Response of bean (Vicia faba L.) plants to low sink demand by measuring the gas exchange rates and chlorophyll a fluorescence kinetics

Background

The decline of photosynthesis in plants under low sink demand is well known. Previous studies focused on the relationship between stomatal conductance (g_s) and net photosynthetic rate (P_n). These studies investigated the effect of changes in Photosystem II (PSII) function on the P_n decline under low sink demand. However, little is known about its effects on different limiting steps of electron transport chain in PSII under this condition.

Methodology/Principal Finding

Two-month-old bean plants were processed by removing pods and flowers (low sink demand). On the 1^st day after low sink demand treatment, a decline of P_n was accompanied by a decrease in g_s and internal-to-ambient CO₂ concentration ratio (C_i/C_a). From the 3^rd to 9^th day, P_n and g_s declined continuously while C_i/C_a ratio remained stable in the treatment. Moreover, these values were lower than that of control. W_k (a parameter reflecting the damage to oxygen evolving complex of the donor side of PSII) values in the treatment were significantly higher than their corresponding control values. However, RC_QA (a parameter reflecting the number of active RCs per excited cross-section of PSII) values in the treatment were significantly lower than control from the 5^th day. From the 11^th to 21^st day, P_n and g_s of the treatment continued to decline and were lower than control. This was accompanied by a decrease of RC_QA, and an increase of W_k. Furthermore, the quantum yield parameters φ_Po, φ_Eo and ψ_Eo in the treatment were lower than in control; however, C_i/C_a values in the treatment gradually increased and were significantly higher than control on the 21^st day.

Conclusions

Stomatal limitation during the early stage, whereas a combination of stomatal and non-stomatal limitation during the middle stage might be responsible for the reduction of P_n under low sink demand. Non-stomatal limitation during the late stages after the removal of the sink of roots and pods may also cause P_n reduction. The non-stomatal limitation was associated with the inhibition of PSII electron transport chain. Our data suggests that the donor side of PSII was the most sensitive to low sink demand followed by the reaction center of PSII. The acceptor side of PSII may be the least sensitive.

Does aphid salivation affect phloem sieve element occlusion in vivo?

To protect against loss of photo-assimilate-rich phloem sap, plants have evolved several mechanisms to plug phloem sieve tubes in response to damage. In many Fabaceae, each sieve element contains a discrete proteinaceous body called a forisome, which, in response to damage, rapidly transforms from a condensed configuration that does not impede the flow of sap to a dispersed configuration that plugs the sieve element. Aphids and other specialized phloem sap feeders can ingest phloem sap from a single sieve element for hours or days, and to do this, they must be able to suppress or reverse phloem plugging. A recent study provided in vitro evidence that aphid saliva can reverse forisome plugs. The present study tested this hypothesis in vivo by inducing forisome plugs which triggered aphids to switch behaviour from phloem sap ingestion to salivation into the sieve element. After salivating into the sieve element for various periods of time, the aphids were instantaneously cryofixed (freeze fixed) in situ on their leaf. The state of the forisome was then determined in the penetrated sieve element and in nearby non-penetrated sieve elements which served as controls for sieve elements not subjected to direct aphid salivation. Forisomes were almost always in close contact with the stylet tips and thus came into direct contact with the saliva. Nonetheless, forisome plugs in the penetrated sieve element did not revert back to a non-plugging state any faster than those in neighbouring sieve elements that were not subjected to direct aphid salivation.

Adjustments in epidermal UV-transmittance of leaves in sun-shade transitions

Epidermal UV transmittance (TUV ) and UV-absorbing compounds were measured in sun and shade leaves of Populus tremuloides and Vicia faba exposed to contrasting light environments under field conditions to evaluate UV acclimation potentials and regulatory roles of photosynthetically active radiation (PAR) and UV in UV-shielding. Within a natural canopy of P. tremuloides, TUV ranged from 4 to 98% and showed a strong nonlinear relationship with mid-day horizontal fluxes of PAR [photon flux density (PFD) = 6-1830 µmol m⁻²  s⁻¹]; similar patterns were found for V. faba leaves that developed under a comparable PFD range. A series of field transfer experiments using neutral-density shade cloth and UV blocking/transmitting films indicated that PAR influenced TUV during leaf development to a greater degree than UV, and shade leaves of both species increased their UV-shielding when exposed to full sun; however, this required the presence of UV, with both UV-A and UV-B required for full acclimation. TUV of sun leaves of both species was largely unresponsive to shade either with or without UV. In most, but not all cases, changes in TUV were associated with alterations in the concentration of whole-leaf UV-absorbing compounds. These results suggest that, (1) moderate-to-high levels of PAR alone during leaf development can induce substantial UV-protection in field-grown plants, (2) mature shade leaves have the potential to adjust their UV-shielding which may reduce the detrimental effects of UV that could occur following sudden exposures to high light and (3) under field conditions, PAR and UV play different roles in regulating UV-shielding during and after leaf development.

Involvement of the sieve element cytoskeleton in electrical responses to cold shocks

This study dealt with the visualization of the sieve element (SE) cytoskeleton and its involvement in electrical responses to local cold shocks, exemplifying the role of the cytoskeleton in Ca(2+)-triggered signal cascades in SEs. High-affinity fluorescent phalloidin as well as immunocytochemistry using anti-actin antibodies demonstrated a fully developed parietal actin meshwork in SEs. The involvement of the cytoskeleton in electrical responses and forisome conformation changes as indicators of Ca(2+) influx was investigated by the application of cold shocks in the presence of diverse actin disruptors (latrunculin A and cytochalasin D). Under control conditions, cold shocks elicited a graded initial voltage transient, ΔV1, reduced by external La(3+) in keeping with the involvement of Ca(2+) channels, and a second voltage transient, ΔV2. Cytochalasin D had no effect on ΔV1, while ΔV1 was significantly reduced with 500 nm latrunculin A. Forisome dispersion was triggered by cold shocks of 4°C or greater, which was indicative of an all-or-none behavior. Forisome dispersion was suppressed by incubation with latrunculin A. In conclusion, the cytoskeleton controls cold shock-induced Ca(2+) influx into SEs, leading to forisome dispersion and sieve plate occlusion in fava bean (Vicia faba).

Enhanced tolerance to drought and salt stresses in transgenic faba bean (Vicia faba L.) plants by heterologous expression of the PR10a gene from potato

We report for the first time that expression of potato PR10a gene in faba bean causes enhanced tolerance to drought and salinity. Grain legumes such as soybean (Glycine max L. Merrill), pea (Pisum sativum L.) and faba bean (Vicia faba L.) are staple sources of protein for human and animal nutrition. Among grain legumes, faba bean is particularly sensitive to abiotic stress (in particular osmotic stress due to lack of water or enhanced soil salinity) and often suffers from severe yield losses. Many stress responsive genes have been reported with an effect on improving stress tolerance in model plants. Pathogenesis-related proteins are expressed by all plants in response to pathogen infection and, in many cases, in response to abiotic stresses as well. The PR10a gene isolated from the potato cultivar Desiree was selected for this study due to its role in enhancing salt and/or drought tolerance in potato, and transferred into faba bean cultivar Tattoo by Agrobacterium tumefaciens-mediated transformation system based upon direct shoot regeneration after transformation of meristematic cells derived from embryo axes. The transgene was under the control of the constitutive mannopine synthase promoter (p-MAS) in a dicistronic binary vector, which also contained luciferase (Luc) gene as scorable marker linked by internal ribosome entry site elements. Fertile transgenic faba bean plants were recovered. Inheritance and expression of the foreign genes were demonstrated by PCR, RT-PCR, Southern blot and monitoring of Luciferase activity. Under drought condition, after withholding water for 3 weeks, the leaves of transgenic plants were still green, while non-transgenic plants (WT) wilted and turned brown. Twenty-four hours after re-watering, the leaves of transgenic plants remained green, while WT plants did not recover. Moreover, the transgenic lines displayed higher tolerance to NaCl stress. Our results suggested that introducing a novel PR10a gene into faba bean could be a promising approach to improve its drought and salt tolerance ability, and that MAS promoter is not only constitutive, but also wound-, auxin/cytokinin- as well as stress-inducible.

Saccharomyces cerevisiae-induced stomatal closure mainly mediated by salicylhydroxamic acid-sensitive peroxidases in Vicia faba

Saccharomyces cerevisiae induced stomatal closure in a dose-dependent manner on Vicia faba L. (cv. Daqingpi). Using pharmacological inhibitors in this study, we found that stomatal closure was completely inhibited by salicylhydroxamic acid (SHAM) and reduced glutathione (GSH), whereas slightly inhibited by diphenyleneiodonium chloride (DPI), suggesting that H2O2 was mostly produced by cell wall peroxidases. The specific NO scavenger (cPTIO), NO synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME) and sodium azide (NaN3; inhibitor of nitrate reductase) prevented yeast-induced stomatal closure, suggesting that NO in guard cells of V. faba is derived from both NOS-like enzyme and nitrate reductase. Results of HgCl2 and β-mercaptoethanol (ME) treatment (as a functional inhibitor of water channels and its reversing agent, respectively) suggest that water channels are involved in yeast-induced stomatal movements. CoCl2 (the blocker of calcium channel), LaCl3 (Ca(2+) antagonist) and EGTA (Ca(2+) chelator) also impaired yeast-induced stomatal closure. Thus, it is concluded that H2O2, NO, water channels and Ca(2+) are involved in yeast-induced stomatal closure.

[Allelopathic effects of extracts from fibrous roots of Coptis chinensis on two leguminous species]

An experiment was carried out to study the allelopathic effects of Coptis chinensis fibrous root extracts (CRE) on the germination and seedling growth of Vicia faba and Pisum sativum in order to alleviate the allelopathic effects and increase land productivity. The seeds of both garden pea (P. sativum) and broad been (V. faba) were germinated in CRE solution of various concentrations, the germination rate, seedling growth and related physiological indexes were measured. The result indicated that there were no significant effects of CRE in low concentrations on seed germination, including both the rate and index, and seed vitality and membrane permeability. With the increment of CRE concentrations, however, the high seed membrane permeability and germination inhibition were observed. For example, the germination rates were reduced by 23.4% (P. sativum) and 9.5% (V. faba), respectively, in CRE solution with 800 mg . L-1. Simultaneously, soluble sugars and the free amino acids in the seeds were lower than those in the control (without CRE) after soaking seeds in CRE solutions. In addition, the seedling growth and nitrate reductase activity were stimulated by CRE at low concentrations in contrast to high concentrations which behaved otherwise and inhibited the nutrient utilization in endosperm. Therefore, the large amount of allelochemicals released from the roots and remains of C. chinensis in soils could inhibit the seed germination and seedling growth of legumes, which may lead to decrease even fail crop yields after growing this medical plant.

Ratiometric monitoring of transient apoplastic alkalinizations in the leaf apoplast of living Vicia faba plants: chloride primes and PM-H+-ATPase shapes NaCl-induced systemic alkalinizations

Transient apoplastic alkalinization has been discussed as a general stress factor, and is thought to represent a root-to-shoot signal that transmits information regarding an ongoing NaCl stress event from the site of the trigger to the distant plant tissue. Surprisingly, despite this importance, a number of gaps exist in our knowledge of NaCl-induced apoplastic pH alkalinization. This study was designed in order to shed light onto the mechanisms responsible for the initiation and transiency of leaf apoplastic alkalinization under conditions of NaCl stress as supplied to roots. An H(+)-sensitive fluorescence probe, in combination with ratiometric microscopy imaging, was used for in planta live recording of leaf apoplastic pH. The use of a nonionic solute demonstrated that the alkalinization is induced in response to ionic, and not osmotic, components of NaCl stress. Tests with Cl(-)- or Na(+)-accompanying counter-ions strengthened the idea that the stress factor itself, namely Cl(-), is transferred from root to shoot and elicits the pH alterations. Investigations with a plasma membrane ATPase inhibitor suggest that ATPase activity influences the course of the alkalinization by having a shaping re-acidifying effect on the alkalinization.

Planting geometry to optimize growth and productivity in faba bean (Vicia faba L.) and soil fertility

Faba bean (Vicia faba L.) responses to alteration of its ambient environment leads to certain modification in the crop phenology, yield attributes and economic yield. To know the extent and pattern of response by faba bean to alterations, a two year field experimentation was carried out with two crop establishment methods (i) flatbed planting (ii) raised bed planting, four planting geometry (i) 30 x 20 cm(ii) 30 x 30 cm (iii) 30 x 45 cm and (iv) 45X45cm and three seeding depth. All the treatment (two crop establishment methods, four planting geometry and three seeding depth) were combined together consisting twenty four treatments, were organized in factorial experiment in complete randomized block design (CRBD) with three replications. Data were recorded on growth and development; yield attributes and yield. Soil analysis was done and finally statistical tool were applied to come in to valid conclusion. Raised bed planting proves superior over flatbed in case of seed yield. Square planting architect with 30 cm apart prove better (3690.9 kg ha(-1)) than other tested planting geometry. Seeding at 10 cm depth showed, significant improvement in seed yield per plant and per ha over other two tested seeding depth. Phosphorus availability was significantly higher in raised bed planting (36.9 kg ha(-1)). However, available K (kg ha(-1)) was significantly influenced by planting geometry and seeding depth. It was maximum (155.2 kg ha(-1)) with 30 x 45 cm plant geometry, proved significantly higher than 30 x 20 cm and 30 x 30 cm and at par with 45 x 45 cm planting.

SB202190 affects cell response to hydroxyurea-induced genotoxic stress in root meristems of Vicia faba

Genotoxic stress caused by a variety of chemical and physical agents may lead to DNA breaks and genome instability. Response to DNA damage depends on ATM/ATR sensor kinases and their downstream proteins, which arrange cell cycle checkpoints. Activation of ATM (ataxia-telangiectasia-mutated)/ATR (ATM and Rad 3-related) signaling pathway triggers cell cycle arrest (by keeping cyclin-Cdk complexes inactive), combined with gamma-phosphorylation of histone H2A.X and induction of DNA repair processes. However, genotoxic stress activates also mitogen-activated protein kinases (MAPKs) which may control the functions of checkpoint proteins both directly, by post-translational modifications, or indirectly, by regulation of their expression. Our results indicate that in root meristem cells of Vicia faba, MAP kinase signaling pathway takes part in response to hydroxyurea-induced genotoxic stress. It is shown that SB202190, an inhibitor of p38 MAP kinase, triggers PCC (premature chromosome condensation) more rapidly, but only if cell cycle checkpoints are alleviated by caffeine. Since SB202190 and, independently, caffeine reduces HU-mediated histone H4 Lys5 acetylation, it may be that there is a cooperation of MAP kinase signaling pathways and ATM/ATR-dependent checkpoints during response to genotoxic stress.

Extracellular hydrogen peroxide, produced through a respiratory burst oxidase/superoxide dismutase pathway, directs ingrowth wall formation in epidermal transfer cells of Vicia faba cotyledons

The intricate, and often polarized, ingrowth walls of transfer cells (TCs) amplify their plasma membrane surface areas to confer a transport function of supporting high rates of nutrient exchange across apo-/symplasmic interfaces. The TC ingrowth wall comprises a uniform wall layer on which wall ingrowths are deposited. Signals and signal cascades inducing trans-differentiation events leading to formation of TC ingrowth walls are poorly understood. Vicia faba cotyledons offer a robust experimental model to examine TC induction as, when placed into culture, their adaxial epidermal cells rapidly (h) and synchronously form polarized ingrowth walls accessible for experimental observations. Using this model, we recently reported findings consistent with extracellular hydrogen peroxide, produced through a respiratory burst oxidase homolog/superoxide dismutase pathway, initiating cell wall biosynthetic activity and providing directional information guiding deposition of the polarized uniform wall. Our conclusions rested on observations derived from pharmacological manipulations of hydrogen peroxide production and correlative gene expression data sets. A series of additional studies were undertaken, the results of which verify that extracellular hydrogen peroxide contributes to regulating ingrowth wall formation and is generated by a respiratory burst oxidase homolog/superoxide dismutase pathway.

[Role of allelopathic compositions in the regulation and development of legume-rhizobial symbiosis]

It was discovered that aromatic compounds isolated from root exudates of three legume species (Pisum sativum L., Vicia faba L. var. major Hartz, and Glycine max L. MERR) and identified as N-phenyl-2-naphthyl amine, dibutyl, and dioctyl esters of orthophthalic acid, which are known to work as negative allelopathic substances, are involved in the regulation of legume-rhizobial symbiosis formation after the inoculation of roots with rhizobia under unfavorable conditions for symbiosis.

Antioxidant and prooxidant effects of lanthanum ions on Vicia faba L. seedlings under cadmium stress, suggesting ecological risk

The present study combined chemical analyses and biological measurements to investigate biphasic effects of La on Cd stress in leaves of Vicia faba seedlings, which were hydroponically cultivated for 15 d in the combination of 6 µM CdCl(2) and 2 to 480 µM La(NO(3))(3), respectively. The results showed that contents of Cd first elevated above and then declined below the 6 µM single Cd treatment when 2 to 30 µM extraneous La were combined. Contents of mineral nutrients altered differentially and became imbalanced. No distinct band was observed in catalase (CAT), guaiacol peroxidase (GPX), or ascorbate peroxidase (APX) patterns, but in superoxide dismutase (SOD) isozymes by the supplementation with 8 to 480 µM of extraneous La. Superoxide dismutase and APX activities changed as a U-shaped curve; however, CAT and GPX changed as an inverted U-shaped curve along with increasing La. Moreover, heat shock protein 70 (HSP 70) production was reduced below the single treatment of Cd at 2 to 8 µM of extraneous La and enhanced thereafter. Thus, La at lower concentrations promoted antioxidation against Cd stress; La at higher concentrations turned to prooxidant effects, implicating potential ecological risk. Heat shock protein 70, combined with the antioxidant enzymes, constitutes an integrative defense system, which can be used to estimate the degree of antioxidation or prooxidation of extraneous La to Cd-induced oxidative stress in the seedlings.

Transient alkalinization in the leaf apoplast of Vicia faba L. depends on NaCl stress intensity: an in situ ratio imaging study

The apoplast is suggested to be involved not only in the response, but also in the perception and transduction of various environmental signals. In this context, apoplastic alkalinization has previously been discussed as a general stress factor caused by abiotic and biotic stress events. In this study, an ion-sensitive fluorescence probe in combination with inverted fluorescence microscopy has been used for in planta monitoring of apoplastic shoot pH during challenging of Vicia faba L. plants by NaCl stress encountered at the roots. We demonstrate that transient increases in leaf apoplastic pH are dependent on the NaCl stress intensity. Moreover, we have visualized spatial pH gradients within the leaf apoplast. Our results indicate that these pH responses are propagated from root to leaf and that this occurs along the apoplast.

Nitric oxide inhibits blue light-induced stomatal opening by regulating the K+ influx in guard cells

Blue light (BL)-induced stomatal opening and nitric oxide (NO)-promoted stomatal closure comprise two main aspects of stomatal regulation. Stomatal movement depends on ion fluxion in guard cells, whereas the physiological roles of BL or NO in regulating ion channel activities remain largely unknown. For gaining further insights into NO function in mediating BL-induced stomatal opening, guard cell protoplasts (GCPs) were patch-clamped in a whole-cell configuration. The results showed that twice BL pulses (100 μmol m⁻² s⁻¹ for 30s) effectively activated inward rectifying K⁺ channels by 67% and 20% in Vicia GCPs, respectively. In contrast, Red light (RL) showed little effect on inward rectifying K⁺ channels. In accord with this, BL also increased inward K⁺ currents by 54% in Arabidopsis thaliana wild type gl1, but not in phot1-5 phot2-1 (BL receptor phototropin deletion mutant). Sodium nitroprusside (SNP, a NO donor), at 100 μM, inhibited BL-dependent K⁺ influx and stomatal opening, which were abolished by c-PTIO (a specific NO scavenger). These results indicated that NO inhibits BL-induced stomatal opening maybe through restricting the K⁺ influx across plasma membrane in guard cells.

Active ROP2 GTPase inhibits ABA- and CO2-induced stomatal closure

ROP GTPases function as molecular switches in diverse cellular processes. Previously, we showed that ROP2 GTPase is activated upon light irradiation, and thereby negatively regulates light-induced stomatal opening. Here we studied the role of ROP2 during stomatal closure. The expression of a constitutively active form of ROP2 (CA-rop2) in Arabidopsis thaliana and Vicia faba resulted in slower and reduced stomatal closure in response to abscisic acid (ABA) and CO(2) . In contrast, the expression of a dominant-negative form of ROP2 (DN-rop2) and the knockout mutation of ROP2 (rop2 KO) promoted ABA-induced stomatal closure in Arabidopsis. As early as 10 min after ABA treatment, ROP2 was inactivated and translocated to the cytoplasm of the stomatal guard cells. To elucidate the mechanism by which active ROP2 suppresses stomatal closure, we monitored endocytotic membrane trafficking, which is regulated by Rho GTPases in animal cells. We found that the endocytosis of plasma membrane (PM), as tracked by FM4-64, was lower in CA-rop2-expressing guard cells than in those of wild-type plants, which suggests that active ROP2 suppresses the endocytotic internalization of PM, a process required for stomatal closure. Together, our results suggest that ROP2 is inactivated by ABA, and that this inactivation is required for the timely stomatal closure.

How does plasmalemma surface area accommodate alterations in guard cell volume during stomatal closing?

During stomatal movement, guard cells undergo considerable and repetitive variations in cell volume and consequently surface area over a period of minutes. Due to limited stretching capability of the plasmalemma, alterations in the surface area must accommodate the volume changes through membrane turnover. Using fluorescence imaging and electrophysiology techniques, extensive studies imply that endocytosis may be a critical mechanism for the plasmalemma turnover. In contrast to the conventional studies, using transmission electronic microscope in combination with laser confocal microscope so that the membrane turnover can be detected without a resolution limitation, our works, recently published in Journal of Experimental Botany, has provided strong evidences that excretion and folding of plasmalemma are critical for the accommodation of the cell volume alterations in intact guard cells in Vicia faba L. These results have opened a new perspective on the mechanism for the membrane turnover during stomatal movement. In this addendum, we further discuss some key issues about the mechanisms for the accommodation of the cell volume alterations during stomatal movements.

Inhibition of dark-induced stomatal closure by fusicoccin involves a removal of hydrogen peroxide in guard cells of Vicia faba

Fusicoccin (FC) treatment prevents dark-induced stomatal closure, the mechanism of which is still obscure. By using pharmacological approaches and laser-scanning confocal microscopy, the relationship between FC inhibition of dark-induced stomatal closure and the hydrogen peroxide (H₂O₂) levels in guard cells in broad bean was studied. Like ascorbic acid (ASA), a scavenger of H₂O₂ and diphenylene iodonium (DPI), an inhibitor of H₂O₂-generating enzyme NADPH oxidase, FC was found to inhibit stomatal closure and reduce H₂O₂ levels in guard cells in darkness, indicating that FC-caused inhibition of dark-induced stomatal closure is related to the reduction of H₂O₂ levels in guard cells. Furthermore, like ASA, FC not only suppressed H₂O₂-induced stomatal closure and H₂O₂ levels in guard cells treated with H₂O₂ in light, but also reopened the stomata which had been closed by darkness and reduced the level of H₂O₂ that had been generated by darkness, showing that FC causes H₂O₂ removal in guard cells. The butyric acid treatment simulated the effects of FC on the stomata treated with H₂O₂ and had been closed by dark, and on H₂O₂ levels in guard cells of stomata treated with H₂O₂ and had been closed by dark, and both FC and butyric acid reduced cytosol pH in guard cells of stomata treated with H₂O₂ and had been closed by dark, which demonstrates that cytosolic acidification mediates FC-induced H₂O₂ removal. Taken together, our results provide evidence that FC causes cytosolic acidification, consequently induces H₂O₂ removal, and finally prevents dark-induced stomatal closure.

Phosphatidic acid inhibits blue light-induced stomatal opening via inhibition of protein phosphatase 1 [corrected]

Stomata open in response to blue light under a background of red light. The plant hormone abscisic acid (ABA) inhibits blue light-dependent stomatal opening, an effect essential for promoting stomatal closure in the daytime to prevent water loss. However, the mechanisms and molecular targets of this inhibition in the blue light signaling pathway remain unknown. Here, we report that phosphatidic acid (PA), a phospholipid second messenger produced by ABA in guard cells, inhibits protein phosphatase 1 (PP1), a positive regulator of blue light signaling, and PA plays a role in stimulating stomatal closure in Vicia faba. Biochemical analysis revealed that PA directly inhibited the phosphatase activity of the catalytic subunit of V. faba PP1 (PP1c) in vitro. PA inhibited blue light-dependent stomatal opening but did not affect red light- or fusicoccin-induced stomatal opening. PA also inhibited blue light-dependent H(+) pumping and phosphorylation of the plasma membrane H(+)-ATPase. However, PA did not inhibit the autophosphorylation of phototropins, blue light receptors for stomatal opening. Furthermore, 1-butanol, a selective inhibitor of phospholipase D, which produces PA via hydrolysis of phospholipids, diminished the ABA-induced inhibition of blue light-dependent stomatal opening and H(+) pumping. We also show that hydrogen peroxide and nitric oxide, which are intermediates in ABA signaling, inhibited the blue light responses of stomata and that 1-butanol diminished these inhibitions. From these results, we conclude that PA inhibits blue light signaling in guard cells by PP1c inhibition, accelerating stomatal closure, and that PP1 is a cross talk point between blue light and ABA signaling pathways in guard cells.

Dynamic regulation of guard cell anion channels by cytosolic free Ca2+ concentration and protein phosphorylation

In guard cells, activation of anion channels (I(anion)) is an early event leading to stomatal closure. Activation of I(anion) has been associated with abscisic acid (ABA) and its elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). However, the dynamics of the action of [Ca(2+)](i) on I(anion) has never been established, despite its importance for understanding the mechanics of stomatal adaptation to stress. We have quantified the [Ca(2+)](i) dynamics of I(anion) in Vicia faba guard cells, measuring channel current under a voltage clamp while manipulating and recording [Ca(2+)](i) using Fura-2 fluorescence imaging. We found that I(anion) rises with [Ca(2+)](i) only at concentrations substantially above the mean resting value of 125 +/- 13 nm, yielding an apparent K(d) of 720 +/- 65 nm and a Hill coefficient consistent with the binding of three to four Ca(2+) ions to activate the channels. Approximately 30% of guard cells exhibited a baseline of I(anion) activity, but without a dependence of the current on [Ca(2+)](i). The protein phosphatase antagonist okadaic acid increased this current baseline over twofold. Additionally, okadaic acid altered the [Ca(2+)](i) sensitivity of I(anion), displacing the apparent K(d) for [Ca(2+)](i) to 573 +/- 38 nm. These findings support previous evidence for different modes of regulation for I(anion), only one of which depends on [Ca(2+)](i), and they underscore an independence of [Ca(2+)](i) from protein (de-)phosphorylation in controlling I(anion). Most importantly, our results demonstrate a significant displacement of I(anion) sensitivity to higher [Ca(2+)](i) compared with that of the guard cell K(+) channels, implying a capacity for variable dynamics between net osmotic solute uptake and loss.

Exogenously applied ascorbic acid ameliorates detrimental effects of NaCl and mannitol stress in Vicia faba seedlings

The adverse effects of either NaCl or mannitol on growth, nitrogen content, and antioxidant system in Vicia faba seedlings were investigated. The role of exogenous ascorbic acid in increasing resistance to these stressors was also evaluated. Thus, with an increase in concentration of either NaCl or mannitol in culture media, a progressively greater significant decrease in percentage germination, in growth parameters, and in nitrogen constituents of the germinating beans, was observed. On the other hand, amide-, nitrate-, and total soluble-N contents appeared to show a progressive significant increase. Exogenous addition of ascorbic acid (4 mM) to the stressful media induced a pronounced significantly increased percentage germination and the growth attributes, whereas nitrogen constituents were variably changed in relation to values maintained in beans treated with either NaCl or mannitol. Furthermore, exogenous addition of ascorbic acid to NaCl or mannitol media induced a significant increase in the contents of ascorbate and glutathione and enzymatic antioxidant activities, in particular, in beans treated with the three highest concentrations of NaCl or mannitol, throughout the period of the experiments (12 days). Thus, ascorbic acid ameliorates the adverse effects of the stressful media; the magnitude of amelioration being a function of the type and the concentration of the stressful agent as well as of the duration of treatment. The importance of the above-mentioned changes in growth and metabolism to stress tolerance in broad bean is discussed.

Alleviation effect of alginate-derived oligosaccharides on Vicia faba root tip cells damaged by cadmium

Cadmium has been shown to prevent Vicia faba growth by inhibiting cell mitosis. In this study we investigated the role of Alginate-derived Oligosaccharides (ADO) in alleviating Vicia faba root tip cells damaged by 6 and 8 mg L(-1) CdCl2. Micronucleus assay and chromosomal aberration assay were used to determine mitotic index, micronucleus frequency and chromosomal aberration frequency. The results showed that micronucleus frequency of Vicia faba root tip cells was inhibited under all the ADO concentrations. Especially, the inhibition ratio of 0.125% ADO highly reached 66.11 and 67.17% in 6 and 8 mg L(-1) CdCl2, respectively. Furthermore, the mitotic index increased (p < 0.05) and chromosomal aberration frequency decreased (p < 0.05) under all the ADO concentrations. This indicated that ADO had a significant alleviation effect on Vicia faba root tip cells damaged by cadmium.

Rapid cooling triggers forisome dispersion just before phloem transport stops

Phloem transport stops transiently within dicot stems that are cooled rapidly, but the cause remains unknown. Now it is known that (1) rapid cooling depolarizes cell membranes giving a transient increase in cytoplasmic Ca(2+), and (2) a rise of free calcium triggers dispersion of forisomes, which then occlude sieve elements (SEs) of fabacean plants. Therefore, we compared the effects of rapid chilling on SE electrophysiology, phloem transport and forisomes in Vicia faba. Forisomes dispersed after rapid cooling with a delay that was longer for slower cooling rates. Phloem transport stopped about 20 s after forisome dispersion, and then transport resumed and forisomes re-condensed within similar time frames. Transport interruption and forisome dispersion showed parallel behaviour--a cooling rate-dependent response, transience and desensitization. Chilling induced both a fast and a slow depolarization of SE membranes, the electrical signature suggesting strongly that the cause of forisome dispersion was the transient promotion of SE free calcium. This apparent block of SEs by dispersed forisomes may be assisted by other Ca(2+)-dependent sealing proteins that are present in all dicots.

Cadmium induction of lipid peroxidation and effects on root tip cells and antioxidant enzyme activities in Vicia faba L

The effects of different concentrations (1-50 microM) of Cd on root growth, cell division and nucleoli in root tip cells, protective enzyme activities and lipid peroxidation in Vicia faba were investigated in order to better understand the processes of Cd-induced senescence. The results indicated that lower concentration of Cd (1 microM) had no obviously influence on the root growth during 24-48 h treatment, but higher concentrations (5-50 microM) inhibited significantly after 48 and 72 h. The mitotic index decreased with increasing of Cd concentration and duration of treatment except for the group exposed to 1 microM Cd. Cd induced c-mitosis, chromosome bridges, chromosome stickiness and lagging chromosomes. The rate of aberrant dividing cells increased with prolonging duration of treatment and increasing of Cd concentration. On nucleolus, some particulates containing the argyrophilic proteins were extruded from the nucleus into the cytoplasm in the cells stressed by Cd and some were scattered in the nucleus. After the treatment with Cd (10 microM Cd, 48 h), the nucleolus did not disaggregate normally and still remain its characteristic structure during metaphase and the particles of similar silver-stained materials were localized on chromosomes. In leaves, Catalase (CAT) activity declined but Peroxidase (POD) activity increased with increasing of the duration of treatment. In roots, CAT activity increased with increasing of the duration of treatment, POD activity increased during early days and then declined. Superoxide dismutase (SOD) activity showed an upward trend with increasing of the duration of treatment after 3 and 6 days, then declined both in leaves and roots (9 days). SOD and POD had highest activities at 50 microM Cd in leaves. CAT activity was lowest at 50 microM Cd. Malondialdehyde (MDA) content increased with the increasing of Cd concentrations and duration of treatment in leaves. In roots, MDA content showed an upward trend with increasing of the duration of treatment at early time and then declined.

The relation between stomatal aperture and gas exchange under consideration of pore geometry and diffusional resistance in the mesophyll

The quantitative relation between stomatal aperture and gas exchange through the stomatal pore can be described by physical models derived from Fick's first law of diffusion. Such models, usually based on a simplified pore geometry, are used to calculate leaf conductance from stomatal pore dimensions or vice versa. In this study a combination of gas-exchange measurements and simultaneous microscopical observations of stomatal apertures was used to empirically determine this relationship. The results show a substantial deviation between measured stomatal conductance and that calculated from the simplified models. The main difference is a much steeper increase of conductance with aperture at small apertures. When the calculation was based on a realistic pore geometry derived from confocal laser scanning microscopy, a good fit to the experimentally found relationship could be obtained if additionally a significant contribution of a mesophyll diffusional resistance was taken into account.