Activation of the Stt7/STN7 Kinase through Dynamic Interactions with the Cytochrome b6f Complex

Photosynthetic organisms have the ability to adapt to changes in light quality by readjusting the cross sections of the light-harvesting systems of photosystem II (PSII) and photosystem I (PSI). This process, called state transitions, maintains the redox poise of the photosynthetic electron transfer chain and ensures a high photosynthetic yield when light is limiting. It is mediated by the Stt7/STN7 protein kinase, which is activated through the cytochrome b₆f complex upon reduction of the plastoquinone pool. Its probable major substrate, the light-harvesting complex of PSII, once phosphorylated, dissociates from PSII and docks to PSI, thereby restoring the balance of absorbed light excitation energy between the two photosystems. Although the kinase is known to be inactivated under high-light intensities, the molecular mechanisms governing its regulation remain unknown. In this study we monitored the redox state of a conserved and essential Cys pair of the Stt7/STN7 kinase and show that it forms a disulfide bridge. We could not detect any change in the redox state of these Cys during state transitions and high-light treatment. It is only after prolonged anaerobiosis that this disulfide bridge is reduced. It is likely to be mainly intramolecular, although kinase activation may involve a transient covalently linked kinase dimer with two intermolecular disulfide bonds. Using the yeast two-hybrid system, we have mapped one interaction site of the kinase on the Rieske protein of the cytochrome b₆f complex.Photosynthetic organisms are subjected to constant changes in light quality and quantity and need to adapt to these changes in order to optimize, on the one hand, their photosynthetic yield, and to minimize photo-oxidative damage on the other. The photosynthetic electron transfer chain consists of photosystem II (PSII), the plastoquinone (PQ) pool, the cytochrome b₆f complex (Cyt b₆f), plastocyanin, and photosystem I (PSI). All of these complexes and components are integrated or closely associated with the thylakoid membrane. The two antenna systems of PSII and PSI capture and direct the light excitation energy to the corresponding reaction centers in which a chlorophyll dimer is oxidized and charge separation occurs across the thylakoid membrane. These processes lead to the onset of electron flow from water on the donor side of PSII to ferredoxin on the acceptor side of PSI coupled with proton translocation across the thylakoid membrane. In order to sustain optimal electron flow along this electron transfer chain, the redox poise needs to be maintained under changing environmental conditions. Several mechanisms have evolved for the maintenance of this redox balance. In the case of over-reduction of the acceptor side of PSI, excess electrons can reduce molecular oxygen through the Mehler reaction to superoxide, which is then converted to hydrogen peroxide by a plastid superoxide dismutase and ultimately to water by a peroxidase (Asada, 2000). Over-reduction of the PQ pool can be alleviated by PTOX, the plastid terminal oxidase responsible for oxidizing PQH₂ to form hydrogen peroxide, which is subsequently converted to water (Carol et al., 1999; Cournac et al., 2000; Wu et al., 1999).In addition to these electron sinks that prevent the over-reduction of the electron transfer chain, the photosynthetic apparatus is able to maintain the redox poise of the PQ pool by readjusting the relative cross sections of the light harvesting systems of PSII and PSI upon unequal excitation of the two photosystems. This readjustment can occur both in the short term through state transitions and in the long term by changing the stoichiometry between PSII and PSI (Bonaventura and Myers, 1969; Murata, 1969; Pfannschmidt, 2003). State transitions occur because of perturbations of the redox state of the PQ pool due to unequal excitation of PSII and PSI, limitations in electron acceptors downstream of PSI, and/or in CO₂ availability. Excess excitation of PSII relative to PSI leads to reduction of the PQ pool and thus favors the docking of PQH₂ to the Qo site of the Cyt b₆f complex. This process activates the Stt7/STN7 protein kinase (Vener et al., 1997; Zito et al., 1999), which is closely associated with this complex and leads to the phosphorylation of some LHCII proteins and to their detachment from PSII and binding to PSI (Depège et al., 2003; Lemeille et al., 2009). Although both Lhcb1 and Lhcb2 are phosphorylated, only the phosphorylated form of Lhcb2 is associated with PSI whereas phosphorylated Lhcb1 is excluded from this complex (Longoni et al., 2015). This state corresponds to state 2. In this way the change in the relative antenna sizes of the two photosystems restores the redox poise of the PQ pool. The process is reversible as over-excitation of PSI relative to PSII leads to the oxidation of the PQ pool and to the inactivation of the kinase. Under these conditions, phosphorylated LHCII associated with PSI is dephosphorylated by the PPH1/TAP38 phosphatase (Pribil et al., 2010; Shapiguzov et al., 2010) and returns to PSII (state 1). It should be noted, however, that a strict causal link between LHCII phosphorylation and its migration from PSII to PSI has been questioned recently by the finding that some phosphorylated LHCII remains associated with PSII supercomplexes and that LHCII serves as antenna for both photosystems under most natural light conditions (Drop et al., 2014; Wientjes et al., 2013).State transitions are important at low light but do not occur under high light because the LHCII kinase is inactivated under these conditions (Schuster et al., 1986). It was proposed that inactivation of the kinase is mediated by the ferredoxin-thioredoxin system and that a disulfide bond in the kinase rather than in the substrate may be the target site of thioredoxin (Rintamäki et al., 1997, 2000). Analysis of the Stt7/STN7 protein sequences indeed reveals the presence of two conserved Cys residues close to the N-terminal end of this kinase, which are conserved in all species examined and both are essential for kinase activity although they are located outside of the kinase catalytic domain (Fig. 1) (Depège et al., 2003; Lemeille et al., 2009). Based on protease protection studies, this model of the Stt7/STN7 kinase proposes that the N-terminal end of the kinase is on the lumen side of the thylakoid membrane separated from the catalytic domain on the stromal side by an unusual transmembrane domain containing several Pro residues (Lemeille et al., 2009). This configuration of the kinase allows its catalytic domain to act on the substrate sites of the LHCII proteins, which are exposed to the stroma. Although in this model the conserved Cys residues in the lumen are on the opposite side from the stromal thioredoxins, it is possible that thiol-reducing equivalents are transferred across the thylakoid membrane through the CcdA and Hcf164 proteins, which have been shown to operate in this way during heme and Cyt b₆f assembly (Lennartz et al., 2001; Page et al., 2004) or through the LTO1 protein (Du et al., 2015; Karamoko et al., 2011).Figure 1.Conserved Cys in the Stt7/STN7 kinase. Alignment of the sequences of the Stt7/STN protein kinase from Selaginella moelendorffii (Sm), Physcomitrella patens (Pp), Oryza sativa (Os), Populus trichocarpa (Pt), Arabidopsis thaliana (At), Chlamydomonas reinhardtii ...Here we have examined the redox state of the Stt7/STN7 kinase during state transitions and after illumination with high light to test the proposed model. We find that the Stt7/STN7 kinase contains a disulfide bridge that appears to be intramolecular and maintained not only during state transitions but also in high light when the kinase is inactive. Although these results suggest at first sight that the disulfide bridge of Stt7/STN7 is maintained during its activation and inactivation, we propose that a transient opening of this bridge occurs during the activation process followed by the formation of an intermolecular disulfide bridge and the appearance of a short-lived, covalently linked kinase dimer.     

Rethinking Guard Cell Metabolism

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere and, therefore, play a pivotal role in regulating CO₂ uptake for photosynthesis as well as water loss through transpiration. Guard cells, which flank the stomata, undergo adjustments in volume, resulting in changes in pore aperture. Stomatal opening is mediated by the complex regulation of ion transport and solute biosynthesis. Ion transport is exceptionally well understood, whereas our knowledge of guard cell metabolism remains limited, despite several decades of research. In this review, we evaluate the current literature on metabolism in guard cells, particularly the roles of starch, sucrose, and malate. We explore the possible origins of sucrose, including guard cell photosynthesis, and discuss new evidence that points to multiple processes and plasticity in guard cell metabolism that enable these cells to function effectively to maintain optimal stomatal aperture. We also discuss the new tools, techniques, and approaches available for further exploring and potentially manipulating guard cell metabolism to improve plant water use and productivity.Stomata are microscopic, adjustable pores on the leaf surface. The evolution of stomata more than 400 million years ago (Edwards et al., 1986, 1992, 1998) helped facilitate the adaptation of plants to a terrestrial environment, where water is typically a limiting resource. Each stoma is composed of two kidney- or dumbbell-shaped guard cells, whose volume changes to adjust pore aperture, allowing plants to simultaneously regulate CO₂ uptake and water loss. This facilitation of gas exchange by stomatal opening is one of the most essential processes in plant photosynthesis and transpiration, affecting plant water use efficiency and agricultural crop yields (Lawson and Blatt, 2014).Plant physiologists have a long history of investigating the behavior of these fascinating structures, reaching back more than a century to the pioneering work of Sir Francis Darwin (Darwin, 1916) and the American botanist Francis Ernest Lloyd (Lloyd, 1908). Major contributions to stomatal research arose from inventing and improving equipment and methods for quantitatively measuring the effects of environmental factors on stomatal pore aperture. After Darwin’s work, it became clear that the stomatal aperture actively responds to changes in the environment and regulates leaf transpiration rates (Meidner, 1987). Over the past century, much has been learned about their structure, development, and physiology.Despite the anatomical simplicity of the stomatal valve, the surrounding guard cells are highly specialized. Guard cells are morphologically distinct from general epidermal cells and possess complex signal transduction networks, elevated membrane ion transport capacity, and modified metabolic pathways. These features allow rapid modulations in guard cell turgor in response to endogenous and environmental signals, promoting the opening and closure of the stomatal pore in time scales of seconds to hours (Assmann and Wang, 2001). A variety of osmotically active solutes contribute to the buildup of stomatal turgor. Potassium (K⁺) and chloride (Cl⁻) act as the main inorganic ions, and malate²⁻ and sucrose (Suc) function as the main organic solutes. Whereas K⁺ and Cl⁻ are taken up from the apoplast, Suc and malate²⁻ can be imported or synthesized internally using carbon skeletons deriving from starch degradation and/or CO₂ fixation in the guard cell chloroplast (Roelfsema and Hedrich, 2005; Vavasseur and Raghavendra, 2005; Lawson, 2009; Kollist et al., 2014). The accumulation of these osmotica lowers the water potential, promoting the inflow of water, the swelling of guard cells, and the opening of the stomatal pore. Most of the ions taken up, or synthesized by guard cells, are sequestered into the vacuole (Barbier-Brygoo et al., 2011). As a result, the guard cell vacuoles undergo dynamic changes in volume and structure, which are crucial for achieving the full amplitude of stomatal movements (Gao et al., 2005; Tanaka et al., 2007; Andrés et al., 2014). During stomatal closure, guard cells reduce their volume through the release of ions into the cell wall and the consequent efflux of water.The transport of osmolytes across the plasma and tonoplast guard cell membranes is energized by H⁺-ATPase activity, which generates a proton motive force by translocating H⁺ ions against their concentration gradient (Blatt, 1987a, 1987b; Thiel et al., 1992; Roelfsema and Hedrich, 2005; Gaxiola et al., 2007). After the pioneering work of Fischer demonstrated the importance of K⁺ uptake in stomatal opening (Fischer, 1968; Fischer and Hsiao, 1968), K⁺ transport became of central interest and has long been considered the essence of stomatal movement regulation. The development of the voltage clamp technique, along with the relative easy acquisition of knockout mutants and transgenics in the model plant Arabidopsis (Arabidopsis thaliana), helped to uncover the precise mechanism and function of K⁺ fluxes in guard cells. It is well established that changes in membrane potential in response to several stimuli (e.g. light/darkness, CO₂, and abscisic acid [ABA]) alter the direction of K⁺ transport (Thiel et al., 1992; Blatt, 2000; Roelfsema et al., 2001, 2002, 2004). During stomatal opening, the activation of the proton pump generates a sufficiently negative electric potential to cause the uptake of K⁺ through the inward-rectifying K⁺ channels (K⁺_in; Fig. 1). During stomatal closure, K⁺ outflow from outward-rectifying K⁺ channels (K⁺_out) results from membrane depolarization (Fig. 2; Blatt, 1988; Schroeder, 1988; Anderson et al., 1992; Sentenac et al., 1992). Besides being gated by opposing changes in voltage, the activation of (K⁺_out) channels is dependent on the extracellular K⁺ concentration, while that of K⁺_in is not (Blatt, 1988, 1992; Roelfsema and Prins, 1997; Dreyer and Blatt, 2009). There is also strong evidence for H⁺-coupled K⁺ symport in guard cells, which could account for up to 50% of total K⁺ uptake during stomatal opening (Blatt and Clint, 1989; Clint and Blatt, 1989; Hills et al., 2012). At least for K⁺_in, the loss of a single-channel gene in Arabidopsis has little or no impact on stomatal movement (Szyroki et al., 2001), showing the redundancy among the different K⁺_in isoforms and of K⁺ transport in general.Open in a separate windowFigure 1.Integration of guard cell carbohydrate metabolism with membrane ion transport during stomatal opening. Sugars in guard cells can be imported from the apoplast, derive from starch breakdown, or be synthesized in the Calvin cycle. These sugars then can be stored as osmotically active solutes in the vacuole or metabolized in the cytosol to yield energy, reducing equivalents, and phosphoenolpyruvate (PEP). PEP can be further metabolized to pyruvate in the mitochondrial tricarboxylic acid (CAC) cycle or used as carbon skeletons for the biosynthesis of malate via PEP carboxylase (PEPC) and NAD-dependent malate dehydrogenase (NAD-MDH). Malate (which also can be imported from the apoplast) and the inorganic ions K⁺ and Cl⁻ accumulate in the vacuole, lowering the guard cell osmotic potential, thereby promoting stomatal opening. ABCB14, ATP-binding cassette transporter B14; AcetylCoA, acetyl-CoA; ALMT, aluminum-activated malate transporter; ATP-PFK, ATP-dependent phosphofructokinase; AttDT, dicarboxylate transporter; cINV, cytosolic invertase; cwINV, cell wall invertase; Fru6P, Fru-6-P; Fru1,6P₂, fructose 1,6-bisphosphate; Gl6P, Glc-6-P; G3P, glyceraldehyde 3-phosphate; iPGAM, phosphoglycerate mutase isoforms; NRGA1, negative regulator of guard cell ABA signaling1; OAA, oxaloacetate; 2-PGA, 2-phosphoglycerate; 3-PGA, 3-phosphoglycerate; PPi-PFK, PPi-dependent Fru-6-P phosphotransferase; STP, monosaccharide/H⁺ cotransporter; SUC, Suc/H⁺ cotransporter; SuSy, Suc synthase; TPT, triose phosphate/phosphate translocator. Compartments are not to scale. The dotted line indicates multiple metabolic steps.Open in a separate windowFigure 2.Proposed pathways of osmolyte dissipation during stomatal closure. While the removal of Cl⁻ and K⁺ is well described in the literature, the fate of Suc and malate during stomatal closure is unclear. Suc can be cleaved by cytosolic invertase (cINV), and the resulting hexoses can be imported into the chloroplast in the form of Glc-6-P (Glc6P). Glc6P is used subsequently for starch biosynthesis. Malate can be removed from the cell via decarboxylation to pyruvate by malic enzyme (ME) and the subsequent complete oxidation in the mitochondrial tricarboxylic acid (CAC) cycle. Alternatively, malate can be converted to PEP via NAD⁺-dependent malate dehydrogenase (NAD-MDH) and PEP carboxykinase (PEPCK). Gluconeogenic conversion of PEP to Glc6P establishes a possible link between malate removal and starch synthesis. Compartments are not to scale. PEP, Phosphoenolpyruvate; OAA; oxaloacetate; STP, monosaccharide/H⁺ cotransporter; SUC, Suc/H⁺ cotransporter; SuSy, Suc synthase; cINV, cytosolic invertase; NRGA1, negative regulator of guard cell ABA signaling1; ALMT, aluminum-activated malate transporter; GPT, Glc-6-P/Pi translocator; cwINV, cell wall invertase; HK, hexokinase; QUAC1, quickly activating anion channel1.Despite the undisputed importance of K⁺ uptake in stomatal opening, the accumulation of K⁺ ions alone cannot account for the increase in osmotic pressure necessary to explain stomatal aperture. Studies from the 1980s by MacRobbie and Fischer demonstrated that Vicia faba guard cells take up approximately 2 pmol of K⁺ during stomatal opening. Assuming that K⁺ uptake is balanced by the accumulation of similar amounts of counter ions (Cl⁻ and/or malate²⁻), the expected increase in stomatal turgor to approximately 3 MPa is less than the 4.5 MPa expected for fully open stomata (Fischer, 1972; MacRobbie and Lettau, 1980a, 1980b; Chen et al., 2012). The realization that other solutes must accumulate in addition to K⁺ salts was one of the major paradigm shifts in stomatal physiology research in the last decades, equal to the discovery of ion channels. Suc was put forward as the most likely candidate for the additional osmoticum to support stomatal opening (MacRobbie, 1987; Tallman and Zeiger, 1988; Talbott and Zeiger, 1993, 1998). Nonetheless, this research area subsequently failed to attract notice commensurate with its importance.In the last few years, the metabolism of starch, sugars and, organic acids in guard cells has seen a rebirth, making this the perfect time to review the developments in this field. In this review, we focus on photosynthetic carbon assimilation and respiratory metabolism in guard cells and provide a historical overview of the subject that highlights the most up-to-date and novel discoveries in guard cell research. We describe the various metabolic pathways separately, but as metabolism is an integrated network, we also discuss their reciprocal and beneficial interactions. Finally, we highlight their connection with the metabolism in the subjacent mesophyll cells and how they integrate with guard cell signal transduction networks and membrane ion transport to regulate stomatal movements. The enzymes and transporters discussed in this review are listed in

Prevalence of Intestinal Protozoans among Schoolchildren in Suburban Areas near Yangon,Myanmar

Although intestinal protozoans are common etiologies of diarrhea, few studies have been conducted in Myanmar. This study planned to investigate the prevalence of Giardia lamblia, Entamoeba coli, Entamoeba histolytica, and Endolimax nana among schoolchildren and their guardians in suburban areas near Yangon, Myanmar. We performed a cross-sectional survey among schoolchildren and their guardians from 7 primary schools in South Dagon and Hlaing Thar Yar districts, Yangon, Myanmar. Stool samples were observed with a microscope after concentration technique and iodine staining. Total 821 stool samples, including 556 from schoolchildren and 265 from guardians, were examined. The median age was 6 years old for schoolchildren and 36 years old for guardians. A 53.1% of the school children and 14.6 % of the guardians were males. The overall prevalence of each intestinal protozoan species was as follows: 3.4% (28/821) for G. lamblia; 3.5% (29/821) for E. coli; 1.2% (10/821) for E. histoytica, and 3.0% for E. nana. This study showed that intestinal protozoans are common in primary schoolchildren and their guardians in suburban areas near Yangon, Myanmar. Health interventions, such as hand washing education, improvement of sanitation, and establishment of water purification systems are urgently needed in this area.     

Biogeography of ‘Cyprinella lutrensis’: intensive genetic sampling from the Pecos River ‘melting pot’ reveals a dynamic history and phylogenetic complexity

Thorough sampling is necessary to delineate lineage diversity for polytypic ‘species’ such as Cyprinella lutrensis. We conducted extensive mtDNA sampling (cytochrome b and ND4) from the Pecos River, Rio Grande, and South Canadian River, New Mexico. Our study focussed on the Pecos River due to its complex geological history and potential to harbour multiple lineages. We used geometric–morphometric, morphometric, and meristic analyses to test for phenotypic divergence and combined nucDNA with mtDNA to test for cytonuclear disequilibrium and combined our sequences with published data to conduct a phylogenetic re‐assessment of the entire C. lutrensis clade. We detected five co‐occurring mtDNA lineages in the Pecos River, but no evidence for cytonuclear disequilibrium or phenotypic divergence. Recognized species were interspersed amongst divergent lineages of ‘C. lutrensis’. Allopatric divergence among drainages isolated in the Late Miocene and Pliocene apparently produced several recognized species and major divisions within ‘C. lutrensis’. Pleistocene re‐expansion and subsequent re‐fragmentation of a centralized lineage founded younger, divergent lineages throughout the Rio Grande basin and Edwards Plateau. There is also evidence of recent introductions to the Rio Grande, Pecos and South Canadian Rivers. Nonetheless, deeply divergent lineages have coexisted since the Pleistocene.     

Role of Autophagy in Capsaicin-Induced Apoptosis in U251 Glioma Cells

In recent years, the role of capsaicin in cancer prevention and treatment has gained people’s attention. However, the mechanism of anti-glioma cells by capsaicin has not been elucidated. Here, we discuss the mechanism of capsaicin in U251 cells. Cell viability was detected by MTT and extracellular LDH measurements, while immunofluorescence was performed to measure changes of LC3 in U251 cells. The expressions of LC3II, Puma-α, Beclin1, P62, Procaspase-3, and P53 were observed by immunoblotting. The cell viability decreased and the punctate patterns of LC3 in U251 cells were observed after Capsaicin treatment. Meanwhile, the expressions of Beclin1, P62, and Puma-α increased. After using 3-MA, the expressions of Beclin1 and Procaspase-3 were reduced while those of P53 and Puma-α increased. The expression of LC3II was increased after Pifithrin-α treatment. Therefore, we believed that capsaicin could induce apoptosis in U251 cells, and the inhibition of autophagy could contribute to apoptosis.     

Off‐road vehicles affect nesting behaviour and reproductive success of American Oystercatchers Haematopus palliatus

As human populations and associated development increase, interactions between humans and wildlife are occurring with greater frequency. The effects of these interactions, particularly on species whose populations are declining, are of great interest to ecologists, conservationists, land managers and natural resource policy‐makers. The American Oystercatcher Haematopus palliatus, a species of conservation concern in the USA, nests on coastal beaches subject to various forms of anthropogenic disturbance, including aircraft overflights, off‐road vehicles and pedestrians. This study assessed the effects of these human disturbances on the incubation behaviour and reproductive success of nesting American Oystercatchers at Cape Lookout National Seashore, on the Atlantic coast of the USA. We expanded on‐going monitoring of Oystercatchers at Cape Lookout National Seashore by supplementing periodic visual observations with continuous 24‐h video and audio recording at nests. Aircraft overflights were not associated with changes in Oystercatcher incubation behaviour, and we found no evidence that aircraft overflights influenced Oystercatcher reproductive success. However, Oystercatchers were on their nests significantly less often during off‐road vehicle and pedestrian events than they were during control periods before the events, and an increase in the number of off‐road vehicles passing a nest during incubation was consistently associated with significant reductions in daily nest survival (6% decrease in daily nest survival for a one‐vehicle increase in the average number of vehicles passing a nest each day; odds ratio = 0.94; 95% confidence interval (CI) 0.90, 0.98) and hatching success (12% decrease in hatching success for a one‐vehicle increase in the average number of vehicles passing a nest each day; odds ratio = 0.88; 95% CI 0.76, 0.97). Management of vehicles and pedestrians in areas of Oystercatcher breeding is important for the conservation of American Oystercatchers.