UV-B exposure enhances senescence of wheat leaves: modulation by photosynthetically active radiation

The alterations in structure and function of photosystem II (PS II) during the senescence of primary leaves of wheat seedlings have been compared with the changes induced by ultraviolet-B (UV-B) radiation in the presence or absence of photosynthetically active radiation (PAR). The results indicated that the senescence-induced loss in pigment content, thylakoid membrane integrity and carotenoid-to-chlorophyll (Car-to-Chl) energy transfer efficiency was intensified by exposure to UV-B radiation. Different parameters for the measurement of PS II activity, such as Chl a fluorescence, O₂-evolution and thermoluminescence intensity, were altered during senescence and these alterations were furthered by UV-B irradiation. The damage of photosynthetic apparatus by UV-B exposure in the presence of PAR was less than the damage in absence of PAR. The activation of molecular defense mechanisms could be a factor in the alleviation of UV-B damage in the presence of PAR.     

Detection of singlet oxygen and superoxide with fluorescent sensors in leaves under stress by photoinhibition or UV radiation

In order to understand the physiological functions of reactive oxygen species (ROS) generated in leaves, their direct measurement in vivo is of special importance. Here we report experiments with two dansyl-based ROS sensors, the singlet oxygen specific DanePy and HO-1889NH, which is reactive to both singlet oxygen and superoxide radicals. Here we report in vivo detection of (1)O(2) and O(2)(-*) by fluorescence quenching of two dansyl-based ROS sensors, the (1)O(2) specific DanePy and HO-1889NH, which was reactive with both (1)O(2) and O(2)(-*). The ROS sensors were administered to spinach leaves through a pinhole, and then the leaves were exposed to either excess photosynthetically active radiation or UV (280-360 nm) radiation. Microlocalization of the sensors' fluorescence and its ROS-induced quenching was followed with confocal laser scanning microscopy and with fluorescence imaging. These sensors were specifically localized in chloroplasts. Quenching analysis indicated that the leaves exposed to strong light produced (1)O(2), but hardly any O(2)(-*). On the other hand, the dominant ROS in UV-irradiated leaves was O(2)(-*), while (1)O(2) was minor.     

Spatio-temporal heterogeneity in Arabidopsis thaliana leaves under drought stress

Using chlorophyll (chl) fluorescence imaging, we studied the effect of mild (MiDS), moderate (MoDS) and severe (SDS) drought stress on photosystem II (PSII) photochemistry of 4-week-old Arabidopsis thaliana. Spatio-temporal heterogeneity in all chl fluorescence parameters was maintained throughout water stress. After exposure to drought stress, maximum quantum yield of PSII photochemistry (F(v)/F(m)) and quantum efficiency of PSII photochemistry (Φ(PSΙΙ)) decreased less in the proximal (base) than in the distal (tip) leaf. The chl fluorescence parameter F(v) /F(m) decreased less after MoDS than MiDS. Under MoDS, the antioxidant mechanism of A. thaliana leaves seemed to be sufficient in scavenging reactive oxygen species, as evident by the decreased lipid peroxidation, the more excitation energy dissipated by non-photochemical quenching (NPQ) and decreased excitation pressure (1-q(p)). Arabidopsis leaves appear to function normally under MoDS, but do not seem to have particular metabolic tolerance mechanisms under MiDS and SDS, as revealed by the level of lipid peroxidation and decreased quantum yield for dissipation after down-regulation in PSII (Φ(NPQ)), indicating that energy dissipation by down-regulation did not function and electron transport (ETR) was depressed. The simultaneous increased quantum yield of non-regulated energy dissipation (Φ(NO)) indicated that both the photochemical energy conversion and protective regulatory mechanism were insufficient. The non-uniform photosynthetic pattern under drought stress may reflect different zones of leaf anatomy and mesophyll development. The data demonstrate that the effect of different degrees of drought stress on A. thaliana leaves show spatio-temporal heterogeneity, implying that common single time point or single point leaf analyses are inadequate.     

Singlet oxygen production in thylakoid membranes during photoinhibition as detected by EPR spectroscopy

Exposure of isolated spinach thylakoids to high intensity illumination (photoinhibition) results in the well-characterized impairment of Photosystem II electron transport, followed by degradation of the D1 reaction centre protein. In the present study we demonstrate that this process is accompanied by singlet oxygen production. Singlet oxygen was detected by EPR spectroscopy, following the formation of stable nitroxide radicals from the trapping of singlet oxygen with a sterically hindered amine TEMP (2,2,6,6-tetramethylpiperidine). There was no detectable singlet oxygen production during anaerob photoinhibition or in the presence of sodium-azide. Comparing the kinetics of the loss of PS II function and D1 protein with that of singlet oxygen trapping suggests that singlet oxygen itself or its radical product initiates the degradation of D1.Abbreviations HEPES 4-(2-hydroxyethyl)-1-piperazine ethanesulphonle acid - PS Photosystem - TEMP 2,2,6,6-tetramethylpiperidine - TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl     

The characterization of photoinhibition and recovery during cold acclimation in Arabidopsis thaliana using chlorophyll fluorescence imaging

The responses to photoinhibition of photosynthesis at low temperature and subsequent recovery were examined in Arabidopsis thaliana (ecotype Columbia) developed at 4°C cold-acclimating conditions, 23°C non-acclimating conditions and for non-acclimated plants shifted to 4°C (cold-shifted). These responses were determined in planta using Chl fluorescence imaging. We show that cold acclimation results in an increased tolerance to photoinhibition in comparison with non-acclimated plants and that growth and development at low temperature is essential for this to occur. Cold-shifted plants were not as tolerant as the cold-acclimated plants. In addition, we demonstrate this tolerance is as a result of growth under high PSII excitation pressure, that can be modulated by growth temperature or growth irradiance. Cold-acclimated and cold-shifted plants fully recover from photoinhibition in the dark, whereas non-acclimated plants show reduced levels of recovery and demonstrate a requirement for light. The role of the PSII repair cycle, PSII quenching centres, and the use of Chl fluorescence imaging to monitor photoinhibitory responses in planta are discussed.     

Guard cell zeaxanthin tracks photosynthetically active radiation and stomatal apertures in Vicia faba leaves*

Zeaxanthin, antheraxanthin and violaxanthin concentrations in guard cells from sonicated abaxial epidermal peels of Vicia faba were measured from dawn to dusk, and compared with concentrations in mesophyll tissue of the same leaves. Measured changes in guard cell zeaxanthin and violaxanthin concentrations indicate that guard cells operate the xanthophyll cycle throughout the day. Mesophyll tissue had no detectable zeaxanthin at dawn, whereas guard cells had 30–50 mmol mol^?1 chlorophyll a+b. On a chlorophyll basis, maximal zeaxanthin levels were 3–4 fold higher in guard cells than in mesophyll cells. Zeaxanthin concentrations tracked levels of photosynthetically active radiation (PAR) in both mesophyll and guard cells. In the mesophyll, most of the zeaxanthin changes occurred in mid-morning and mid-afternoon. In guard cells, zeaxanthin concentrations changed nearly linearly with PAR in the early morning and late afternoon, and closely tracked PAR levels throughout the day. Guard cell zeaxanthin concentrations were also closely correlated with stomatal apertures. The close relationship between zeaxanthin concentrations and PAR levels in guard cells indicates that zeaxanthin is well suited to function as a molecular photosensor in stomatal movements.     

Singlet oxygen scavenging activity of tocopherol and plastochromanol in Arabidopsis thaliana: relevance to photooxidative stress

In the present study, singlet oxygen (¹O₂) scavenging activity of tocopherol and plastochromanol was examined in tocopherol cyclase‐deficient mutant (vte1) of Arabidopsis thaliana lacking both tocopherol and plastochromanol. It is demonstrated here that suppression of tocopherol and plastochromanol synthesis in chloroplasts isolated from vte1 Arabidopsis plants enhanced ¹O₂ formation under high light illumination as monitored by electron paramagnetic resonance spin‐trapping spectroscopy. The exposure of vte1 Arabidopsis plants to high light resulted in the formation of secondary lipid peroxidation product malondialdehyde as determined by high‐pressure liquid chromatography. Furthermore, it is shown here that the imaging of ultra‐weak photon emission known to reflect oxidation of lipids was unambiguously higher in vte1 Arabidopsis plants. Our results indicate that tocopherol and plastochromanol act as efficient ¹O₂ scavengers and protect effectively lipids against photooxidative damage in Arabidopsis plants.     

Interception of photosynthetically active radiation by cacti of different morphology

Summary Ribbing, orientation of cladodes, and variations in stem height were examined theoretically and in the field to see whether the morphology of various cacti could be interpreted as adaptations for intercepting photosynthetically active radiation (PAR). The total daytime PAR incident on different parts of the stem was related to the nocturnal increase in tissue acidity of these Crassulacean acid metabolism plants. The acidity change was 90% saturated for a total daytime PAR of about 22 mol m^-2, which indicates that the vertical stems of cacti are often light-limited in the desert.PAR interception by a cactus with ribs of various depths was simulated. Ribbing led to more surface area but a lower PAR per unit area, so net carbon gain was little influenced.Although the cladodes of Opuntia basilaris Engelm. and Bigel. var. basilaris were randomly oriented, those of another platyopuntia, O. chlorotica Engelm. and Bigel, preferentially faced northsouth. Compared to facing east-west, cladodes facing N-S would receive 52% more PAR at the winter solstice, a time when water is generally available and overheating is not a problem in the Mojave desert habitat of O. chlorotica.The maximum height of Stenocereus gummosus (Engelm.) Gibs. & Horak varied from 0.7 m in northern Baja California (31°52N) to 4.1 m at 23°47N. Stem height was positively correlated (r ²=0.93) with the height of the surrounding vegetation and negatively correlated (r ²=0.91) with the PAR 1 m above the ground. The great plasticity in stem height permits S. gummosus to obtain sufficient PAR in the presence of subtropical trees in the southern part of its range and yet expend less biomass to reach unobstructed PAR in the northern part of its range, where the surrounding vegetation is much shorter.     

Vitamin E protects against photoinhibition and photooxidative stress in Arabidopsis thaliana

Vitamin E is considered a major antioxidant in biomembranes, but little evidence exists for this function in plants under photooxidative stress. Leaf discs of two vitamin E mutants, a tocopherol cyclase mutant (vte1) and a homogentisate phytyl transferase mutant (vte2), were exposed to high light stress at low temperature, which resulted in bleaching and lipid photodestruction. However, this was not observed in whole plants exposed to long-term high light stress, unless the stress conditions were extreme (very low temperature and very high light), suggesting compensatory mechanisms for vitamin E deficiency under physiological conditions. We identified two such mechanisms: nonphotochemical energy dissipation (NPQ) in photosystem II (PSII) and synthesis of zeaxanthin. Inhibition of NPQ in the double mutant vte1 npq4 led to a marked photoinhibition of PSII, suggesting protection of PSII by tocopherols. vte1 plants accumulated more zeaxanthin in high light than the wild type, and inhibiting zeaxanthin synthesis in the vte1 npq1 double mutant resulted in PSII photoinhibition accompanied by extensive oxidation of lipids and pigments. The single mutants npq1, npq4, vte2, and vte1 showed little sensitivity to the stress treatments. We conclude that, in cooperation with the xanthophyll cycle, vitamin E fulfills at least two different functions in chloroplasts at the two major sites of singlet oxygen production: preserving PSII from photoinactivation and protecting membrane lipids from photooxidation.     

水稻冠层光合有效辐射的时空分布特征

以2个不同株型水稻品种为材料,设置高、中、低3个施氮水平,利用SunScan冠层分析仪于灌浆期系统测定了不同施氮水平下不同株型水稻品种植株形态和冠层内光合有效辐射（PAR）的时空分布状况.结果表明：施氮量对水稻株高、穗弯曲度和茎叶夹角有明显影响;群体叶面积的垂直分布呈中部>上部>下部的分布特征,最大分层叶面积指数（LAI）出现在0.60相对高度处.冠层内平均PAR透光率从顶部向下递减,且在冠层上中部递减迅速,下部递减缓慢;平均PAR透光率随施氮量的增加而递减;平均PAR透光率日变化表现为早晚较低,中午较高;平均PAR透光率随向下累积LAI的增加呈指数递减,群体消光系数K的日变化表现为早晚较高,中午较低,灌浆期的K值介于0.35～0.50.水稻冠层内PAR的三维空间分布表现为冠层上中部水平面上PAR透光率高,光斑面积大;下部水平面上PAR透光率低,光斑少;同一冠层高度水平面上的PAR光强呈不均匀分布.株型紧凑的水稻品种,冠层透光率高,透光率日变化大,群体消光系数小.     

Analysis of photosynthetically active radiation under various sky conditions in Wuhan,Central China

Observations of photosynthetically active radiation (PAR) and global solar radiation (G) at Wuhan, Central China during 2005–2012 were first reported to investigate PAR variability at different time scales and its PAR fraction (F _p) under different sky conditions. Both G irradiances (I _g) and PAR irradiances (I _p) showed similar seasonal features that peaked in values at noon during summer and reached their lower values in winter. F _p reached higher values during either sunrise or sunset; lower values of F _p appeared at local noon because of the absorption effects of water vapor and clouds on long-wave radiation. There was an inverse relationship between clearness index (K _t) and F _p; the maximum I _p decreased by 22.3 % (39.7 %) when sky conditions changed from overcast to cloudless in summer (winter); solar radiation was more affected by cloudiness than the seasonal variation in cloudy skies when compared with that in clear skies. The maximum daily PAR irradiation (R _p) was 11.89 MJ m^?2 day^?1 with an annual average of 4.85 MJ m^?2 day^?1. F _p was in the range of 29–61.5 % with annual daily average value being about 42 %. Meanwhile, hourly, daily, and monthly relationships between R _p and G irradiation (R _g) under different sky conditions were investigated. It was discovered that cloudy skies were the dominated sky condition in this region. Finally, a clear-sky PAR model was developed by analyzing the dependence of PAR irradiances on optical air mass under various sky conditions for the whole study period in Central China, which will lay foundations for ecological process study in the near future.     

Gravitropism in the starch excess mutant of Arabidopsis thaliana

Amyloplasts are hypothesized to play a key role in the cellular mechanisms of gravity perception in plants. While previous studies have examined the effects of starch deficiency on gravitropic sensitivity, in this paper, we report on gravitropism in plants with a greater amount of starch relative to the normal wild type. Thus, we have studied the sex1 (starch excess) mutant of Arabidopsis thaliana, which accumulates extra starch because it is defective in a protein involved in the regulation of starch mobilization. Compared to the wild type (WT), sex1 seedlings contained excess starch in cotyledons, hypocotyls, the root-hypocotyl transition zone, the body of the root, root hairs, and in peripheral rootcap cells. Sedimented amyloplasts were found in both the WT and in sex1 in the rootcap columella and in the endodermis of stems, hypocotyls, and petioles. In roots, the starch content and amyloplast sedimentation in central columella cells and the gravitropic sensitivity were comparable in sex1 and the WT. However, in hypocotyls, the sex1 mutant was much more sensitive to gravity during light-grown conditions compared to the WT. This difference was correlated to a major difference in size of plastids in gravity-perceiving endodermal cells between the two genotypes (i.e., sex1 amyloplasts were twice as big). These results are consistent with the hypothesis that only very large changes in starch content relative to the WT affect gravitropic sensitivity, thus indicating that wild-type sensing is not saturated.     

Imaging photosynthesis in wounded leaves of Arabidopsis thaliana

Chlorophyll fluorescence imaging provides a non-invasive and non-destructive means with which to measure photosynthesis. This technique has been used, in combination with 14CO2 feeding, to study the spatial and temporal changes in source-sink relationships which occur in mechanically wounded leaves of Arabidopsis thaliana. Twenty-four hours after wounding, cells proximal to the wound margin showed a rapid induction of PhiII upon illumination (a measure of the efficiency of photosystem II photochemistry) whilst cells more distal to the wound margin exhibited a much slower induction of PhiII and a large, transient increase in NPQ (a measure of the rate constant for non-photochemical energy dissipation within the light-harvesting antenna). These results are indicative of an increase in sink strength in the vicinity of the wound and this was confirmed by the retention of 14C photosynthate in this region. It has been hypothesized that wound-induced cell wall (apoplastic) invertase (cwINV) activity plays a central role in generating localized increases in sink strength in stressed plant tissue and that hexose sugars generated by the sucrolytic activity of cwINV may act as a signal regulating gene expression. Enzyme activity measurements, quantitative RT-PCR, and T-DNA insertional mutagenesis have been used to determine that expression of AtcwINV1 is responsible for all induced cwINV activity in mechanically wounded leaves. Whilst inactivation of this gene abolished wound-induced cwINV activity, it did not affect localized alterations in source-sink relationships of wounded leaves or wound-regulated gene expression. The signals that may regulate source-sink relationships and signalling in wounded leaves are discussed.     

Abscisic aldehyde oxidase in leaves of Arabidopsis thaliana

Abscisic acid (ABA) is a plant hormone involved in seed development and responses to various environmental stresses. Oxidation of abscisic aldehyde is the last step of ABA biosynthesis and is catalysed by aldehyde oxidase (EC 1.2.3.1). We have reported the occurrence of three isoforms of aldehyde oxidase, AOalpha, AObeta and AOgamma, in Arabidopsis thaliana seedlings, but none oxidized abscisic aldehyde. Here we report a new isoform, AOdelta, found in rosette leaf extracts, which efficiently oxidizes abscisic aldehyde. AO delta was specifically recognized by antibodies raised against a recombinant peptide encoded by AAO3, one of four Arabidopsis aldehyde oxidase genes (AAO1, AAO2, AAO3 and AAO4). Functionally expressed AAO3 protein in the yeast Pichia pastoris showed a substrate preference very similar to that of rosette AOdelta. These results indicate that AOdelta is encoded by AAO3. AOdelta produced in P. pastoris exhibited a very low Km value for abscisic aldehyde (0.51 microM), and the oxidation product was determined by gas chromatography-mass spectrometry to be ABA. Northern analysis showed that AAO3 mRNA is highly expressed in rosette leaves. When the rosette leaves were detached and exposed to dehydration, AAO3 mRNA expression increased rapidly within 3 h of the treatment. These results suggest that AOdelta, the AAO3 gene product, acts as an abscisic aldehyde oxidase in Arabidopsis rosette leaves.     

UV radiation reduces epidermal cell expansion in Arabidopsis thaliana leaves without altering cellular microtubule organization

Upon chronic UV treatment pavement cell expansion in Arabidopsis leaves is reduced, implying alterations in symplastic and apoplastic properties of the epidermal cells. In this study, the effect of UV radiation on microtubule patterning is analysed, as microtubules are thought to serve as guiding rails for the cellulose synthase complexes depositing cellulose microfibrils. Together with hemicelluloses, these microfibrils are regarded as the load-bearing components of the cell wall. Leaves of transgenic plants with fluorescently tagged microtubules (GFP-TUA6) were as responsive to UV as wild type plants. Despite the UV-induced reduction in cell elongation, confocal microscopy revealed that cellular microtubule arrangements were seemingly not affected by the UV treatments. This indicates an unaltered deposition of cellulose microfibrils in the presence of UV radiation. Therefore, we surmise that the reduction in cell expansion in UV-treated leaves is most probably due to changes in cell wall loosening and/or turgor pressure.Key words: arabidopsis, cell expansion, GFP-TUA6, leaf development, microtubule cytoskeleton, UV radiationPhotosynthetic functions such as solar light capture and carbon fixation are highly evolved features of plant leaves. To fulfil these functions in an optimal way, leaf development needs to be tuned to environmental conditions. Leaves are continuously exposed and subjected to environmental influences, which serve as co-regulators of leaf and plant development.¹ This ability of plants to adapt, secures the plant''s survival, even under non-optimal conditions. An example of a regulatory environmental parameter is solar light, indispensable for photosynthesis but potentially causing photoinhibition and/or UV-radiation stress. The highly energetic ultraviolet B (UV-B) rays of short wavelengths (280–315 nm) can both cause damage, as well as induce a range of specific metabolic and morphogenic plant responses. It was reported before that exposure to low dose UV radiation reduces Arabidopsis leaf size due to a decreased cell size.² Expansion of leaf epidermal cells of Arabidopsis thaliana is the combined action of promotion and restriction of growth, resulting in the typical irregular sinuous pavement cells. It has been postulated that cellulose microfibrils are responsible for generating a force opposing isotropic expansion by creating neck regions in between outgrowing lobes.³ As the microtubule cytoskeleton is believed to serve as guiding rails for the cellulose synthase complexes (CESAs),⁴ the deposition of the cellulose fibrils is intimately linked to the cortical microtubule arrangement. We have studied the UV-effect on microtubule organisation in leaf epidermal cells whose expansion had decreased upon this UV radiation. Microtubules in the adaxial pavement cells of the fourth leaf were monitored on several successive days in a transgenic line containing GFP fused to tubulin A6.⁵ The chronic UV treatment was started on day 0 when the plants were 2 weeks old, using UV exposure conditions as described in reference ². First the responsiveness of the GFP-TUA6 plants to UV radiation was evaluated. Similar to wild type (WT) plants,² the GFP-TUA6 plants had smaller leaves following 8 days of UV treatment (t-test, p < 0.01) (Fig. 1). This was caused by a significant reduction in the generalized cell area average of all measured cells, irrespective of the location within the leaf (Fig. 1; t-test, p < 0.01). In more detail, the average cell area within the base, middle and top zones of the GFP-TUA6 leaf was systematically lower in UV-treated leaves from 8 days after the treatment started onwards (data not shown).Open in a separate windowFigure 1Effect of UV radiation on leaf and cell area after different days of UV radiation. Open asterisks indicate a statistically significant difference in leaf area between UV-treated and control plants, black asterisks indicate statistically significant difference in cell area (t-test, *p < 0.05, **p < 0.01, ***p < 0.001). Error bars indicate the standard error for five different leaves at all measured time-points and 600, 170 and 180 cells at day 0, 8 and 12 respectively.As GFP-TUA6 leaves were as responsive to UV radiation as wild type leaves, confocal microscopy was used to visualize the organisation of the cortical microtubules facing the outer periclinal wall of the adaxial epidermis. No clear difference in microtubule (re)organization could be detected during the development of pavement cells, and throughout the UV treatment period. As shown in Figure 2 at day 2, pavement cells with comparable areas are similarly shaped in control and UV-irradiated plants and contain similar microtubule arrangements (Fig. 2 and marked cells). This means that microtubule organization is not directly affected by the UV exposure and that shape development proceeds in an analoguous manner as under control conditions. This lack of alteration in the microtubule arrangement can be observed for cells at the leaf tip, which were already in the process of lobe formation at the start of the exposure period, as well as for cells at the leaf base. Under our growth conditions, and in the monitored leaf number 4, cell proliferation still took place in this part of the leaf and lobes only started to appear on the cell surface. As microtubules are linked to the deposition of cellulose microfibrils, it can be assumed that no alterations in cellulose deposition occur upon UV treatment either. We can therefore conclude that the process of lobe formation and microtubule patterning is not impeded and that only the extent of cell expansion is restricted upon UV exposure.Open in a separate windowFigure 2Microtubule pattern in control and UV-exposed leaves visualized using GFP-TUA6 and confocal microscopy. Both images are from cells at the mid zone of the fourth leaf at day 2. Microtubules are similarly arranged in equally shaped and sized cells of control and UV-exposed leaves. The marked cells show a pattern whereby the tubules are centred in the neck regions between two outgrowing lobes.According to the Lockhart equation,⁶ cell (wall) growth is modulated by wall biomechanics and turgor pressure. Concerning turgor pressure, no clear differences in this factor between UV-exposed and control plants of Lactuca sativa L.⁷ and Pisum sativum⁸ could be observed, reinforcing the idea that especially the modulation of cell wall properties is the main factor causing the observed UV-induced reduction in cell expansion. Some reports indicate differential expression of wall loosening enzymes like expansins or xyloglucan endotransglycosylase/hydrolases (XTHs),^9,10 or cell wall strengthening enzymes as particular peroxidases⁷ after UV exposure. Another key event could involve UV-mediated changes in the phenylpropanoid pathway, which may cause changes in the lignin biosynthesis. As shown by the literature^11–14 lignin may well be an important modulator of cell wall architecture in Arabidopsis and therefore alterations in lignin synthesis could form the basis for morphological modifications. Further research on the cell wall properties of UV-treated plants may resolve this uncertainty.As a general conclusion we can state that the patterning of microtubules is not altered, but that alterations in cell wall composition or arrangements are the most plausible candidates for the observed reduction in pavement cell expansion upon chronic UV treatment.     

Control of seed development in Arabidopsis thaliana by atmospheric oxygen

Seed development is known to be inhibited completely when plants are grown in oxygen concentrations below 5·1 kPa, but apart from reports of decreased seed weight little is known about embryogenesis at subambient oxygen concentrations above this critical level. Arabidopsis thaliana (L.) Heynh. plants were grown full term under continuous light in premixed atmospheres with oxygen partial pressures of 2·5, 5·1, 10·1, 16·2 and 21·3 kPa O₂, 0·035 kPa CO₂ and the balance nitrogen. Seeds were harvested for germination tests and microscopy when siliques had yellowed. Seed germination was depressed in O₂ treatments below 16·2 kPa, and seeds from plants grown in 2·5 kPa O₂ did not germinate at all. Fewer than 25% of the seeds from plants grown in 5·1 kPa oxygen germinated and most of the seedlings appeared abnormal. Light and scanning electron microscopic observation of non-germinated seeds showed that these embryos had stopped growing at different developmental stages depending upon the prevailing oxygen level. Embryos stopped growing at the heart-shaped to linear cotyledon stage in 5·1 kPa O₂, at around the curled cotyledon stage in 10·1 kPa O₂, and at the premature stage in 16·2 kPa O₂. Globular and heart-shaped embryos were observed in sectioned seeds from plants grown in 2·5 kPa O₂. Tissue degeneration caused by cell autolysis and changes in cell structure were observed in cotyledons and radicles. Transmission electron microscopy of mature seeds showed that storage substances, such as protein bodies, were reduced in subambient oxygen treatments. The results demonstrate control of embryo development by oxygen in Arabidopsis .     

Respiratory acclimation in Arabidopsis thaliana leaves at low temperature

Acclimation of 25 degrees C-grown Arabidopsis thaliana at 5 degrees C resulted in a marked increase of leaf respiration in darkness (Rd) measured at 5 degrees C. Rd was particularly high in leaves developed at 5 degrees C. Leaf respiration (non-photorespiratory intracellular decarboxylation) in the light (Rl) also increased during cold acclimation, but less so than did Rd. The ratio Rd/Pt (Pt - true photosynthesis) was higher in more acclimated or cold-developed leaves, while the ratio Rl/Pt remained unchanged. In cold-acclimated leaves, Rl did not correlate with 3-phosphoglycerate and pyruvate nor with hexose phosphate pools in the cytosol. Rl in A. thaliana leaves was probably not limited by the substrate during cold acclimation. Under the conditions tested, Rd was more sensitive to low temperature stress than Rl.     

Cryptic chlorophyll breakdown in non-senescent green Arabidopsis thaliana leaves

Photosynthesis Research - Chlorophyll (Chl) breakdown is a diagnostic visual process of leaf senescence, which furnishes phyllobilins (PBs) by the PAO/phyllobilin pathway. As Chl breakdown disables...     

Dynamic behavior of clathrin in Arabidopsis thaliana unveiled by live imaging

Clathrin-coated vesicles (CCV) are necessary for selective transport events, including receptor-mediated endocytosis on the plasma membrane and cargo molecule sorting in the trans-Golgi network (TGN). Components involved in CCV formation include clathrin heavy and light chains and several adaptor proteins that are conserved among plants. Clathrin-dependent endocytosis has been shown to play an integral part in plant endocytosis. However, little information is known about clathrin dynamics in living plant cells. In this study, we have visualized clathrin in Arabidopsis thaliana by tagging clathrin light chain with green fluorescent protein (CLC-GFP). Quantitative evaluations of colocalization demonstrate that the majority of CLC-GFP is localized to the TGN, and a minor population is associated with multivesicular endosomes and the Golgi trans-cisternae. Live imaging further demonstrated the presence of highly dynamic clathrin-positive tubules and vesicles, which appeared to mediate interactions between the TGNs. CLC-GFP is also targeted to cell plates and the plasma membrane. Although CLC-GFP colocalizes with a dynamin isoform at the plasma membrane, these proteins exhibit distinct distributions at newly forming cell plates. This finding indicates independent functions of CLC (clathrin light chains) and dynamin during the formation of cell plates. We have also found that brefeldin A and wortmannin treatment causes distinctly different alterations in the dynamics and distribution of clathrin-coated domains at the plasma membrane. This could account for the different effects of these drugs on plant endocytosis.