Freek S. Posthumus

Expression and activity of sulfate transporters and APS reductase in curly kale in response to sulfate deprivation and re-supply.

Both activity and expression of sulfate transporters and APS reductase in plants are modulated by the sulfur status of the plant. To examine the regulatory mechanisms in curly kale (Brassica oleracea L.), the sulfate supply was manipulated by the transfer of seedlings to sulfate-deprived conditions, which resulted in an up to 3-fold increase in the sulfate uptake capacity by the root, accompanied by an induction of transcript abundances of the Group 1 and 4 sulfate transporters in root and shoot. Upon sulfate re-supply, there was no correlation between the activity and expression of the sulfate transporters. Despite the decrease in the abundance of the sulfate transporter transcripts, especially at the onset of the sulfate re-supply, the sulfate uptake capacity was affected very little for up to 96h. There was no relationship between changes in the sulfate or thiol content and activity and expression of the sulfate transporters. Thus, their significance as regulatory signal compounds remains unresolved. The activity and expression of APS reductase, which was enhanced strongly only in the shoots of sulfate-deprived plants, and rapidly decreased again upon sulfate re-supply, corresponded with changes in thiol content, consistent with this pool having a role as a regulatory signal.

Impact of Suboptimal Temperature on Growth, Photosynthesis, Leaf Pigments and Carbohydrates of Domestic and High-altitude Wild Lycopersicon Species

The impact of near-optimal (25/20 degrees C) and suboptimal (16/14 degrees C) day/night temperatures on growth, photosynthesis, pigment composition and carbohydrate content was compared between domestic and high-altitude wild Lycopersicon species. When related to the relative shoot growth rate (RSGR) measured at optimal temperature, genotypes of the domestic tomato (L. esculentum (L.) Mill. cv. Abunda and cv. Large Red Cherry (LRC) showed a stronger inhibition of RSGR at suboptimal temperature than the high-altitude wild species L. peruvianum Mill. LA 385 and L. hirsutum Humb. & Bonpl. LA 1777. The initiation rare of new leaves was 2.1-fold faster in all species at 25/20 degrees C than at 16/14 degrees C. In contrast to the other genotypes, the leaf area of suboptimally grown Abunda plants was 28 % smaller than the area of leaves that were fully expanded at optimal temperature. In all species, specific leaf area (SLA) at 16/14 degrees C was 17-26 % lower than at 25/20 degrees C. The percentage of leaf dry matter increased in response to growth ar suboptimal temperature. This increase was higher in L. esculentum genotype Abunda (99 %) than in genotype LRC (38 %), and the wild species L. peruvianum (50 %) and L. hirsutum (38 %), which could be attributed to inter- and intra-specific differences in starch accumulation of 16/14 degrees C-grown leaves. Only in both L. esculentum genotypes, net photosynthetic rate at growth irradiance (A(225)) and at light saturation (A(sat)) was 14 to 30 % lower in leaves grown and measured at suboptimal temperature, compared with leaves grown and measured at optimal temperature (25 degrees C). Chlorophyll (Chl) a fluorescence measurements indicated that the decrease of A225 in leaves of suboptimally grown L. esculentum plants was paralleled by a decrease in the quantum yield of photosystem II electron transport (Phi(PSII)), which could be mainly attributed to a decrease in the photochemical quenching component (q(P)). In all species, the nonphotochemical quenching component (NPQ) was 2 to 4-fold higher at 16/14 degrees C. Growth temperature hardly affected Chi content on a leaf area basis, whereas the content of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) on a Chi basis was ca. 1.5-fold higher in 16/14 degrees C-grown leaves. The epoxidation state of the xanthophyll cycle pool was only slightly lower in suboptimal leaves due to the moderate growth irradiance.

Efficiency of nitrate uptake in spinach: impact of external nitrate concentration and relative growth rate on nitrate influx and efflux

Regulation of nitrate influx and efflux in spinach (Spinacia oleracea L., cv. Subito), was studied in short-term label experiments with 13N- and 15N-nitrate. Nitrate fluxes were examined in relation to the N demand for growth, defined as relative growth rate (RGR) times plant N concentration. Plants were grown at different nitrate concentrations (0.8 and 4 mM), with mineral composition of growth and uptake solutions identical.Nitrate influx, efflux and net nitrate uptake rate (NNUR) were independent of the external nitrate concentration, despite differences in internal nitrate concentration. At both N regimes, NNUR was adequate to meet the N demand for growth. RGR-related signals predominantly determined the nitrate fluxes. At high RGR (0.25 g g-1 day-1), nitrate influx was 20 to 40% lower and nitrate efflux was 50 to 70% lower than at lower RGR (0.17 g g-1 day-1); efflux:influx ratio (E:I) declined from 0.5 at low RGR to 0.2 at higher RGR. Thus, the efficiency of NNUR substantially increased with increasing RGR. Differences in nitrate translocation between morning and afternoon coincided with differences in nitrate efflux, which is in accordance with the suggested regulation of nitrate efflux by the root cytoplasmic nitrate concentration.

Copper exposure interferes with the regulation of the uptake, distribution and metabolism of sulfate in Chinese cabbage.

Exposure of Chinese cabbage (Brassica pekinensis) to enhanced Cu(2+) concentrations (1-10 microM) resulted in leaf chlorosis, a loss of photosynthetic capacity and lower biomass production at > or = 5 microM. The decrease in pigment content was likely not the consequence of degradation, but due to hindered chloroplast development upon Cu exposure. The Cu content of the root increased with the Cu(2+) concentration (up to 40-fold), though only a minor proportion (4%) was transferred to the shoot. The nitrate uptake by the root was substantially reduced at > or = 5 microM Cu(2+). The nitrogen content of the root was affected little at lower Cu(2+) levels, whereas that in the shoot was decreased at > or = 5 microM Cu(2+). Cu affected the uptake, distribution and metabolism of sulfate in Chinese cabbage. The total sulfur content of the shoot was increased at > or = 2 microM Cu(2+), which could be attributed mainly to an increase in sulfate content. Moreover, there was a strong increase in water-soluble non-protein thiol content in the root and, to a lesser extent, in the shoot at > or = 1 microM, which could only partially be ascribed to a Cu-induced enhancement of the phytochelatin content. The nitrate uptake by the root was substantially reduced at > or = 5 microM Cu(2+), coinciding with a decrease in biomass production. However, the activity of the sulfate transporters in the root was slightly enhanced at 2 and 5 microM Cu(2+), accompanied by enhanced expression of the Group 1 high affinity transporter Sultr1;2, and the Group 4 transporters Sultr4;1 and Sultr4;2. In the shoot, there was an induction of expression of Sultr4;2 at 5 and 10 microM Cu(2+). The expression of APS reductase was affected little in the root and shoot up to 10 microM Cu(2+). The upregulation of the sulfate transporters may be due not only to greater sulfur demand at higher Cu levels, but also the consequence of interference by Cu with the signal transduction pathway regulating the expression and activity of the sulfate transporters.

Effects of NaCl salinity on 15N-nitrate fluxes and specific root length in the halophyte Plantago maritima L.

The effect of salinity on nitrate influx, efflux, nitrate net uptake rate and net nitrogen translocation to the shoot was assessed in a 15N steady state labelling experiment in the halophyte Plantago maritima L. raised for 14 days on solution supplied with 50, 100 and 200 mol m−3 sodium chloride or without sodium chloride. Additionally, salinity induced changes in root morphology were determined. Specific root length increased upon exposure to elevated sodium chloride concentrations due to variations in biomass allocation and length growth of the tap root. Changes in root morphology, however, had a minor effect on nitrate fluxes when expressed on a root fresh weight basis. The decreased rate of nitrate net uptake in plants grown on elevated levels of sodium chloride was almost entirely due to a decrease in nitrate influx. Expressed as a proportion of influx, nitrate efflux remained unchanged and was even lower at the highest salinity level. At all sodium chloride concentrations applied the initial rate of nitrogen net translocation to the shoot decreased relative to the rate of nitrate net uptake. It is concluded that under steady state conditions the negative effect of sodium chloride on the rate of nitrate net uptake at non growth-limiting salinity levels was due to the interaction between sodium chloride and nitrate transporters in the root plasma membrane and/or processes mediating the translocation of nitrogen compounds, possibly nitrate, to the shoot.

Growth under UV-B radiation increases tolerance to high-light stress in pea and bean plants

Pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) plants were exposed to enhanced levels of UV-B radiation in a growth chamber. Leaf discs of UV-B treated and control plants were exposed to high-light (HL) stress (PAR: 1200 μmol m−2 s−1) to study whether pre-treatment with UV-B affected the photoprotective mechanisms of the plants against photoinhibition. At regular time intervals leaf discs were taken to perform chlorophyll a fluorescence and oxygen evolution measurements to assess damage to the photosystems. Also, after 1 h of HL treatment the concentration of xanthophyll cycle pigments was determined. A significantly slower decline of maximum quantum efficiency of PSII (Fv/Fm), together with a slower decline of oxygen evolution during HL stress was observed in leaf discs of UV-B treated plants compared to controls in both plant species. This indicated an increased tolerance to HL stress in UV-B treated plants. The total pool of xanthophyll cycle pigments was increased in UV-B treated pea plants compared to controls, but in bean no significant differences were found between treatments. However, in bean plants thiol concentrations were significantly enhanced by UV-B treatment, and UV-absorbing compounds increased in both species, indicating a higher antioxidant capacity. An increased leaf thickness, together with increases in antioxidant capacity could have contributed to the higher protection against photoinhibition in UV-B treated plants.

Copper toxicity in Chinese cabbage is not influenced by plant sulphur status, but affects sulphur metabolism-related gene expression and the suggested regulatory metabolites

The toxicity of high copper (Cu) concentrations in the root environment of Chinese cabbage (Brassica pekinensis) was little influenced by the sulphur nutritional status of the plant. However, Cu toxicity removed the correlation between sulphur metabolism-related gene expression and the suggested regulatory metabolites. At high tissue Cu levels, there was no relation between sulphur metabolite levels viz. total sulphur, sulphate and water-soluble non-protein thiols, and the expression and activity of sulphate transporters and expression of APS reductase under sulphate-sufficient or-deprived conditions, in the presence or absence of H2 S. This indicated that the regulatory signal transduction pathway of sulphate transporters was overruled or by-passed upon exposure to elevated Cu concentrations.

Metabolism of atmospheric sulfur gases in onion

The impact of atmospheric sulfur gases was studied in onion (Allium cepa L.). The occurrence of toxic effects of H2S in onion depended not only on the atmospheric H2S level but also on the duration of the exposure. Prolonged exposure of onion to ≥ 0.3 µl l−1 H2S resulted in a strong reduction in shoot biomass production. H2S exposure resulted in a decrease in the organic N/S ratio at all levels (0.15 to 0.6 µl l−1), which could be attributed to an increase in the pool of secondary sulfur compounds and not to changes in the sulfolipid content. The latter even decreased upon H2S exposure when expressed on a lipid basis. SO2 exposure resulted in an enhanced content of sulfate and total sulfur in the shoot, whereas roots were not affected. In contrast to exposure to H2S, SO2 exposure did not result in an increase in non-protein organic (secondary) sulfur compounds, which showed that these compounds only were a sink pool for reduced atmospheric sulfur, when both the uptake of sulfate by the roots and its reduction in the shoot were by-passed.

Impact of Enhanced Copper Levels and Sulfate Deprivation on the Uptake and Metabolism of Sulfate in Chinese Cabbage (Brassica pekinensis)

Exposure of Chinese cabbage (Brassica pekinensis) to an enhanced Cu2+ level (4 μM) resulted in a reduced plant biomass production and an increased shoot to root ratio at both sulfate-sufficient and sulfate-deprived conditions. However, sulfate deprivation had a more rapid negative effect on plant biomass production than an enhanced Cu2+ level, The expression and activity of the sulfate transporters and the expression of APS reductase in Chinese cabbage were rapidly up-regulated (already after 1 or 2 days) upon exposure to 4 μM Cu2+, sulfate deprivation and their combination. Though the impact of sulfate deprivation on the expression and activity of the sulfate transporters was hardly further affected by Cu2+.

Interaction Between Atmospheric and Pedospheric Sulfur Nutrition in Eucalyptus camaldulensis

Plants are able to use gaseous sulfurous air pollutants as a sulfur source for growth, especially under circumstances where the sulfur supply to the roots is limited. In Eucalyptus camaldulensis seedlings there was a direct interaction between atmospheric H2S exposure and the uptake of sulfate by the root. At an ample sulfate supply, exposure of E. camaldulensis to 0.2 μl l−1 H2S for 1 week resulted in a decrease of the sulfate uptake capacity of the root (up to 70%). Sulfate deprivation resulted in an up to sixfold increase in the sulfate uptake capacity of the root, however, it was completely alleviated upon H2S exposure. At all conditions, the sulfate uptake capacity was strongly related to the sulfate concentration in the root. Evidently, upon H2S exposure E. camaldulensis transferred from sulfate taken up by the root to sulfide absorbed by the shoot as sulfur source for growth. Despite that down-regulation of the sulfate uptake capacity occurred upon H2S exposure there was no direct metabolic control of the uptake and utilization of foliarly absorbed H2S. H2S exposure resulted in enhanced levels of total and organic sulfur (and nitrogen) in the shoot, whereas the N/S ratio strongly decreased.