Bundle Sheath Chloroplasts Research Articles

Chloroplast NADH dehydrogenase-like complex-mediated cyclic electron flow is the main electron transport route in C4 bundle sheath cells.

The superior productivity of C4 plants is achieved via a metabolic C4 cycle which acts as a CO2 pump across mesophyll and bundle sheath (BS) cells and requires an additional input of energy in the form of ATP. The importance of chloroplast NADH dehydrogenase-like complex (NDH) operating cyclic electron flow (CEF) around Photosystem I (PSI) for C4 photosynthesis has been shown in reverse genetics studies but the contribution of CEF and NDH to cell-level electron fluxes remained unknown. We have created gene-edited Setaria viridis with null ndhO alleles lacking functional NDH and developed methods for quantification of electron flow through NDH in BS and mesophyll cells. We show that CEF accounts for 84% of electrons reducing PSI in BS cells and most of those electrons are delivered through NDH while the contribution of the complex to electron transport in mesophyll cells is minimal. A decreased leaf CO2 assimilation rate and growth of plants lacking NDH cannot be rescued by supplying additional CO2. Our results indicate that NDH-mediated CEF is the primary electron transport route in BS chloroplasts highlighting the essential role of NDH in generating ATP required for CO2 fixation by the C3 cycle in BS cells.

Kranz anatomical and biochemical characterization of C4 photosynthesis in an aphyllous shrub Calligonum comosum (Polygonaceae)

Calligonum is a shrubby and aphyllous C4 eudicot lineage, employing green cylindrical shoots for carbon acquisition; however, comprehensive assessments of C4 photosynthesis within the genus are limited. One species, C. comosum, grows in Jordan, offering an opportunity for evaluating features of C4 photosynthesis in Calligonum. Light microscopy showed that C. comosum exhibits a variation of the classical salsoloid Kranz type (Calligonum variant). Transmission electron microscopy revealed traits typical of the NAD-malic enzyme (NAD-ME) subtype. Bundle sheath cells had numerous large mitochondria and mitochondria:chloroplast ratios were greater than those of mesophyll cells. Moreover, bundle sheath chloroplasts had developed grana, whereas mesophyll chloroplasts were deficient of grana. Enzyme activity and western blotting further supported NAD-ME as the principal decarboxylase. Immunolocalization of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPC) and pyruvate-phosphate dikinase (PPdK) showed strict intercellular compartmentation patterns, with Rubisco confined to bundle sheath and PEPC and PPdK to mesophyll. Labeling for PEPC was observed in both the mesophyll and hypodermis, while PPdK was restricted to the mesophyll. We suggest classifying C. comosum as a NAD-ME subtype C4 species and propose a mechanism of C4 photosynthesis, suggesting a tractable C4 model for further insights and studies on the biochemical intricacy of C4 photosynthesis.

The role of chloroplast movement in C4 photosynthesis: a theoretical analysis using a three-dimensional reaction-diffusion model for maize.

Chloroplasts movement within mesophyll cells in C4 plants is hypothesized to enhance the CO2 concentrating mechanism, but this is difficult to verify experimentally. A three-dimensional (3D) leaf model can help analyse how chloroplast movement influences the operation of the CO2 concentrating mechanism. The first volumetric reaction-diffusion model of C4 photosynthesis that incorporates detailed 3D leaf anatomy, light propagation, ATP and NADPH production, and CO2, O2 and bicarbonate concentration driven by diffusional and assimilation/emission processes was developed. It was implemented for maize leaves to simulate various chloroplast movement scenarios within mesophyll cells: the movement of all mesophyll chloroplasts towards bundle sheath cells (aggregative movement) and movement of only those of interveinal mesophyll cells towards bundle sheath cells (avoidance movement). Light absorbed by bundle sheath chloroplasts relative to mesophyll chloroplasts increased in both cases. Avoidance movement decreased light absorption by mesophyll chloroplasts considerably. Consequently, total ATP and NADPH production and net photosynthetic rate increased for aggregative movement and decreased for avoidance movement compared with the default case of no chloroplast movement at high light intensities. Leakiness increased in both chloroplast movement scenarios due to the imbalance in energy production and demand in mesophyll and bundle sheath cells. These results suggest the need to design strategies for coordinated increases in electron transport and Rubisco activities for an efficient CO2 concentrating mechanism at very high light intensities.

Light-Dependent Reactions of Photosynthesis in Mesophyll and Bundle Sheath Chloroplasts of C4 Plant Maize. How Our Views Have Changed in Recent Years

Abstract Plants experience a range of light intensities and qualities in their environment. Leaves are subjected to spatial and temporal gradients in incident light, which has major consequences in the photosynthetic carbon assimilation. Plants acclimate to light by developing a range of mechanisms, from adjustments in leaf morphology to changes in the photosynthetic apparatus stoichiometry. In C4 plants, light intensity is a major limiting factor for photosynthesis at optimum temperatures. Under limiting light, it is not clear if all of factors (e.g., temperature, mineral nutrition, water supply) are co-limiting or if there is one primary limitation. Differences in light quality and intensity have a profound impact on C4 photosynthesis, where pathways require metabolic coordination of the mesophyll and bundle sheath cycles. Changes in the linear versus cyclic electron flux in maize (NADP-malic enzyme C4 subtype) in the mesophyll and bundle sheath chloroplasts in response to light may lead to an imbalance in the coordination of the C3 and C4 pathways. Additionally, the rearrangement of the thylakoid complexes of both types of chloroplasts in maize optimizes the light energy distribution between the mesophyll and bundle sheath cells and may also participate in energy dissipation. This review aims to highlight the changes in the understanding of the functions of photosystem II in maize bundle sheath chloroplasts and the role of super and megacomplexes in the thylakoids.

Starch phosphorylase 2 is essential for cellular carbohydrate partitioning in maize

Carbohydrate partitioning is essential for plant growth and development, and its hindrance will result in excess accumulation of carbohydrates in source tissues. Most of the related mutants in maize (Zea mays L.) display impaired whole-plant sucrose transport, but other mechanisms affecting carbohydrate partitioning have seldom been reported. Here, we characterized chlorotic leaf3 (chl3), a recessive mutation causing leaf chlorosis with starch accumulation excessively in bundle sheath chloroplasts, suggesting that chl3 is defective in carbohydrate partitioning. Positional cloning revealed that the chl3 phenotype results from a frameshift mutation in ZmPHOH, which encodes starch phosphorylase 2. Two mutants in ZmPHOH exhibited the same phenotype as chl3, and both alleles failed to complement the chl3 mutant phenotype in an allelism test. Inactivation of ZmPHOH in chl3 leaves reduced the efficiency of transitory starch conversion, resulting in increased leaf starch contents and altered carbohydrate metabolism patterns. RNA-seq revealed the transcriptional downregulation of genes related to photosynthesis and carbohydrate metabolism in chl3 leaves compared to the wild type. Our results demonstrate that transitory starch remobilization is very important for cellular carbohydrate partitioning in maize, in which ZmPHOH plays an indispensable role.

Chloroplastic photoprotective strategies differ between bundle sheath and mesophyll cells in maize (Zea mays L.) Under drought.

Bundle sheath cells play a crucial role in photosynthesis in C4 plants, but the structure and function of photosystem II (PSII) in these cells is still controversial. Photoprotective roles of bundle sheath chloroplasts at the occurrence of environmental stresses have not been investigated so far. Non-photochemical quenching (NPQ) of chlorophyll a fluorescence is the photoprotective mechanism that responds to a changing energy balance in chloroplasts. In the present study, we found a much higher NPQ in bundle sheath chloroplasts than in mesophyll chloroplasts under a drought stress. This change was accompanied by a more rapid dephosphorylation of light-harvesting complex II (LHCII) subunits and a greater increase in PSII subunit S (PsbS) protein abundance than in mesophyll cell chloroplasts. Histochemical staining of reactive oxygen species (ROS) suggested that the high NPQ may be one of the main reasons for the lower accumulation of ROS in bundle sheath chloroplasts. This may maintain the stable functioning of bundle sheath cells under drought condition. These results indicate that the superior capacity for dissipation of excitation energy in bundle sheath chloroplasts may be an environmental adaptation unique to C4 plants.

How Light Reactions of Photosynthesis in C4 Plants Are Optimized and Protected under High Light Conditions.

Most C4 plants that naturally occur in tropical or subtropical climates, in high light environments, had to evolve a series of adaptations of photosynthesis that allowed them to grow under these conditions. In this review, we summarize mechanisms that ensure the balancing of energy distribution, counteract photoinhibition, and allow the dissipation of excess light energy. They secure effective electron transport in light reactions of photosynthesis, which will lead to the production of NADPH and ATP. Furthermore, a higher content of the cyclic electron transport components and an increase in ATP production are observed, which is necessary for the metabolism of C4 for effective assimilation of CO2. Most of the data are provided by studies of the genus Flaveria, where species belonging to different metabolic subtypes and intermediate forms between C3 and C4 are present. All described mechanisms that function in mesophyll and bundle sheath chloroplasts, into which photosynthetic reactions are divided, may differ in metabolic subtypes as a result of the different organization of thylakoid membranes, as well as the different demand for ATP and NADPH. This indicates that C4 plants have plasticity in the utilization of pathways in which efficient use and dissipation of excitation energy are realized.

Salt tolerance in relation to elemental concentrations in leaf cell vacuoles and chloroplasts of a C4 monocotyledonous halophyte.

Halophytes accumulate and sequester high concentrations of salt in vacuoles while maintaining lower levels of salt in the cytoplasm. The current data on cellular and subcellular partitioning of salt in halophytes are, however, limited to only a few dicotyledonous C3 species. Using cryo‐scanning electron microscopy X‐ray microanalysis, we assessed the concentrations of Na, Cl, K, Ca, Mg, P and S in various cell types within the leaf‐blades of a monocotyledonous C4 halophyte, Rhodes grass (Chloris gayana). We also linked, for the first time, elemental concentrations in chloroplasts of mesophyll and bundle sheath cells to their ultrastructure and photosynthetic performance of plants grown in nonsaline and saline (200 mM NaCl) conditions. Na and Cl accumulated to the highest levels in xylem parenchyma and epidermal cells, but were maintained at lower concentrations in photosynthetically active mesophyll and bundle sheath cells. Concentrations of Na and Cl in chloroplasts of mesophyll and bundle sheath cells were lower than in their respective vacuoles. No ultrastructural changes were observed in either mesophyll or bundle sheath chloroplasts, and photosynthetic activity was maintained in saline conditions. Salinity tolerance in Rhodes grass is related to specific cellular Na and Cl distributions in leaf tissues, and the ability to regulate Na and Cl concentrations in chloroplasts.

Engineering chloroplast development in rice through cell-specific control of endogenous genetic circuits.

SummaryThe engineering of C4 photosynthetic activity into the C3 plant rice has the potential to nearly double rice yields. To engineer a two‐cell photosynthetic system in rice, the rice bundle sheath (BS) must be rewired to enhance photosynthetic capacity. Here, we show that BS chloroplast biogenesis is enhanced when the transcriptional activator, Oryza sativa Cytokinin GATA transcription factor 1 (OsCGA1), is driven by a vascular specific promoter. Ectopic expression of OsCGA1 resulted in increased BS chloroplast planar area and increased expression of photosynthesis‐associated nuclear genes (PhANG), required for the biogenesis of photosynthetically active chloroplasts in BS cells of rice. A further refinement using a DNAse dead Cas9 (dCas9) activation module driven by the same cell‐type specific promoter, directed enhanced chloroplast development of the BS cells when gRNA sequences were delivered by the dCas9 module to the promoter of the endogenous OsCGA1 gene. Single gRNA expression was sufficient to mediate the transactivation of both the endogenous gene and a transgenic GUS reporter fused with OsCGA1 promoter. Our results illustrate the potential for tissue‐specific dCas9‐activation and the co‐regulation of genes needed for multistep engineering of C4 rice.

Upregulation of bundle sheath electron transport capacity under limiting light in C4Setaria viridis

SUMMARYC4 photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO2 concentration at the site of CO2 fixation. C4 plants benefit from high irradiance but their efficiency decreases under shade, causing a loss of productivity in crop canopies. We investigated shade acclimation responses of Setaria viridis, a model monocot of NADP‐dependent malic enzyme subtype, focussing on cell‐specific electron transport capacity. Plants grown under low light (LL) maintained CO2 assimilation rates similar to high light plants but had an increased chlorophyll and light‐harvesting‐protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen‐evolving activity and the PSII/PSI ratio were enhanced in LL BS cells, indicating a higher capacity for linear electron flow. Abundances of PSI, ATP synthase, Cytochrome b 6 f and the chloroplast NAD(P)H dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were also increased in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts were associated with a more oxidised plastoquinone pool in LL plants and the formation of PSII – light‐harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results suggest that the supramolecular composition of PSII in BS cells is adjusted according to the redox state of the plastoquinone pool. This discovery contributes to the understanding of the acclimation of PSII activity in C4 plants and will support the development of strategies for crop improvement, including the engineering of C4 photosynthesis into C3 plants.

Rubisco production in maize mesophyll cells through ectopic expression of subunits and chaperones.

C4 plants, such as maize, strictly compartmentalize Rubisco to bundle sheath chloroplasts. The molecular basis for the restriction of Rubisco from the more abundant mesophyll chloroplasts is not fully understood. Mesophyll chloroplasts transcribe the Rubisco large subunit gene and, when normally quiescent transcription of the nuclear Rubisco small subunit gene family is overcome by ectopic expression, mesophyll chloroplasts still do not accumulate measurable Rubisco. Here we show that a combination of five ubiquitin promoter-driven nuclear transgenes expressed in maize leads to mesophyll accumulation of assembled Rubisco. These encode the Rubisco large and small subunits, Rubisco assembly factors 1 and 2, and the assembly factor Bundle sheath defective 2. In these plants, Rubisco large subunit accumulates in mesophyll cells, and appears to be assembled into a holoenzyme capable of binding the substrate analog CABP (carboxyarabinitol bisphosphate). Isotope discrimination assays suggest, however, that mesophyll Rubisco is not participating in carbon assimilation in these plants, most probably due to a lack of the substrate ribulose 1,5-bisphosphate and/or Rubisco activase. Overall, this work defines a minimal set of Rubisco assembly factors in planta and may help lead to methods of regulating the C4 pathway.

Changes in leaf blade morphology and anatomy caused by clomazone and saflufenacil in Setaria viridis, a model C4 plant

Clomazone and saflufenacil are herbicides extensively used worldwide to weed control. We studied the effects of these two herbicides on morphoanatomical parameters of Setaria viridis. Plants were sprayed with four concentrations of each herbicide (clomazone: 500, 1000, 1500 and 2000 g of active ingredient (ai) ha−1 and saflufenacil: 49, 98, 147 and 196 g ai ha−1) besides control (without spraying) 20 days after transplantation. The experimental design was completely randomized with five replicates per treatment. Pigment content, visible injuries, morphological and ultrastructural changes were evaluated. No signs of tolerance to either of the tested herbicides were observed. Clomazone caused a decrease in photosynthetic pigment content over time, mostly in young leaves, leading to an “albino” like appearance. There was a reduction in the number of grana in the chloroplasts of mesophyll cells (MC) in necrotic areas. Saflufenacil reduced chlorophyll content, impairing energy absorption in the antenna complex. Injuries to foliar tissues, such as necrosis and depigmentation, were visible as early as 24 h after herbicide application. Bundle sheath chloroplasts (BSC) and MC were completely deformed. The data support the use of S. viridis as a model plant for studies on herbicide effects in C4 monocots.

A tribute to Maarib (Darwish Lutfi Bakri) Bazzaz (1940–2020): the one who proved the existence of “new” chlorophylls in plants

We present here a tribute to Maarib Bazzaz (1940-2020), formerly Maarib Darwish Lutfi Bakri, who was an innovative and a highly inquisitive plant physiologist of her time. Her research achievements include exploitation of a highly productive mutant of maize (ON8147), of differences between mesophyll and bundle sheath chloroplasts of maize, and of chlorophyll (Chl) biosynthesis. Above all, she provided convincing scientific proof that, in addition to the usual monovinyl Chl a and monovinyl Chl b, divinyl Chl a (4-vinyl-4-desethyl-Chl a) and divinyl Chl b (4-vinyl-4-desethyl-Chl b) exist in plants. We also include here reminiscences by some of her contemporaries. Particularly noteworthy is how effortlessly Maarib pursued her deep interest in science while raising two wonderful children. She was always curious about life’s mysteries and determined to search for answers in a thorough and logical manner. Above all and at all times, she was very kind to others.

Assessment of leaf ultrastructure offers insights into mechanisms regulating sugarcane performance under low-phosphorus stress

Detailed studies of plant responses to phosphorus (P) deficiency might reveal opportunities to improve sugarcane performance under nutrient stress, termed as P use efficiency. Then, we aimed through transmission electron microscopy to offer insights into the effects of varying P supply on leaf ultrastructural traits influencing photosynthesis and leaf area formation. Plants of the genotype RB86-7515 were cultivated under varying nutrient supply (0.0125, 0.05, 0.2 and 0.8 mM P) for 4 months. Diagnostic mature leaves were sampled to evaluate ultrastructural organization and P concentration. Leaf area and plant dry mass responded positively to increasing P rates and maximum values were attained at 0.55 mM P. Leaf P concentration increased as a function of nutrient supply, and the greatest area of diagnostic leaf and whole plant growth was associated with 1.80 g kg−1 P. Under severe deficiency (relative growth ≤ 30% and leaf P ≤ 0.90 g kg−1) generated with 0.0125- and 0.05-mM P, transmission electron microscopy revealed pronounced damages to leaf ultrastructure. In mesophyll cells, the changes were more intense under the lowest P rate, and included disintegration of the middle lamella, collapse of plasma membrane and distortion of thylakoids. Bundle sheath chloroplasts of the severely P-deficient plants exhibited, compared with those under 0.2- and 0.8-mM P, disorganization of lamellar membrane system. Collectively, these ultrastructural alterations offer insights into how P deficiency affects whole plant photosynthetic capacity and provide possible target traits at canopy level related to sugarcane P use efficiency.

Biochemical properties and ultrastructure of mesophyll and bundle sheath thylakoids from maize (Zea mays) chloroplasts.

A characteristic feature of C4 plants is the differentiation of the photosynthetic leaf tissues into two distinct cell types: mesophyll (M) and bundle sheath (BS) cells. We have investigated several biochemical parameters, including pigment composition, polypeptide patterns, fluorescence at 77K, the activity of photosystems and ultrastructure of mesophyll and bundle sheath chloroplasts of maize (Zea mays L.) plants. It is shown that the BS chloroplasts have ~2-fold higher chlorophyll a/b ratio than M chloroplasts, 6.15 and 3.12 respectively. The PSI apoprotein (68 kDa) was more abundant in BS than in M thylakoids. Polypeptides belonging to PSII core antenna, are in similar amounts in both types of membranes, but the 45kDa band is more intensive in M thylakoids. Polypeptides in the region of 28-24 kDa of the light-harvesting complex of PSII (LHCII) are also present in both types of chloroplasts, though their amounts are reduced in BS thylakoids. The chlorophyll fluorescence emission spectra in M cells showed the presence of three bands at 686, 695 and 735 nm characteristics of LHCII, PSII core and PSI complexes, respectively. However, in the fluorescence spectrum of agranal plastids, there are almost traces of the band at 695 nm, which belongs to the PSII core complex. The research results revealed that the photochemical activity of PSII in BS chloroplasts is ~5 times less than in the chloroplasts of M cells. The highest PSI activity was found in maize BS chloroplasts.

Light stress-induced chloroplast movement and midday depression of photosynthesis in sorghum leaves

ABSTRACTPlants are exposed to high light intensity, high leaf temperatures and high air-to-leaf water vapor pressure deficit (ALVPD) during the day. These environmental stresses cause stomatal closure and photoinhibitory damage, leading to midday depression of photosynthesis. Chloroplast positioning is essential for the efficient operation of photosynthesis. However, chloroplast behavior before, during, and even after the midday depression of photosynthesis remains unknown. We investigated changes in the intracellular positioning of chloroplasts and photosynthetic traits under a diurnal pattern of light. Sorghum leaves were exposed to a 12-h regime of light mimicking the natural light environment, with constant leaf temperature and ALVPD. Net photosynthetic rate (Pn) showed a diurnal pattern, and midday depression in Pn was observed at 3.8 h of irradiation. Depression in Pn was attributed to stomatal limitation because the decrease in Pn was in accordance with the decrease in stomatal conductance. The maximum efficiency of photosystem II decreased with the increase in light intensity and remained low after 12 h of irradiation. Bundle sheath chloroplasts swelled after 8 h of irradiation, representing the accumulation of starch. Conversely, mesophyll chloroplasts exhibited avoidance response after 4 h of irradiation, and the avoidance position was maintained during the remainder of the daytime. These data suggest that chloroplasts are subject to light stress during and after the midday depression of photosynthesis. The intensity of natural light is excessive for most of the day and this light stress induces chloroplast avoidance response and depression of photosynthesis.

Photosynthesis and organization of maize mesophyll and bundle sheath thylakoids of plants grown in various light intensities

The aim of this study was to investigate the acclimation of photosynthetic apparatus of maize to different light intensities during growth. Plants were grown at irradiance of 80 (LL), 200 (ML) and 600 (HL) μmol photons m−2 s−1. Our main interest was to understand how the light intensity influences the organization/function of thylakoid complexes in mesophyll (M) and bundle sheath (BS) chloroplasts. We have investigated several physiological parameters including rate of photosynthesis and respiration, the activity of photosystems, fluorescence in RT and at 77 K, phosphorylation of PSII proteins, level of proteins, and organization of thylakoid complexes. We suggest that BS chloroplasts are the main light target. Adjustment of the light absorption and distribution between both photosystems in BS is related to the organization of supercomplexes according to the phosphorylation level. The high rate of phosphorylation of LHCII in BS thylakoids is probably related to the absence of Lhcb3 protein and its replacement by Lhcb1 or Lhcb2 that undergo phosphorylation. In BS thylakoids, for all light conditions the LHCII proteins are bound to PSI, and a high pool of phosphorylated free trimers is present. We conclude that the investigated M and BS chloroplasts use different mechanisms of adjustment and optimization of their function, depending on the optimal or adverse irradiance conditions prevailing during the growth, and that these mechanisms are associated with different light penetration across the leaf. These changes induced by light in both types of chloroplasts are important in the regulation of chloroplast membrane flexibility and thus its function.

Bundle sheath chloroplast volume can house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling.

C4 leaves confine Rubisco to bundle sheath cells. Thus, the size of bundle sheath compartments and the total volume of chloroplasts within them limit the space available for Rubisco. Rubisco activity limits photosynthesis at low temperatures. C3 plants counter this limitation by increasing leaf Rubisco content, yet few C4 species do the same. Because C3 plants usually outperform C4 plants in chilling environments, it has been suggested that there is insufficient chloroplast volume available in the bundle sheath of C4 leaves to allow such an increase in Rubisco at low temperatures. We investigated this potential limitation by measuring bundle sheath and mesophyll compartment volumes and chloroplast contents, as well as leaf thickness and inter-veinal distance, in three C4 Andropogoneae grasses: two crops (Zea mays and Saccharum officinarum) and a wild, chilling-tolerant grass (Miscanthus × giganteus). A wild C4 Paniceae grass (Alloteropsis semialata) was also included. Despite significant structural differences between species, there was no evidence of increased bundle sheath chloroplast volume per leaf area available to the chilling-tolerant species, relative to the chilling-sensitive ones. Maximal theoretical photosynthetic capacity of the leaf far exceeded the photosynthetic rates achieved even at low temperatures. C4 bundle sheath cells therefore have the chloroplast volume to house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling.

Maize bundle sheath chloroplasts - a unique model of permanent State 2

The effect of high intensity of far red (FR) light on the energy distribution between PSII and PSI, measured as a change in the fluorescence emission in mesophyll (M) and bundle sheath (BS) chloroplasts of Zea mays was investigated in this paper. In bundle sheath thylakoids, FR light (730önm) significantly stimulates 77öK fluorescence of PSII but does not change the PSI fluorescence. In mesophyll thylakoids, the FR light used after the white light increased and decreased the PSII and PSI fluorescence, respectively. This indicates that in M chloroplasts light quality controls photosynthetic state transitions. The blue native (BN) PAGE, followed by SDS PAGE demonstrated that LHCII proteins associated with PSI were phosphorylated in the BS thylakoids in FR light. In M chloroplasts FR-light induced dephosphorylation of LHCII and the detachment of antenna from PSI occurred. In the BS thylakoids, the FR light initiated dephosphorylation of free and aggregated LHCII antenna, the dephosphorylated antenna can bound to PSII, thereby strongly increase its fluorescence, while part of LHCII pool stays associated with PSI. Thus aggregates of LHCII tended to be resolubilized in FR light and this allowed LHCII to connect to PSII, without changes in the pool of PSI-LHCI-LHCII. Our data indicate that PSI is locked in State 2 in agranal bundle sheath chloroplasts of maize. These results allow us to better understand the complexity of regulatory mechanisms of state transitions activated in response to light changes in M and BS chloroplasts of maize.

Reactive oxygen species and redox regulation in mesophyll and bundle sheath cells of C4 plants

Redox regulation, antioxidant defence, and reactive oxygen species (ROS) signalling are critical in performing and tuning metabolic activities. However, our concepts have mostly been developed for C3 plants since Arabidopsis thaliana has been the major model for research. Efforts to convert C3 plants to C4 to increase yield (such as IRRI's C4 Rice Project) entail a better understanding of these processes in C4 plants. Various photosynthetic enzymes that take part in light reactions and carbon reactions are regulated via redox components, such as thioredoxins as redox transmitters and peroxiredoxins. Hence, understanding redox regulation in the mesophyll and bundle sheath chloroplasts of C4 plants is of paramount importance: it appears impossible to utilize efficient C4 photosynthesis without understanding its exact redox needs and the regulation mechanisms used during light reactions. In this review, we discuss current knowledge on redox regulation in C3 and C4 plants, with special emphasis on the mesophyll and bundle sheath differences that are found in C4. In these two cell types in C4 plants, linear and cyclic electron transport in the chloroplasts operate differentially when compared to C3 chloroplasts, changing the redox needs of the cell. Therefore, our focus is on photosynthetic light reactions, ROS production dynamics, antioxidant defence, and thiol-based redox regulation, with the aim of providing an overview of our current knowledge.