IntroductionFaidherbia albida (Del.) A. Chev. (syn. Acacia albida: Mimosoideae) is a leguminous tree species widely distributed in Africa. It is present in Sudanese and Sahelian zones that are affected by a long dry season. The species is distributed over territories with an annual rainfall ranging from 50 to 1500 mm (Fagg & Barnes 1990). In eastern, central and southern Africa, it occurs naturally along riverbanks on alluvial soils. The best known peculiarity of F. albida is its reverse phenology (Wickens 1969). Trees are in leaf, growing and fruiting during the dry season, whereas leaves are shed after the first rains and growth resumes only at the end of the wet season. This phenology is advantageous for agroforestry, because competition with associated crops growing during the wet season is minimized. Faidherbia albida trees are rather vigor- 3. Radial trunk growth ceased before the end of the dry season and could have been affected by the moderate drought stress. However, leafiness remained constant during the dry season. Leaf shedding occurred after the first rains and was probably independent of drought. 4. Faidherbia albida displayed large transpiration rates under favourable conditions but the ratio of sapflow to Penman evapotranspiration and the soil-to-leaf specific hydraulic conductance decreased severely towards the end of the dry season. 5. Roots of F. albida were distributed through the weathered rock, down to a depth of 7 m, and vanished in the vicinity of a permanent water-table. The isotopic composition of oxygen in the xylem sap (δ 18 O) remained very close to the values recorded in the water-table during the course of the year. Phreatophytism thus explained the maintenance of growth and transpiration during the dry season. Nevertheless, during early rains, δ 18 O of sap switched towards the composition of the superficial soil layers, indicating facultative phreatophytism. 6. Reverse phenology, low density and depth of water uptake of F. albida indicated a low competition with annual crops for water; the fraction of annual rainfall used by the trees was estimated to remain below 5%.

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The process of thyroid hormone synthesis, which occurs in the lumen of the thyroid follicles, results from an oxidative reaction leading, as side effects, to the multimerization of thyroglobulin (TG), the prothyroid hormone. Although hormone synthesis is a continuous process, the amount of Tg multimers is relatively constant. Here, we investigated the role of two molecular chaperones, protein disulfide isomerase (PDI) and immunoglobulin heavy chain-binding protein (BiP), present in the follicular lumen, on the multimerization process due to oxidation using both native Tg and its N-terminal domain (NTD). In vitro, PDI decreased multimerization of Tg and even suppressed the formation of NTD multimers. Under the same conditions, BiP was able to bind to Tg and NTD multimers but did not affect the process of multimerization. Associating BiP with PDI did not enhance the ability of PDI to limit the formation of multimers produced by oxidation. However, when BiP and PDI were reacted together with the multimeric forms and for a longer time (48 h), BiP greatly increased the efficiency of PDI. Accordingly, these two molecular chaperones probably act sequentially on the reduction of the intermolecular disulfide bridges. In the thyroid, a similar process may also be effective and participate in limiting the amount of Tg multimers present in the colloid. These results suggest that extracellular molecular chaperones play a similar role to that occurring in the endoplasmic reticulum and, furthermore, take part in the control of multimerization and aggregation of proteins formed by oxidation.

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