Energy Transfer from Phycobilisomes to Photosystems of Nostoc flagelliforme Born. et Flah. During the Rewetting Course and Its Physiological Significance

During the non-frost season, the condensation of dew makes Nostocflagelliforme Born. et Flah., a highly drought-tolerant terrestrial cyanobacterium, frequently undergo rehydration-dehydration.Rehydration begins in the dark at night. After rewetting in the dark, photochemical activity and the structure of photosystem (PS) Ⅱ were not recovered at all; the structure of PSⅠ, energy transfer in phycobilisomes, and energy transfer from phycobilisomes to PSⅠ were recovered within 5 min, as in the light. The recovery of energy transfer from phycobilisomes to PSⅡ was light dependent and energy transfer from phycobilisomes to PSⅡ was only partially recovered in the dark. These results suggest that the two-trigger control (water and light) of photosynthetic recovery may make N. flagelliforme avoid unnecessary energy consumption and, at the same time, the partial recovery of energy transfer from phycobilisomes to PSⅡ in the dark could help N. flagelliforme accumulate more photosynthetic products during the transient period of rehydrationdehydration.     

Dissection of genetic overlap of salt tolerance QTLs at the seedling and tillering stages using backcross introgression lines in rice

QTLs for salt-tolerance(ST)related traits at the seedling and tillering stages were identified using 99 BC2F8 introgression lines(IL)derived from a cross between IR64(indica)as a recurrent parent and Binam(japonica)from Iran as the donor parent.Thirteen QTLs affecting survival days of seedlings(SDS), score of salt toxicity of leaves(SST),shoot K concentration(SKC)and shoot Na concentration(SNC) at the seedling stage and 22 QTLs underlying fresh weight of shoots(FW),tiller number per plant(TN) and plant height(PH)at the tillering stage were identified.Most QTLs detected at the tillering stage showed obvious differential expression to salt stress and were classified into three types based on their differential behaviors.Type I included 11 QTLs which were expressed only under the non-stress condition.Type II included five QTLs expressed in the control and the salt stress conditions,and three of them(QPh5,QPh8 and QTn9)had similar quantity and the same direction of gene effect,suggesting their expression was less influenced by salt stress.Type III included six QTLs which were detectable only under salt stress,suggesting that these QTLs were apparently induced by the stress.Thirteen QTLs affecting trait difference or trait stability of ILs between the stress and non-stress conditions were identified and the Binam alleles at all loci except QPh4,QTn2 and QFw2a decreased trait difference.The three QTLs less influenced by the stress and 13 QTLs affecting trait stability were considered as ST QTLs which contributed to ST.Comparing the distribution of QTLs detected at the seedling and tillering stages,most(69%)of them were genetically independent.Only four were the same or adjacent regions on chromosomes 1,2,8 and 11 harboring ST QTLs detected at the two stages,suggesting that partial genetic overlap of ST across the two stages occurs.It is likely,therefore,to develop ST rice variety for both stages by pyramiding of ST QTLs of different stages or selection against the overlapping QTLs between the two stages via marker-assisted selection(MAS).     

Overexpression of a tobacco small G protein gene NtRop1 causes salt sensitivity and hydrogen peroxide production in transgenic plants

The small GTPases of Rop/Rho family is central regulators of important cellular processes in plants. Tobacco small G protein gene NtRop1 has been isolated; however, its roles in stress responses were unknown. In the present study, the genomic sequence of NtRop1 was cloned, which has seven exons and six introns, similar to the Rop gene structure from Arabidopsis. The NtRop1 gene was constitutively expressed in the different organs whereas the other six Rop genes from tobacco had differential expression patterns. The expression of the NtRop1 gene was moderately induced by methyl viologen, NaCl, and ACC treatments, but slightly inhibited by ABA treatment, with no significant induction by NAA treatment. The transgenic Arabidopsis plants overexpressing the NtRop1 showed increased salt sensitivity as can be seen from the reduced root growth and elevated relative electrolyte leakage. The hydrogen peroxide production was also promoted in the NtRop1-trangenic plants in comparison with wild type plants. These results imply that the NtRop1 may confer salt sensitivity through activation of H2O2 production during plant response to salt stress.     

Boron and calcium increasePisum sativum seed germination and seedling development under salt stress

A beneficial effect of B and Ca application on symbiotic interaction between legume and rhizobia under saline conditions has recently been shown, suggesting conventional agricultural practices to increase crop salt tolerance. However, nothing is known about application of both nutrients on early events of legume development under salt stress, prior to the establishment of a symbiotic interaction. Therefore, the effects of different levels of B (from 9.3 to 93μM B) and Ca (from 0.68 to 5.44 mM Ca) on seed germination, root elongation, plant development, and mineral composition of pea (Pisum sativum L. cv. Argona) grown under 0 to 150 mM NaCl, were analysed. Development of plants previously germinated in the presence of salt was more impaired than that of plants put under salt stress once seeds were germinated. A NaCl concentration of 75 mM and 150 mM inhibited pea seed germination and seedling growth. The addition of either extra B or extra Ca to the germination solution prevented the reduction caused by 75 mM NaCl but not that of 150 mM NaCl. However, root elongation and plant development under salt stress (75 mM NaCl) was enhanced only by addition of both B and Ca. When plants were cultivated in the absence of external N, N content in roots and shoots originating from seeds was diminished by salt and enhanced by B and Ca, suggesting a role of these nutrients in remobilisation of seed nutrient stores. Salinity also led to an extremely high concentration of Na⁺ ions, and to a decrease of B and Ca concentrations. This can be overcome by addition of both nutrients, increasing salt tolerance of developing pea plants. The necessity of nutritional studies to increase crop production in saline soils is discussed and proposed.     

Overexpression of Rice Sphingosine-1-Phoshpate Lyase Gene OsSPL1 in Transgenic Tobacco Reduces Salt and Oxidative Stress Tolerance

Sphingolipids, including sphingosine-1-phosphate (S1P), have been shown to function as signaling mediators to regulate diverse aspects of plant growth, development, and stress response. In this study, we performed functional analysis of a rice (Oryza sativa) S1P lyase gene OsSPL1 in transgenic tobacco plants and explored its possible involvement in abiotic stress response. Overexpression of OsSPL1 in transgenic tobacco resulted in enhanced sensitivity to exogenous abscisic acid (ABA), and decreased tolerance to salt and oxidative stress, when compared with the wild type. Furthermore, the expression levels of some selected stress-related genes in OsSPL1-overexpressing plants were reduced after application of salt or oxidative stress, indicating that the altered responsiveness of stress-related genes may be responsible for the reduced tolerance in OsSPL1-overexpressing tobacco plants under salt and oxidative stress. Our results suggest that rice OsSPL1 plays an important role in abiotic stress responses.     

Inositol Polyphosphate Phosphatidylinositol 5-Phosphatase9 （At5PTase9） Controls Plant Salt Tolerance by Regulating Endocytosis

Phosphatidylinositol 5-phosphatases （5PTases） components of membrane trafficking system. Recently, we that hydrolyze the 5＇ position of the inositol ring are key reported that mutation in AtSPTase7 gene reduced produc- tion of reactive oxygen species （ROS） and decreased expression of stress-responsive genes, resulting in increased salt sensitivity. Here, we describe an even more salt-sensitive 5ptase mutant, At5ptase9, which also hydrolyzes the 5＇ phos- phate groups specifically from membrane-bound phosphatidylinositides. Interestingly, the mutants were more tolerant to osmotic stress. We analyzed the main cellular processes that may be affected by the mutation, such as production of ROS, influx of calcium, and induction of salt-response genes. The At5ptase9 mutants showed reduced ROS produc- tion and Ca2~ influx, as well as decreased fluid-phase endocytosis. Inhibition of endocytosis by phenylarsine oxide or Tyrphostin A23 in wild-type plants blocked these responses. Induction of salt-responsive genes in wild-type plants was also suppressed by the endocytosis inhibitors. Thus, inhibition of endocytosis in wild-type plants mimicked the salt stress responses, observed in the AtSptase9 mutants. In summary, our results show a key non-redundant role of At5PTase7 and 9 isozymes, and underscore the localization of membrane-bound Ptdlns in regulating plant salt tolerance by coordinating the endocytosis, ROS production, Ca2＋ influx, and induction of stress-responsive genes.     

Effects of Glycerol on the Fluorescence Spectra and Chloroplast Ultrastructure of Phaeodactylum tricornutum （Bacillariophyta）

Responses of the photosynthetic activity of Phaeodactylum tricornutum （Bacillariophyta） to organic carbon glycerol were investigated. The growth rate, photosynthetic pigments, 77 K fluorescence spectra, and chloroplast ultrastructure of P. tricornutum were examined under photoautotrophic, mixotrophic, and photoheterotrophic conditions. The results showed that the specific growth rate was the fastest under mixotrophic conditions. The cell photosynthetic pigment content and values of Chl a/Chl c were reduced under mixotrophic and photoheterotrophic conditions. The value of carotenoid/Chl a was enhanced under mixotrophic conditions, but was decreased under photoheterotrophic conditions. In comparison with photoautotrophic conditions, the fluorescence emission peaks and fluorescence excitation peaks were not shifted. The relative fluorescence of photosystem （PS） Ⅰ and PS Ⅱ and the values of F685/F710 and F685/F738 were decreased. Chloroplast thylakoid pairs were less packed under mixotrophic and photoheterotrophic conditions. There was a strong correlation between degree of chloroplast thylakoid packing and the excitation energy kept in PS Ⅱ. These results suggested that the PS Ⅱ activity was reduced by glycerol under mixotrophic conditions, thereby leading to repression of the photosynthetic activity.     

Production of Polyamines Is Enhanced by Endogenous Abscisic Acid in Maize Seedlings Subjected to Salt Stress

It is known that salt stress and exogenously applied abscisic acid （ABA） can enhance the polyamine content in plants and that salt stress itself can lead to an increase in endogenous ABA production. In the present study, the relationships between salt-induced ABA and polyamine accumulation were inves- tigated using ABA-deficient mutant （vp5/vp5） maize （Zea mays L.） seedlings and ABA and polyamine biosynthesis inhibitors. The results show that reduced endogenous ABA levels, as a result of either the mutation or by using a chemical inhibitor （sodium tungstate）, also reduced the accumulation of polyamines in salt-stressed leaves of maize seedlings. The polyamine synthesis inhibitors D-arginine and α- difluoromethylornithine also reduced the polyamine content of the leaves of maize seedling under salt stress. Both ABA and polyamine enhanced the dry weight accumulation of salt-stressed seedlings and also increased the activities of the two dominant tonoplast membrane enzymes, H^＋-ATPase and H^＋-PPase, when plants were under salt stress. The results suggest that salt stress induces an increase in endogenous ABA levels, which then enhances polyamine synthesis. Such responses may increase a plant＇s tolerance to salt.     

Seed Treatment with Auxins Modulates Growth and Ion Partitioning in Salt-stressed Wheat Plants

Experiments were performed to determine whether seed priming with different concentrations (100, 150, and 200 mg/L) ofauxins (indoleacetic acid (IAA), indolebutyric acid (IBA), or their precursor tryptophane (Trp)) could alter salinity inducedperturbances in salicylic acid and ion concentrations and, hence, growth in wheat (Triticum aestivum L.) cultivars, namelyM. H.-97 (salt intolerant) and Inqlab-91 (salt tolerant). Primed and non-primed seeds were sown in Petri dishes in a growthroom, as well as in a field treated with 15 dS/m NaCl salinity. All priming agents, except IBA, increased the final germinationpercentage in both cultivars. The seedlings of either cultivar raised from Trp-treated seeds had greater dry biomass whenunder salt stress. In field experiments, Trp priming was much more effective in mediating the increase in grain yield,irrespective of the cultivar, under salt stress. The alleviatory effect of Trp was found to be associated with reduced uptakeof Na~ in the roots and subsequent translocation to the shoots, as well as increased partitioning of Ca~(2 )in the roots ofsalt-stressed wheat plants. Plants of both cultivars raised from Trp-and IAA-treated seeds accumulated free salicylic acidin their leaves when under salt stress. Overall, the Trp priming-induced improvement in germination and the subsequentgrowth of wheat plants could be related to ion homeostasis when under salt stress. The possible involvement of salicylicacid in the Trp priming-induced better growth under conditions of salt stress is discussed.     

Abscisic acid-dependent PMT1 expression regulates salt tolerance by alleviating abscisic acid-mediated reactive oxygen species production in Arabidopsis

Phosphocholine(PCho) is an intermediate metabolite of nonplastid plant membranes that is essential for salt tolerance. However, how PCho metabolism modulates response to salt stress remains unknown. Here, we characterize the role of phosphoethanolamine N-methyltransferase 1(PMT1) in salt stress tolerance in Arabidopsis thaliana using a T-DNA insertional mutant, geneediting alleles, and complemented lines. The pmt1 mutants showed a severe inhibition of root elongation when exposed to salt stress,...     

Thermal dissipation during photosynthetic induction and subsequent dark recovery as measured by photoacoustic signals

The thermal photoacoustic signal (279 Hz) and the chlorophyll (Chl) fluorescence of radish cotyledons (Raphanus sativus L.) were measured simultaneously. The signals were recorded during a photosynthetic induction with actinic radiation of different quantum fluence rates [20, 200, and 1200 μmol(PAR-quantum) m^-2 s^-1]. The rise of these signals upon irradiation saturating photosynthesis was followed in the steady state of the induction and during the subsequent dark-recovery (i.e., in dark periods of 1, 5, 15, and 45 min after the induction). From these values various parameters (e.g., quantum yield, photochemical loss, different types of quenching coefficients) were calculated. The results show that heat dissipation detected by photoacoustic measurements is neither low, constant, nor always parallel to Chl fluorescence. Therefore, the thermal signal should always be measured in order to fully understand the way leaves convert energy taken up by PAR absorption. This helps in the interpretation of photosynthesis under different natural and anthropogenic conditions (stress and damage effects). This revised version was published online in September 2006 with corrections to the Cover Date.     

Na~ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress

The response of halophyte arrowleaf saltbush(Atriplex triangularis Willd)plants to a gradient of salt stress were investigatedwith hydroponically cultured seedlings.Under salt stress,both the Na~ uptake into root xylem and negative pressures inxylem vessels increased with the elevation of salinity(up to 500 mol/m~3)in the root environment.However,the increment innegative pressures in root xylem far from matches the decrease in the osmotic potential of the root bathing solutions,evenwhen the osmotic potential of xylem sap is taken into consideration.The total water potential of xylem sap in arrowleafsaltbush roots was close to the osmotic potential of root bathing solutions when the salt stress was low,but a progressivelyincreased gap between the water potential of xylem sap and the osmotic potential of root bathing solutions was observedwhen the salinity in the root environment was enhanced.The maximum gap was 1.4 MPa at a salinity level of 500 mol/m~3without apparent dehydration of the tested plants.This discrepancy could not be explained with the current theories inplant physiology.The radial reflection coefficient of root in arrowleaf saltbush decreased with the enhanced salt stress wasand accompanied by an increase in the Na~ uptake into xylem sap.However,the relative Na~ in xylem exudates based onthe corresponding NaCl concentration in the root bathing solutions showed a tendency of decrease.The results showedthat the reduction in the radial reflection coefficient of roots in the arrowleaf saltbush did not lead to a mass influx of NaClinto xylem when the radial reflection coefficient of the root was considerably small;and that arrowleaf saltbush could usesmall xylem pressures to counterbalance the salt stresses,either with the uptake of large amounts of salt,or with thedevelopment of xylem pressures dangerously negative.This strategy could be one of the mechanisms behind the highresistance of arrowleaf saltbush plants to salt stress.     

Promotion by 5-Aminolevulinic Acid of Germination of Pakchoi （Brassica campestris ssp. chinensis var. communis Tsen et Lee） Seeds Under Salt Stress

The seed germination and seedling growth of pakchoi （Brassica campestris ssp. chinensis var. communis Tsen et Lee cv. Hanxiao） were not significantly inhibited until the concentration of NaCl was increased to 150 mmol/L. Treatment of pakchoi seeds with exogenous 5-aminolevulinic acid （ALA）, at concentrations ranging from 0.01 to 10.00 mg/L, promoted seed germination when seeds were stressed by salinity, whereas levulinic acid （LA）, an inhibitor of ALA dehydrase, significantly inhibited seed germination and seedling growth, suggesting that metabolism of ALA into porphyrin compounds was necessary for seed germination and seedling growth. Determination of respiratory rate during seed germination showed that ALA increased seed respiration under both normal conditions and salt stress. Furthermore, salt stress decreased levels of endogenous ALA, as well as heme, in etiolated seedlings. More salt-tolerant cultivars of pakchoi contained higher relative levels of endogenous ALA and heme under conditions of salt stress. These results indicate that salt stress may inhibit the biosynthesis of endogenous ALA and then heme, which is necessary for seed germination, and treatment of seeds with exogenous ALA prior to germination may be associated with the biosynthesis of heme.     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient during freezing and recovery in winter wheat

Chlorophyll (Chl) a fluorescence measurements as evaluators of plant freezing tolerance are frequently insufficiently sensitive to detect the early metabolic changes that are initiated following exposure to freezing temperatures. Using cold-acclimated winter wheat, I analysed the polyphasic transience (from 50 μs to 1 s) of Chl a fluorescence. This enabled detailed studies of the progressive energy flows and efficiencies within the photosystem 2 (PS2) complex that ensue following initial exposure to freezing temperatures right through to the plant recovery stage. The initial consequences of mild frosts that may cause primary damage involve a disturbance to the energy transfer subsequent to Q_A (the primary quinone electron acceptor of PS2). Lower freezing temperatures, on the other hand, may deter energy flow between the PS2 reaction centre (RC), Chl, and Q_A. All primary damage could only be repaired partially. Further freezing-triggered dysfunction of the electron transfer between the PS2 RCs and Q_A was connected with secondary damage that could lead to PS2 deactivation. Both primary and secondary freezing damages were reflected in decreased PI_ABS, the Performance Index based on equal absorption that characterizes all energy bifurcations in PS2. PI_ABS also differentiated cultivars with contrasting freezing-tolerance either subsequent to the onset of freezing or during the recovery stage. In contrast, the potential quantum yield of PS2 (F_v/F_m), which characterizes efficiency of energy trapping in the PS2 RCs, was only different in cultivars with contrasting freezing-tolerance during the recovery stage.     

Evaluation of Genetic and Epigenetic Modification in Rapeseed （Brassica napus） Induced by Salt Stress

Salinity is an important limiting environmental factor for rapeseed production worldwide. In this study, we assessed the extent and pattern of DNA damages caused by salt stress in rapeseed plants. Amplified fragment length polymorphism （AFLP） analysis revealed dose-related increases in sequence alterations in plantlets exposed to 10-1000 mmol/L sodium chloride. In addition, individual plantlets exposed to the same salt concentration showed different AFLP and selected region amplified polymorphism banding patterns. These observations suggested that DNA mutation in response to salt stress was random in the genome and the effect was dose-dependant. DNA methylation changes in response to salt stress were also evaluated by methylation sensitive amplified polymorphism （MSAP）. Three types of MSAP bands were recovered. Type Ⅰ bands were observed with both isoschizomers Hpa Ⅱ and Msp Ⅰ, while type Ⅱ and type Ⅲ bands were observed only with Hpa Ⅱ and Msp Ⅰ, respectively. Extensive changes in types of MSAP bands after NaCI treatments were observed, including appearance and disappearance of type Ⅰ, Ⅱ and Ⅲ bands, as well as exchanges between either type Ⅰand type Ⅱ or type Ⅰ and type Ⅲ bands. An increase of 0.2-17.6% cytosine methylated CCGG sites were detected in plantlets exposed to 10- 200 mmol/L salt compared to the control, and these changes included both de novo methylation and demethylation events. Nine methylation related fragments were also recovered and sequenced, and one sharing a high sequence homology with the ethylene responsive element binding factor was identified. These results demonstrated clear DNA genetic and epigenetic alterations in planUets as a response to salt stress, and these changes may suggest a mechanism for plants adaptation under salt stress.