Phototransformation de la protochlorophyllide en chlorophyllide et les effets des facteurs externes chez des feuilles d'orge etiolees

In this work the effect of the phosphatase inhibitor sodium fluoride on pigment-POR complexes in etiolated barley leaves before and after a flash-illumination was studied. Before the flash, fluoride decreased the ratio between photoactive Pchlide fluorescing at 657 nm (Pchlide 657) and non-photoactive Pchlide emitting at 633 nm (Pchlide 633). After the flash, both the blue spectral shift of Chlide (Shibata shift) and the photoactive Pchlide regeneration occurring normally in non-treated illuminated leaves were inhibited by the fluoride treatment. Light-dependent dephosphorylation of the two phosphoserine-containing proteins having apparent molecular weights of 31 and 41 kDa was detected in etioplast inner membranes (EPIMs) isolated from shortly illuminated leaves. This dephosphrylation was prevented in illuminated leaves pre-treated with fluoride suggesting that these two proteins may play a role in the Shibata shift process and/or formation of the photoactive Pchlide. A low level of serine phosphorylation within the POR protein itself detected by the phosphoserine antibody, however, did not depend on the light-dark transition. The association of POR with isolated EPIMs appeared not to be dependent on protein (de)phosphorylation since light-induced dissociation of the POR from the EPIMs was not prevented by the phosphatase inhibitor.; POR protein function is not only controlled by light but also depends on other environmental factors. We showed that the photochemical reduction of Pchlide to Chlide is highly resistant to dehydration of the leaves. Following this reaction POR releases Chlide and re-binds remaining Pchlide pool for another round of catalysis even in highly dehydrated leaves. However, reformation of the photo-transformable complexes between POR and Pchlide was very limited though POR amount was stable during the stress treatment. This suggested an impairment of the new Pchlide synthesis process. Furthermore, dehydration induced significant alteration of the Pchlide photoreduction-initiated but light-independent Chlide transformation reactions including the complex enzymatic process called the Shibata shift.; Our study concerning long-term effects of drought-stress on the assembly process of the functional PSII complexes in etiolated leaves exposed to continuous light for greening showed that, with respect to the PSII function, the plant acclimates to the stress conditions by favoring the assembly of the PSII outer antenna light-harvesting complex protein (LHCP) trimers. (Abstract shortened by UMI.)...