Reorganization of Thylakoid Components during Chloroplast Development in Higher Plants after Transfer to Darkness

Abstract

It was shown earlier that in etiolated bean (Phaseolus rulgaris, var. red kidney) leaves exposed to continuous light for a short time and then transferred to darkness a reorpnization of their photosystem II (PSII) unit components occurs. This reorpnization involves disornization of the light-harvesting complex of PSII (LHC-II), destruction of its chlorophyll b and the 25 kilodalton polypeptide, and reuse of its chlorophyll a for the formation of additional, small in size, PSII units (ArgyroudiAkoyunoglou, Akoyunoglou, Kaloskas, Akoyunoglou 1982 Plant Physiol 70: 1242-1248. The present study further shows that parallel to the PSII unit reorganization a reorganization of the PSI unit components also occurs: upon transfer to darkness the 24, 23, and 21 kilodalton polypeptides, components of the light-harvesting complex of PSI (LHC-I), are decreased, the 69 kilodalton polypeptide, component of the chlorophyll a-rich P700-protein complex (CPI), is increased and new smallsized PSI units are formed. Concomitantly, the cytochromeJlchlorophyll and the cytochrome b/chlorophyll ratios are gradually increased. This suggests that the concentration of the electron transport components is also modulated in darkness to allow for adequate electron flow to occur between the newly synthesized PSII and PSI units.Young etiolated bean leaves exposed to CL2 accumulate Chl a and Chl b without lag-phase (3), and they form complete (large in size) photosynthetic units. The concentration of the PSII and PSI units per chloroplast increases with the time of exposure to CL, until it reaches the concentration of the mature chloroplast.Under certain conditions, but always in cases where the thylakoid is still in the process of development, the thylakoid components seem to undergo a process of reorganization (2, 4). Such a case has been noticed recently in the PSII unit components of etiolated bean leaves exposed to CL for a short time and then transferred to darkness (1 1): after transfer to darkness Chl accumulation stops completely; however, the growth of the leaf continues, and thus the Chl per g fresh weight decreases.

The effect of the dark interval in intermittent light on thylakoid development: photosynthetic unit formation and light harvesting protein accumulation

“…Chloroplasts used for P700 determination were isolated as described before (Akoyunoglou and Akoyunoglou 1985). The thylakoids were washed with 0.05M Tricine--NaOH, PH 7.3 and resuspended in the same buffer containing 0.5% Triton X-100 to have 20/ag Chl/ml.…”

“…To explain these results it has been proposed (Akoyunoglou 1982, Akoyunoglou and Argyroudi-Akoyunoglou 1986) that one of the factors, which regulate the assembly of the various thylakoid components into active pigment-protein complexes, is the rate of Chl accumulation; during greening in the LDC the various thylakoid components are synthesized at different rates; the component synthesized at the lowest rate becomes a limiting factor in thylakoid development. As already known, Chl is essential for the stabilization of a great number of thylakoid polypeptides; this is achieved through binding to the polypeptides and formation of complexes.…”

“…Under such conditions, therefore, there is a competition, between the diferent polypeptides, for the small amount of Chl present. The reaction center polypeptides seem to have higher affinity for Chl than the LHC-I and LHC-II polypeptides (Argyroudi- Akoyunoglou et al 1982, Akoyunoglou andAkoyunoglou 1985); thus the CPI and CPa core complexes are formed mainly, while the LHC-I and LHC-II apoproteins, not being able to be stabilized, are digested. Higher rate of Chl accumulation would be, expected, therefore, to allow some Chl binding to occur on the LHC-I and LHC-II apoproteins resulting in their stabilization.…”

Chloramphenicol‐induced stabilization of light‐harvesting complexes in thylakoids during development

“…Similarly, CAP treatment of plants upon transfer to darkness after brief pre-exposure to CL prevents degradation of LHC apoproteins and the PS unit size remains unaffected in the dark. Long pre-exposure to CL has also been shown to prevent degradation of LHC polypeptides after transfer to darkness [7,9]; in this case the number of PS units formed in CL has already attained that of the mature chloroplast and no RC synthesis occurs in the dark. In all these cases, therefore, in the absence of RC protein synthesis the LHC apoproteins are stabilized.…”

“…Under these conditions, the LHC apoprotein formed in CL was drastically reduced in the dark [6,7], while new small-sized PS units were formed [7][8][9]. Since this was found to occur only in plants which are still in the process of development and have the need/capacity to form new PS units to attain the FEBS number of mature chloroplasts, the effect was attributed to a reorganization process [7][8][9] rather than to increased LHC apoprotein turnover in the dark [6] …”