Aménagement de terroirs au Yatenga (Nord-Ouest du Burkina Faso) : quatre années de gestion conservatoire de l'eau et de la fertilité des sols (GCES) : bilan et perspective

Ce document fait le bilan de 4 années de collaboration originale entre un chercheur spécialisé dans la lutte antiérosive et un agronome coopérant chargé de l'aménagement de terroir en milieu soudano sahélien très pauvre. Après un diagnostic de la situation régionale, des propositions ont été faite pour gérer l'eau, la biomasse, les matières organiques et la fertilité des sols. Un suivi des innovations a été réalisé avec l'aide de stagiaires et de missions courtes qui a permis la formation des responsables locaux et l'évolution des innovations en fonction des réactions des paysans. L'accent a été mis sur les contrats d'aménagement intégré à l'échelle de quartiers de culture, sur la régénération des parcours, sur l'aménagement d'impluvium, le stockage du ruissellement en citerne et la création de petits jardins intensifs, sur l'implantation de microbarrages perméables (haies vives en lignes d'herbes ou de cailloux) pour réduire l'énergie du ruissellement, sur le stockage de fumier/compost et de fourrages, ainsi que sur l'aménagement de ravines, de pistes rurales et de bas fonds. L'effort a porté sur des techniques simples, peu coûteuses, réalisables par les paysans et vise la restauration de la fertilité de leurs terres, l'infiltration et la fumure pour valoriser les aménagements. Des suggestions ont été faites pour le projet vivrier du Nord Yatenga tant sur la plan des aménagements que de la recherche d'accompagnement. (Résumé d'auteur

Adsorption and desorption dynamics of citric acid anions in soil

The functional role of organic acid anions (e.g. citrate, oxalate, malonate, etc) in soil has been intensively investigated with special focus either on (i) microbial respiration and soil carbon dynamics, (ii) nutrient solubilization, or (iii) metal detoxification. Considering the potential impact of sorption processes on the functional significance of these effects, comparatively little is known about the adsorption and desorption dynamics of organic acid anions in soils. The aim of this study therefore was to experimentally characterize the adsorption and desorption dynamics of organic acid anions in different soils using citrate as a model carboxylate. Results showed that both adsorption and desorption processes were fast, reaching a steady state equilibrium solution concentration within approximately 1 hour. However, for a given total soil citrate concentration(ctot) the steady state value obtained was critically dependent on the starting conditions of the experiment (i.e. whether most of the citrate was initially present in solution (cl) or held on the solid phase (cs)). Specifically, desorption-led processes resulted in significantly lower equilibrium solution concentrations than adsorption led processes indicating time-dependent sorption hysteresis. As it is not possible to experimentally distinguish between different sorption pools in soil (i.e. fast, slow, irreversible adsorption/desorption), a new dynamic hysteresis model was developed that relies only on measured soil solution concentrations. The model satisfactorily explained experimental data and was able to predict dynamic adsorption and desorption behaviour. To demonstrate its use we applied the model to two relevant scenarios (exudation and microbial degradation), where the dynamic sorption behaviour of citrate occurs. Overall, this study highlights the complex nature of citrate sorption in soil and concludes that existing models need to incorporate both a temporal and sorption hysteresis component to realistically describe the role and fate of organic acids in soil processes

La G.C.E.S. : une nouvelle stratégie de lutte anti-érosive appliquée à l'aménagement de terroirs en zone soudano-sahélienne du Burkina-Faso

Face à la dégradation de la végétation, des sols et du réseau hydrographique de la région soudano-sahélienne d'Afrique occidentale, et face à l'échec des méthodes d'équipement rural, les auteurs proposent une approche où la participation des intéressés intervient dès le stade des définitions des projets. Il faut donc définir avec les paysans les risques d'érosion et les moyens traditionnels améliorés de gérer l'eau (citernes, travail du sol, haies vives, cordons de pierres) et de restaurer la fertilité des sols (Zaï, Zaï forestier, amélioration des fumiers, paillage et agroforesterie). Les auteurs analysent alors comment organiser l'aménagement de terroirs villageois dans l'espace (parcelles individuelles, versants, petits bassins) et dans le temps (3 étapes : sensibilisation, expérimentation + démonstration, généralisation au terroir). (Résumé d'auteur

The Psb27 protein facilitates manganese cluster assembly in photosystem II

Photosystem II (PSII) is a large membrane protein complex that uses light energy to convert water to molecular oxygen. This enzyme undergoes an intricate assembly process to ensure accurate and efficient positioning of its many components. It has been proposed that the Psb27 protein, a lumenal extrinsic subunit, serves as a PSII assembly factor. Using a psb27 genetic deletion strain (Δpsb27) of the cyanobacterium Synechocystis sp. PCC 6803, we have defined the role of the Psb27 protein in PSII biogenesis. While the Psb27 protein was not essential for photosynthetic activity, various PSII assembly assays revealed that the Δpsb27 mutant was defective in integration of the Mn4Ca1Clx cluster, the catalytic core of the oxygen-evolving machinery within the PSII complex. The other lumenal extrinsic proteins (PsbO, PsbU, PsbV, and PsbQ) are key components of the fully assembled PSII complex and are important for the water oxidation reaction, but we propose that the Psb27 protein has a distinct function separate from these subunits. We show that the Psb27 protein facilitates Mn4Ca1Cl x cluster assembly in PSII at least in part by preventing the premature association of the other extrinsic proteins. Thus, we propose an exchange of lumenal subunits and cofactors during PSII assembly, in that the Psb27 protein is replaced by the other extrinsic proteins upon assembly of the Mn4Ca1Clx cluster. Furthermore, we show that the Psb27 protein provides a selective advantage for cyanobacterial cells under conditions such as nutrient deprivation where Mn4Ca 1Clx cluster assembly efficiency is critical for survival. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc

The extrinsic proteins of Photosystem II

Years of genetic, biochemical, and structural work have provided a number of insights into the oxygen evolving complex (OEC) of Photosystem II (PSII) for a variety of photosynthetic organisms. However, questions still remain about the functions and interactions among the various subunits that make up the OEC. After a brief introduction to the individual subunits Psb27, PsbP, PsbQ, PsbR, PsbU, and PsbV, a current picture of the OEC as a whole in cyanobacteria, red algae, green algae, and higher plants will be presented. Additionally, the role that these proteins play in the dynamic life cycle of PSII will be discussed. © 2006 Springer Science+Business Media B.V

The PsbP domain protein 1 functions in the assembly of lumenal domains in photosystem I

Photosystem I (PS I) is a multisubunit membrane protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase. The PsbP domain protein 1 (PPD1; At4g15510) is located in the thylakoid lumen of plant chloroplasts and is essential for photoautotrophy, functioning as a PS I assembly factor. In this work, RNAi was used to suppress PPD1 expression, yielding mutants displaying a range of phenotypes with respect to PS I accumulation and function. These PPD1 RNAi mutants showed a loss of assembled PS I that was correlated with loss of the PPD1 protein. In the most severely affected PPD1 RNAi lines, the accumulated PS I complexes exhibited defects in electron transfer from plastocyanin to the oxidized reaction center P 700+. The defects in PS I assembly in the PPD1 RNAi mutants also had secondary effects with respect to the association of light-harvesting antenna complexes to PS I. Because of the imbalance in photosystem function in the PPD1 RNAi mutants, light-harvesting complex II associated with and acted as an antenna for the PS I complexes. These results provide new evidence for the role of PPD1 in PS I biogenesis, particularly as a factor essential for proper assembly of the lumenal portion of the complex. © 2014 by The American Society for Biochemistry and Molecular Biology, Inc

Absence of the PsbQ protein results in destabilization of the PsbV protein and decreased oxygen evolution activity in cyanobacterial photosystem II

We have previously reported that cyanobacterial photosystem II (PS II) contains a protein homologous to PsbQ, the extrinsic 17-kDa protein found in higher plant and green algal PS II (Kashino, Y., Lauber, W. M., Carroll, J. A., Wang, Q., Whitmarsh, J., Satoh, K., and Pakrasi, H. B. (2002) Biochemistry 41, 8004-8012) and that it has regulatory role(s) on the water oxidation machinery (Thornton, L. E., Ohkawa, H., Roose, J. L., Kashino, Y., Keren, N., and Pakrasi, H. B. (2004) Plant Cell 16, 2164-2175). In this work, the localization and the function of PsbQ were assessed using the cyanobacterium Synechocystis sp. PCC 6803. From the predicted sequence, cyanobacterial PsbQ is expected to be a lipoprotein on the luminal side of the thylakoid membrane. Indeed, experiments in this work show that upon Triton X-114 fractionation of thylakoid membranes, PsbQ partitioned in the hydrophobic phase, and trypsin digestion revealed that PsbQ was highly exposed to the luminal space of thylakoid membranes. Detailed functional assays were conducted on the psbQ deletion mutant (ΔpsbQ) to analyze its water oxidation machinery. PS II complexes purified from ΔpsbQ mutant cells had impaired oxygen evolution activity and were remarkably sensitive to NH2OH, which indicates destabilization of the water oxidation machinery. Additionally, the cytochrome c550 (PsbV) protein partially dissociated from purified ΔpsbQ PS II complexes, suggesting that PsbQ contributes to the stability of PsbV in cyanobacterial PS II. Therefore, we conclude that the major function of PsbQ is to stabilize the PsbV protein, thereby contributing to the protection of the catalytic Mn 4-Ca1-Clx cluster of the water oxidation machinery. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc

A genetically tagged Psb27 protein allows purification of two consecutive photosystem II (PSII) assembly intermediates in Synechocystis 6803, a cyanobacterium

Photosystem II (PSII) is a large membrane bound molecular machine that catalyzes light-driven oxygen evolution from water. PSII constantly undergoes assembly and disassembly because of the unavoidable damage that results from its normal photochemistry. Thus, under physiological conditions, in addition to the active PSII complexes, there are always PSII subpopulations incompetent of oxygen evolution, but are in the process of undergoing elaborate biogenesis and repair. These transient complexes are difficult to characterize because of their low abundance, structural heterogeneity, and thermodynamic instability. In this study, we show that a genetically tagged Psb27 protein allows for the biochemical purification of two monomeric PSII assembly intermediates, one with an unprocessed form of D1 (His27ΔctpAPSII) and a second one with a mature form of D1 (His27PSII). Both forms were capable of light-induced charge separation, but unable to photooxidize water, largely because of the absence of a functional tetramanganese cluster. Unexpectedly, there was a significant amount of the extrinsic lumenal PsbO protein in the His27PSII, but not in the His27ΔctpAPSII complex. In contrast, two other lumenal proteins, PsbU and PsbV, were absent in both of these PSII intermediate complexes. Additionally, the only cytoplasmic extrinsic protein, Psb28 was detected in His27PSII complex. Based on these data, we have presented a refined model of PSII biogenesis, illustrating an important role of Psb27 as a gate-keeper during the complex assembly process of the oxygen-evolving centers in PSII. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc