Effect of moderate and high light on photosystem II function in Arabidopsis thaliana depleted in digalactosyl-diacylglycerol.

The response of the heat-sensitive dgd1-2 and dgd1-3 Arabidopsis mutants depleted in the galactolipid DGDG to photoinhibition of chloroplasts photosystem II was studied to verify if there is a relationship between heat stress vulnerability due to depletion in DGDG and the susceptibility to photoinhibitory damage. Non-photochemical quenching (NPQ) is known to dissipate excessive absorbed light energy as heat to protect plants against photodamage. The main component of NPQ is dependent of the transthylakoid pH gradient and is modulated by zeaxanthin (Zx) synthesis. These processes together with chlorophyll fluorescence induction were used to characterize the response of the genotypes. The mutants were more sensitive to photoinhibition to a small extent but this was more severe for dgd1-3 especially at high light intensity. It was deduced that DGDG was not a main factor to influence photoinhibition but other lipid components could affect PSII sensitivity towards photoinhibition in relation to the physical properties of the thylakoid membrane. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Functional aspects of the photosynthetic light reactions in heat stressed Arabidopsis deficient in digalactosyl-diacylglycerol.

Plants are often submitted, in their natural environment, to various abiotic stresses such as heat stress. However, elevated temperature has a detrimental impact on overall plant growth and development. We have examined the physiological response of the dgd1-2 and dgd1-3 Arabidopsis mutants lacking 30-40% of digalactosyl-diacylglycerol (DGDG) exposed to heat constraint. These mutants, which grow similarly to wild type under normal conditions, were previously reported to be defective in basal thermotolerance as measured by cotyledon development. However their functional properties were not described. Chlorophyll fluorescence measurements and absorbance changes at 820nm were used to monitor photosystem II (PSII) and PSI activity, respectively. It was observed that both mutants have similar photosystem activities with some differences. The mutants were less able to use near saturation light energy and elicited higher rates of cyclic PSI electron flow compare to wild type. Arabidopsis leaves exposed to short-term (5min) mild (40°C) or strong (44°C) heat treatment have shown a decline in the operating effective quantum yield of PSII and in the proportion of active PSI reaction centers. However, cyclic PSI electron flow was enhanced. The establishment of the energy-dependent non-photochemical quenching of chlorophyll fluorescence was accelerated but its decline under illumination was inhibited. Furthermore, heat stress affected the process implicated in the redistribution of light excitation energy between the photosystems known as the light state transitions. All the effects of heat stress mentioned above were more intense in the mutant leaves with dgd1-3 being even more susceptible. The decreased DGDG content of the thylakoid membranes together with other lipid changes are proposed to influence the thermo-sensitivity of the light reactions of photosynthesis towards heat stress.

Abolition of photosystem I cyclic electron flow in Arabidopsis thaliana following thermal-stress.

Heat tolerance of Arabidopsis thaliana (WT) and its mutants, crr2-2, lacking NADPH-dehydrogenase (Ndh-pathway), and pgr5, deficient in proton gradient regulation and/or ferredoxin-quinone-reductase (FQR-pathway), was studied from 30 to 46°C. Chlorophyll fluorescence revealed that thermal damage to photosystem II (PSII) was maximal in WT plants following short-term exposure of leaves to moderate or high temperature stress. Thermal stress impaired the photosynthetic electron flow at oxidizing and reducing sides of PSII. This was deduced from the transformation of temperature dependent OJIP to OKP patterns, changes in the relative amplitudes of K-step fluorescence rise and F(v)/F(o) ratio. The amplitude of the K-peak that corresponds to the magnitude of damage to the oxygen evolving complex (OEC) in crr2-2 mutants was about 50% of that observed in WT plants exposed to 46°C. The damage to OEC in pgr5 mutants was relatively smaller and thus their PSII complexes were more heat tolerant. P700 oxidation-reduction kinetics following heat-stress revealed that photosystem I (PSI) complexes remained oxidizable either with 10-ms multiple turn-over flashes or far-red illumination but the complementary cyclic electron flow around PSI (CEF) was abolished in both mutants. With further increase in incubation temperature, CEF was fully suppressed even in WT. Thus, P700 turn-over was not enhanced following thermal stress. Furthermore, the experimental data predicts the onset of pseudocyclic electron transport with molecular oxygen as terminal acceptor in crr2-2 and pgr5 mutants but not in wild type Arabidopsis subjected to severe thermal-stress.

Proteome analysis of apical and basal regions of poplar stems under gravitropic stimulation.

Gravity is a constant force guiding the direction of plant growth. In young poplar stem, reorientation of the apical region is mainly obtained by differential growth of elongating primary tissues. At the base, where elongation is achieved but where the cambium is active, reorientation is due to asymmetrical formation of reaction wood. After 45 min of gravistimulation, the stem showed no reorientation, but 1 week later, reaction wood was observed at the base of the stem. To determine the molecular mechanisms taking place at the top and base of the stem, after 45 min or 1 week of inclination, the changes induced in protein accumulation were studied by two-dimensional polyacrylamide gel electrophoresis and quantitatively analyzed using image analysis software. Around 300 protein spots were reproducibly detected and analyzed. Forty percent of these proteins showed significant changes after inclination. Mass spectrometry analysis of 135 spots led to the identification of 60 proteins involved in a wide range of activities and metabolisms. Very different patterns of protein expression were obtained according to conditions tested, highlighting the complexity of gravitropic responses. Our results suggest that primary and secondary tissues present specific mechanisms to sense reorientation and to respond to inclination. Some selected proteins are discussed.

Anatomical and biochemical changes during adventitious rooting of apple rootstocks MM 106 cultured in vitro.

Adventitious rooting in microcuttings of Malus rootstocks MM106 was studied as regards their histological and biochemical aspects. Microcuttings from shoots raised in Murashige and Skoog's (1962) medium were transferred into a rooting medium containing IBA in the dark, then fixed 0, 3, 5, 7 and 10 days after. Some cambial zone and adjacent phloem cells became dense cytoplasm, nuclei with prominent nucleoli and the first cell divisions were observed at day 3. Meristemoids became individualized, consisting of densely staining cells (with enlarged nucleoli) formed outside the xylem by day 5. Identifiable root primordia with a conical shape and several cell layers were present at day 7. Roots with organized tissue system emerged from the stem 10 days after the root induction treatment. From these histological observations, it can be established that the rooting induction stage ended before day 3. The initiation stage, with the first histological modifications to the formation of meristemoids, would correspond to the transient increase of our biochemical marker (peroxidase activity) until day 5. The best rooting percentage obtained with cultures in the presence of auxin during 5 days confirms this hypothesis. The expression of rooting can then take place.

Sensitivity of two wheat species's seeds (Triticum durum, variety Karim and Triticum aestivum, variety Salambô) to heat constraint during germination.

The aim of this study is determine the effect of different temperature regimes on germination of wheat seeds and early germination events. Germination is very sensitive to environment conditions, particularly the temperature. Physiological and biochemical responses of wheat seed germination during time at various temperatures (5, 15, 25, 35 and 45 degrees C) have shown that optimal temperature (25 degrees C) favorites a good aptitude to germinate, whereas low (5 degrees C) and high temperature (45 degrees C) were extend the delay of germination. Residuary dry matter's mass of germinating, both Karim and Salambô wheat species, Seeds at 5 and 45 degrees C decreased lightly. This show a less mobilization of reserves to embryo. The activity of totals solubles peroxidases changes with temperature of wheat seed germination. At low (5 degrees C) and high (45 degrees C) temperatures, for both two cultivars, peroxidase activity decreases during germination. With exception, during imbibition (2 h), only 45 degrees C involves a high stimulation of this activity for Karim cultivar. This increase is considered as a biochemical response to high temperature. In control temperature (25 degrees C), we have obtained a light increase in peroxidase activity in comparison with that of dry seeds for both these two wheat species. At the end of this study, we have studied the effect of heat stress on totals proteins content. Nevertheless, both for these two wheat cultivars, at 5 degrees C there is no a markedly change in proteins amount during germination. In control condition (25 degrees C) of germination, there is a slight decrease in this content. Germination under high temperature, for Karim variety, induces a rapid synthesis of Heat Shock Proteins (HSPs) and concomitantly a weak degradation of normal proteins. The opposite phenomenon was observed for Salambô variety (weak synthesis of HSPs and important degradation of normal proteins).

Iron-superoxide dismutase and monodehydroascorbate reductase transcripts accumulate in response to internode rubbing in tomato.

A cDNA encoding an iron-superoxide dismutase (Fe-SOD) was isolated by RACE-PCR from a Lycopersicon esculentum cDNA library. The Fe-SOD cDNA consists of a 746-bp open reading frame and is predicted to encode a protein of 249 amino acids with a calculated molecular mass of 27.9 kDa. The deduced amino acid sequence was very similar to other plant Fe-SODs and a potential chloroplastic targeting was found. To study the induction of oxidative burst in response to mechanical stimulation, the accumulation of Fe-SOD and monodehydroascorbate reductase (MDHAR) mRNAs was analysed in response to young growing internode rubbing in tomato plants. Northern analyses show that Fe-SOD mRNA and MDHAR mRNA accumulated in tomato internodes 10 min after the mechanical stimulation. These results suggest that reactive oxygen species are early involved in the response of a plant to a mechanical stimulation, such as rubbing. The nucleotide sequence data reported in this paper will appear in the NCBI Nucleotide Sequence Databases under the accession number AY262025.