Differential changes in the steady state levels of thylakoid membrane proteins during senescence in Cucumis sativus cotyledons

Chloroplast structure and function is known to alter during foliar senescen ce. Besides, the alterations in the structural organisation of thylakoid me mbranes changes in the steady state levels of thylakoid membrane proteins o ccur due to leaf ageing. We monitored temporal changes in some of the speci fic proteins of thylakoid membrane protein complexes by western blotting in the Cucumis sativus cotyledons as a function of the cotyledon age. We obse rved that the levels of D1 and D2 proteins of photosystem II started declin ing at the early stages of senescence of Cucumis cotyledons and continued t o decline with the progress of cotyledon age. Similarly the level of Cyt f of Cyt b(6)/f complex declined rapidly with progress of senescence in these cotyledons. The reaction centre proteins of photosystem I were relatively found to be more stable than that of photosystem II reaction centre protein s reflecting possibly the disorganisation of photosystem II prior to photos ystem I. The 33 kDa extrinsic protein (MSP) of oxygen evolving complex, the LHCII apoprotein and the beta -subunit of ATPsynthase showed the declined levels with the progress of cotyledon age. However, the extents of loss of these proteins were not as high as the reaction centre proteins of photosys tem II and the Cyt f. These results provide that during senescence, protein s of thylakoid membranes degrade in a specific temporal sequence and thereb y affect the temporal photochemical functions in Cucumis sativus cotyledons .