Phot1 and phot2 mediate blue light regulation of stomatal opening

A CRY1-HY5-MYB signaling cascade fine-tunes guard cell reactive oxygen species levels and triggers stomatal opening

Stomatal opening facilitates CO2 uptake and causes water loss via transpiration. Compared with the considerable progress made toward understanding phototropin-mediated blue light (BL) signaling in guard cells, the significance of cryptochromes (CRYs) in stomatal opening and their downstream elements remain largely unknown. Here, we show that 3 homologous MYB transcription factor genes, namely MYB11, MYB12, and MYB111, are rapidly transactivated in guard cells during the dark-to-light transition in Arabidopsis (Arabidopsis thaliana). Genetic characterization of myb mutants demonstrates that these proteins specifically mediate light-induced stomatal opening by promoting local flavonol accumulation, thereby controlling reactive oxygen species homeostasis in guard cells. In response to light, activation of the plasma membrane H+-ATPase is inhibited in the myb11 myb12 myb111 triple mutant, compromising transmembrane K+ influx in the mutant guard cells. Furthermore, we demonstrate that MYB11/12/111 expression in guard cells upon illumination is induced by a CRY1-specific signaling cascade involving ELONGATED HYPOCOTYL 5 (HY5), a direct transcriptional activator of these MYBs. Overall, our work reveals a mechanism by which the CRY1-HY5-MYB module facilitates light-induced stomatal opening, providing evidence that flavonoid metabolism in guard cells is crucial for plant stress tolerance.

Lighting the path: how light signaling regulates stomatal movement and plant immunity

Stomata, the small pores on the surfaces of plant leaves and stems, are crucial for gas exchange and also play a role in defense against pathogens. Stomatal movement is influenced not only by surrounding light conditions but also by the presence of foliar pathogens. Certain light wavelengths such as blue or high irradiance red light cause stomatal opening, making it easier for bacteria to enter through opened stomata and causing disease progression in plants. Illumination with blue or intense red light autophosphorylates phototropin, a blue light photoreceptor protein kinase, that in turn activates a signaling cascade to open the stomata. Undoubtedly stomatal defense is a fascinating aspect of plant immunology, especially in plant-foliar pathogen interactions. During these interactions, stomata fundamentally serve as entry points for intrusive pathogens and initiate the plant defense signaling cascade. This review highlights how light-activated photoreceptors such as cryptochromes (CRYs), phytochromes (phys), and UV-receptors (UVRs) influence stomatal movement and defense signaling after foliar pathogen intrusion. It also explores the link between stomatal defense, light signaling, and plant immunity, which is vital for safeguarding crops against pathogens.

Membrane-localized orientation of NONPHOTOTROPIC HYPOCOTYL3 affects the necessity of its phosphorylation for phototropism

NONPHOTOTROPIC HYPOCOTYL3 (NPH3) is a key regulator of hypocotyl phototropism under both low- and high-intensity blue light (LBL/HBL), mediating phototropin1 (phot1) and phot2 signaling. NPH3 undergoes dephosphorylation and is released from the plasma membrane (PM) upon blue light irradiation. However, how its phosphorylation status and PM localization mediate phot1 and phot2 signaling in Arabidopsis (Arabidopsis thaliana) remains elusive. In this study, we found that fusing NPH3 with GFP at its C-terminus (N3G) impaired its release from the PM, a defect exacerbated by a phosphorylation-deficient mutation, resulting in a dephosphorylated NPH3-GFP (N3AG). Unlike N3G, transgenic lines expressing N3AG exhibited defective hypocotyl phototropism under HBL, which could be rescued by myristoylation at the N-terminus of N3AG (mN3AG), indicating that NPH3 phosphorylation is not essential for HBL-induced phototropic responses when it is artificially anchored at the PM via its N-terminus. Furthermore, genetic analysis revealed that N3AG anchored to the PM by its N-terminus (as in mN3AG) only rescues phot1-mediated HBL responses, which require RPT2. However, N3AG failed to regulate phot2-mediated HBL signaling, regardless of its PM orientation. Taken together, our results revealed that NPH3 phosphorylation is essential for phot2-mediated hypocotyl phototropism under HBL, but is not required for phot1-mediated HBL signaling when the NPH3 N-terminus is PM-anchored.

The blue-light receptor CRY1 serves as a switch to balance photosynthesis and plant defense

Plant stomata open in response to blue light, allowing gas exchange and water transpiration. However, open stomata are potential entry points for pathogens. Whether plants can sense pathogens and mount defense responses upon stomatal opening and how blue-light cues are integrated to balance growth-defense trade-offs are poorly characterized. We show that the Arabidopsis blue-light photoreceptor CRYPTOCHROME 1 (CRY1) mediates various aspects of immunity, including pathogen-triggered stomatal closure as well as activation of plant immunity through a typical light-responsive protein LATE UPREGULATED IN RESPONSE TO HYALOPERONOSPORA PARASITICA (LURP1). LURP1 undergoes N-terminal palmitoylation in the presence of bacterial flagellin, prompting a change in subcellular localization from the cytoplasm to plasma membrane, where it enhances the activity of the receptor FLAGELLIN SENSING 2 (FLS2) to mediate plant defense. Collectively, these findings reveal that blue light regulates stomatal defense and highlight the dual functions of CRY1 in photosynthesis and immunity.

Metabolic modeling reveals distinct roles of sugars and carboxylic acids in stomatal opening as well as unexpected carbon fluxes

Guard cell metabolism is crucial for stomatal dynamics, but a full understanding of its role is hampered by experimental limitations and the flexible nature of the metabolic network. To tackle this challenge, we constructed a time-resolved stoichiometric model of guard cell metabolism that accounts for energy and osmolyte requirements and which is integrated with the mesophyll. The model resolved distinct roles for starch, sugars, and malate in guard cell metabolism and revealed several unexpected flux patterns in central metabolism. During blue light-mediated stomatal opening, starch breakdown was the most efficient way to generate osmolytes with downregulation of glycolysis allowing starch-derived glucose to accumulate as a cytosolic osmolyte. Maltose could also accumulate as a cytosolic osmoticum, although this made the metabolic system marginally less efficient. The metabolic energy for stomatal opening was predicted to be derived independently of starch, using nocturnally accumulated citrate which was metabolized in the tricarboxylic acid cycle to malate to provide mitochondrial reducing power for ATP synthesis. In white light-mediated stomatal opening, malate transferred reducing equivalents from guard cell photosynthesis to mitochondria for ATP production. Depending on the capacity for guard cell photosynthesis, glycolysis showed little flux during the day but was crucial for energy metabolism at night. In summary, our analyses have corroborated recent findings in Arabidopsis guard cell research, resolved conflicting observations by highlighting the flexibility of guard cell metabolism, and proposed new metabolic flux modes for further experimental testing.

Spatiotemporal bifurcation of HY5-mediated blue-light signaling regulates wood development during secondary growth

Plants have evolved photoreceptors to optimize their development during primary growth, including germination, hypocotyl elongation, cotyledon opening, and root growth, allowing them to adapt to challenging light conditions. The light signaling transduction pathway during seedling establishment has been extensively studied, but little molecular evidence is available for light-regulated secondary growth, and how light regulates cambium-derived tissue production remains largely unexplored. Here, we show that CRYPTOCHROME (CRY)-dependent blue light signaling and the subsequent attenuation of ELONGATED HYPOCOTYL 5 (HY5) movement to hypocotyls are key inducers of xylem fiber differentiation in Arabidopsis thaliana. Using grafted chimeric plants and hypocotyl-specific transcriptome sequencing of light signaling mutants under controlled light conditions, we demonstrate that the perception of blue light by CRYs in shoots drives secondary cell wall (SCW) deposition at xylem fiber cells during the secondary growth of hypocotyls. We propose that HY5 is a blue light-responsive mobile protein that inhibits xylem fiber formation via direct transcriptional repression of NAC SECONDARY WALL THICKENING PROMOTING 3 (NST3). CRYs retain HY5 in the nucleus, impede its long-distance transport from leaf to hypocotyl, and they initiate NST3-driven SCW gene expression, thereby triggering xylem fiber production. Our findings shed light on the long-range CRYs-HY5-NST3 signaling cascade that shapes xylem fiber development, highlighting the activity of HY5 as a transcriptional repressor during secondary growth.

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.

Subfamily C7 Raf-like kinases MRK1, RAF26, and RAF39 regulate immune homeostasis and stomatal opening in Arabidopsis thaliana

The calcium-dependent protein kinase CPK28 regulates several stress pathways in multiple plant species. Here, we aimed to discover CPK28-associated proteins in Arabidopsis thaliana. We used affinity-based proteomics and identified several potential CPK28 binding partners, including the C7 Raf-like kinases MRK1, RAF26, and RAF39. We used biochemistry, genetics, and physiological assays to gain insight into their function. We define redundant roles for these kinases in stomatal opening, immune-triggered reactive oxygen species (ROS) production, and resistance to a bacterial pathogen. We report that CPK28 associates with and trans-phosphorylates RAF26 and RAF39, and that MRK1, RAF26, and RAF39 are active kinases that localize to endomembranes. Although Raf-like kinases share some features with mitogen-activated protein kinase kinase kinases (MKKKs), we found that MRK1, RAF26, and RAF39 are unable to trans-phosphorylate any of the 10 Arabidopsis mitogen-activated protein kinase kinases (MKKs). Overall, our study suggests that C7 Raf-like kinases associate with and are phosphorylated by CPK28, function redundantly in stomatal opening and immunity, and possess substrate specificities distinct from canonical MKKKs.

Warming triggers stomatal opening by enhancement of photosynthesis and ensuing guard cell CO2 sensing, whereas higher temperatures induce a photosynthesis-uncoupled response

Plants integrate environmental stimuli to optimize photosynthesis vs water loss by controlling stomatal apertures. However, stomatal responses to temperature elevation and the underlying molecular genetic mechanisms remain less studied. We developed an approach for clamping leaf-to-air vapor pressure difference (VPDleaf) to fixed values, and recorded robust reversible warming-induced stomatal opening in intact plants. We analyzed stomatal temperature responses of mutants impaired in guard cell signaling pathways for blue light, abscisic acid (ABA), CO2, and the temperature-sensitive proteins, Phytochrome B (phyB) and EARLY-FLOWERING-3 (ELF3). We confirmed that phot1-5/phot2-1 leaves lacking blue-light photoreceptors showed partially reduced warming-induced stomatal opening. Furthermore, ABA-biosynthesis, phyB, and ELF3 were not essential for the stomatal warming response. Strikingly, Arabidopsis (dicot) and Brachypodium distachyon (monocot) mutants lacking guard cell CO2 sensors and signaling mechanisms, including ht1, mpk12/mpk4-gc, and cbc1/cbc2 abolished the stomatal warming response, suggesting a conserved mechanism across diverse plant lineages. Moreover, warming rapidly stimulated photosynthesis, resulting in a reduction in intercellular (CO2). Interestingly, further enhancing heat stress caused stomatal opening uncoupled from photosynthesis. We provide genetic and physiological evidence that the stomatal warming response is triggered by increased CO2 assimilation and stomatal CO2 sensing. Additionally, increasing heat stress functions via a distinct photosynthesis-uncoupled stomatal opening pathway.

Illuminating stomatal responses to red light: establishing the role of Ci-dependent versus -independent mechanisms in control of stomatal behaviour

The stomatal response to red light appears to link stomatal conductance (gs) with photosynthetic rates. Initially, it was suggested that changes in intercellular CO2 concentration (Ci) provide the main cue via a Ci-dependent response. However, evidence for Ci-independent mechanisms suggests an additional, more direct relationship with photosynthesis. While both Ci-dependent and -independent mechanisms clearly function in stomatal red light responses, little is known regarding their relative contribution. The present study aimed to quantify the relative magnitude of Ci-dependent and -independent mechanisms on the stomatal red light response, to characterize their interplay and to assess the putative link between plastoquinone redox state and Ci-independent stomatal responses. Red light response curves measured at a range of Ci values for wild-type Arabidopsis (Col-0) and the CO2 hyposensitive mutant ca1ca4 allowed deconvolution of Ci-dependent and -independent pathways. Surprisingly, we observed that both mechanisms contribute equally to stomatal red light responses, but Ci-independent stomatal opening is suppressed at high Ci. The present data are also consistent with the involvement of the plastoquinone redox state in coordinating the Ci-independent component. Overall, it seems that while Ci-independent mechanisms are distinct from responses to Ci, interplay between these two pathways is important to facilitate effective coordination between gs and photosynthesis.

Recent Advances in Understanding the Regulatory Mechanism of Plasma Membrane H+-ATPase through the Brassinosteroid Signaling Pathway

The polyhydroxylated steroid phytohormone brassinosteroid (BR) controls many aspects of plant growth, development and responses to environmental changes. Plasma membrane (PM) H+-ATPase, the well-known PM proton pump, is a central regulator in plant physiology, which mediates not only plant growth and development, but also adaptation to stresses. Recent studies highlight that PM H+-ATPase is at least partly regulated via the BR signaling. Firstly, the BR cell surface receptor BRASSINOSTEROID-INSENSITIVE 1 (BRI1) and multiple key components of BR signaling directly or indirectly influence PM H+-ATPase activity. Secondly, the SMALL AUXIN UP RNA (SAUR) gene family physically interacts with BRI1 to enhance organ development of Arabidopsis by activating PM H+-ATPase. Thirdly, RNA-sequencing (RNA-seq) assays showed that the expression of some SAUR genes is upregulated under the light or sucrose conditions, which is related to the phosphorylation state of the penultimate residue of PM H+-ATPase in a time-course manner. In this review, we describe the structural and functional features of PM H+-ATPase and summarize recent progress towards understanding the regulatory mechanism of PM H+-ATPase by BRs, and briefly introduce how PM H+-ATPase activity is modulated by its own biterminal regions and the post-translational modifications.

Enhancing strawberry resilience to saline, alkaline, and combined stresses with light spectra: impacts on growth, enzymatic activity, nutrient uptake, and osmotic regulation

BACKGROUND

This study examines the effects of various complementary light spectra on the growth, development, antioxidant activity, and nutrient absorption in strawberry plants under stress conditions. Light-emitting diodes (LEDs) were used to provide specific wavelengths, including monochromatic blue (460 nm), monochromatic red (660 nm), a dichromatic mix of blue and red (1:3 ratio), full-spectrum white light (400-700 nm), and ambient light as a control (no LED treatment). The stress treatments applied were: control (no stress), salinity (80 mM NaCl), alkalinity (40 mM NaHCO₃), and a combined salinity/alkalinity condition.

RESULTS

Our results indicated that complementary light spectra, especially red and blue/red, helped mitigate the adverse effects of stress on plant growth and development. These spectra improved plant tolerance by enhancing the activity of polyphenol oxidase and peroxidase enzymes and increasing starch accumulation in the leaves. Furthermore, under stress conditions, red and blue-red light significantly boosted fruit anthocyanin levels. Although stress elevated antioxidant activity, supplementary light reduced this activity by alleviating stress compared to ambient light. While stress led to increased Na and Cl ion concentrations in leaves, treatments with blue, red, and blue-red light minimized these harmful effects and promoted the absorption of beneficial ions such as K, Mg, Fe, and Cu.

CONCLUSIONS

Adjusting light quality significantly influences the morphology and physiology of strawberry plants, underscoring the role of specific light spectra in promoting optimal growth under stress conditions.

CLINICAL TRIAL NUMBER

Not applicable.

Fungal toxin fusicoccin enhances plant growth by upregulating 14-3-3 interaction with plasma membrane H+-ATPase

Fusicoccin-A (FC-A) is a diterpene glucoside produced by a pathogenic fungus. Since its discovery, FC-A has been widely recognized as a phytotoxin that induces stomatal opening and leaf wilting, eventually leading to plant death. In this study, we present the first evidence that FC-A enhances plant growth by stabilizing the protein-protein interaction between plasma membrane (PM) H+-ATPase and 14-3-3 in guard cells. Long-term treatment of Arabidopsis plants with FC-A resulted in ~ 30% growth enhancement. Structurally similar fusicoccin-J (FC-J) showed a similar degree of growth-promotion activity as FC-A, whereas the more hydrophilic fusicoccin-H (FC-H) exhibited no effect on plant growth, indicating that the enhancement of plant growth observed with FC-A and FC-J involves upregulation of the protein-protein interaction between PM H+-ATPase and 14-3-3 in guard cells, which promotes stomatal opening and photosynthesis.

Unusual photodynamic characteristics of the light-oxygen-voltage domain of phototropin linked to terrestrial adaptation of Klebsormidium nitens

Phototropin (Phot), a blue light-sensing LOV domain protein, mediates blue light responses and is evolutionarily conserved across the green lineage. Klebsormidium nitens, a green terrestrial alga, presents a valuable opportunity to study adaptive responses from aquatic to land habitat transitions. We determined the crystal structure of Klebsormidium nitens Phot LOV1 domain (KnLOV1) in the dark and engineered different mutations (R60K, Q122N, and D33N) to modulate the lifetime of the photorecovery cycle. We observed unusual, slow recovery kinetics in the wild-type KnLOV1 domain (τ = 41 ± 3 min) compared to different mutants (R60K: τ = 2.0 ± 0.1 min, Q122N: τ = 1.7 ± 0.1 min, D33N: τ = 9.6 ± 0.1 min). Crystal structures of wild-type KnLOV1 and mutants revealed subtle but critical changes near the protein chromophore that is responsible for modulating protein dark recovery time. Our findings shed light on the unique structural and biochemical characteristics of the newly studied KnLOV1 and its evolutionary importance for phototropin-mediated physiology.

The PATROL1 function in roots contributes to the increase in shoot biomass

MAIN CONCLUSION

PATOL1 contributes to increasing biomass not only by effective stomatal movement but also by root meristematic activity. PATROL1 (PROTON ATPase TRANSLOCATION CONTROL 1), a protein with a MUN domain, is involved in the intercellular trafficking of AHA1 H+-ATPase to the plasma membrane in guard cells. This allows for larger stomatal opening and more efficient photosynthesis, leading to increased biomass. Although PATROL1 is expressed not only in stomata but also in other tissues of the shoot and root, the role in other tissues than stomata has not been determined yet. Here, we investigated PATROL1 functions in roots using a loss-of-function mutant and an overexpressor. Cytological observations revealed that root meristematic size was significantly smaller in the mutant resulting in the short primary root. Grafting experiments showed that the shoot biomass of the mutant scion was increased when it grafted onto wild-type or overexpressor rootstocks. Conversely, grafting of the overexpressor scion shoot enhanced the growth of the mutant rootstock. The leaf temperatures of the grafted plants were consistent with those of their respective genotypes, indicating cell-autonomous behavior of stomatal movement and independent roles of PATROL1 in plant growth. Moreover, plasma membrane localization of AHA1 was not altered in root epidermal cells in the patrol1 mutant implying existence of a different mode of PATROL1 action in roots. Thus PATROL1 plays a role in root meristem and contributes to increase shoot biomass.

Freezing treatment under light conditions leads to a dramatic enhancement of freezing tolerance in cold-acclimated Arabidopsis

Overwintering plants survive subzero temperatures by cold acclimation (CA), wherein they acquire freezing tolerance through short-term exposure to low temperatures above 0°C. The freezing tolerance of CA plants increases when they are subsequently exposed to mild subzero temperatures, a phenomenon known as second-phase cold hardening (2PH). Here, we explored the molecular mechanism and physiological conditions of 2PH. The results show that, compared with supercooling, a freezing treatment during 2PH after CA enhanced the freezing tolerance of Arabidopsis. This required CA as a pretreatment, and was designated as second-phase freezing acclimation (2PFA). Light increased the effect of 2PFA to enhance freezing tolerance after CA. C-repeat binding factor and cold-regulated genes were downregulated by light during the 2PFA treatment, a different transcription profile from that during CA. The freezing tolerance of 2PFA plants was decreased by the presence of the photosynthetic electron transfer inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea during the 2PFA treatment. Compared with wild-type plants, phototropin1,2 and phyb mutants showed lower freezing tolerance after 2PFA treatment. These results show that exposure to freezing after CA increases freezing tolerance as a secondary process, and that freezing under light conditions further increases freezing tolerance via pathways involving photoreceptors and photosynthetic electron transfer.

Up-regulation of non-photochemical quenching improves water use efficiency and reduces whole-plant water consumption under drought in Nicotiana tabacum

Water supply limitations will likely impose increasing restrictions on future crop production, underlining a need for crops that use less water per mass of yield. Water use efficiency (WUE) therefore becomes a key consideration in developing resilient and productive crops. In this study, we hypothesized that it is possible to improve WUE under drought conditions via modulation of chloroplast signals for stomatal opening by up-regulation of non-photochemical quenching (NPQ). Nicotiana tabacum plants with strong overexpression of the PsbS gene encoding PHOTOSYSTEM II SUBUNIT S, a key protein in NPQ, were grown under differing levels of drought. The PsbS-overexpressing lines lost 11% less water per unit CO2 fixed under drought and this did not have a significant effect on plant size. Depending on growth conditions, the PsbS-overexpressing lines consumed from 4-30% less water at the whole-plant level than the corresponding wild type. Leaf water and chlorophyll contents showed a positive relation with the level of NPQ. This study therefore provides proof of concept that up-regulation of NPQ can increase WUE, and as such is an important step towards future engineering of crops with improved performance under drought.

Continuous Blue Light Treatment Enhances the Nutritional Value of Hydroponically Grown Eruca vesicaria L. by Improving Ascorbic Acid Biosynthesis

This study investigates the effect of continuous blue light (CBL) treatment on quality-related metabolites, focusing on ascorbic acid (AsA) accumulation in hydroponically grown Eruca vesicaria (L.). Plants were subjected to CBL treatment, consisting of 24-h exposure to constant-intensity blue light (48 μmol m-2 s-1) and 12-h exposure to the remaining spectrum (192 μmol m-2 s-1). The activities of key enzymes in AsA biosynthesis and recycling were analyzed, including L-galactono-1,4-lactone dehydrogenase (GalLDh), monodehydroascorbate reductase (MDhAR), dehydroascorbate reductase (DhAR), and ascorbate peroxidase (APX). The results showed a significant increase in AsA accumulation of 65.9% during the "day" and 69.1% during the "night" phases under CBL compared to controls. GalLDh activity increased by 20% during the "day phase" in CBL-treated plants. APX activity also rose significantly under CBL conditions, by 101% during the "day" and 75.6% during the "night". However, this did not affect dehydroascorbic acid levels or the activities of MDhAR and DhAR. These findings highlight the potential of tailored light treatments to enhance the nutraceutical content of horticultural species, offering valuable insights for sustainably improving food quality in controlled-environment agriculture (CEA) systems and understanding the roles of blue light in ascorbic acid biosynthesis.

Young guard cells function dynamically despite low mechanical anisotropy but gain efficiency during stomatal maturation in Arabidopsis thaliana

Stomata are pores at the leaf surface that enable gas exchange and transpiration. The signaling pathways that regulate the differentiation of stomatal guard cells and the mechanisms of stomatal pore formation have been characterized in Arabidopsis thaliana. However, the process by which stomatal complexes develop after pore formation into fully mature complexes is poorly understood. We tracked the morphogenesis of young stomatal complexes over time to establish characteristic geometric milestones along the path of stomatal maturation. Using 3D-nanoindentation coupled with finite element modeling of young and mature stomata, we found that despite having thicker cell walls than young guard cells, mature guard cells are more energy efficient with respect to stomatal opening, potentially attributable to the increased mechanical anisotropy of their cell walls and smaller changes in turgor pressure between the closed and open states. Comparing geometric changes in young and mature guard cells of wild-type and cellulose-deficient plants revealed that although cellulose is required for normal stomatal maturation, mechanical anisotropy appears to be achieved by the collective influence of cellulose and additional wall components. Together, these data elucidate the dynamic geometric and biomechanical mechanisms underlying the development process of stomatal maturation.

PIF transcriptional regulators are required for rhythmic stomatal movements

Stomata govern the gaseous exchange between the leaf and the external atmosphere, and their function is essential for photosynthesis and the global carbon and oxygen cycles. Rhythmic stomata movements in daily dark/light cycles prevent water loss at night and allow CO2 uptake during the day. How the actors involved are transcriptionally regulated and how this might contribute to rhythmicity is largely unknown. Here, we show that morning stomata opening depends on the previous night period. The transcription factors PHYTOCHROME-INTERACTING FACTORS (PIFs) accumulate at the end of the night and directly induce the guard cell-specific K+ channel KAT1. Remarkably, PIFs and KAT1 are required for blue light-induced stomata opening. Together, our data establish a molecular framework for daily rhythmic stomatal movements under well-watered conditions, whereby PIFs are required for accumulation of KAT1 at night, which upon activation by blue light in the morning leads to the K+ intake driving stomata opening.

Functional comparison of phototropin from the liverworts Apopellia endiviifolia and Marchantia polymorpha

Phototropin (phot) is a blue light (BL) receptor and thermosensor that mediates chloroplast movements in plants. Liverworts, as early-diverging plant species, have a single copy of PHOT gene, and the phot protein in each liverwort activates the signaling pathway adapted to its specific growing environment. In this study, we functionally compared phot from two different liverworts species: Apopellia endiviifolia (Aephot) and Marchantia polymorpha (Mpphot). The BL-dependent photochemical activity of Aephot was similar to that of Mpphot, whereas the thermochemical activity of Aephot was lower than that of Mpphot. Therefore, the phot-mediated signaling pathways of the two plant species may differ more in response to temperature than to BL. Furthermore, we analyzed the functional compatibility of Aephot and Mpphot in chloroplast movements by transiently expressing AePHOT or MpPHOT. The transient expression of AePHOT did not mediate chloroplast movement in M. polymorpha, showing the incompatibility of Aephot with the signaling pathway of M. polymorpha. By contrast, the transient expression of MpPHOT mediated chloroplast movement in A. endiviifolia, indicating the compatibility of Mpphot with the signaling pathway of A. endiviifolia. Our findings reveal both functional similarities and differences between Aephot and Mpphot proteins from the closely related liverworts.

Mechanical advantage makes stomatal opening speed a function of evaporative demand

Stomatal opening in the light, observed in nearly all vascular land plants, is essential for providing access to atmospheric CO2 for photosynthesis. The speed of stomatal opening in the light is critical for maximizing carbon gain in environments in which light intensity changes, yet we have little understanding of how other environmental signals, particularly evaporative demand driven by vapor pressure deficit (VPD) influences the kinetics of this response. In angiosperms, and some fern species from the family Marsileaceae, a mechanical interaction between the guard cells and the epidermal cells determines the aperture of the pore. Here, we examine whether this mechanical interaction influences the speed of stomatal opening in the light. To test this, we investigated the speed of stomatal opening in response to light across a range of VPDs in seven plant species spanning the evolutionary diversity of guard cell and epidermal cell mechanical interactions. We found that stomatal opening speed is a function of evaporative demand in angiosperm species and Marsilea, which have guard cell and epidermal cell mechanical interactions. Stomatal opening speeds did not change across a range of VPD in species of gymnosperm and fern, which do not have guard cell mechanical interactions with the epidermis. We find that guard cell and epidermal cell mechanical interactions may play a key role in regulating stomatal responsiveness to light. These results provide valuable insight into the adaptive relevance of mechanical advantage.

A phytochrome/phototropin chimeric photoreceptor promotes growth of fern gametophytes under limited light conditions

Many ferns thrive even in low-light niches such as under an angiosperm forest canopy. However, the shade adaptation strategy of ferns is not well understood. Phytochrome 3/neochrome (phy3/neo) is an unconventional photoreceptor, found in the fern Adiantum capillus-veneris, that controls both red and blue light-dependent phototropism and chloroplast photorelocation, which are considered to improve photosynthetic efficiency in ferns. Here we show that phy3/neo localizes not only at the plasma membrane but also in the nucleus. Since both phototropism and chloroplast photorelocation are mediated by membrane-associated phototropin photoreceptors, we speculated that nucleus-localized phy3/neo possesses a previously undescribed biological function. We reveal that phy3/neo directly interacts with Adiantum cryptochrome 3 (cry3) in the nucleus. Plant cryptochromes are blue light receptors that transcriptionally regulate photomorphogenesis; therefore, phy3/neo may function via cry3 to synchronize light-mediated development with phototropism and chloroplast photorelocation to promote fern growth under low-light conditions. Furthermore, we demonstrate that phy3/neo regulates the expression of the Cyclin-like gene AcCyc1 and promotes prothallium expansion growth. These findings provide insight into the shade adaptation strategy of ferns and suggest that phy3/neo plays a substantial role in the survival and growth of ferns during the tiny gametophytic stage under low-light conditions, such as those on the forest floor.

Identification of microRNAs and their target genes associated with chasmogamous and cleistogamous flower development in Viola prionantha

MAIN CONCLUSION

Differentially expressed microRNAs were found associated with the development of chasmogamous and cleistogamous flowers in Viola prionantha, revealing potential roles of microRNAs in the developmental evolution of dimorphic flowers. In Viola prionantha, chasmogamous (CH) flowers are induced by short daylight, while cleistogamous (CL) flowers are triggered by long daylight. How environmental factors and microRNAs (miRNAs) affect dimorphic flower formation remains unknown. In this study, small RNA sequencing was performed on CH and CL floral buds at different developmental stages in V. prionantha, differentially expressed miRNAs (DEmiRNAs) were identified, and their target genes were predicted. In CL flowers, Viola prionantha miR393 (vpr-miR393a/b) and vpr-miRN3366 were highly expressed, while in CH flowers, vpr-miRN2005, vpr-miR172e-2, vpr-miR166m-3, vpr-miR396f-2, and vpr-miR482d-2 were highly expressed. In the auxin-activated signaling pathway, vpr-miR393a/b and vpr-miRN2005 could target Vpr-TIR1/AFB and Vpr-ARF2, respectively, and other DEmiRNAs could target genes involved in the regulation of transcription, e.g., Vpr-AP2-7. Moreover, Vpr-UFO and Vpr-YAB5, the main regulators in petal and stamen development, were co-expressed with Vpr-TIR1/AFB and Vpr-ARF2 and showed lower expression in CL flowers than in CH flowers. Some V. prionantha genes relating to the stress/defense responses were co-expressed with Vpr-TIR1/AFB, Vpr-ARF2, and Vpr-AP2-7 and highly expressed in CL flowers. Therefore, in V. prionantha, CH-CL flower development may be regulated by the identified DEmiRNAs and their target genes, thus providing the first insight into the formation of dimorphic flowers in Viola.

Nanoparticle Effects on Ice Plant Mineral Accumulation under Different Lighting Conditions and Assessment of Hazard Quotients for Human Health

Nanotechnologies can improve plant growth, protect it from pathogens, and enrich it with bioactive and mineral substances. In order to fill the lack of knowledge about the combined environmental effects of lighting and nanoparticles (NPs) on plants, this study is designed to investigate how different HPS and LED lighting combined with CuO and ZnO NPs influence the elemental composition of ice plants (Mesembryanthemum crystallinum L.). Plants were grown in hydroponic systems with LED and HPS lighting at 250 ± 5 μmol m-2 s-1 intensity, sprayed with aqueous suspensions of CuO (40 nm, 30 ppm) and ZnO (35-45 nm, 800 ppm) NPs; their elemental composition was measured using an ICP-OES spectrometer and hazard quotients were calculated. LED lighting combined with the application of ZnO NPs significantly affected Zn accumulation in plant leaves. Cu accumulation was higher when plants were treated with CuO NPs and HPS illumination combined. The calculated hazard quotients showed that the limits are not exceeded when applying our selected concentrations and growth conditions on ice plants. In conclusion, ice plants had a more significant positive effect on the accumulation of macro- and microelements under LED lighting than HPS. NPs had the strongest effect on the increase in their respective microelements.

Light-induced stomatal opening requires phosphorylation of the C-terminal autoinhibitory domain of plasma membrane H+-ATPase

Plasma membrane H+-ATPase provides the driving force for light-induced stomatal opening. However, the mechanisms underlying the regulation of its activity remain unclear. Here, we show that the phosphorylation of two Thr residues in the C-terminal autoinhibitory domain is crucial for H+-ATPase activation and stomatal opening in Arabidopsis thaliana. Using phosphoproteome analysis, we show that blue light induces the phosphorylation of Thr-881 within the C-terminal region I, in addition to penultimate Thr-948 in AUTOINHIBITED H+-ATPASE 1 (AHA1). Based on site-directed mutagenesis experiments, phosphorylation of both Thr residues is essential for H+ pumping and stomatal opening in response to blue light. Thr-948 phosphorylation is a prerequisite for Thr-881 phosphorylation by blue light. Additionally, red light-driven guard cell photosynthesis induces Thr-881 phosphorylation, possibly contributing to red light-dependent stomatal opening. Our findings provide mechanistic insights into H+-ATPase activation that exploits the ion transport across the plasma membrane and light signalling network in guard cells.

Phosphorylation of plasma membrane H+-ATPase Thr881 participates in light-induced stomatal opening

Plasma membrane (PM) H+-ATPase is crucial for light-induced stomatal opening and phosphorylation of a penultimate residue, Thr948 (pen-Thr, numbering according to Arabidopsis AHA1) is required for enzyme activation. In this study, a comprehensive phosphoproteomic analysis using guard cell protoplasts from Vicia faba shows that both red and blue light increase the phosphorylation of Thr881, of PM H+-ATPase. Light-induced stomatal opening and the blue light-induced increase in stomatal conductance are reduced in transgenic Arabidopsis plants expressing mutant AHA1-T881A in aha1-9, whereas the blue light-induced phosphorylation of pen-Thr is unaffected. Auxin and photosynthetically active radiation induce the phosphorylation of both Thr881 and pen-Thr in etiolated seedlings and leaves, respectively. The dephosphorylation of phosphorylated Thr881 and pen-Thr are mediated by type 2 C protein phosphatase clade D isoforms. Taken together, Thr881 phosphorylation, in addition of the pen-Thr phosphorylation, are important for PM H+-ATPase function during physiological responses, such as light-induced stomatal opening in Arabidopsis thaliana.

Stomatal responses to VPD utilize guard cell intracellular signaling components

Stomatal pores, vital for CO2 uptake and water loss regulation in plants, are formed by two specialized guard cells. Despite their importance, there is limited understanding of how guard cells sense and respond to changes in vapor pressure difference (VPD). This study leverages a selection of CO2 hyposensitive and abscisic acid (ABA) signaling mutants in Arabidopsis, including heterotrimeric G protein mutants and RLK (receptor-like kinase) mutants, along with a variety of canola cultivars to delve into the intracellular signaling mechanisms prompting stomatal closure in response to high VPD. Stomatal conductance response to step changes in VPD was measured using the LI-6800F gas exchange system. Our findings highlight that stomatal responses to VPD utilize intracellular signaling components. VPD hyposensitivity was particularly evident in mutants of the ht1 (HIGH LEAF TEMPERATURE1) gene, which encodes a protein kinase expressed mainly in guard cells, and in gpa1-3, a null mutant of the sole canonical heterotrimeric Gα subunit, previously implicated in stomatal signaling. Consequently, this research identifies a nexus in the intricate relationships between guard cell signal perception, stomatal conductance, environmental humidity, and CO2 levels.

Arabidopsis COP1 guides stomatal response in guard cells through pH regulation

Plants rely on precise regulation of their stomatal pores to effectively carry out photosynthesis while managing water status. The Arabidopsis CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a critical light signaling repressor, is known to repress stomatal opening, but the exact cellular mechanisms remain unknown. Here, we show that COP1 regulates stomatal movement by controlling the pH levels in guard cells. cop1-4 mutants have larger stomatal apertures and disrupted pH dynamics within guard cells, characterized by increased vacuolar and cytosolic pH and reduced apoplastic pH, leading to abnormal stomatal responses. The altered pH profiles are attributed to the increased plasma membrane (PM) H+-ATPase activity of cop1-4 mutants. Moreover, cop1-4 mutants resist to growth defect caused by alkali stress posed on roots. Overall, our study highlights the crucial role of COP1 in maintaining pH homeostasis of guard cells by regulating PM H+-ATPase activity, and demonstrates how proton movement affects stomatal movement and plant growth.

PHOTOTROPIN1 lysine 526 functions to enhance phototropism in Arabidopsis

MAIN CONCLUSION

After blue-light exposure, ubiquitination of PHOTOTROPIN1 lysine 526 enhances phototropic responses. Arabidopsis blue-light photoreceptor, PHOTOTROPIN1 (PHOT1) mediates a series of blue-light responses that function to optimize photosynthesis efficiency. Blue-light sensing through the N-terminal sensory domain activates the C-terminal kinase activity of PHOT1, resulting in autophosphorylation. In addition to phosphorylation, PHOT1 lysine residue 526 (Lys526), after blue-light exposure, was found to carry a double glycine attachment, indicative of a possible ubiquitination modification. The functionality of PHOT1 Lys526 was investigated by reverse genetic approaches. Arginine replacements of PHOT1 Lys526, together with Lys527, complemented phot1-5 phot2-1 double mutant with attenuated phototropic bending, while blue-light responses: leaf expansion and stomatal opening, were restored to wild type levels. Transgenic seedlings were not different in protein levels of phot1 Lys526 527Arg than the wild type control, suggesting the reduced phototropic responses was not caused by reduction in protein levels. Treating the transformants with proteosome inhibitor, MG132, did not restore phototropic sensitivity. Both transgenic protein and wild type PHOT1 also had similar dark recovery of kinase activity, suggesting that phot1 Lys526 527Arg replacement did not affect the protein stability to cause the phenotype. Together, our results indicate that blocking Lys526 ubiquitination by arginine substitution may have caused the reduced phototropic phenotype. Therefore, the putative ubiquitination on Lys526 functions to enhance PHOT1-mediated phototropism, rather than targeting PHOT1 for proteolysis.

Illuminating plant water dynamics: the role of light in leaf hydraulic regulation

Light intensity and quality influence photosynthesis directly but also have an indirect effect by increasing stomatal apertures and enhancing gas exchange. Consequently, in areas such as the upper canopy, a high water demand for transpiration and temperature regulation is created. This paper explores how light intensity and the natural high Blue-Light (BL) : Red-Light (RL) ratio in these areas, is important for controlling leaf hydraulic conductance (Kleaf ) by BL signal transduction, increasing water permeability in cells surrounding the vascular tissue, in supporting the enormous water demands. Conversely, shaded inner-canopy areas receive less radiation, have lower water and cooling demands, and exhibit reduced Kleaf due to diminished intensity and BL induction. Intriguingly, shaded leaves display higher water-use efficiency (compared with upper-canopy) due to decreased transpiration and cooling requirements while the presence of RL supports photosynthesis.

Phototropin2 3'UTR overlaps with the AT5G58150 gene encoding an inactive RLK kinase

BACKGROUND

This study examines the biological implications of an overlap between two sequences in the Arabidopsis genome, the 3'UTR of the PHOT2 gene and a putative AT5G58150 gene, encoded on the complementary strand. AT5G58150 is a probably inactive protein kinase that belongs to the transmembrane, leucine-rich repeat receptor-like kinase family. Phot2 is a membrane-bound UV/blue light photoreceptor kinase. Thus, both proteins share their cellular localization, on top of the proximity of their loci.

RESULTS

The extent of the overlap between 3'UTR regions of AT5G58150 and PHOT2 was found to be 66 bp, using RACE PCR. Both the at5g58150 T-DNA SALK_093781C (with insertion in the promoter region) and 35S::AT5G58150-GFP lines overexpress the AT5G58150 gene. A detailed analysis did not reveal any substantial impact of PHOT2 or AT5G58150 on their mutual expression levels in different light and osmotic stress conditions. AT5G58150 is a plasma membrane protein, with no apparent kinase activity, as tested on several potential substrates. It appears not to form homodimers and it does not interact with PHOT2. Lines that overexpress AT5G58150 exhibit a greater reduction in lateral root density due to salt and osmotic stress than wild-type plants, which suggests that AT5G58150 may participate in root elongation and formation of lateral roots. In line with this, mass spectrometry analysis identified proteins with ATPase activity, which are involved in proton transport and cell elongation, as putative interactors of AT5G58150. Membrane kinases, including other members of the LRR RLK family and BSK kinases (positive regulators of brassinosteroid signalling), can also act as partners for AT5G58150.

CONCLUSIONS

AT5G58150 is a membrane protein that does not exhibit measurable kinase activity, but is involved in signalling through interactions with other proteins. Based on the interactome and root architecture analysis, AT5G58150 may be involved in plant response to salt and osmotic stress and the formation of roots in Arabidopsis.

IGT/LAZY genes are differentially influenced by light and required for light-induced change to organ angle

BACKGROUND

Plants adjust their growth orientations primarily in response to light and gravity signals. Considering that the gravity vector is fixed and the angle of light incidence is constantly changing, plants must somehow integrate these signals to establish organ orientation, commonly referred to as gravitropic set-point angle (GSA). The IGT gene family contains known regulators of GSA, including the gene clades LAZY, DEEPER ROOTING (DRO), and TILLER ANGLE CONTROL (TAC).

RESULTS

Here, we investigated the influence of light on different aspects of GSA phenotypes in LAZY and DRO mutants, as well as the influence of known light signaling pathways on IGT gene expression. Phenotypic analysis revealed that LAZY and DRO genes are collectively required for changes in the angle of shoot branch tip and root growth in response to light. Single lazy1 mutant branch tips turn upward in the absence of light and in low light, similar to wild-type, and mimic triple and quadruple IGT mutants in constant light and high-light conditions, while triple and quadruple IGT/LAZY mutants show little to no response to changing light regimes. Further, the expression of IGT/LAZY genes is differentially influenced by daylength, circadian clock, and light signaling.

CONCLUSIONS

Collectively, the data show that differential expression of LAZY and DRO genes are required to enable plants to alter organ angles in response to light-mediated signals.

GIGANTEA Unveiled: Exploring Its Diverse Roles and Mechanisms

GIGANTEA (GI) is a conserved nuclear protein crucial for orchestrating the clock-associated feedback loop in the circadian system by integrating light input, modulating gating mechanisms, and regulating circadian clock resetting. It serves as a core component which transmits blue light signals for circadian rhythm resetting and overseeing floral initiation. Beyond circadian functions, GI influences various aspects of plant development (chlorophyll accumulation, hypocotyl elongation, stomatal opening, and anthocyanin metabolism). GI has also been implicated to play a pivotal role in response to stresses such as freezing, thermomorphogenic stresses, salinity, drought, and osmotic stresses. Positioned at the hub of complex genetic networks, GI interacts with hormonal signaling pathways like abscisic acid (ABA), gibberellin (GA), salicylic acid (SA), and brassinosteroids (BRs) at multiple regulatory levels. This intricate interplay enables GI to balance stress responses, promoting growth and flowering, and optimize plant productivity. This review delves into the multifaceted roles of GI, supported by genetic and molecular evidence, and recent insights into the dynamic interplay between flowering and stress responses, which enhance plants' adaptability to environmental challenges.

Phosphoproteome analyses pinpoint the F-box protein SLOW MOTION as a regulator of warm temperature-mediated hypocotyl growth in Arabidopsis

Hypocotyl elongation is controlled by several signals and is a major characteristic of plants growing in darkness or under warm temperature. While already several molecular mechanisms associated with this process are known, protein degradation and associated E3 ligases have hardly been studied in the context of warm temperature. In a time-course phosphoproteome analysis on Arabidopsis seedlings exposed to control or warm ambient temperature, we observed reduced levels of diverse proteins over time, which could be due to transcription, translation, and/or degradation. In addition, we observed differential phosphorylation of the LRR F-box protein SLOMO MOTION (SLOMO) at two serine residues. We demonstrate that SLOMO is a negative regulator of hypocotyl growth, also under warm temperature conditions, and protein-protein interaction studies revealed possible interactors of SLOMO, such as MKK5, DWF1, and NCED4. We identified DWF1 as a likely SLOMO substrate and a regulator of warm temperature-mediated hypocotyl growth. We propose that warm temperature-mediated regulation of SLOMO activity controls the abundance of hypocotyl growth regulators, such as DWF1, through ubiquitin-mediated degradation.

Disruption of the FMN-A524 interaction cascade and Glu513-induced collapse of the hydrophobic barrier promotes light-induced Jα-helix unfolding in AsLOV2

The C-terminal Jα-helix of the Avena sativa's Light Oxygen and Voltage (AsLOV2) protein, unfolds on exposure to blue light. This characteristic seeks relevance in applications related to engineering novel biological photoswitches. Using molecular dynamics simulations and the Markov state modeling (MSM) approach we provide the mechanism that explains the stepwise unfolding of the Jα-helix. The unfolding was resolved into seven steps represented by the structurally distinguishable states distributed over the initiation and the post initiation phases. Whereas, the initiation phase occurs due to the collapse of the interaction cascade FMN-Q513-N492-L480-W491-Q479-V520-A524, the onset of the post initiation phase is marked by breaking of the hydrophobic interactions between the Jα-helix and the Iβ-strand. This study indicates that the displacement of N492 out of the FMN binding pocket, not necessarily requiring Q513, is essential for the initiation of the Jα-helix unfolding. Rather, the structural reorientation of Q513 activates the protein to cross the hydrophobic barrier and enter the post initiation phase. Similarly, the structural deviations in N482, rather than its integral role in unfolding, could enhance the unfolding rates. Furthermore, the MSM studies on the wild-type and the Q513 mutant, provide the spatiotemporal roadmap that lay out the possible pathways of structural transition between the dark and the light states of the protein. Overall, the study provides insights useful to enhance the performance of AsLOV2-based photoswitches.

The light spectrum differentially influences morphology, physiology and metabolism of Chrysanthemum × morifolium without affecting biomass accumulation

The development of light emitting diodes (LED) gives new possibilities to use the light spectrum to manipulate plant morphology and physiology in plant production and research. Here, vegetative Chrysanthemum × morifolium were grown at a photosynthetic photon flux density of 230 μmol m-2  s-1 under monochromatic blue, cyan, green, and red, and polychromatic red:blue or white light with the objective to investigate the effect on plant morphology, gas exchange and metabolic profile. After 33 days of growth, branching and leaf number increased from blue to red light, while area per leaf, leaf weight fraction, flavonol index, and stomatal density and conductance decreased, while dry matter production was mostly unaffected. Plants grown under red light had decreased photosynthesis performance compared with blue or white light-grown plants. The primary and secondary metabolites, such as organic acids, amino acids and phenylpropanoids (measured by non-targeted metabolomics of polar metabolites), were regulated differently under the different light qualities. Specifically, the levels of reduced ascorbic acid and its oxidation products, and the total ascorbate pool, were significantly different between blue light-grown plants and plants grown under white or red:blue light, which imply photosynthesis-driven alterations in oxidative pressure under different light regimens. The overall differences in plant phenotype, inflicted by blue, red:blue or red light, are probably due to a shift in balance between regulatory pathways controlled by blue light receptors and/or phytochrome. Although morphology, physiology, and metabolism differed substantially between plants grown under different qualities of light, these changes had limited effects on biomass accumulation.

Genome-wide association study unveils ascorbate regulation by PAS/LOV PROTEIN during high light acclimation

Varying light conditions elicit metabolic responses as part of acclimation with changes in ascorbate levels being an important component. Here, we adopted a genome-wide association-based approach to characterize the response in ascorbate levels on high light (HL) acclimation in a panel of 315 Arabidopsis (Arabidopsis thaliana) accessions. These studies revealed statistically significant SNPs for total and reduced ascorbate under HL conditions at a locus in chromosome 2. Ascorbate levels under HL and the region upstream and within PAS/LOV PROTEIN (PLP) were strongly associated. Intriguingly, subcellular localization analyses revealed that the PLPA and PLPB splice variants co-localized with VITAMIN C DEFECTIVE2 (VTC2) and VTC5 in both the cytosol and nucleus. Yeast 2-hybrid and bimolecular fluorescence complementation analyses revealed that PLPA and PLPB interact with VTC2 and that blue light diminishes this interaction. Furthermore, PLPB knockout mutants were characterized by 1.5- to 1.7-fold elevations in their ascorbate levels, whereas knockout mutants of the cry2 cryptochromes displayed 1.2- to 1.3-fold elevations compared to WT. Our results collectively indicate that PLP plays a critical role in the elevation of ascorbate levels, which is a signature response of HL acclimation. The results strongly suggest that this is achieved via the release of the inhibitory effect of PLP on VTC2 upon blue light illumination, as the VTC2-PLPB interaction is stronger under darkness. The conditional importance of the cryptochrome receptors under different environmental conditions suggests a complex hierarchy underpinning the environmental control of ascorbate levels. However, the data we present here clearly demonstrate that PLP dominates during HL acclimation.

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.

Sink-source imbalance triggers delayed photosynthetic induction: Transcriptomic and physiological evidence

Sink-source imbalance causes accumulation of nonstructural carbohydrates (NSCs) and photosynthetic downregulation. However, despite numerous studies, it remains unclear whether NSC accumulation or N deficiency more directly decreases steady-state maximum photosynthesis and photosynthetic induction, as well as underlying gene expression profiles. We evaluated the relationship between photosynthetic capacity and NSC accumulation induced by cold girdling, sucrose feeding, and low nitrogen treatment in Glycine max and Phaseolus vulgaris. In G. max, changes in transcriptome profiles were further investigated, focusing on the physiological processes of photosynthesis and NSC accumulation. NSC accumulation decreased the maximum photosynthetic capacity and delayed photosynthetic induction in both species. In G. max, such photosynthetic downregulation was explained by coordinated downregulation of photosynthetic genes involved in the Calvin cycle, Rubisco activase, photochemical reactions, and stomatal opening. Furthermore, sink-source imbalance may have triggered a change in the balance of sugar-phosphate translocators in chloroplast membranes, which may have promoted starch accumulation in chloroplasts. Our findings provide an overall picture of photosynthetic downregulation and NSC accumulation in G. max, demonstrating that photosynthetic downregulation is triggered by NSC accumulation and cannot be explained solely by N deficiency.

Cell type-specific proteomics uncovers a RAF15-SnRK2.6/OST1 kinase cascade in guard cells

Multicellular organisms such as plants contain various cell types with specialized functions. Analyzing the characteristics of each cell type reveals specific cell functions and enhances our understanding of organization and function at the organismal level. Guard cells (GCs) are specialized epidermal cells that regulate the movement of the stomata and gaseous exchange, and provide a model genetic system for analyzing cell fate, signaling, and function. Several proteomics analyses of GC are available, but these are limited in depth. Here we used enzymatic isolation and flow cytometry to enrich GC and mesophyll cell protoplasts and perform in-depth proteomics in these two major cell types in Arabidopsis leaves. We identified approximately 3,000 proteins not previously found in the GC proteome and more than 600 proteins that may be specific to GC. The depth of our proteomics enabled us to uncover a guard cell-specific kinase cascade whereby Raf15 and Snf1-related kinase2.6 (SnRK2.6)/OST1(open stomata 1) mediate abscisic acid (ABA)-induced stomatal closure. RAF15 directly phosphorylated SnRK2.6/OST1 at the conserved Ser175 residue in its activation loop and was sufficient to reactivate the inactive form of SnRK2.6/OST1. ABA-triggered SnRK2.6/OST1 activation and stomatal closure was impaired in raf15 mutants. We also showed enrichment of enzymes and flavone metabolism in GC, and consistent, dramatic accumulation of flavone metabolites. Our study answers the long-standing question of how ABA activates SnRK2.6/OST1 in GCs and represents a resource potentially providing further insights into the molecular basis of GC and mesophyll cell development, metabolism, structure, and function.

An Update on Toll-like Receptor 2, Its Function and Dimerization in Pro- and Anti-Inflammatory Processes

While a certain level of inflammation is critical for humans to survive infection and injury, a prolonged inflammatory response can have fatal consequences. Pattern recognition Toll-like receptors (TLRs) are key players in the initiation of an inflammatory process. TLR2 is one of the most studied pattern recognition receptors (PRRs) and is known to form heterodimers with either TLR1, TLR4, TLR6, and TLR10, allowing it to recognize a wide range of pathogens. Although a large number of studies have been conducted over the past decades, there are still many unanswered questions regarding TLR2 mechanisms in health and disease. In this review, we provide an up-to-date overview of TLR2, including its homo- and heterodimers. Furthermore, we will discuss the pro- and anti-inflammatory properties of TLR2 and recent findings in prominent TLR2-associated infectious and neurodegenerative diseases.

Unveiling the dark side of guard cell metabolism

Evidence suggests that guard cells have higher rate of phosphoenolpyruvate carboxylase (PEPc)-mediated dark CO2 assimilation than mesophyll cells. However, it is unknown which metabolic pathways are activated following dark CO2 assimilation in guard cells. Furthermore, it remains unclear how the metabolic fluxes throughout the tricarboxylic acid (TCA) cycle and associated pathways are regulated in illuminated guard cells. Here we carried out a13C-HCO3 labelling experiment in tobacco guard cells harvested under continuous dark or during the dark-to-light transition to elucidate principles of metabolic dynamics downstream of CO2 assimilation. Most metabolic changes were similar between dark-exposed and illuminated guard cells. However, illumination altered the metabolic network structure of guard cells and increased the 13C-enrichment in sugars and metabolites associated to the TCA cycle. Sucrose was labelled in the dark, but light exposure increased the 13C-labelling and leads to more drastic reductions in the content of this metabolite. Fumarate was strongly labelled under both dark and light conditions, while illumination increased the 13C-enrichment in pyruvate, succinate and glutamate. Only one 13C was incorporated into malate and citrate in either dark or light conditions. Our results indicate that several metabolic pathways are redirected following PEPc-mediated CO2 assimilation in the dark, including gluconeogenesis and the TCA cycle. We further showed that the PEPc-mediated CO2 assimilation provides carbons for gluconeogenesis, the TCA cycle and glutamate synthesis and that previously stored malate and citrate are used to underpin the specific metabolic requirements of illuminated guard cells.

Shift in the Light Quality of Night Interruption Affects Flowering and Morphogenesis of Petunia hybrida

Petunia hybrida Hort. "Easy Wave Pink", a qualitative long-day plant (LDP), was investigated to study the effects of the night interruption light (NIL) provided by light-emitting diodes (LEDs) quality shifting on the morphogenesis, blooming, and transcription of photoreceptor genes. Plants were grown in a closed-type plant factory employing white (W) LEDs at an intensity of 180 μmol·m-2·s-1 PPFD provided for short day (SD, 10 h light, 14 h dark), long day (LD, 16 h light, 8 h dark), or SD with 4 h night interruption (NI) with LEDs at an intensity of 10 μmol·m-2·s-1 PPFD. The NIL quality was shifted from one light spectrum to another after the first 2 h of NI. Light treatments consisting of all possible pairings of W, far-red (Fr), red (R), and blue (B) light were tested. The SD and LD were referenced as the control, while 12 NI treatments involved altering LED NIL qualities, as follows: from R to B (NI-RB), from B to R (NI-BR), from Fr to R (NI-FrR), from R to Fr (NI-RFr), from Fr to B (NI-FrB), from B to Fr (NI-BFr), from B to W (NI-BW), from W to B (NI-WB), from W to Fr (NI-WFr), from Fr to W (NI-FrW), from W to R (NI-WR), and from R to W (NI-RW). The NI-RFr resulted in the longest shoots, while the NI-WR and NI-RW resulted in the shortest shoots. NI-WR, NI-RW, NI-BW, NI-WB, NI-RFr, NI-RB, NI-BR, and LD all exhibited flowering. High-level expressions of photoreceptor genes were confirmed in the NI-RFr, NI-FrR, NI-BFr, NI-RW, and NI-WR treatments. Morphogenesis and blooming were both impacted by the photoperiod. The first NIL had no effects on the flowering or the morphogenesis, but the second NIL had a profound impact on both.

The Role of Blue and Red Light in the Orchestration of Secondary Metabolites, Nutrient Transport and Plant Quality

Light is a fundamental environmental parameter for plant growth and development because it provides an energy source for carbon fixation during photosynthesis and regulates many other physiological processes through its signaling. In indoor horticultural cultivation systems, sole-source light-emitting diodes (LEDs) have shown great potential for optimizing growth and producing high-quality products. Light is also a regulator of flowering, acting on phytochromes and inducing or inhibiting photoperiodic plants. Plants respond to light quality through several light receptors that can absorb light at different wavelengths. This review summarizes recent progress in our understanding of the role of blue and red light in the modulation of important plant quality traits, nutrient absorption and assimilation, as well as secondary metabolites, and includes the dynamic signaling networks that are orchestrated by blue and red wavelengths with a focus on transcriptional and metabolic reprogramming, plant productivity, and the nutritional quality of products. Moreover, it highlights future lines of research that should increase our knowledge to develop tailored light recipes to shape the plant characteristics and the nutritional and nutraceutical value of horticultural products.

Identification and improvement of isothiocyanate-based inhibitors on stomatal opening to act as drought tolerance-conferring agrochemicals

Stomatal pores in the plant epidermis open and close to regulate gas exchange between leaves and the atmosphere. Upon light stimulation, the plasma membrane (PM) H⁺-ATPase is phosphorylated and activated via an intracellular signal transduction pathway in stomatal guard cells, providing a primary driving force for the opening movement. To uncover and manipulate this stomatal opening pathway, we screened a chemical library and identified benzyl isothiocyanate (BITC), a Brassicales-specific metabolite, as a potent stomatal-opening inhibitor that suppresses PM H⁺-ATPase phosphorylation. We further developed BITC derivatives with multiple isothiocyanate groups (multi-ITCs), which demonstrate inhibitory activity on stomatal opening up to 66 times stronger, as well as a longer duration of the effect and negligible toxicity. The multi-ITC treatment inhibits plant leaf wilting in both short (1.5 h) and long-term (24 h) periods. Our research elucidates the biological function of BITC and its use as an agrochemical that confers drought tolerance on plants by suppressing stomatal opening.

Small RNA-mediated silencing of phototropin suppresses the induction of photoprotection in the green alga Chlamydomonas reinhardtii

Small RNAs (sRNAs) form complexes with Argonaute proteins and bind to transcripts with complementary sequences to repress gene expression. sRNA-mediated regulation is conserved in a diverse range of eukaryotes and is involved in the control of various physiological functions. sRNAs are present in the unicellular green alga Chlamydomonas reinhardtii, and genetic analyses revealed that the core sRNA biogenesis and action mechanisms are conserved with those of multicellular organisms. However, the roles of sRNAs in this organism remain largely unknown. Here, we report that Chlamydomonas sRNAs contribute to the induction of photoprotection. In this alga, photoprotection is mediated by LIGHT HARVESTING COMPLEX STRESS-RELATED 3 (LHCSR3), whose expression is induced by light signals through the blue-light receptor phototropin (PHOT). We demonstrate here that sRNA-defective mutants showed increased PHOT abundance leading to greater LHCSR3 expression. Disruption of the precursor for two sRNAs predicted to bind to the PHOT transcript also increased PHOT accumulation and LHCSR3 expression. The induction of LHCSR3 in the mutants was enhanced by light containing blue wavelengths, but not by red light, indicating that the sRNAs regulate the degree of photoprotection via regulation of PHOT expression. Our results suggest that sRNAs are involved not only in the regulation of photoprotection but also in biological phenomena regulated by PHOT signaling.

Moonlight Is Perceived as a Signal Promoting Genome Reorganization, Changes in Protein and Metabolite Profiles and Plant Growth

Rhythmic exposure to moonlight has been shown to affect animal behavior, but its effects on plants, often observed in lunar agriculture, have been doubted and often regarded as myth. Consequently, lunar farming practices are not well scientifically supported, and the influence of this conspicuous environmental factor, the moon, on plant cell biology has hardly been investigated. We studied the effect of full moonlight (FML) on plant cell biology and examined changes in genome organization, protein and primary metabolite profiles in tobacco and mustard plants and the effect of FML on the post-germination growth of mustard seedlings. Exposure to FML was accompanied by a significant increase in nuclear size, changes in DNA methylation and cleavage of the histone H3 C-terminal region. Primary metabolites associated with stress were significantly increased along with the expression of stress-associated proteins and the photoreceptors phytochrome B and phototropin 2; new moon experiments disproved the light pollution effect. Exposure of mustard seedlings to FML enhanced growth. Thus, our data show that despite the low-intensity light emitted by the moon, it is an important environmental factor perceived by plants as a signal, leading to alteration in cellular activities and enhancement of plant growth.

Light-induced stomatal opening in Arabidopsis is negatively regulated by chloroplast-originated OPDA signaling

Stomatal movement is orchestrated by diverse signaling cascades and metabolic activities in guard cells. Light triggers the opening of the pores through the phototropin-mediated pathway, which leads to the activation of plasma membrane H⁺-ATPase and thereby facilitates potassium accumulation through K_in⁺ channels. However, it remains poorly understood how phototropin signaling is fine-tuned to prevent excessive stomatal opening and consequent water loss. Here, we show that the stomatal response to light is negatively regulated by 12-oxo-phytodienoic acid (OPDA), an oxylipin metabolite produced through enzymatic oxygenation of polyunsaturated fatty acids (PUFAs). We identify a set of phospholipase-encoding genes, phospholipase (PLIP)1/2/3, which are transactivated rapidly in guard cells upon illumination in a phototropin-dependent manner. These phospholipases release PUFAs from the chloroplast membrane, which is oxidized by guard-cell lipoxygenases and further metabolized to OPDA. The OPDA-deficient mutants had wider stomatal pores, whereas mutants containing elevated levels of OPDA showed the opposite effect on stomatal aperture. Transmembrane solute fluxes that drive stomatal aperture were enhanced in lox6-1 guard cells, indicating that OPDA signaling ultimately impacts on activities of proton pumps and K_in⁺ channels. Interestingly, the accelerated stomatal kinetics in lox6-1 leads to increased plant growth without cost in water or macronutrient use. Together, our results reveal a new role for chloroplast membrane oxylipin metabolism in stomatal regulation. Moreover, the accelerated stomatal opening kinetics in OPDA-deficient mutants benefits plant growth and water use efficiency.

Discovery of 2,6-Dihalopurines as Stomata Opening Inhibitors: Implication of an LRX-Mediated H+-ATPase Phosphorylation Pathway

Stomata are pores in the leaf epidermis of plants and their opening and closing regulate gas exchange and water transpiration. Stomatal movements play key roles in both plant growth and stress responses. In recent years, small molecules regulating stomatal movements have been used as a powerful tool in mechanistic studies, as well as key players for agricultural applications. Therefore, the development of new molecules regulating stomatal movement and the elucidation of their mechanisms have attracted much attention. We herein describe the discovery of 2,6-dihalopurines, AUs, as a new stomatal opening inhibitor, and their mechanistic study. Based on biological assays, AUs may involve in the pathway related with plasma membrane H⁺-ATPase phosphorylation. In addition, we identified leucine-rich repeat extensin proteins (LRXs), LRX3, LRX4 and LRX5 as well as RALF, as target protein candidates of AUs by affinity based pull down assay and molecular dynamics simulation. The mechanism of stomatal movement related with the LRXs-RALF is an unexplored pathway, and therefore further studies may lead to the discovery of new signaling pathways and regulatory factors in the stomatal movement.