Senescence induced structural reorganization of thylakoid membranes in Cucumis sativus cotyledons; LHC II involvement in reorganization of thylakoid membranes

We report the formation and appearance of loosely stacked extended grana li ke structures along with plastoglobuli in the chloroplasts isolated from 27 -day old senescing cucumber cotyledons. The origin and the nature of these extended grana structures have not been elucidated earlier. We isolated Pho tosystem I complexes from 6-day-old control and 27-day-old senescing cotyle dons. The chlorophyll a/b ratio of the isolated Photosystem I complex obtai ned from 6-day cotyledons was 5-5.5 as against a ratio of 2.9 was found in Photosystem I complexes obtained from 27-day-old senescing cotyledons. We a lso found that the presence of LHC II in the Photosystem I complexes isolat ed from 27-day cotyledonary chloroplasts. The presence of LHC II in Photosy stem I complexes in senescing and not in control samples, clearly suggest t he detachment and diffusion of LHC II complexes from stacked grana region t o Photosystem I enriched stroma lamellar region thereby, forming loose diso rganized extended grana structures seen in the transmission electron micros cope. Furthermore, we show that under in vitro condition the senescing coty ledon chloroplasts exhibited lower extent of light induced phosphorylation of LHC II than the control samples suggesting a possible irreversible phosp horylation and diffusion of LHC II in vivo during the progress of senescenc e in Cucumis cotyledons. From these findings, we suggest that the senescenc e induced phosphorylation of LHC II and its migration towards Photosystem I may be a programmed one some how causing the destruction of the thylakoid membrane. The released membrane components may be stored in the plastoglobu li prior to their mobilization to the younger plant parts.