Differentiation and development of bundle sheath and mesophyll thylakoids in maize. Thylakoid polypeptide composition, phosphorylation, and organization of photosystem II

Photosynthetic electron flow, polypeptide pattern, presence of chlorophyll-protein complexes, and phosphorylation of thylakoid polypeptides have been investigated in differentiated mesophyll (M) and bundle sheath (B) thylakoids of the C4 plant Zea mays. The polypeptide pattern of M thylakoids and their photosynthetic electron flow are comparable to those of other green plants. B thylakoids exhibit only photosystem I (PSI) activity, contain only traces of the PSII light harvesting (LHCII) polypeptide, do not bind [3H] diuron, and lack polypeptides of the water-oxidation complex of PSII and the herbicide binding 32-kDa polypeptide, as detected by specific antibodies. However, B thylakoids possess a partially active PSII reaction center, as demonstrated by light-dependent reduction of silicomolybdate with 1,5-diphenylcarbazide (DPC) as an electron donor, and the presence of the PSII reaction center polypeptides of 44-47 kDa. Only one chlorophyll a-protein complex, corresponding to the PSI reaction center-core antenna, was detectable in B thylakoids, as opposed to chlorophyll a and chlorophyll a,b-protein complexes present in M thylakoids. The light-dependent, membrane-bound kinase activity present in M thylakoids could not be detected in B thylakoids which, nevertheless, contain a protein kinase able to phosphorylate casein. A total of 19 differences between the electrophoretic pattern of B and M thylakoid polypeptides were observed. The mRNA coding for the LHCII polypeptide is primarily, if not exclusively, localized in M cells. The development of PSII complex precedes that of PSI during the differentiation of B and M chloroplasts in expanding leaves of light-grown plants and during the greening of dark-grown etiolated seedlings. The differentiation of the maize leaf into cells programmed to form B or M chloroplasts does not require light. In light-grown plants, the differentiation of B and M thylakoids occurred progressively from the base of the leaf and was completed at 4-5 cm from the leaf base.     

Oxidative damage to thylakoid proteins in water-stressed leaves of wheat (Triticum aestivum)

The production of reactive oxygen species in the chloroplast may increase under water deficit. To determine if this causes oxidative damage to the photosynthetic apparatus, we analyzed the accumulation of oxidatively damaged proteins in thylakoids of water-stressed wheat ( Triticum aestivum L.) leaves. Water stress was imposed on 4-week-old plants by withholding watering for 10 days to reach a soil water potential of about −2.0 MPa. In thylakoids of water-stressed leaves there was an increase in oxidative damage, particularly in polypeptides of 68, 54, 41 and 24 kDa. High molecular mass oxidized (probably cross-linked) proteins accumulated in chloroplasts of droughted leaves. Oxidative damage was associated with a substantial decrease in photosynthetic electron transport activity and photosystem II (PSII) efficiency (F_v/F_m). Treatment of stressed leaves with l -galactono-1,4-lactone (GL) increased their ascorbic acid content and enhanced photochemical and non-photochemical quenching of chlorophyll fluorescence. GL reduced oxidative damage to photosynthetic proteins of droughted plants, but it reverted the decrease in electron transport activity and PSII efficiency only partially, suggesting that other factors also contributed to loss of photosystem activity in droughted plants. Increasing the ascorbic acid content of leaves might be an effective strategy to protect thylakoid membranes from oxidative damage in water-stressed leaves.     

The SCO2 protein disulphide isomerase is required for thylakoid biogenesis and interacts with LHCB1 chlorophyll a/b binding proteins which affects chlorophyll biosynthesis in Arabidopsis seedlings

The process of chloroplast biogenesis requires a multitude of pathways and processes to establish chloroplast function. In cotyledons of seedlings, chloroplasts develop either directly from proplastids (also named eoplasts) or, if germinated in the dark, via etioplasts, whereas in leaves chloroplasts derive from proplastids in the apical meristem and are then multiplied by division. The snowy cotyledon 2, sco2, mutations specifically disrupt chloroplast biogenesis in cotyledons. SCO2 encodes a chloroplast-localized protein disulphide isomerase, hypothesized to be involved in protein folding. Analysis of co-expressed genes with SCO2 revealed that genes with similar expression patterns encode chloroplast proteins involved in protein translation and in chlorophyll biosynthesis. Indeed, sco2-1 accumulates increased levels of the chlorophyll precursor, protochlorophyllide, in both dark grown cotyledons and leaves. Yeast two-hybrid analyses demonstrated that SCO2 directly interacts with the chlorophyll-binding LHCB1 proteins, being confirmed in planta using bimolecular fluorescence complementation (BIFC). Furthermore, ultrastructural analysis of sco2-1 chloroplasts revealed that formation and movement of transport vesicles from the inner envelope to the thylakoids is perturbed. SCO2 does not interact with the signal recognition particle proteins SRP54 and FtsY, which were shown to be involved in targeting of LHCB1 to the thylakoids. We hypothesize that SCO2 provides an alternative targeting pathway for light-harvesting chlorophyll binding (LHCB) proteins to the thylakoids via transport vesicles predominantly in cotyledons, with the signal recognition particle (SRP) pathway predominant in rosette leaves. Therefore, we propose that SCO2 is involved in the integration of LHCB1 proteins into the thylakoids that feeds back on the regulation of the tetrapyrrole biosynthetic pathway and nuclear gene expression.     

Mevalonate activating enzymes and phosphatases in higher plants

The pH optima of mevalonate kinase and phosphatases in green leaves, cotyledons and chloroplasts of French bean, and in green leaves and chloroplasts of maize, have been studied. Whereas in chloroplasts the pH optimum for mevalonate kinase is at pH 7·5 with little or no activity at pH 5·5, there is with leaf and cotyledon preparations appreciable activity at the lower pH. Under some circumstances isoelectric focusing studies have given fractions showing mevalonate kinase activity at only pH 7·5 or 5·5. Acid phosphatase and ATPase activity in preparations is maximal at pH 5·5 and is much reduced in the presence of high levels of phosphate. Other investigations reported concern the stability of mevalonate kinase and phosphatase activity at pH 5·5 and 7·5 on ageing of extracts, and the activity of mevalonate kinase on greening of etiolated French bean cotyledons. The influence of metal cofactors and fluoride on mevalonate kinase and phosphatase are reported.     

The Effect of Root Hypoxia and Iron Deficiency on the Photosynthesis, Biochemical Composition, and Structure of Pea Chloroplasts

The combined effect of root hypoxia and iron deficiency on biochemical composition, photosynthetic indices, and structure of pea (Pisum sativum L.) chloroplasts were investigated. Both factors suppressed chlorophyll accumulation and leaf photosynthetic activity, causing chlorosis. It was shown, that iron deficiency reduced more severe the light-harvesting complexes of photosystems (PS), and root hypoxia, the reaction center complexes of the photosystem I (PSI) and photosystem II (PSII). The combined action of both factors was stronger than the effect of each factor. However, even in yellow and almost white leaves, chloroplasts contained small amounts of all pigment–protein complexes and maintained weak photosynthetic activity, although their structure was poorly developed and comprised only vesicles and small thylakoids capable to form contacts and small grana. The conclusion is that the mechanisms of root hypoxia and iron deficiency destructive action are different and these factors differently and independently influenced leaf chloroplasts.     

Phosphorylation of PSII proteins in maize thylakoids in the presence of Pb ions

Lead is potentially toxic to all organisms including plants. Many physiological studies suggest that plants have developed various mechanisms to contend with heavy metals, however the molecular mechanisms remain unclear. We studied maize plants in which lead was introduced into detached leaves through the transpiration stream. The photochemical efficiency of PSII, measured as an Fv/Fm ratio, in the maize leaves treated with Pb was only 10% lower than in control leaves. The PSII activity was not affected by Pb ions in mesophyll thylakoids, whereas in bundle sheath it was reduced. Protein phosphorylation in mesophyll and bundle sheath thylakoids was analyzed using mass spectrometry and protein blotting before and after lead treatment. Both methods clearly demonstrated increase in phosphorylation of the PSII proteins upon treatment with Pb²⁺, however, the extent of D1, D2 and CP43 phosphorylation in the mesophyll chloroplasts was clearly higher than in bundle sheath cells. We found that in the presence of Pb ions there was no detectable dephosphorylation of the strongly phosphorylated D1 and PsbH proteins of PSII complex in darkness or under far red light. These results suggest that Pb²⁺ stimulates phosphorylation of PSII core proteins, which can affect stability of the PSII complexes and the rate of D1 protein degradation. Increased phosphorylation of the PSII core proteins induced by Pb ions may be a crucial protection mechanism stabilizing optimal composition of the PSII complexes under metal stress conditions. Our results show that acclimation to Pb ions was achieved in both types of maize chloroplasts in the same way. However, these processes are obviously more complex because of different metabolic status in mesophyll and bundle sheath chloroplasts.     

Developmental changes in energy dissipation in etiolated wheat seedlings during the greening process

We studied the developmental changes in photosynthetic and respiration rates and thermal dissipation processes connected with chloroplasts and mitochondria activity in etiolated wheat (Triticum aestivum L., var. Irgina) seedlings during the greening process. Etioplasts gradually developed into mature chloroplasts under continuous light [190 μmol(photon) m^?2 s^?1] for 48 h in 5-day-dark-grown seedlings. The net photosynthetic rate of irradiated leaves became positive after 6 h of illumination and increased further. The first two hours of de-etiolation were characterized by low values of maximum (F_v/F_m) and actual photochemical efficiency of photosystem II (PSII) and by a coefficient of photochemical quenching in leaves. F_v/F_m reached 0.8 by the end of 24 h-light period. During greening, energy-dependent component of nonphotochemical quenching of chlorophyll fluorescence, violaxanthin cycle (VXC) operation, and lipoperoxidation activity changed in a similar way. Values of these parameters were the highest at the later phase of de-etiolation (4–12 h of illumination). The respiration rate increased significantly after 2 h of greening and it was the highest after 4–6 h of illumination. It was caused by an increase in alternative respiration (AP) capacity. The strong, positive linear correlation was revealed between AP capacity and heat production in greening tissues. These results indicated that VXC in chloroplasts and AP in mitochondria were intensified as energy-dissipating systems at the later stage of greening (after 4 h), when most of prolamellar bodies converted into thylakoids, and they showed the greatest activity until the photosynthetic machinery was almost completely developed.     

Changes in photosynthetic metabolism induced by tobamovirus infection in Nicotiana benthamiana studied in vivo by thermoluminescence

* In thylakoids from Nicotiana benthamiana infected with the pepper mild mottle virus (PMMoV), a decreased amount of the PsbP and PsbQ proteins of photosystem II and different proteins of the Calvin cycle have been previously observed. We used thermoluminescence to study the consequences in vivo. * Measurements on unfrozen discs from symptomatic and asymptomatic leaves of plants infected by two tobamovirus PMMoV-S and PMMoV-I strains were compared with homologous samples in control plants. * Thermoluminescence emission did not reveal noticeable alteration of PSII electron transfer activity in infected symptomatic leaves. In these leaves, the relative intensity of the 'afterglow' emission indicated an increase of the NADPH + ATP assimilatory potential, contrasting with its decrease in asymptomatic leaves. High-temperature thermoluminescence, as a result of peroxides, increased in symptomatic and asymptomatic leaves. * In young infected leaves, PSII activity is preserved, producing a high assimilatory potential. Older asymptomatic leaves export more nutrients towards young infected leaves. This depresses their assimilatory potential and weakens their defence mechanisms against reactive oxygen species, resulting in higher peroxide content.     

The Arabidopsis PsbO2 protein regulates dephosphorylation and turnover of the photosystem II reaction centre D1 protein

The extrinsic photosystem II (PSII) protein of 33 kDa (PsbO), which stabilizes the water-oxidizing complex, is represented in Arabidopsis thaliana (Arabidopsis) by two isoforms. Two T-DNA insertion mutant lines deficient in either the PsbO1 or the PsbO2 protein were retarded in growth in comparison with the wild type, while differing from each other phenotypically. Both PsbO proteins were able to support the oxygen evolution activity of PSII, although PsbO2 was less efficient than PsbO1 under photoinhibitory conditions. Prolonged high light stress led to reduced growth and fitness of the mutant lacking PsbO2 as compared with the wild type and the mutant lacking PsbO1. During a short period of treatment of detached leaves or isolated thylakoids at high light levels, inactivation of PSII electron transport in the PsbO2-deficient mutant was slowed down, and the subsequent degradation of the D1 protein was totally inhibited. The steady-state levels of in vivo phosphorylation of the PSII reaction centre proteins D1 and D2 were specifically reduced in the mutant containing only PsbO2, in comparison with the mutant containing only PsbO1 or with wild-type plants. Phosphorylation of PSII proteins in vitro proceeded similarly in thylakoid membranes from both mutants and wild-type plants. However, dephosphorylation of the D1 protein occurred much faster in the thylakoids containing only PsbO2. We conclude that the function of PsbO1 in Arabidopsis is mostly in support of PSII activity, whereas the interaction of PsbO2 with PSII regulates the turnover of the D1 protein, increasing its accessibility to the phosphatases and proteases involved in its degradation.     

Ultrastructural localization of diaminobenzidine photooxidation in etiochloroplasts

Summary Photooxidation of diaminobenzidine (DAB) has been used to detect ultrastructurally the photosynthetic activity (photosystem I) in the thylakoids of etiochloroplasts in greening bean leaves. The result of this photooxidation is a dark (osmiophilic) deposit which accumulates at first in certain portions of the primary thylakoids. In the course of further greening this area enlarges more and more and at last all the thylakoids become uniformly dark. It has been shown that the beginning of the appearance of the DAB deposits and the speed of their accumulation in the thylakoids largely depends on the experimental conditions of the plants: in leaves maintained in damp atmosphere the first DAB deposits appear between the first and the second hour of greening, while in those kept in dry air this does not happen until three or more hours in light.The tubules of the transformed prolamellar bodies in etiochloroplast—at least at the beginning of their dispersion — do not react with DAB. The tubules formedde novo after a period of darkness in young chloroplasts remain also without DAB deposits.     

Chloroplast protection in greening leaves

Changes in photosynthetic activity, leaf pigments and the activities of enzymes that scavenge damaging oxygen species in chloroplasts were followed during the greening of 8-day-old etiolated pea (Pisum sativum L. cv. Meteor) seedlings. Accumulation of chlorophyll and carotenoids was accompanied by development of photosynthetic activity. Carotenoids present in etiolated leaves, and the high ratio of carotenoid to chlorophyll detected during the early hours of greening are suggested to provide important protection against singlet oxygen. Superoxide dismutase, ascor-bate peroxidase and glutathione reductase, which scavenge superoxide and hydrogen peroxide in chloroplasts, are present at high activities in etiolated leaves and throughout greening. The mechanisms by which developing chloroplasts may generate damaging oxygen species, and the role of these scavengers during greening is discussed.     

Dephosphorylation of photosystem II reaction center proteins in plant photosynthetic membranes as an immediate response to abrupt elevation of temperature

Kinetic studies of protein dephosphorylation in photosynthetic thylakoid membranes revealed specifically accelerated dephosphorylation of photosystem II (PSII) core proteins at elevated temperatures. Raising the temperature from 22 degrees C to 42 degrees C resulted in a more than 10-fold increase in the dephosphorylation rates of the PSII reaction center proteins D1 and D2 and of the chlorophyll a binding protein CP43 in isolated spinach (Spinacia oleracea) thylakoids. In contrast the dephosphorylation rates of the light harvesting protein complex and the 9-kD protein of the PSII (PsbH) were accelerated only 2- to 3-fold. The use of a phospho-threonine antibody to measure in vivo phosphorylation levels in spinach leaves revealed a more than 20-fold acceleration in D1, D2, and CP43 dephosphorylation induced by abrupt elevation of temperature, but no increase in light harvesting protein complex dephosphorylation. This rapid dephosphorylation is catalyzed by a PSII-specific, intrinsic membrane protein phosphatase. Phosphatase assays, using intact thylakoids, solubilized membranes, and the isolated enzyme, revealed that the temperature-induced lateral migration of PSII to the stroma-exposed thylakoids only partially contributed to the rapid increase in the dephosphorylation rate. Significant activation of the phosphatase coincided with the temperature-induced release of TLP40 from the membrane into thylakoid lumen. TLP40 is a peptidyl-prolyl cis-trans isomerase, which acts as a regulatory subunit of the membrane phosphatase. Thus dissociation of TLP40 caused by an abrupt elevation in temperature and activation of the membrane protein phosphatase are suggested to trigger accelerated repair of photodamaged PSII and to operate as possible early signals initiating other heat shock responses in chloroplasts.     

Effect of Grapevine Leafroll on the Photosynthesis of Field Grown Grapevine Plants (Vitis vinifera L. cv. Lagrein)

We have studied the effect of grapevine leafroll infection on some features of the thylakoids from field grown grapevine (Vitis vinifera L.) leaves. Changes in photosynthetic pigments, soluble proteins, ribulose‐1,5‐bisphosphate carboxylase (RuBP), nitrate reductase, photosynthetic activities and thylakoid membrane proteins were investigated. The level of total chlorophyll (Chl) and carotenoids were reduced in virus‐infected leaves. Similar results were also observed for soluble proteins and RuBP case activity. The in vivo nitrate reductase activity was significantly reduced in infected leaves. Virus infection considerably decreased leaf net photosynthetic rate (P_n), stomatal conductance (g_s) and transpiration rate (E) in grapevine leaves. When various photosynthetic activities were followed in isolated thylakoids, virus infection caused marked inhibition of whole chain and photosystem (PS) II activity while the inhibition of PSI activity was only marginal. The artificial exogenous electron donors, diphenyl carbazide and hydroxylamine (NH₂OH) significantly restored the loss of PSII activity in infected leaves. The same results were obtained when F_v/F_m was evaluated by Chl fluorescence measurements. The marked loss of PSII activity in infected leaves could be due to the loss of 47, 43, 33, 28–25, 23 and 17 kDa polypeptides. It is concluded that virus infection inactivates the donor side of PSII. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the water‐splitting complex was diminished significantly in infected leaves.     

Changes in some thylakoid membrane proteins and pigments upon desiccation of the resurrection plant Haberlea rhodopensis

The changes in some proteins involved in the light reactions of photosynthesis of the resurrection plant Haberlea rhodopensis were examined in connection with desiccation. Fully hydrated (control) and completely desiccated plants (relative water content (RWC) 6.5%) were used for thylakoid preparations. The chlorophyll (Chl) a to Chl b ratios of thylakoids isolated from control and desiccated leaves were very similar, which was also confirmed by measuring their absorption spectra. HPLC analysis revealed that β-carotene content was only slightly enhanced in desiccated leaves compared with the control, but the zeaxanthin level was strongly increased. Desiccation of H. rhodopensis to an air-dried state at very low light irradiance led to a little decrease in the level of D1, D2, PsbS and PsaA/B proteins in thylakoids, but a relative increase in LHC polypeptides. To further elucidate whether the composition of the protein complexes of the thylakoid membranes had changed, we performed a separation of solubilized thylakoids on sucrose density gradients. In contrast to spinach, Haberlea thylakoids appeared to be much more resistant to the same solubilization procedure, i.e. complexes were not separated completely and complexes of higher density were found. However, the fractions analyzed provided clear evidence for a move of part of the antenna complexes from PSII to PSI when plants became desiccated. This move was also confirmed by low temperature emission spectra of thylakoids.Overall, the photosynthetic proteins remained comparatively stable in dried Haberlea leaves when plants were desiccated under conditions similar to their natural habitat. Low light during desiccation was enough to induce a rise in the xanthophyll zeaxanthin and β-carotene. Together with the extensive leaf shrinkage and some leaf folding, increased zeaxanthin content and the observed shift in antenna proteins from PSII to PSI during desiccation of Haberlea contributed to the integrity of the photosynthetic apparatus, which is important for rapid recovery after rehydration.     

Effect of copper deficiency on photosynthetic electron transport in wheat plants

Copper deficiency in wheat ( Triticum aestivum L. cv. Nazareno Stramppeli) markedly affects photosynthetic activity. Flag leaves of copper-deficient plants showed a 50% reduction of the photosynthetic rate expressed as mg CO₂ dm⁻²h⁻¹. The activities of PSI and PSII, determined for isolated chloroplasts, as well as fluorescence measurements on intact leaves of copper-deficient plants, indicated a low activity of photosynthetic electron transport. Ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity was not affected by copper deficiency but copper deficiency affected the chloroplast ultrastructure, especially at the level of grana, where a disorganization of thylakoids is evident.     

The Heterogeneity of Structural and Functional Photosynthetic Characteristics of Mesophyll Chloroplasts in Various Parts of Mature or Senescing Leaf Blade of Two Maize (Zea Mays L.) Genotypes

Differences in ultrastructural parameters of mesophyll cell (MC) chloroplasts, contents of photosynthetic pigments, and photochemical activities of isolated MC chloroplasts were studied in the basal, middle, and apical part of mature or senescing leaf blade of two maize genotypes. A distinct heterogeneity of leaf blade was observed both for structural and functional characteristics of chloroplasts. In both mature and senescing leaves the shape of MC chloroplasts changed from flat one in basal part of leaf to nearly spherical one in leaf apex. The volume density of granal thylakoids decreased from leaf base to apex in both types of leaves examined, while the amount of intergranal thylakoids increased in mature leaves but decreased in senescing leaves. The most striking heterogeneity was found for the quantity of plastoglobuli, which strongly increased with the increasing distance from leaf base. The differences in chloroplast ultrastructure were accompanied by differences in other photosynthetic characteristics. The Hill reaction activity and activity of photosystem 1 of isolated MC chloroplasts decreased from leaf base to apex in mature leaves. Apical part of senescing leaf blade was characterised by low contents of chlorophyll (Chl) a and Chl b, whereas in mature leaves, the content of Chls as well as the content of total carotenoids (Car) slightly increased from basal to apical leaf part. This was reflected also in the ratio Chl (a+b)/total Car; the ratio of Chl a/b did not significantly differ between individual parts of leaf blade. Both genotypes examined differed in the character of developmental gradient observed along whole length of leaf blade.     

Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity

Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.     

Immunogold localization of a transglutaminase related to grana development in different maize cell types

Summary A comparative study of the subcellular localization of a plant transglutaminase (TGase; EC 2.3.2.13) in various in vivo and in vitro maize cell types was carried out with a polyclonal antibody raised against a 58 kDa TGase purified fromHelianthus tuberosus leaves. Immunocytochemical staining, followed by electron microscopy, showed that this enzyme was markedly present in the grana-appressed thylakoids of mature chloroplasts of the lightexposed cells. Moreover, during embryogénie callus chloroplast differentiation, the abundance of TGase in the grana-appressed thylakoids depended on the degree of grana development and was greater than in mature leaf chloroplasts. In addition to the 58 kDa form, two other forms of the protein (of 77 and 34 kDa) were obtained by Western blot. The 77 kDa form might correspond to the inactive form and was immunodetected in dense vesicles observed in dark-grown embryogenie callus cells. In adult leaves, the enzyme was also markedly present in the grana-appressed thylakoids of the mesophyll cell chloroplasts, though very scarce and dispersed in the bundle-sheath cell chloroplasts (which do not contain grana). The concordance of these localizations with those described for the light-harvesting antenna proteins of the photosystem II suggests that it is possible that this TGase has a functional role in photosynthesis, perhaps modulating the photosynthetic efficiency and the absorption of excess light by means of polyamine conjugation to the antenna proteins.     

Insensitivity of Water-Oxidation and Photosystem II Activity in Tomato to Chilling Temperatures

Chilling tomato plants (Lycopersicon esculentum Mill. cv. Rutgers and cv. Floramerica) in the dark resulted in a sizable inhibition in the rate of light- and CO₂-saturated photosynthesis. However, at low light intensity, the inhibition disappeared and the absolute quantum yield of CO₂ reduction was diminished only slightly. The quantum yield of photosystem II (PSII) electron flow was 18% lower when measured in chloroplasts isolated from chilled leaves than in chloroplasts isolated from unchilled leaves. Even though the maximum rate of PSII turnover in these chloroplasts was 12% lower subsequent to chilling, it was in all cases two or more times that required to support the light- and CO₂-saturated rate of photosynthesis measured in the attached leaf. The concentration of active PSII centers in chloroplasts isolated from leaves either before or after chilling was determined by measurement of the products of water oxidation from a series of saturating flashes short enough to turnover the electron transport carriers only a single time. There was no significant change in the concentration of active PSII centers due to dark chilling.

It was concluded that PSII activity and water oxidation capacity are not significantly impaired in tomato by chilling in the dark and therefore are not primary aspects of the inhibition of CO₂ reduction observed in attached leaves.