Senescence induced structural reorganization of thylakoid membranes in Cucumis sativus cotyledons; LHC II involvement in reorganization of thylakoid membranes
Jss. Prakash et al., Senescence induced structural reorganization of thylakoid membranes in Cucumis sativus cotyledons; LHC II involvement in reorganization of thylakoid membranes, PHOTOSYN R, 68(2), 2001, pp. 153-161
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.