Light-dependent pH changes in leaves of C3 plants : I. Recording pH changes in various cellular compartments by fluorescent probes

The interplay of post-translational protein modifications in Arabidopsis leaves during photosynthesis induction

Diurnal dark to light transition causes profound physiological changes in plant metabolism. These changes require distinct modes of regulation as a unique feature of photosynthetic lifestyle. The activities of several key metabolic enzymes are regulated by light-dependent post-translational modifications (PTM) and have been studied at depth at the level of individual proteins. In contrast, a global picture of the light-dependent PTMome dynamics is lacking, leaving the response of a large proportion of cellular function undefined. Here, we investigated the light-dependent metabolome and proteome changes in Arabidopsis rosettes in a time resolved manner to dissect their kinetic interplay, focusing on phosphorylation, lysine acetylation, and cysteine-based redox switches. Of over 24 000 PTM sites that were detected, more than 1700 were changed during the transition from dark to light. While the first changes, as measured 5 min after onset of illumination, occurred mainly in the chloroplasts, PTM changes at proteins in other compartments coincided with the full activation of the Calvin-Benson cycle and the synthesis of sugars at later timepoints. Our data reveal connections between metabolism and PTM-based regulation throughout the cell. The comprehensive multiome profiling analysis provides unique insight into the extent by which photosynthesis reprograms global cell function and adds a powerful resource for the dissection of diverse cellular processes in the context of photosynthetic function.

Mesophyll conductance exerts a significant limitation on photosynthesis during light induction

Past studies have established mesophyll diffusion conductance to CO₂ (g_m ) as a variable and significant limitation to plant photosynthesis under steady-state conditions. However, the role of g_m in influencing photosynthesis (A) during the transient period of light induction is largely unknown. We combined gas exchange measurements with laser-enabled carbon isotope discrimination measurements to assess g_m during photosynthetic induction, using Arabidopsis as the measurement species. Our measurements revealed three key findings: (1) we found that the rate at which g_m approached steady state during induction was not necessarily faster than the induction rate of the carboxylation process, contradictory to what has been suggested in previous studies; (2) g_m displayed a strong and consistent coordination with A under both induction and steady-state settings, hinting that the mechanism driving g_m -A coupling does not require physiological stability as a prerequisite; and (3) photosynthetic limitation analysis of our data revealed that when integrated over the entire induction period, the relative limitation of A imposed by g_m can be as high as > 35%. The present study provides the first demonstration of the important role of g_m in limiting CO₂ assimilation during photosynthetic induction, thereby pointing to a need for more research attention to be devoted to g_m in future induction studies.

Simultaneously measuring pulse-amplitude-modulated (PAM) chlorophyll fluorescence of leaves at wavelengths shorter and longer than 700 nm

PAM fluorescence of leaves of cherry laurel (Prunus laurocerasus L.) was measured simultaneously in the spectral range below 700 nm (sw) and above 700 nm (lw). A high-sensitivity photodiode was employed to measure the low intensities of sw fluorescence. Photosystem II (PSII) performance was analyzed by the saturation pulse method during a light response curve with subsequent dark phase. The sw fluorescence was more variable, resulting in higher PSII photochemical yields compared to lw fluorescence. The variations between sw and lw data were explained by different levels of photosystem I (PSI) fluorescence: the contribution of PSI fluorescence to minimum fluorescence (F₀) was calculated to be 14% at sw wavelengths and 45% at lw wavelengths. With the results obtained, the validity of an earlier method for the quantification of PSI fluorescence (Genty et al. in Photosynth Res 26:133-139, 1990, https://doi.org/10.1007/BF00047085 ) was reconsidered. After subtracting PSI fluorescence from all fluorescence levels, the maximum PSII photochemical yield (F_V/F_M) in the sw range was 0.862 and it was 0.883 in the lw range. The lower F_V/F_M at sw wavelengths was suggested to arise from inactive PSII reaction centers in the outermost leaf layers. Polyphasic fluorescence transients (OJIP or OI₁I₂P kinetics) were recorded simultaneously at sw and lw wavelengths: the slowest phase of the kinetics (IP or I₂P) corresponded to 11% and 13% of total variable sw and lw fluorescence, respectively. The idea that this difference is due to variable PSI fluorescence is critically discussed. Potential future applications of simultaneously recording fluorescence in two spectral windows include studies of PSI non-photochemical quenching and state I-state II transitions, as well as measuring the fluorescence from pH-sensitive dyes simultaneously with chlorophyll fluorescence.

A tribute to Ulrich Heber (1930-2016) for his contribution to photosynthesis research: understanding the interplay between photosynthetic primary reactions, metabolism and the environment

The dynamic and efficient coordination of primary photosynthetic reactions with leaf energization and metabolism under a wide range of environmental conditions is a fundamental property of plants involving processes at all functional levels. The present historical perspective covers 60 years of research aiming to understand the underlying mechanisms, linking major breakthroughs to current progress. It centers on the contributions of Ulrich Heber who had pioneered novel concepts, fundamental methods, and mechanistic understanding of photosynthesis. An important first step was the development of non-aqueous preparation of chloroplasts allowing the investigation of chloroplast metabolites ex vivo (meaning that the obtained results reflect the in vivo situation). Later on, intact chloroplasts, retaining their functional envelope membranes, were isolated in aqueous media to investigate compartmentation and exchange of metabolites between chloroplasts and external medium. These studies elucidated metabolic interaction between chloroplasts and cytoplasm during photosynthesis. Experiments with isolated intact chloroplasts clarified that oxygenation of ribulose-1.5-bisphosphate generates glycolate in photorespiration. The development of non-invasive optical methods enabled researchers identifying mechanisms that balance electron flow in the photosynthetic electron transport system avoiding its over-reduction. Recording chlorophyll a (Chl a) fluorescence allowed one to monitor, among other parameters, thermal energy dissipation by means of 'nonphotochemical quenching' of the excited state of Chl a. Furthermore, studies both in vivo and in vitro led to basic understanding of the biochemical mechanisms of freezing damage and frost tolerance of plant leaves, to SO₂ tolerance of tree leaves and dehydrating lichens and mosses.

A Reliable and Non-destructive Method for Monitoring the Stromal pH in Isolated Chloroplasts Using a Fluorescent pH Probe

The proton gradient established by the pH difference across a biological membrane is essential for many physiological processes, including ATP synthesis and ion and metabolite transport. Currently, ionophores are used to study proton gradients, and determine their importance to biological functions of interest. Because of the lack of an easy method for monitoring the proton gradient across the inner envelope membrane of chloroplasts (ΔpH_env), whether the concentration of ionophores used can effectively abolish the ΔpH_env is not proven for most experiments. To overcome this hindrance, we tried to setup an easy method for real-time monitoring of the stromal pH in buffered, isolated chloroplasts by using fluorescent pH probes; using this method the ΔpH_env can be calculated by subtracting the buffer pH from the measured stromal pH. When three fluorescent dyes, BCECF-AM [2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester], CFDA-SE [5(6)-Carboxyfluorescein diacetate succinimidyl ester] and SNARF-1 carboxylic acid acetate succinimidyl ester were incubated with isolated chloroplasts, BCECF-AM and CFDA-SE, but not the ester-formed SNARF-1 were taken up by chloroplasts and digested with esterase to release high levels of fluorescence. According to its relatively higher pKa value (6.98, near the physiological pH of the stroma), BCECF was chosen for further development. Due to shielding of the excitation and emission lights by chloroplast pigments, the ratiometric fluorescence of BCECF was highly dependent on the concentration of chloroplasts. By using a fixed concentration of chloroplasts, a highly correlated standard curve of pH to the BCECF ratiometric fluorescence with an r-square value of 0.98 was obtained, indicating the reliability of this method. Consistent with previous reports, the light-dependent formation of ΔpH_env can be detected ranging from 0.15 to 0.33 pH units upon illumination. The concentration of the ionophore nigericin required to collapse the ΔpH_env was then studied. The establishment of a non-destructive method of monitoring the stromal pH will be valuable for studying the roles of the ΔpH_env in chloroplast physiology.

From horse thief to professor: confessions of a plant physiologist

Can 50 years of research, performed between ignorance and the wish to know, and executed between hope, despair, satisfaction and pain, be compressed into an abstract? What has been done in more than 50 years may be expressed in four words: it was worth it. If I had another life, I would do it again. In the beginning of my career, life was an enigma. It still is. Molecular details of the workings of life had been largely unknown when I began. Now, at the end, I still wish to know details: how is light, master of life, manipulated to either support life, when photosynthesis is possible, or to protect it when light endangers it. What is the molecular and the physical nature of the biological mechanisms which control both, energy conservation and energy dissipation, in photosynthesis?

Modulation of the vacuolar H+-ATPase by adenylates as basis for the transient CO2-dependent acidification of the leaf vacuole upon illumination

Using tonoplast vesicles, we have investigated the activity of the vacuolar H+-ATPase which is the dominant proton pump at the tonoplast of mesophyll cells. Bafilomycin-sensitive ATP hydrolysis or acidification of tonoplast vesicles in the presence of ATP were measured at varying ATP, ADP and Pi concentrations, and in the presence of oxidized or reduced glutathione. Increased ATP/ADP ratios as reported for the extrachloroplast cytoplasm during the induction phase of photosynthesis at high or low CO2 (P. Gardeström, Biochim. Biophys. Acta 1183 (1993) 327-332) increased the activity of the V-ATPase in simulation experiments with vesicles. Depending on reported subsequent decreases in cytoplasmic ATP/ADP ratios in the presence of high or low CO2, the ATPase activity of tonoplast vesicles changed in simulation experiments to lower values. More than 10 mM phosphate was required to decrease the ATPase activity in vesicles significantly at ATP/ADP ratios of 3 or higher, indicating that ATPase activity is controlled more by ratios of ATP to ADP than by phosphorylation potentials (ATP)/(ADP)(Pi). Oxidized glutathione was inhibitory. The results permit interpretation of the observation that on illumination of previously darkened leaves the pH of the vacuoles of mesophyll cells decreases indicating energized transport of protons across the tonoplast into acidic vacuoles, and that the extent of vacuolar acidification depends on the CO2 concentration of the surrounding air (Z.-H. Yin, S. Neimanis, U. Heber, Planta 182 (1990) 253-261). We conclude that short term control of tonoplast ATPase activity in leaves during dark/light transients can essentially be understood on the basis of reported changes in cytoplasmic ATP/ADP ratios, with a possible participation of redox modulation.

Patterns of phosphoenolpyruvate carboxylase activity and cytosolic pH during light activation and dark deactivation in C3 and C 4 plants

The rate and extent of light activation of PEPC may be used as another criterion to distinguish C3 and C4 plants. Light stimulated phosphoenolypyruvate carboxylase (PEPC) in leaf discs of C4 plants, the activity being three times greater than that in the dark but stimulation of PEPC was limited about 30% over the dark-control in C3 species. The light activation of PEPC in leaves of C3 plants was complete within 10 min, while maximum activation in C4 plants required illumination for more than 20 min, indicating that the relative pace of PEPC activation was slower in C4 plants than in C3 plants. Similarly, the dark-deactivation of the enzyme was also slower in leaves of C4 than in C3 species. The extent of PEPC stimulation in the alkaline pH range indicated that the dark-adapted form of the C4 enzyme is very sensitive to changes in pH. The pH of cytosol-enriched cell sap extracted from illuminated leaves of C4 plants was more alkaline than that of dark-adapted leaves. The extent of such light-dependent alkalization of cell sap was three times higher in C4 leaves than in C3 plants. The course of light-induced alkalization and dark-acidification of cytosol-enriched cell sap was markedly similar to the pattern of light activation and dark-deactivation of PEPC in Alternanthera pungens, a C4 plant. Our report provides preliminary evidence that the photoactivation of PEPC in C4 plants may be mediated at least partially by the modulation of cytosolic pH.

PAM fluorometer based on medium-frequency pulsed Xe-flash measuring light: A highly sensitive new tool in basic and applied photosynthesis research

A newly developed modulation fluorometer is described which employs repetitive 1 μs Xe-flashes for excitation light. Similar to the standard PAM Chlorophyll Fluorometer, which uses 1 μs LED pulses for measuring light, the integrated measuring light intensity is sufficiently low to monitor the dark-fluorescence level, Fo. The maximal fluorescence yield, Fm, can be determined with high selectivity upon application of a saturating light pulse. The Xe-PAM displays exceptionally high sensitivity, enabling quenching analysis at chlorophyll concentrations as low as 1 μg/l, thus allowing to assess photosynthesis of phytoplankton in natural waters like lakes, rivers and oceans. Due to high flexibility in the choice of excitation and emission wavelengths, this system also provides the experimental basis for a thorough study of fluorescence and photosynthesis properties of various algae classes with differing antenna organisation. By appropriate modifications, the instrument may as well be used to measure with great sensitivity and selectivity other types of fluorescence (e.g. NADPH-fluorescence), as well as light-scattering and absorbance changes.

Regulatory phosphorylation of Sorghum leaf phosphoenolpyruvate carboxylase. Identification of the protein-serine kinase and some elements of the signal-transduction cascade

The phosphoenolpyruvate (PPrv) carboxylase isozyme involved in C4 photosynthesis undergoes a day/night reversible phosphorylation process in leaves of the C4 plant, Sorghum. Ser8 of the target enzyme oscillates between a high (light) and a low (dark) phosphorylation status. Both in vivo and in vitro, phosphorylation of dark-form carboxylase was accompanied by an increase in the apparent Ki of the feedback inhibitor L-malate and an increase in Vmax. Feeding detached leaves various photosynthetic inhibitors, i.e. 3-(3,4-dichlorophenyl)-1,1-dimethylurea, gramicidin and DL-glyceraldehyde, prevented PPrv carboxylase phosphorylation in the light, thus suggesting that the cascade involves the photosynthetic apparatus as the light signal receptor, and presumably has the electron transfer chain and the Calvin-Benson cycle as components in the signal-transduction chain. Two protein-serine kinases capable of phosphorylating PPrv carboxylase in vitro have been partially purified from light-adapted leaves. One was isolated on a calmodulin-Sepharose column; it was calcium-dependent but did not require calmodulin for activity. The other was purified on a blue-dextran-agarose column and the only Me2+ required for activity was Mg2+. In reconstituted phosphorylation assays, only the latter caused the expected decrease in malate sensitivity of PPrv carboxylase suggesting that this protein is the genuine PPrv-carboxylase-kinase. Desalted extracts from light-adapted leaves possessed a considerably greater phosphorylation capacity with immunopurified dephosphorylated PPrv carboxylase as substrate than did dark extracts. This light stimulation was insensitive to type 2A protein phosphatase inhibitors, okadaic acid and microcystin-LR, which suggests that the kinase is a controlled step in the cascade which leads to phosphorylation of PPrv carboxylase. The higher phosphorylation capacity of light-adapted leaf tissue was nullified by pretreatment with the cytosolic protein synthesis inhibitor, cycloheximide. Thus, protein turnover is involved as part of the mechanism controlling the activity of the kinase purified on blue-dextran-agarose. However, no information is available with respect to the specific nature of the link between the above-mentioned light transducing steps and the protein kinase that achieves the physiological response. Finally, the in vivo phosphorylation site (Ser8) in the N-terminal region of the C4 type Sorghum PPrv carboxylase is also present in a non-photosynthetic form of the Sorghum enzyme (Ser7), as deduced by cDNA sequence analysis.

Light-dependent pH changes in leaves of C3 plants : II. Effect of CO2 and O 2 on the cytosolic and the vacuolar pH

Illumination of leaves of C3 plants caused cytosolic alkalization and vacuolar acidification in the mesophyll cells. Both phenomena were particularly pronounced when CO2 was absent, were suppressed by CO2, and were related to the activation state of the photosynthetic apparatus. The cytosolic alkalization reaction has at least two major components. Trivalent cytosolic phosphoglycerate must be protonated before it can be transferred into the chloroplasts for reduction. Pumping of protons from the cytosol into the vacuole also contributes to cytosolic alkalization. The dependence of light scattering by chloroplast thylakoids on the energy fluence rate was closely related to that of vacuolar acidification under different conditions for chloroplast energization. This indicates (i) transport of energy from the chloroplasts to the cytosol in the light and (ii) use of this energy for the transport of protons into the vacuoles. The light-dependent vacuolar acidification is interpreted to be caused by the increase in the activity of a proton-translocating enzyme of the tonoplast. The decrease of vacuolar acidification during photosynthetic carbon reduction or photorespiration is indicative of decreased cytosolic energization. In low light, the light-dependent vacuolar acidification was stimulated in the absence of CO2 when photorespiration was inhibited. The data do not support the view that photorespiration is capable of increasing the cytosolic energy state in the light.