movement

Starch metabolism in guard cells plays a central role in regulating stomatal movement in response to light, elevated ambient CO2 and potentially other abiotic and biotic factors. Here, we discuss how various guard cell signal transduction pathways converge to promote rearrangements in guard cell starch metabolism for efficient stomatal responses, an essential physiological process that sustains plant productivity and stress tolerance. We suggest manipulation of guard cell starch dynamics as a previously overlooked strategy to improve stomatal behavior under changing environmental conditions.

Levels of photoactivated phototropin modulate signal transmission during the chloroplast accumulation response

Chloroplasts accumulate in regions of plant cells exposed to irradiation to maximize light reception for efficient photosynthesis. This response is mediated by the blue-light receptor phototropin. Upon the perception of blue light, phototropin is photoactivated, an unknown signal is transmitted from the photoactivated phototropin to distant chloroplasts, and the chloroplasts begin their directional movement. How activated phototropin initiates this signal transmission is unknown. Here, using the liverwort Marchantia polymorpha, we analysed whether increased photoactive phototropin levels mediate signal transmission and chloroplast behaviour during the accumulation response. The signal transmission rate was higher in transgenic cells overexpressing phototropin than in wild-type cells. However, the chloroplast directional movement was similar between wild-type and transgenic cells. Consistent with the observation, increasing the amount of photoactivated phototropin through higher blue-light intensity also accelerated signal transmission but did not affect chloroplast behaviour in wild-type cells. Photoactivation of phototropin under weak blue-light led to the greater protein level of phosphorylated phototropin in cells overexpressing phototropin than in wild-type cells, whereas the autophosphorylation level within each phototropin molecule was similar. These results indicate that the abundance of photoactivated phototropin modulates the signal transmission rate to distant chloroplasts but does not affect chloroplast behaviour during the accumulation response.

Chloroplast-actin filaments decide the direction of chloroplast avoidance movement under strong light in Arabidopsis thaliana

Chloroplast-actin (cp-actin) filaments are crucial for light-induced chloroplast movement, and appear in the front region of moving chloroplasts when visualized using GFP-mouse Talin. They are short and thick, exist between a chloroplast and the plasma membrane, and move actively and rapidly compared to cytoplasmic long actin filaments that run through a cell. The average period during which a cp-actin filament was observed at the same position was less than 0.5 s. The average lengths of the cp-actin filaments calculated from those at the front region of the moving chloroplast and those around the chloroplast periphery after stopping the movement were almost the same, approximately 0.8 µm. Each cp-actin filament is shown as a dotted line consisting of 4-5 dots. The vector sum of cp-actin filaments in a moving chloroplast is parallel to the moving direction of the chloroplast, suggesting that the direction of chloroplast movement is regulated by the vector sum of cp-actin filaments. However, once the chloroplasts stopped moving, the vector sum of the cp-actin filaments around the chloroplast periphery was close to zero, indicating that the direction of movement was undecided. To determine the precise structure of cp-actin filaments under electron microscopy, Arabidopsis leaves and fern Adiantum capillus-veneris gametophytes were frozen using a high-pressure freezer, and observed under electron microscopy. However, no bundled microfilaments were found, suggesting that the cp-actin filaments were unstable even under high-pressure freezing.

CHLOROPLAST UNUSUAL POSITIONING 1 is a plant-specific actin polymerization factor regulating chloroplast movement

Plants have unique responses to fluctuating light conditions. One such response involves chloroplast photorelocation movement, which optimizes photosynthesis under weak light by the accumulation of chloroplasts along the periclinal side of the cell, which prevents photodamage under strong light by avoiding chloroplast positioning toward the anticlinal side of the cell. This light-responsive chloroplast movement relies on the reorganization of chloroplast actin (cp-actin) filaments. Previous studies have suggested that CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1) is essential for chloroplast photorelocation movement as a regulator of cp-actin filaments. In this study, we conducted comprehensive analyses to understand CHUP1 function. Functional, fluorescently tagged CHUP1 colocalized with and was coordinately reorganized with cp-actin filaments on the chloroplast outer envelope during chloroplast movement in Arabidopsis thaliana. CHUP1 distribution was reversibly regulated in a blue light- and phototropin-dependent manner. X-ray crystallography revealed that the CHUP1-C-terminal domain shares structural homology with the formin homology 2 (FH2) domain, despite lacking sequence similarity. Furthermore, the CHUP1-C-terminal domain promoted actin polymerization in the presence of profilin in vitro. Taken together, our findings indicate that CHUP1 is a plant-specific actin polymerization factor that has convergently evolved to assemble cp-actin filaments and enables chloroplast photorelocation movement.

Melatonin Modulates Tomato Root Morphology by Regulating Key Genes and Endogenous Hormones

Melatonin plays a vital role in plant growth and development. In this study, we treated hydroponically grown tomato roots with various concentrations of exogenous melatonin (0, 10, 30, and 50 μmol·L-1). We utilized root scanning and microscopy to examine alterations in root morphology and cell differentiation and elucidated the mechanism by which melatonin regulates these changes through the interplay with endogenous hormones and relevant genes. The results showed that for melatonin at concentrations ranging between 10 and 30 μmol·L-1, the development of lateral roots were significantly stimulated, the root hair growth was enhanced, and biomass accumulation and root activity were increased. Furthermore, we elucidated that melatonin acts as a mediator for the expression of genes, such as SlCDKA1, SlCYCA3;1, SlARF2, SlF3H, and SlKT1, which are involved in the regulation of root morphology changes. Additionally, we observed that melatonin influences the levels of endogenous hormones, including ZT, GA3, IAA, ABA, and BR, which subsequently impact the root morphology development of tomato roots. In summary, this study shows that tomato root morphology can be promoted by the optimal concentration of exogenous melatonin (10-30 μmol·L-1).

Visualizing the dynamics of plant energy organelles

Plant organelles predominantly rely on the actin cytoskeleton and the myosin motors for long-distance trafficking, while using microtubules and the kinesin motors mostly for short-range movement. The distribution and motility of organelles in the plant cell are fundamentally important to robust plant growth and defense. Chloroplasts, mitochondria, and peroxisomes are essential organelles in plants that function independently and coordinately during energy metabolism and other key metabolic processes. In response to developmental and environmental stimuli, these energy organelles modulate their metabolism, morphology, abundance, distribution and motility in the cell to meet the need of the plant. Consistent with their metabolic links in processes like photorespiration and fatty acid mobilization is the frequently observed inter-organellar physical interaction, sometimes through organelle membranous protrusions. The development of various organelle-specific fluorescent protein tags has allowed the simultaneous visualization of organelle movement in living plant cells by confocal microscopy. These energy organelles display an array of morphology and movement patterns and redistribute within the cell in response to changes such as varying light conditions, temperature fluctuations, ROS-inducible treatments, and during pollen tube development and immune response, independently or in association with one another. Although there are more reports on the mechanism of chloroplast movement than that of peroxisomes and mitochondria, our knowledge of how and why these three energy organelles move and distribute in the plant cell is still scarce at the functional and mechanistic level. It is critical to identify factors that control organelle motility coupled with plant growth, development, and stress response.

UV-B responses in the spotlight: Dynamic photoreceptor interplay and cell-type specificity

Plants are constantly exposed to a multitude of external signals, including light. The information contained within the full spectrum of light is perceived by a battery of photoreceptors, each with specific and shared signalling outputs. Recently, it has become clear that UV-B radiation is a vital component of the electromagnetic spectrum, guiding growth and being crucial for plant fitness. However, given the large overlap between UV-B specific signalling pathways and other photoreceptors, understanding how plants can distinguish UV-B specific signals from other light components deserves more scrutiny. With recent evidence, we propose that UV-B signalling and other light signalling pathways occur within distinct tissues and cell-types and that the contribution of each pathway depends on the type of response and the developmental stage of the plant. Elucidating the precise site(s) of action of each molecular player within these signalling pathways is key to fully understand how plants are able to orchestrate coordinated responses to light within the whole plant body. Focusing our efforts on the molecular study of light signal interactions to understand plant growth in natural environments in a cell-type specific manner will be a next step in the field of photobiology.

Red-Light Transmittance Changes in Variegated Pelargonium zonale-Diurnal Variation in Chloroplast Movement and Photosystem II Efficiency

Chloroplast movement rapidly ameliorates the effects of suboptimal light intensity by accumulating along the periclinal cell walls, as well as the effects of excess light by shifting to the anticlinal cell walls. These acclimation responses are triggered by phototropins located at the plasma membrane and chloroplast envelope. Here, we used a recently developed non-invasive system sensitive to very small changes in red light leaf transmittance to perform long-term continuous measurements of dark-light transitions. As a model system, we used variegated Pelargonium zonale leaves containing green sectors (GS) with fully developed chloroplasts and achlorophyllous, white sectors (WS) with undifferentiated plastids, and higher phototropin expression levels. We observed biphasic changes in the red-light transmittance and oscillations triggered by medium intensities of white light, described by a transient peak preceded by a constant decrease in transmittance level. A slight change in red-light transmittance was recorded even in WS. Furthermore, the chloroplast position at lower light intensities affected the rapid light curves, while high light intensity decreased saturated electron transport, maximum quantum efficiency of photosystem II, and increased non-photochemical quenching of chlorophyll fluorescence and epidermal flavonoids. Our results extend the knowledge of light-dependent chloroplast movements and thus contribute to a better understanding of their role in regulating photosynthesis under fluctuating light conditions.

Statistical analysis of organelle movement using state-space models

BACKGROUND

Organelle motility is essential for the correct cellular function of various eukaryotic cells. In plant cells, chloroplasts move towards the intracellular area irradiated by a weak light to maximise photosynthesis. To initiate this process, an unknown signal is transferred from the irradiated area to distant chloroplasts. Quantification of this chloroplast movement has been performed using visual estimations that are analyst-dependent and labour-intensive. Therefore, an objective and faster method is required.

RESULTS

In this study, we developed the cellssm package of R ( https://github.com/hnishio/cellssm.git ), which is a user-friendly tool for state-space modelling to statistically analyse the directional movement of cells or organelles. Our method showed a high accuracy in estimating the start time of chloroplast movement in the liverwort Marchantia polymorpha over a short period. The tool indicated that chloroplast movement accelerates during transport to the irradiated area and that signal transfer speed is uneven within a cell. We also developed a method to estimate the common dynamics among multiple chloroplasts in each cell, which clarified different characteristics among cells.

CONCLUSIONS

We demonstrated that state-space modelling is a powerful method to understand organelle movement in eukaryotic cells. The cellssm package can be applied to various directional movements (both accumulation and avoidance) at cellular and subcellular levels to estimate the true transition of states behind the time-series data.

Dark-induced chloroplast relocation depends on actin filaments in the liverwort Apopellia endiviifolia along with the light- and cold-induced relocations

Chloroplasts move to the periclinal walls of cells under weak light to harness light energy for photosynthesis and to anticlinal walls to avoid strong light. These responses involve the cytoskeleton components microtubules and/or actin filaments. In the dark, chloroplasts move to the anticlinal cell walls bordering neighbouring cells (dark-positioning response), but this response in various plants normally requires a prolonged dark incubation period, which has hampered analysis. However, we recently demonstrated the dark-positioning response that can be induced after a short period of dark incubation in the liverwort Apopellia endiviifolia. Here, we investigated whether the cytoskeleton components function in the dark-positioning response of A. endiviifolia cells. Microtubules and actin filaments were fluorescently visualised in A. endiviifolia cells and were disrupted following treatment with the microtubule and actin filament polymerisation inhibitors. The dark-positioning response was unaffected in the cells with disrupted microtubules. By contrast, the dark-positioning response was inhibited by the disruption of actin filaments. The disruption of actin filaments also restricted chloroplast mobility during light- and cold-dependent chloroplast movements in A. endiviifolia. Therefore, the dark-positioning response of A. endiviifolia depends solely on an actin filament-associated motility mechanism, as do the light- and cold-dependent chloroplast responses.

Leaf optical properties and photosynthesis of fern species with a wide range of divergence time in relation to mesophyll anatomy

BACKGROUND AND AIMS

For a comprehensive understanding of the mechanisms of changing plant photosynthetic capacity during plant evolutionary history, knowledge of leaf gas exchange and optical properties are essential, both of which relate strongly to mesophyll anatomy. Although ferns are suitable for investigating the evolutionary history of photosynthetic capacity, comprehensive research of fern species has yet to be undertaken in this regard.

METHODS

We investigated leaf optical properties, gas exchange and mesophyll anatomy of fern species with a wide range of divergence time, using 66 ferns from natural habitats and eight glasshouse-grown ferns. We used a spectroradiometer and an integrating sphere to measure light absorptance and reflectance by the leaves.

KEY RESULTS

The more newly divergent fern species had a thicker mesophyll, a larger surface area of chloroplasts facing the intercellular airspaces (Sc), thicker cell walls and large light absorptance. Although no trend with divergence time was obtained in leaf photosynthetic capacity on a leaf-area basis, when the traits were expressed on a mesophyll-thickness basis, trends in leaf photosynthetic capacity became apparent. On a mesophyll-thickness basis, the more newly divergent species had a low maximum photosynthesis rate, accompanied by a low Sc.

CONCLUSIONS

We found a strong link between light capture, mesophyll anatomy and photosynthesis rate in fern species for the first time. The thick mesophyll of the more newly divergent ferns does not necessarily relate to the high photosynthetic capacity on a leaf-area basis. Rather, the thick mesophyll accompanied by thick cell walls allowed the ferns to adapt to a wider range of environments through increasing leaf toughness, which would contribute to the diversification of fern species.

Enhancement of the CRISPR/Cas9-Based Genome Editing System in Lettuce (Lactuca sativa L.) Using the Endogenous U6 Promoter

The CRISPR/Cas9 system has been widely applied as a precise gene-editing tool for studying gene functions as well as improving agricultural traits in various crop plants. Here, we optimized a gene-editing system in lettuce (Lactuca sativa L.) using the endogenous U6 promoter and proved that the PHOT2 gene is a versatile target gene. We isolated the LsU6-10 promoter from 10 U6 snRNA genes identified from the lettuce genome database for comparison with the AtU6-26 promoter that has been used to drive sgRNAs in lettuce. Two CRISPR/Cas9 vectors were constructed using the LsU6-10 and AtU6-26 promoters to drive sgRNA361 to target the PHOT2 gene. The chloroplast avoidance response was defective in lettuces with biallelic mutations in the targeted PHOT2 gene, as in the Arabidopsis phot2 mutant. The PHOT2 gene mutations were stably heritable from the R0 to R2 generations, and the high gene-editing efficiency enabled the selection of transgene-free lines in the R1 generation and the establishment of independent phot2 mutants in the R2 generation. Our results suggest that the LsU6-10 promoter is more effective than the AtU6-26 promoter in driving sgRNA for the CRISPR/Cas9 system in lettuce and that PHOT2 is a useful target gene to verify gene editing efficiency without any detrimental effects on plant growth, which is often a consideration in conventional target genes.

Chloroplasts in plant cells show active glassy behavior under low-light conditions

Plants have developed intricate mechanisms to adapt to changing light conditions. Besides phototropism and heliotropism (differential growth toward light and diurnal motion with respect to sunlight, respectively), chloroplast motion acts as a fast mechanism to change the intracellular structure of leaf cells. While chloroplasts move toward the sides of the plant cell to avoid strong light, they accumulate and spread out into a layer on the bottom of the cell at low light to increase the light absorption efficiency. Although the motion of chloroplasts has been studied for over a century, the collective organelle motion leading to light-adapting self-organized structures remains elusive. Here, we study the active motion of chloroplasts under dim-light conditions, leading to an accumulation in a densely packed quasi-2D layer. We observe burst-like rearrangements and show that these dynamics resemble systems close to the glass transition by tracking individual chloroplasts. Furthermore, we provide a minimal mathematical model to uncover relevant system parameters controlling the stability of the dense configuration of chloroplasts. Our study suggests that the meta-stable caging close to the glass transition in the chloroplast monolayer serves a physiological relevance: Chloroplasts remain in a spread-out configuration to increase the light uptake but can easily fluidize when the activity is increased to efficiently rearrange the structure toward an avoidance state. Our research opens questions about the role that dynamical phase transitions could play in self-organized intracellular responses of plant cells toward environmental cues.

The action of enhancing weak light capture via phototropic growth and chloroplast movement in plants

To cope with fluctuating light conditions, terrestrial plants have evolved precise regulation mechanisms to help optimize light capture and increase photosynthetic efficiency. Upon blue light-triggered autophosphorylation, activated phototropin (PHOT1 and PHOT2) photoreceptors function solely or redundantly to regulate diverse responses, including phototropism, chloroplast movement, stomatal opening, and leaf positioning and flattening in plants. These responses enhance light capture under low-light conditions and avoid photodamage under high-light conditions. NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) and ROOT PHOTOTROPISM 2 (RPT2) are signal transducers that function in the PHOT1- and PHOT2-mediated response. NPH3 is required for phototropism, leaf expansion and positioning. RPT2 regulates chloroplast accumulation as well as NPH3-mediated responses. NRL PROTEIN FOR CHLOROPLAST MOVEMENT 1 (NCH1) was recently identified as a PHOT1-interacting protein that functions redundantly with RPT2 to mediate chloroplast accumulation. The PHYTOCHROME KINASE SUBSTRATE (PKS) proteins (PKS1, PKS2, and PKS4) interact with PHOT1 and NPH3 and mediate hypocotyl phototropic bending. This review summarizes advances in phototropic growth and chloroplast movement induced by light. We also focus on how crosstalk in signaling between phototropism and chloroplast movement enhances weak light capture, providing a basis for future studies aiming to delineate the mechanism of light-trapping plants to improve light-use efficiency.

Cytological, genetic and transcriptomic characterization of a cucumber albino mutant

Photosynthesis, a fundamental process for plant growth and development, is dependent on chloroplast formation and chlorophyll synthesis. Severe disruption of chloroplast structure results in albinism of higher plants. In the present study, we report a cucumber albino alc mutant that presented white cotyledons under normal light conditions and was unable to produce first true leaf. Meanwhile, alc mutant could grow creamy green cotyledons under dim light conditions but died after exposure to normal light irradiation. No chlorophyll and carotenoid were detected in the alc mutant grown under normal light conditions. Using transmission electron microscopy, impaired chloroplasts were observed in this mutant. The genetic analysis indicated that the albino phenotype was recessively controlled by a single locus. Comparative transcriptomic analysis between the alc mutant and wild type revealed that genes involved in chlorophyll metabolism and the methylerythritol 4-phosphate pathway were affected in the alc mutant. In addition, three genes involved in chloroplast development, including two FtsH genes and one PPR gene, were found to have negligible expression in this mutant. The quality of RNA sequencing results was further confirmed by real-time quantitative PCR analysis. We also examined 12 homologous genes from alc mutant in other plant species, but no genetic variation in the coding sequences of these genes was found between alc mutant and wild type. Taken together, we characterized a cucumber albino mutant with albinism phenotype caused by chloroplast development deficiency and this mutant can pave way for future studies on plastid development.

Molecular insights into the phototropin control of chloroplast movements

Chloroplast movements are controlled by ultraviolet/blue light through phototropins. In Arabidopsis thaliana, chloroplast accumulation at low light intensities and chloroplast avoidance at high light intensities are observed. These responses are controlled by two homologous photoreceptors, the phototropins phot1 and phot2. Whereas chloroplast accumulation is triggered by both phototropins in a partially redundant manner, sustained chloroplast avoidance is elicited only by phot2. Phot1 is able to trigger only a small, transient chloroplast avoidance, followed by the accumulation phase. The source of this functional difference is not fully understood at either the photoreceptor or the signalling pathway levels. In this article, we review current understanding of phototropin functioning and try to dissect the differences that result in signalling to elicit two distinct chloroplast responses. First, we focus on phototropin structure and photochemical and biochemical activity. Next, we analyse phototropin expression and localization patterns. We also summarize known photoreceptor systems controlling chloroplast movements. Finally, we focus on the role of environmental stimuli in controlling phototropin activity. All these aspects impact the signalling to trigger chloroplast movements and raise outstanding questions about the mechanism involved.

Overdominant expression of related genes of ion homeostasis improves K+ content advantage in hybrid tobacco leaves

BACKGROUND

Potassium(K+) plays a vital role in improving the quality of tobacco leaves. However, how to improve the potassium content of tobacco leaves has always been a difficult problem in tobacco planting. K+ content in tobacco hybrid is characterized by heterosis, which can improve the quality of tobacco leaves, but its underlying molecular genetic mechanisms remain unclear.

RESULTS

Through a two-year field experiment, G70×GDH11 with strong heterosis and K326×GDH11 with weak heterosis were screened out. Transcriptome analyses revealed that 80.89% and 57.28% of the differentially expressed genes (DEGs) in the strong and weak heterosis combinations exhibited an overdominant expression pattern, respectively. The genes that up-regulated the overdominant expression in the strong heterosis hybrids were significantly enriched in the ion homeostasis. Genes involved in K+ transport (KAT1/2, GORK, AKT2, and KEA3), activity regulation complex (CBL-CIPK5/6), and vacuole (TPKs) genes were overdominant expressed in strong heterosis hybrids, which contributed to K+ homeostasis and heterosis in tobacco leaves.

CONCLUSIONS

K+ homeostasis and accumulation in tobacco hybrids were collectively improved. The overdominant expression of K+ transport and homeostasis-related genes conducted a crucial role in the heterosis of K+ content in tobacco leaves.

Has breeding altered the light environment, photosynthetic apparatus, and photosynthetic capacity of wheat leaves?

Whether photosynthesis has improved with increasing yield in major crops remains controversial. Research in this area has often neglected to account for differences in light intensity experienced by cultivars released in different years. Light intensity is expected to be positively associated with photosynthetic capacity and the resistance of the photosynthetic apparatus to high light but negatively associated with light-utilization efficiency under low light. Here, we analyzed the light environment, photosynthetic activity, and protein components of leaves of 26 winter wheat cultivars released during the past 60 years in China. Over time, light levels on flag leaves significantly decreased due to architectural changes, but photosynthetic rates under high or low light and the resistance of the photosynthetic apparatus to high light remained steady, contrary to expectations. We propose that the difference between the actual and expected trends is due to breeding. Specifically, breeding has optimized photosynthetic performance under high light rather than low light. Moreover, breeding selectivity altered the stoichiometry of several proteins related to dynamic photosynthesis, canopy light distribution, and photoprotection. These results indicate that breeding has significantly altered the photosynthetic mechanism in wheat and its response to the light environment. These changes likely have helped increase wheat yields.

The role of photosynthesis related pigments in light harvesting, photoprotection and enhancement of photosynthetic yield in planta

Photosynthetic pigments are an integral and vital part of all photosynthetic machinery and are present in different types and abundances throughout the photosynthetic apparatus. Chlorophyll, carotenoids and phycobilins are the prime photosynthetic pigments which facilitate efficient light absorption in plants, algae, and cyanobacteria. The chlorophyll family plays a vital role in light harvesting by absorbing light at different wavelengths and allowing photosynthetic organisms to adapt to different environments, either in the long-term or during transient changes in light. Carotenoids play diverse roles in photosynthesis, including light capture and as crucial antioxidants to reduce photodamage and photoinhibition. In the marine habitat, phycobilins capture a wide spectrum of light and have allowed cyanobacteria and red algae to colonise deep waters where other frequencies of light are attenuated by the water column. In this review, we discuss the potential strategies that photosynthetic pigments provide, coupled with development of molecular biological techniques, to improve crop yields through enhanced light harvesting, increased photoprotection and improved photosynthetic efficiency.

Development of quantitative and concise measurement method of oxygen in fine bubble dispersion

Fine bubbles (FBs) have attracted significant attention in several research fields. Although some reports have argued that FB dispersion is useful as an oxygen (gas) carrier, only a few reports have examined its properties as an oxygen carrier using experimental data. As one of the reasons for this, there are no standard methods for measuring the oxygen content in FB dispersions. Conventional oxygen measurement methods have certain drawbacks in accuracy or speed; thus, it is difficult to use oxygen content as the primary outcome. In this study, we introduce a Clark-type polarographic oxygen electrode device (OXYG1-PLUS) for oxygen measurement, allowing the dilution of FB dispersion without the influence of ambient air and the adhesion of FBs on the electrode surface due to its special shape. First, the accuracy of our dilution method was evaluated using pure water as a sample, and it was confirmed that our method could measure with an accuracy of ±0.5 mg/L from the results with conventional dissolved oxygen meters. Second, the oxygen content in FB dispersion was evaluated with our method and a chemical titration method (Winkler’s method), and it was found that our method could measure the oxygen content in FB dispersions quantitively. This method satisfies the easiness (4 steps) and quickness (within 8 min) for a wide range of oxygen contents (0 to 332 mg/L, theoretical range) with low coefficient variation (< 4.7%) and requires a small sample volume (50–500 μL); thus, it is a useful method for measuring the oxygen in FB dispersions.

Maize Golden2-like transcription factors boost rice chloroplast development, photosynthesis, and grain yield

Chloroplasts are the sites for photosynthesis, and two Golden2-like factors act as transcriptional activators of chloroplast development in rice (Oryza sativa L.) and maize (Zea mays L.). Rice OsGLK1 and OsGLK2 are orthologous to maize ZmGLK1 (ZmG1) and ZmGLK2 (ZmG2), respectively. However, while rice OsGLK1 and OsGLK2 act redundantly to regulate chloroplast development in mesophyll cells, maize ZmG1 and ZmG2 are functionally specialized and expressed in different cell-specific manners. To boost rice chloroplast development and photosynthesis, we generated transgenic rice plants overexpressing ZmG1 and ZmG2, individually or simultaneously, with constitutive promoters (pZmUbi::ZmG1 and p35S::ZmG2) or maize promoters (pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2). Both ZmG1 and ZmG2 genes were highly expressed in transgenic rice leaves. Moreover, ZmG1 and ZmG2 showed coordinated expression in pZmG1::ZmG1/pZmG2::ZmG2 plants. All Golden2-like (GLK) transgenic plants had higher chlorophyll and protein contents, Rubisco activities and photosynthetic rates per unit leaf area in flag leaves. However, the highest grain yields occurred when maize promoters were used; pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2 transgenic plants showed increases in grain yield by 51%, 47%, and 70%, respectively. In contrast, the pZmUbi::ZmG1 plant produced smaller seeds without yield increases. Transcriptome analysis indicated that maize GLKs act as master regulators promoting the expression of both photosynthesis-related and stress-responsive regulatory genes in both rice shoot and root. Thus, by promoting these important functions under the control of their own promoters, maize GLK1 and GLK2 genes together dramatically improved rice photosynthetic performance and productivity. A similar approach can potentially improve the productivity of many other crops.

Effect of Photoconversion Coatings for Greenhouses on Electrical Signal-Induced Resistance to Heat Stress of Tomato Plants

The use of photoconversion coatings is a promising approach to improving the quality of light when growing plants in greenhouses in low light conditions. In this work, we studied the effect of fluoropolymer coatings, which produce photoconversion of UV-A radiation and violet light into blue and red light, on the growth and resistance to heat stress of tomato plants (Solanum lycopersicum L.). The stimulating effect of the spectrum obtained as a result of photoconversion on plant growth and the activity of the photosynthesis process are shown. At the same time, the ability to withstand heat stress is reduced in plants grown under a photoconversion coating. Stress electrical signals, which normally increase resistance, in such plants have a much weaker protective effect on the photosynthetic apparatus. The observed effects are apparently explained by a decrease in the concentration of H₂O₂ in plants grown using photoconversion technologies, which leads to a shift in the development program towards increased productivity to the detriment of the protective function. Thus, when using photoconversion technologies in agricultural practice, it is necessary to pay increased attention to maintaining stable conditions during plant cultivation.

Light quality, oxygenic photosynthesis and more

Oxygenic photosynthesis takes place in thylakoid membranes (TM) of cyanobacteria, algae, and higher plants. It begins with light absorption by pigments in large (modular) assemblies of pigment-binding proteins, which then transfer excitation energy to the photosynthetic reaction centers of photosystem (PS) I and PSII. In green algae and plants, these light-harvesting protein complexes contain chlorophylls (Chls) and carotenoids (Cars). However, cyanobacteria, red algae, and glaucophytes contain, in addition, phycobiliproteins in phycobilisomes that are attached to the stromal surface of TM, and transfer excitation energy to the reaction centers via the Chl a molecules in the inner antennas of PSI and PSII. The color and the intensity of the light to which these photosynthetic organisms are exposed in their environment have a great influence on the composition and the structure of the light-harvesting complexes (the antenna) as well as the rest of the photosynthetic apparatus, thus affecting the photosynthetic process and even the entire organism. We present here a perspective on 'Light Quality and Oxygenic Photosynthesis', in memory of George Christos Papageorgiou (9 May 1933-21 November 2020; see notes a and b). Our review includes (1) the influence of the solar spectrum on the antenna composition, and the special significance of Chl a; (2) the effects of light quality on photosynthesis, measured using Chl a fluorescence; and (3) the importance of light quality, intensity, and its duration for the optimal growth of photosynthetic organisms.

Guard-Cell-Specific Expression of Phototropin2 C-Terminal Fragment Enhances Leaf Transpiration

Phototropins (phot1 and phot2) are plant-specific blue light receptors that mediate chloroplast movement, stomatal opening, and phototropism. Phototropin is composed of the N-terminus LOV1 and LOV2 domains and the C-terminus Ser/Thr kinase domain. In previous studies, 35-P2CG transgenic plants expressing the phot2 C-terminal fragment–GFP fusion protein (P2CG) under the control of 35S promoter showed constitutive phot2 responses, including chloroplast avoidance response, stomatal opening, and reduced hypocotyl phototropism regardless of blue light, and some detrimental growth phenotypes. In this study, to exclude the detrimental growth phenotypes caused by the ectopic expression of P2C and to improve leaf transpiration, we used the PHOT2 promoter for the endogenous expression of GFP-fused P2C (GP2C) (P2-GP2C) and the BLUS1 promoter for the guard-cell-specific expression of GP2C (B1-GP2C), respectively. In P2-GP2C plants, GP2C expression induced constitutive phototropin responses and a relatively dwarf phenotype as in 35-P2CG plants. In contrast, B1-GP2C plants showed the guard-cell-specific P2C expression that induced constitutive stomatal opening with normal phototropism, chloroplast movement, and growth phenotype. Interestingly, leaf transpiration was significantly improved in B1-GP2C plants compared to that in P2-GP2C plants and WT. Taken together, this transgenic approach could be applied to improve leaf transpiration in indoor plants.

Chloroplast relocation movement in the liverwort Apopellia endiviifolia

Changes in the subcellular localisation of chloroplasts help optimise photosynthetic activity under different environmental conditions. In many plants, this movement is mediated by the blue-light photoreceptor phototropin. A model organism with simple phototropin signalling that allows clear observation of chloroplasts would facilitate the study of chloroplast relocation movement. Here, we examined this process in the simple thalloid liverwort Apopellia endiviifolia. Transverse sections of the thallus tissue showed uniformly developed chloroplasts and no air chambers; these characteristics enable clear observation of chloroplasts and analysis of their movements under a fluorescence stereomicroscope. At 22°C, the chloroplasts moved to the anticlinal walls of cells next to the neighbouring cells in the dark (dark-positioning response), whereas they moved towards weak light (accumulation response) and away from strong light (avoidance response). When the temperature was reduced to 5°C, the chloroplasts moved away from weak light (cold-avoidance response). Hence, both light- and temperature-dependent chloroplast relocation movements occur in A. endiviifolia. Notably, the accumulation, avoidance and cold-avoidance responses were induced under blue-light but not under red-light. These results suggest that phototropin is responsible for chloroplast relocation movement in A. endiviifolia and that the characteristics are similar to those in the model liverwort Marchantia polymorpha. RNA sequencing and Southern blot analysis identified a single copy of the PHOTOTROPIN gene in A. endiviifolia, indicating that a simple phototropin signalling pathway functions in A. endiviifolia. We conclude that A. endiviifolia has great potential as a model system for elucidating the mechanisms of chloroplast relocation movement.

Regulation of plant phototropic growth by NPH3/RPT2-like substrate phosphorylation and 14-3-3 binding

Polarity underlies all directional growth responses in plants including growth towards the light (phototropism). The plasma-membrane associated protein, NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) is a key determinant of phototropic growth which is regulated by phototropin (phot) AGC kinases. Here we demonstrate that NPH3 is directly phosphorylated by phot1 within a conserved C-terminal consensus sequence (RxS) that is necessary to promote phototropism and petiole positioning in Arabidopsis. RxS phosphorylation also triggers 14-3-3 binding combined with changes in NPH3 phosphorylation and localisation status. Mutants of NPH3 that are unable to bind or constitutively bind 14-3-3 s show compromised functionality consistent with a model where phototropic curvature is established by signalling outputs arising from a gradient of NPH3 RxS phosphorylation across the stem. Our findings therefore establish that NPH3/RPT2-Like (NRL) proteins are phosphorylation targets for plant AGC kinases. Moreover, RxS phosphorylation is conserved in other members of the NRL family, suggesting a common mechanism of regulating plant growth to the prevailing light environment.

Phototropin 1 Mediates High-Intensity Blue Light-Induced Chloroplast Accumulation Response in a Root Phototropism 2-Dependent Manner in Arabidopsis phot2 Mutant Plants

Phototropins, namely, phototropin 1 (phot1) and phototropin 2 (phot2), mediate chloroplast movement to maximize photosynthetic efficiency and prevent photodamage in plants. Phot1 primarily functions in chloroplast accumulation process, whereas phot2 mediates both chloroplast avoidance and accumulation responses. The avoidance response of phot2-mediated chloroplasts under high-intensity blue light (HBL) limited the understanding of the function of phot1 in the chloroplast accumulation process at the HBL condition. In this study, we showed that the phot2 mutant exhibits a chloroplast accumulation response under HBL, which is defective when the root phototropism 2 (RPT2) gene is mutated in the phot2 background, mimicking the phenotype of the phot1 phot2 double mutant. A further analysis revealed that the expression of RPT2 was induced by HBL and the overexpression of RPT2 could partially enhance the chloroplast accumulation response under HBL. These results confirmed that RPT2 also participates in regulating the phot1-mediated chloroplast accumulation response under HBL. In contrast, RPT2 functions redundantly with neural retina leucine zipper (NRL) protein for chloroplast movement 1 (NCH1) under low-light irradiation. In addition, no chloroplast accumulation response was detected in the phot2 jac1 double mutant under HBL, which has been previously observed in phot2 rpt2 and phot1 phot2 double mutants. Taken together, our results indicated that phot1 mediates the HBL-induced chloroplast accumulation response in an RPT2-dependent manner and is also regulated by j-domain protein required for chloroplast accumulation response 1 (JAC1).

OsMORF9 is necessary for chloroplast development and seedling survival in rice

Chloroplasts are closely associated with the growth and development of higher plants. Accumulating evidence has revealed that the multiple organellar RNA editing factors (MORF) family of proteins influences plastidic and mitochondrial development through post-transcriptional regulation. However, the role of MORFs in regulating the development of chloroplasts in rice is still unclear. The OsMORF9 gene belongs to a small family of 7 genes in rice and is highly expressed in young leaves. We used the CRISPR/Cas9 system to mutate OsMORF9. The resulting knockout lines osmorf9-1 and osmorf9-2 exhibited an albino seedling lethal phenotype. Besides, the expression of many plastid-encoded genes involved in photosynthesis, the biogenesis of plastidic ribosomes and the editing and splicing of specific plastidic RNA molecules were severely affected in these two OsMORF9 mutants. Furthermore, yeast two-hybrid analysis revealed that OsMORF9 could interact with OsSLA4 and DUA1 which are members of the pentatricopeptide repeat (PPR) family of proteins. Analysis of subcellular localization of OsMORF9 also suggested that it might function in chloroplasts. The findings from the present study demonstrated the critical role of OsMORF9 in the biogenesis of chloroplast ribosomes, chloroplast development and seedling survival. This therefore provides new insights on the function of MORF proteins in rice.

The new kid on the block: a dominant-negative mutation of phototropin1 enhances carotenoid content in tomato fruits

Phototropins, the UVA-blue light photoreceptors, endow plants to detect the direction of light and optimize photosynthesis by regulating positioning of chloroplasts and stomatal gas exchange. Little is known about their functions in other developmental responses. A tomato Non-phototropic seedling1 (Nps1) mutant, bearing an Arg495His substitution in the vicinity of LOV2 domain in phototropin1, dominant-negatively blocks phototropin1 responses. The fruits of Nps1 mutant were enriched in carotenoids, particularly lycopene, compared with its parent, Ailsa Craig. On the contrary, CRISPR/CAS9-edited loss of function phototropin1 mutants displayed subdued carotenoids compared with the parent. The enrichment of carotenoids in Nps1 fruits is genetically linked with the mutation and exerted in a dominant-negative fashion. Nps1 also altered volatile profiles with high levels of lycopene-derived 6-methyl 5-hepten2-one. The transcript levels of several MEP and carotenogenesis pathway genes were upregulated in Nps1. Nps1 fruits showed altered hormonal profiles with subdued ethylene emission and reduced respiration. Proteome profiles showed a causal link between higher carotenogenesis and increased levels of protein protection machinery, which may stabilize proteins contributing to MEP and carotenogenesis pathways. The enhancement of carotenoid content by Nps1 in a dominant-negative fashion offers a potential tool for high lycopene-bearing hybrid tomatoes.

Phytoremediation of aniline by Salix babylonica cuttings: Removal, accumulation, and photosynthetic response

Aniline, a synthetic compound widely used in industrial and pesticide production, is a potential environmental pollutant. The removal of aniline is extremely important to minimize threats to human health and the surrounding environment. The objectives of this study were to investigate the removal efficiency and physiological response of Salix. babylonica cuttings to aniline pollution. Photosynthesis, chlorophyll fluorescence, spectral reflectance and the concentration of aniline in leaves, stems and roots were analysed. The experiment showed that S. babylonica has a strong removal effect on aniline wastewater. Cuttings from S. babylonica stems and roots played an important role in accumulating aniline. However, this increase in aniline concentration was dose dependent and was not always linear. With increasing aniline concentration in S. babylonica was increasingly stressed, with negative impacts on photosynthesis, chlorophyll fluorescence and spectral reflectance index in S. babylonica leaves. These results indicate that non-stomatal limitations are the main reason for the reduction in Pn in S. babylonica leaves due to chlorophyll structure destruction under aniline stress. In addition, aniline concentrations result in an unbalanced distribution of excitation energy between the two light systems, thereby hindering photosynthetic electron transfer and restricting the efficient operation of photosynthesis. Salix babylonica can endure moderate concentrations of aniline and has potential for the phyto-management of aniline-polluted wastewater, although further studies are needed using polluted wastewater.

Leaf photosynthetic and anatomical insights into mechanisms of acclimation in rice in response to long-term fluctuating light

Long-term fluctuating light (FL) conditions are very common in natural environments. The physiological and biochemical mechanisms for acclimation to FL differ between species. However, most of the current conclusions regarding acclimation to FL were made based on studies in algae or Arabidopsis thaliana. It is still unclear how rice (Oryza sativa L.) integrate multiple physiological changes to acclimate to long-term FL. In this study, we found that rice growth was repressed under long-term FL. By systematically measuring phenotypes and physiological parameters, we revealed that: (a) under short-term FL, photosystem I (PSI) was inhibited, while after 1-7 days of long-term FL, both PSI and PSII were inhibited. Higher acceptor-side limitation in electron transport and higher overall nonphotochemical quenching (NPQ) explained the lower efficiencies of PSI and PSII, respectively. (b) An increase in pH differences across the thylakoid membrane and a decrease in thylakoid proton conductivity revealed a reduction of ATP synthase activity. (c) Using electron microscopy, we showed a decrease in membrane stacking and stomatal opening after 7 days of FL treatment. Taken together, our results show that electron flow, ATP synthase activity and NPQ regulation are the major processes determining the growth performance of rice under long-term FL conditions.

Arabidopsis Phototropins Participate in the Regulation of Dark-Induced Leaf Senescence

Senescence is the final stage of plant development, affecting individual organs or the whole organism, and it can be induced by several environmental factors, including shading or darkness. Although inevitable, senescence is a complex and tightly regulated process, ensuring optimal remobilization of nutrients and cellular components from senescing organs. Photoreceptors such as phytochromes and cryptochromes are known to participate in the process of senescence, but the involvement of phototropins has not been studied to date. We investigated the role of these blue light photoreceptors in the senescence of individually darkened Arabidopsis thaliana leaves. We compared several physiological and molecular senescence markers in darkened leaves of wild-type plants and phototropin mutants (phot1, phot2, and phot1phot2). In general, all the symptoms of senescence (lower photochemical activity of photosystem II, photosynthetic pigment degradation, down-regulation of photosynthetic genes, and up-regulation of senescence-associated genes) were less pronounced in phot1phot2, as compared to the wild type, and some also in one of the single mutants, indicating delayed senescence. This points to different mechanisms of phototropin operation in the regulation of senescence-associated processes, either with both photoreceptors acting redundantly, or only one of them, phot1, playing a dominant role.

A comprehensive review on the influence of light on signaling cross-talk and molecular communication against phyto-microbiome interactions

Generally, plant growth, development, and their productivity are mainly affected by their growth rate and also depend on environmental factors such as temperature, pH, humidity, and light. The interaction between plants and pathogens are highly specific. Such specificity is well characterized by plants and pathogenic microbes in the form of a molecular signature such as pattern-recognition receptors (PRRs) and microbes-associated molecular patterns (MAMPs), which in turn trigger systemic acquired immunity in plants. A number of Arabidopsis mutant collections are available to investigate molecular and physiological changes in plants under the presence of different light conditions. Over the past decade(s), several studies have been performed by selecting Arabidopsis thaliana under the influence of red, green, blue, far/far-red, and white light. However, only few phenotypic and molecular based studies represent the modulatory effects in plants under the influence of green and blue lights. Apart from this, red light (RL) actively participates in defense mechanisms against several pathogenic infections. This evolutionary pattern of light sensitizes the pathologist to analyze a series of events in plants during various stress conditions of the natural and/or the artificial environment. This review scrutinizes the literature where red, blue, white, and green light (GL) act as sensory systems that affects physiological parameters in plants. Generally, white and RL are responsible for regulating various defense mechanisms, but, GL also participates in this process with a robust impact! In addition to this, we also focus on the activation of signaling pathways (salicylic acid and jasmonic acid) and their influence on plant immune systems against phytopathogen(s).

FMT, a protein that affects mitochondrial distribution, interacts with translation-related proteins in Arabidopsis thaliana

Two translation-related proteins are identified as FMT-interacting proteins. However, FMT, unlike mutants of other CLU genes in fly and human, has no clear impact on the accumulation of mitochondrial proteins. Organelle distribution is critical for effective metabolism and stress response and is controlled by various environmental factors. Clustered mitochondria (CLU) superfamily genes affect mitochondrial distribution and their disruptions cause mitochondria to cluster within a cell in various species including yeast, fly, mammals and Arabidopsis. In Arabidopsis thaliana, Friendly mitochondria (FMT) is a CLU gene that is required for normal mitochondrial distribution, but its molecular function is unclear. Here, we demonstrate that FMT interacts with some translation-related proteins (translation initiation factor eIFiso4G1 and glutamyl-tRNA synthetase OVA9), as well as itself. We also show FMT forms dynamic particles in the cytosol that sometimes move with mitochondria, and their movements are mainly controlled by actin filaments but also by microtubules. Similar results have been reported for animal CLU orthologs. However, an fmt mutant, unlike animal clu mutants, did not show any clear decrease of nuclear-encoded mitochondrial protein levels. This difference may reflect a functional divergence of FMT from other CLU superfamily genes.

Phototropin 1 and 2 Influence Photosynthesis, UV-C Induced Photooxidative Stress Responses, and Cell Death

Phototropins are plasma membrane-associated photoreceptors of blue light and UV-A/B radiation. The Arabidopsis thaliana genome encodes two phototropins, PHOT1 and PHOT2, that mediate phototropism, chloroplast positioning, and stomatal opening. They are well characterized in terms of photomorphogenetic processes, but so far, little was known about their involvement in photosynthesis, oxidative stress responses, and cell death. By analyzing phot1, phot2 single, and phot1phot2 double mutants, we demonstrated that both phototropins influence the photochemical and non-photochemical reactions, photosynthetic pigments composition, stomata conductance, and water-use efficiency. After oxidative stress caused by UV-C treatment, phot1 and phot2 single and double mutants showed a significantly reduced accumulation of H₂O₂ and more efficient photosynthetic electron transport compared to the wild type. However, all phot mutants exhibited higher levels of cell death four days after UV-C treatment, as well as deregulated gene expression. Taken together, our results reveal that on the one hand, both phot1 and phot2 contribute to the inhibition of UV-C-induced foliar cell death, but on the other hand, they also contribute to the maintenance of foliar H₂O₂ levels and optimal intensity of photochemical reactions and non-photochemical quenching after an exposure to UV-C stress. Our data indicate a novel role for phototropins in the condition-dependent optimization of photosynthesis, growth, and water-use efficiency as well as oxidative stress and cell death response after UV-C exposure.

Impaired chloroplast positioning affects photosynthetic capacity and regulation of the central carbohydrate metabolism during cold acclimation

Photosynthesis and carbohydrate metabolism of higher plants need to be tightly regulated to prevent tissue damage during environmental changes. The intracellular position of chloroplasts changes due to a changing light regime. Chloroplast avoidance and accumulation response under high and low light, respectively, are well known phenomena, and deficiency of chloroplast movement has been shown to result in photodamage and reduced biomass accumulation. Yet, effects of chloroplast positioning on underlying metabolic regulation are less well understood. Here, we analysed photosynthesis together with metabolites and enzyme activities of the central carbohydrate metabolism during cold acclimation of the chloroplast unusual positioning 1 (chup1) mutant of Arabidopsis thaliana. We compared cold acclimation under ambient and low light and found that maximum quantum yield of PSII was significantly lower in chup1 than in Col-0 under both conditions. Our findings indicated that net CO₂ assimilation in chup1 is rather limited by biochemistry than by photochemistry. Further, cold-induced dynamics of sucrose phosphate synthase differed significantly between both genotypes. Together with a reduced rate of sucrose cycling derived from kinetic model simulations our study provides evidence for a central role of chloroplast positioning for photosynthetic and metabolic acclimation to low temperature.

CIPK23 regulates blue light-dependent stomatal opening in Arabidopsis thaliana

Phototropins (phot1 and phot2) are plant blue light receptor kinases that function to mediate phototropism, chloroplast movement, leaf flattening, and stomatal opening in Arabidopsis. Considerable progress has been made in understanding the mechanisms associated with phototropin receptor activation by light. However, the identities of phototropin signaling components are less well understood by comparison. In this study, we specifically searched for protein kinases that interact with phototropins by using an in vitro screening method (AlphaScreen) to profile interactions against an Arabidopsis protein kinase library. We found that CBL-interacting protein kinase 23 (CIPK23) interacts with both phot1 and phot2. Although these interactions were verified by in vitro pull-down and in vivo bimolecular fluorescence complementation assays, CIPK23 was not phosphorylated by phot1, as least in vitro. Mutants lacking CIPK23 were found to exhibit impaired stomatal opening in response to blue light but no deficits in other phototropin-mediated responses. We further found that blue light activation of inward-rectifying K⁺ (K⁺ _in ) channels was impaired in the guard cells of cipk23 mutants, whereas activation of the plasma membrane H⁺ -ATPase was not. The blue light activation of K⁺ _in channels was also impaired in the mutant of BLUS1, which is one of the phototropin substrates in guard cells. We therefore conclude that CIPK23 promotes stomatal opening through activation of K⁺ _in channels most likely in concert with BLUS1, but through a mechanism other than activation of the H⁺ -ATPase. The role of CIPK23 as a newly identified component of phototropin signaling in stomatal guard cells is discussed.

Phototropin- and photosynthesis-dependent mitochondrial positioning in Arabidopsis thaliana mesophyll cells

Mitochondria are frequently observed in the vicinity of chloroplasts in photosynthesizing cells, and this association is considered necessary for their metabolic interactions. We previously reported that, in leaf palisade cells of Arabidopsis thaliana, mitochondria exhibit blue-light-dependent redistribution together with chloroplasts, which conduct accumulation and avoidance responses under the control of blue-light receptor phototropins. In this study, precise motility analyses by fluorescent microscopy revealed that the individual mitochondria in palisade cells, labeled with green fluorescent protein, exhibit typical stop-and-go movement. When exposed to blue light, the velocity of moving mitochondria increased in 30 min, whereas after 4 h, the frequency of stoppage of mitochondrial movement markedly increased. Using different mutant plants, we concluded that the presence of both phototropin1 and phototropin2 is necessary for the early acceleration of mitochondrial movement. On the contrary, the late enhancement of stoppage of mitochondrial movement occurs only in the presence of phototropin2 and only when intact photosynthesis takes place. A plasma-membrane ghost assay suggested that the stopped mitochondria are firmly adhered to chloroplasts. These results indicate that the physical interaction between mitochondria and chloroplasts is cooperatively mediated by phototropin2- and photosynthesis-dependent signals. The present study might add novel regulatory mechanism for light-dependent plant organelle interactions.

Impairment of chloroplast movement reduces growth and delays reproduction of Arabidopsis thaliana in natural and controlled conditions

PREMISE

The importance of chloroplast movement for plant growth in constant, controlled light and of nonphotochemical quenching (NPQ) in variable, natural light are known. Here we concurrently investigated growth and reproduction of several Arabidopsis thaliana mutants to assess the relative importance of photoprotection via chloroplast movement and NPQ.

METHODS

Plants were grown outdoors (natural conditions) or in a growth chamber with variable light and chilling temperatures (controlled conditions). Phenotypic growth and reproductive variables were determined at set times before maturity in wild-type (WT) and phot1, phot2, phot1phot2 (e.g., impaired chloroplast movement, stomatal conductance, leaf flattening), chup1 (impaired chloroplast movement), and npq1 (reduced NPQ) plants.

RESULTS

Mutants were most adversely affected in natural conditions, with phot1phot2 and chup1 most severely impacted. These mutants bolted later and produced fewer leaves and siliques, less leaf biomass, and fewer secondary inflorescences than WT. In controlled conditions, leaf traits of these mutants were unaffected, but phot1phot2 bolted later and produced fewer secondary inflorescences and siliques than WT. For most variables, there were significant interactions between growth conditions and plant genotype. Many variables were correlated, but those relationships changed with growth conditions and genotype.

CONCLUSIONS

Phenotypic variables at the time of the harvest were strongly affected by growth conditions and genotype. In natural conditions, phot1phot2 and chup1 mutants were most adversely affected, demonstrating the importance of chloroplast movement. In controlled conditions, only phot1phot2 was consistently affected, also emphasizing the important, pleiotropic effects of phototropins. In both conditions, NPQ was less important.

Full sunlight acclimation mechanisms in Riccia discolor thalli: Assessment at morphological, anatomical, and biochemical levels

Light occupies a central position in regulating development of plants. Either little or excess of light could be harmful for plants. Since bryophytes are shade loving organisms, they must adapt to function in fluctuating light regimes. Therefore, the aim of this study was to investigate acclimatory responses of Riccia discolor thalli grown under full sunlight, and were compared with shade grown thalli (control). Length, width, and fresh mass of thallus were significantly lower (by 27, 41 and 37%, respectively) but endogenous nitric oxide content (by 81%) and nitric oxide synthase like activity (by 58%) were higher in full sunlight grown thalli than shade grown thalli. Number of rhizoids was greater in shade but length and width of rhizoids were higher (by 36 and 25%, respectively) in full sunlight grown thalli. The content of carotenoids was higher (by 34%) in full sunlight grown thalli. In full sunlight grown thalli, chloroplasts exhibited avoidance movement but in shade grown thalli they exhibited accumulation movement. Photosynthetic yields were higher in shade grown thalli. Among energy fluxes, ABS/RC did not vary but DI₀/RC was higher (by 12%) in full sunlight grown thalli. Reactive oxygen species and damage were greater in full sunlight grown thalli despite enhanced levels of antioxidants i.e. superoxide dismutase (by 66%) and catalase (by 34%). Overall results suggest that full sunlight acclimation in Riccia discolor thalli occurred at various levels in which endogenous NO plays a positive role.

Rethinking the Influence of Chloroplast Movements on Non-photochemical Quenching and Photoprotection

Under blue light, plant chloroplasts relocate to different areas of the cell. The photoreceptor phototropin2 (phot2) mediates the chloroplast movement mechanism under excess blue light alongside the chloroplast unusual positioning1 (chup1) protein. Recently, it has been proposed that leaf transmittance changes associated with chloroplast relocation affect measurements of nonphotochemical quenching (NPQ), resulting in kinetic differences due to these movements (termed "qM"). We evaluated these claims using Arabidopsis (Arabidopsis thaliana) knock-out mutants lacking either phot2 or chup1 and analyzed the kinetics of both the onset and recovery of NPQ under equivalent intensities of both red and blue light. We also evaluated the photoprotective ability of chloroplast movements both during the early onset of photoinhibition and under sustained excess light. We monitored photoinhibition using the chlorophyll fluorescence parameter of photochemical quenching in the dark, which measures the redox state of Q_A within PSII without any of the complications of traditional F _v /F _m measurements. While there were noticeable differences between the responses under red and blue light, the chloroplast movement mechanism had no effect on the rate or amplitude of NPQ onset or recovery. Therefore, we were unable to replicate the "qM" component and its corresponding influence on the kinetics of NPQ in Arabidopsis grown under "shade" conditions. Furthermore, chloroplast relocation had no effect on the high-light tolerance of these plants. These data cast doubt upon the existence of a chloroplast movement-dependent component of NPQ Therefore, the influence of chloroplast movements on photoprotection should be thoroughly reevaluated.

Light-induced chloroplast movements in Oryza species

Light-induced chloroplast movements control efficient light utilization in leaves, and thus, are essential for leaf photosynthesis and biomass production under fluctuating light conditions. Chloroplast movements have been intensively analyzed using wild-type and mutant plants of Arabidopsis thaliana. The molecular mechanism and the contribution to biomass production were elucidated. However, the knowledge of chloroplast movements is very scarce in other plant species, especially grass species including crop plants. Because chloroplast movements are efficient strategy to optimize light capture in leaves and thus promote leaf photosynthesis and biomass, analysis of chloroplast movements in crops is required for biomass production. Here, we analyzed chloroplast movements in a wide range of cultivated and wild species of genus Oryza. All examined Oryza species showed the blue-light-induced chloroplast movements. However, O. sativa and its ancestral species O. rufipogon, both of which are AA-genome species and usually grown in open condition where plants are exposed to full sunlight, showed the much weaker chloroplast movements than Oryza species that are usually grown under shade or semi-shade conditions, including O. officinalis, O. eichingeri, and O. granulata. Further detailed analyses of different O. officinalis accessions, including sun, semi-shade, and shade accessions, indicated that the difference in chloroplast movement strength between domesticated rice plants and wild species might result from the difference in habitat, and the shape of mesophyll chlorenchyma cells. The findings of this study provide useful information for optimizing Oryza growth conditions, and lay the groundwork for improving growth and yield in staple food crop Oryza sativa.

The cold-induced switch in direction of chloroplast relocation occurs independently of changes in endogenous phototropin levels

When exposed to fluctuating light intensity, chloroplasts move towards weak light (accumulation response), and away from strong light (avoidance response). In addition, cold treatment (5°C) induces the avoidance response even under weak-light conditions (cold-avoidance response). These three responses are mediated by the phototropin (phot), which is a blue-light photoreceptor and has also been reported to act as a thermosensory protein that perceives temperature variation. Our previous report indicated that cold-induced changes in phot biochemical activity initiate the cold-avoidance response. In this study, we further explored the induction mechanism of the cold-avoidance response in the liverwort Marchantia polymorpha and examined the relationship between changes in the amount of phot and the induction of the cold-avoidance response. The switch between the accumulation and avoidance responses occurs at a so-called 'transitional' light intensity. Our physiological experiments revealed that a cold-mediated decrease in the transitional light intensity leads to the induction of the cold-avoidance response. While artificial overexpression of phot decreased the transitional light intensity as much as cold treatment did, the amount of endogenous phot was not increased by cold treatment in wild-type M. polymorpha. Taken together, these findings show that the cold-avoidance response is initiated by a cold-mediated reduction of the transitional light intensity, independent of the amount of endogenous phot. This study provides a clue to understanding the mechanism underlying the switch in direction of chloroplast relocation in response to light and temperature.