Structural and functional integrity of Sulla carnosa photosynthetic apparatus under iron deficiency conditions

The abundance of calcareous soils makes bicarbonate-induced iron (Fe) deficiency a major problem for plant growth and crop yield. Therefore, Fe-efficient plants may constitute a solution for use on calcareous soils. We investigated the ability of the forage legume Sulla carnosa (Desf.) to maintain integrity of its photosynthetic apparatus under Fe deficiency conditions. Three treatments were applied: control, direct Fe deficiency and bicarbonate-induced Fe deficiency. At harvest, all organs of deficient plants showed severe growth inhibition, the effect being less pronounced under indirect Fe deficiency. Pigment analysis of fully expanded leaves revealed a reduction in concentrations of chlorophyll a, chlorophyll b and carotenoids under Fe deficiency. Electron transport rate, maximum and effective quantum yield of photosystem II (PSII), photochemical quenching (qP), non-photochemical quenching (qN) as well as P700 activity also decreased significantly in plants exposed to direct Fe deficiency, while qN was not affected. The effects of indirect Fe deficiency on the same parameters were less pronounced in bicarbonate-treated plants. The relative abundances of thylakoid proteins related to PSI (PsaA, Lhca1, Lhca2) and PSII (PsbA, Lhcb1) were also more affected under direct than indirect Fe deficiency. We conclude that S. carnosa can maintain the integrity of its photosynthetic apparatus under bicarbonate-induced Fe deficiency, preventing harmful effects to both photosystems under direct Fe deficiency. This suggests a high capacity of this species not only to take up Fe in the presence of bicarbonate (HCO₃^- ) but also to preferentially translocate absorbed Fe towards leaves and prevent its inactivation.

Photoinhibition of photosystem I in a pea mutant with altered LHCII organization

Comparative analysis of in vivo chlorophyll fluorescence imaging revealed that photosystem II (PSII) photochemical efficiency (Fv/Fm) of leaves of the Costata 2/133 pea mutant with altered pigment composition and decreased level of oligomerization of the light harvesting chlorophyll a/b-protein complexes (LHCII) of PSII (Dobrikova et al., 2000; Ivanov et al., 2005) did not differ from that of WT. In contrast, photosystem I (PSI) activity of the Costata 2/133 mutant measured by the far-red (FR) light inducible P700 (P700(+)) signal exhibited 39% lower steady state level of P700(+), a 2.2-fold higher intersystem electron pool size (e(-)/P700) and higher rate of P700(+) re-reduction, which indicate an increased capacity for PSI cyclic electron transfer (CET) in the Costata 2/133 mutant than WT. The mutant also exhibited a limited capacity for state transitions. The lower level of oxidizable P700 (P700(+)) is consistent with a lower amount of PSI related chlorophyll protein complexes and lower abundance of the PsaA/PsaB heterodimer, PsaD and Lhca1 polypeptides in Costata 2/133 mutant. Exposure of WT and the Costata 2/133 mutant to high light stress resulted in a comparable photoinhibition of PSII measured in vivo, although the decrease of Fv/Fm was modestly higher in the mutant plants. However, under the same photoinhibitory conditions PSI photochemistry (P700(+)) measured as ΔA820-860 was inhibited to a greater extent (50%) in the Costata 2/133 mutant than in the WT (22%). This was accompanied by a 50% faster re-reduction rate of P700(+) in the dark indicating a higher capacity for CET around PSI in high light treated mutant leaves. The role of chloroplast thylakoid organization on the stability of the PSI complex and its susceptibility to high light stress is discussed.

Genetic decrease in fatty acid unsaturation of phosphatidylglycerol increased photoinhibition of photosystem I at low temperature in tobacco leaves

Leaves of transgenic tobacco plants with decreased levels of fatty acid unsaturation in phosphatidylglycerol (PG) exhibited a slightly lower level of the steady state oxidation of the photosystem I (PSI) reaction center P700 (P700(+)) than wild-type plants. The PSI photochemistry of wild-type plants was only marginally affected by high light treatments. Surprisingly, all plants of transgenic lines exhibited much higher susceptibility to photoinhibition of PSI than wild-type plants. This was accompanied by a 2.5-fold faster re-reduction rate of P700(+) in the dark, indicating a higher capacity for cyclic electron flow around PSI in high light treated transgenic leaves. This was associated with a much higher intersystem electron pool size suggesting over-reduction of the PQ pool in tobacco transgenic lines with altered PG unsaturation compared to wild-type plants. The physiological role of PG unsaturation in PSI down-regulation and modulation of the capacity of PSI-dependent cyclic electron flows and distribution of excitation light energy in tobacco plants under photoinhibitory conditions at low temperatures is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Restricted capacity for PSI-dependent cyclic electron flow in ΔpetE mutant compromises the ability for acclimation to iron stress in Synechococcus sp. PCC 7942 cells

Exposure of wild type (WT) and plastocyanin coding petE gene deficient mutant (ΔpetE) of Synechococcus cells to low iron growth conditions was accompanied by similar iron-stress induced blue-shift of the main red Chl a absorption peak and a gradual decrease of the Phc/Chl ratio, although ΔpetE mutant was more sensitive when exposed to iron deficient conditions. Despite comparable iron stress induced phenotypic changes, the inactivation of petE gene expression was accompanied with a significant reduction of the growth rates compared to WT cells. To examine the photosynthetic electron fluxes in vivo, far-red light induced P700 redox state transients at 820nm of WT and ΔpetE mutant cells grown under iron sufficient and iron deficient conditions were compared. The extent of the absorbance change (ΔA(820)/A(820)) used for quantitative estimation of photooxidizable P700(+) indicated a 2-fold lower level of P700(+) in ΔpetE compared to WT cells under control conditions. This was accompanied by a 2-fold slower re-reduction rate of P700(+) in the ΔpetE indicating a lower capacity for cyclic electron flow around PSI in the cells lacking plastocyanin. Thermoluminescence (TL) measurements did not reveal significant differences in PSII photochemistry between control WT and ΔpetE cells. However, exposure to iron stress induced a 4.5 times lower level of P700(+), 2-fold faster re-reduction rate of P700(+) and a temperature shift of the TL peak corresponding to S(2)/S(3)Q(B)(-) charge recombination in WT cells. In contrast, the iron-stressed ΔpetE mutant exhibited only a 40% decrease of P700(+) and no significant temperature shift in S(2)/S(3)Q(B)(-) charge recombination. The role of mobile electron carriers in modulating the photosynthetic electron fluxes and physiological acclimation of cyanobacteria to low iron conditions is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

The lack of LHCII proteins modulates excitation energy partitioning and PSII charge recombination in Chlorina F2 mutant of barley

Analysis of the partitioning of absorbed light energy within PSII into fractions utilized by PSII photochemistry (ØPSII), thermally dissipated via ΔpH-and zeaxanthin-dependent energy quenching (ØNPQ) and constitutive non-photochemical energy losses (ØNO) was performed in wild type and F2 mutant of barley. The estimated energy partitioning of absorbed light to various pathways indicated that the fraction of ØPSII was slightly higher, while the proportion of thermally dissipated energy through ØNPQ was 38% lower in F2 mutant than in WT. In contrast, ØNO, i.e. the fraction of absorbed light energy dissipated by additional quenching mechanism(s) was 34% higher in F2 mutant. The increased proportion of ØNO correlated with narrowing the temperature gap (ΔT M) between S2/3QB- and S2QA- charge recombinations in F2 mutant as revealed by thermoluminescence measurements. We suggest that this would result in increased probability for an alternative non-radiative P680+QA- radical pair recombination pathway for energy dissipation within the reaction centre of PSII (reaction center quenching) and that this additional quenching mechanism might play an important role in photoprotection when the capacity for the primary, zeaxanthin-dependent non-photochemical quenching (ØNPQ) and state transitions pathways are restricted in the absence of LHCII polypeptides in F2 mutant.

Seasonal responses of photosynthetic electron transport in Scots pine (Pinus sylvestris L.) studied by thermoluminescence

The potential of photosynthesis to recover from winter stress was studied by following the thermoluminescence (TL) and chlorophyll fluorescence changes of winter pine needles during the exposure to room temperature (20 degrees C) and an irradiance of 100 micromol m(-2) s(-1). TL measurements of photosystem II (PSII) revealed that the S(2)Q(B)(-) charge recombinations (the B-band) were shifted to lower temperatures in winter pine needles, while the S(2)Q(A)(-) recombinations (the Q-band) remained close to 0 degrees C. This was accompanied by a drastically reduced (65%) PSII photochemical efficiency measured as F(v)/ F(m,) and a 20-fold faster rate of the fluorescence transient from F(o) to F(m) as compared to summer pine. A strong positive correlation between the increase in the photochemical efficiency of PSII and the increase in the relative contribution of the B-band was found during the time course of the recovery process. The seasonal dynamics of TL in Scots pine needles studied under field conditions revealed that between November and April, the contribution of the Q- and B-bands to the overall TL emission was very low (less than 5%). During spring, the relative contribution of the Q- and B-bands, corresponding to charge recombination events between the acceptor and donor sides of PSII, rapidly increased, reaching maximal values in late July. A sharp decline of the B-band was observed in late summer, followed by a gradual decrease, reaching minimal values in November. Possible mechanisms of the seasonally induced changes in the redox properties of S(2)/S(3)Q(B)(-) recombinations are discussed. It is proposed that the lowered redox potential of Q(B) in winter needles increases the population of Q(A)(-), thus enhancing the probability for non-radiative P680(+)Q(A)(-) recombination. This is suggested to enhance the radiationless dissipation of excess light within the PSII reaction center during cold acclimation and during cold winter periods.

Low-temperature stress and photoperiod affect an increased tolerance to photoinhibition in Pinus banksiana seedlings

The capacity to develop tolerance to photoinhibition of photosynthesis was assessed in jack pine seedlings (Pinus banksiana Lamb.). Photoinhibition induced at 5 °C in control jack pine seedlings grown at 20 °C was saturated above an irradiance of 1000 μmol ∙ m−2 ∙ s−1 but was detectable at an irradiance as low as 25 μmol ∙ m−2 ∙ s−1. However, 20 °C seedlings shifted to 5 °C were 2-fold more tolerant to photoinhibition than 20 °C unshifted control seedlings, as detected by either the light-dependent decrease in photochemical efficiency or the apparent quantum yield of O2 evolution. The extent of this tolerance of photoinhibition was dependent upon time, photoperiod, and irradiance during exposure to the low-temperature shift. Furthermore, the tolerance of photoinhibition was correlated with anthocyanin accumulation in 20 °C grown seedlings shifted to 5 °C. In addition, seedlings shifted to 5 °C and an 8-h photoperiod exhibited a 2-fold higher yield of photosystem II electron transport, which was associated with an increased capacity to keep QA, the first stable quinone electron acceptor of photosystem II, oxidized at high irradiance. This was consistent with a 2-fold higher rate of photosynthesis on a chlorophyll basis. We propose that the combination of light attenuation by anthocyanin in the epidermis and enhanced rates of photosynthesis may, in part, account for the reduced sensitivity of jack pine to photoinhibition at low temperature. Key words: anthocyanin, light attenuation, low temperature, Pinus banksiana Lamb, (jack pine), photosynthesis, photoinhibition, photoperiod.

Major sclerotial polypeptides of psychrophilic fungi: temperature regulation of in vivo synthesis in vegetative hyphae

Western blot analysis of the major sclerotial polypeptides of the psychrophilic species Myriosclerotinia borealis (W51) and Coprinus psychromorbidus (LRS131) indicated that these polypeptides were not present in vegetative hyphae during growth at permissive temperature (5 °C), but significant accumulations were observed in hyphae upon prolonged exposure to nonpermissive temperature (25 °C). In contrast, low levels of sclerotial polypeptides were detected in the vegetative hyphae of Typhula incarnata (W29) and Typhula idahoensis (W21). We show, for the first time, that the in vivo synthesis of the major sclerotial polypeptides was induced when vegetative hyphae of M. borealis and C. psychromorbidus were shifted from 5 to 10 °C for 12 h. In contrast, vegetative hyphae of T. idahoensis and T. incarnata appeared to synthesize low levels of sclerotial polypeptides constitutively at 5 °C. Furthermore, a shift from 5 to 10 °C had little effect on the synthesis of major sclerotial polypeptides in the Typhula species. Prior exposure of vegetative hyphae from all species to 25 °C for 2 days caused a marked reduction in the capacity to synthesize sclerotial polypeptides. However, vegetative hyphae of T. incarnata synthesized a new polypeptide of 35 kDa that had not been detected previously. Antiserum to low molecular mass maize heat-shock polypeptides cross reacted with the major sclerotial polypeptides of T. idahoensis only. We conclude that the more psychrophilic species examined, M. borealis (W51) and C. psychromorbidus (LRS131), exhibit temperature-induced synthesis and accumulation of sclerotial polypeptides in vegetative hyphae. In contrast, sclerotial polypeptides of the less psychrophilic Typhula species appear to be expressed constitutively in vegetative hyphae.

Major polypeptides associated with differentiation in psychrophilic fungi

Major polypeptides were observed upon one-dimensional sodium dodecyl sulfate gel electrophoresis of sclerotial extracts of the following psychrophiles: Myriosclerotinia borealis, Coprinus psychromorbidus, Typhula idahoensis, and Typhula incarnata. In general, the number, molecular mass, and relative proportion of these major sclerotial polypeptides varied considerably from species to species. Furthermore, in the case of M. borealis, the major sclerotial polypeptide did not appear to be an artifact of culturing conditions since a major polypeptide of similar molecular mass was also present in sclerotia of M. borealis collected from the field. Generally, the major sclerotial polypeptides were visible in the sclerotial initials but were not apparent in the vegetative hyphae. Thus, these major sclerotial polypeptides appear to be expressed as a function of sclerotial development. Electrophoresis of protein extracts of T. idahoensis and T. incarnata initially solubilized either in sodium dodecyl sulfate or urea sample buffer indicated that the type of denaturant initially used had a profound influence on the relative proportions of the major polypeptides and the overall polypeptide profile. Isoelectric focusing of sclerotial extracts indicated that the isoelectric points of the major sclerotial polypeptides of M. borealis ranged from 6.2 to 6.7, whereas the values of the major sclerotial polypeptides of the other three species were basic and ranged from 7.0 to 7.7.

The effects of temperature on the growth and polypeptide composition of several snow mold species

Myriosclerotinia borealis (W51), Coprinus spp. (13W1 and 14W2), Typhula idahoensis (W21), and Typhula incarnata (W29) were incubated in the dark on a defined agar medium at permissive (4 °C) and nonpermissive temperatures (22 and 30 °C). Isolates of Coprinus spp. and Typhula spp. required higher temperatures than M. borealis to arrest vegetative growth completely. The effects of incubation at permissive and nonpermissive temperatures on the polypeptide compositions of M. borealis, Coprinus spp., T. idahoensis (W21), and T. incarnata (W29) were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The results indicated that the number of polypeptides in the polypeptide complement of M. borealis and both Typhula species decreased significantly during incubation at nonpermissive temperatures. In contrast, Coprinus sp. (13W1) showed no significant change in the number of polypeptides observed during incubation at nonpermissive temperatures. Furthermore, there appeared to be an increase in the relative proportion of at least three polypeptides during incubation of Coprinus at nonpermissive temperatures. The significance of these species-dependent responses to nonpermissive growth temperatures is discussed.

Light microscopic and polypeptide analyses of sclerotia from mesophilic and psychrophilic pathogenic fungi

The structure and polypeptide composition of sclerotia of three mesophilic pathogenic fungi, Sclerotinia sclerotiorum, Sclerotium rolfsii, and Botrytis cinerea, and one psychrophilic snow mold, Myriosclerotinia borealis, were compared. The sclerotia of S. sclerotiorum and B. cinerea were black, round, hard structures which were composed of three areas: the rind, the cortex, and the medulla. Both the cortical and medullary areas of these sclerotia exhibited intensely stained inclusions. In contrast, sclerotia of M. borealis were not present as discrete entities but coalesced near the central point of inoculation. These black sclerotial masses were composed of thin-walled, pseudoparenchymal cells tightly packed together to form two distinct areas: a rind and a medullarylike region. No inclusions were evident in the medulla of cells of M. borealis sclerotia. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of sclerotial extracts of mesophilic fungi indicated the presence of major polypeptides. The polypeptide complement of B. cinerea contained a single, major polypeptide with an apparent molecular mass of 33 to 36 kDa. Similarly, sclerotia of S. sclerotiorum contained one major polypeptide of approximately 36 kDa in addition to one minor polypeptide of about 18 kDa. However, sclerotia of S. rolfsii contained a major polypeptide of about 16 kDa. Sclerotia of M. borealis contained a major polypeptide of 32 to 36 kDa and a minor polypeptide of about 16 kDa.

Acclimation of winter rye to cold-hardening temperatures results in an increased capacity for photosynthetic electron transport

Photosynthetic electron transport and its partial reactions were measured as a function of light intensity and temperature in isolated chloroplasts from cold-hardened and unhardened rye (Secale cereale L. cv. Puma). Chloroplasts from cold-hardened rye plants exhibited light-saturated rates for whole chain electron transport that were about 1.4-fold higher at 25 °C than those observed in chloroplasts from unhardened rye plants. This was correlated with light-saturated rates of electron transport through photosystem I that were 1.6-fold higher in cold-hardened chloroplasts than in unhardened chloroplasts. These results could not be attributed to a differential uncoupling of electron transport, nor to a differential production of a superoxide free radical which would stimulate net O2 consumption. Electron transport through photosystem II of cold-hardened and unhardened chloroplasts exhibited a differential sensitivity to temperature. The ratio of light-saturated rates in cold-hardened chloroplasts to light-saturated rates in unhardened chloroplasts increased from 0.87 to 1.40 when the temperature was decreased from 26 to 4.5 °C. The locus of this differential temperature sensitivity appeared to reside on the O2 evolving side of photosystem II prior to the site of electron donation by diphenylcarbazide. On the basis of the estimates of the number of photons required for half the maximal rates of electron transport, it was concluded that the efficiency of light utilization by photosystem I and II was unaffected by growth temperature. However, the estimated efficiency for light utilization did increase with a decrease in reaction temperature for chloroplasts from both cold-hardened and unhardened ‘Puma’ rye. Thus, it is concluded that growth at cold-hardening temperatures resulted in an increase in the capacity for photosynthetic electron transport, but did not affect the quantum efficiency for this process.

An appropriate physiological control for environmental temperature studies: comparative growth kinetics of winter rye

The accurate interpretation of physiological and biochemical alterations observed in plants grown under contrasting environmental conditions requires knowledge of their relative physiological ages. For this purpose, we compared the growth kinetics of winter rye (Secale cereale L. cv. Puma) at nonhardening and cold-hardening temperatures. Growth at nonhardening temperatures was characterized by a 10-day lag phase with the attainment of maximum growth after about 28 days. Growth at cold-hardening temperatures resulted in an extension of the lag phase to about 21 days with maximum growth being attained after 56 days. The calculated growth coefficient at cold-hardening temperatures was 35–40% of that at nonhardening temperatures. This relationship was consistent with growth parameters such as leaf dry weight, fresh weight, and area, but not with plant height. Although total leaf dry weight and total number of leaves per plant did not differ between nonhardened and cold-hardened plants at maximum growth, total leaf area per plant and stretched plant height was 3- to 4-times greater in nonhardened than in cold-hardened plants. This resulted in a fourfold increase in leaf dry weight per leaf area during growth at low temperature in contrast to the maintenance of a constant ratio during growth at nonhardening conditions. The increase in this ratio during low temperature growth was, in part, accounted for by a decrease in water content and an increase in cytoplasmic content. These results were confirmed by the investigation of growth on an individual leaf basis. However, the growth response of leaves 1 and 2 differed from that of leaves 3 and 4 when the leaf dry weight: leaf area ratio was measured as a function of time at cold-hardening temperatures. This indicates that the stage of leaf development influences its growth response to an altered environment. The results of the development of leaf freezing tolerance indicated that maximum vegetative growth appeared to coincide with maximum freezing tolerance of leaves from cold-hardened plants (−22 °C) but not of leaves from unhardened plants (−11 °C).

Growth and development at cold-hardening temperatures. Chlorophyll–protein complexes and thylakoid membrane polypeptides

Chlorophyll–protein complexes of thylakoid membranes from rye plants (Secale cereale L. cv. Puma) grown at warm and cold-hardening temperatures were investigated by gel electrophoresis. Complex IV from cold-grown tissue was detectable in the presence of dodecyl sulfate if and only if solubilization and electrophoresis were performed at 4 °C, whereas complex IV from warm-grown material was detectable if membrane solubilization and electrophoresis were performed at either 4 or 23 °C in the presence of dodecyl sulfate. In the presence of octyl-β-D-glucopyranoside, the chlorophyll–protein complexes from cold-grown tissue were less stable at 23 °C than those from warm-grown tissue. Regardless of the detergent used, there was always more oligomer of the light-harvesting complex present in samples prepared from thylakoid membranes of warm-grown tissue than those from membranes of cold-grown tissue. It is concluded that the pigment–protein interaction in those complexes associated with photosystem II and the light-harvesting chlorophyll a/b – protein complex has been altered upon growth and development at cold-hardening temperatures.