Photoinhibition and D1 protein degradation in mesophyll and agranal bundle sheath thylakoids of maize

Abstract

Susceptibility of photosystem II complex (PSII) to photoinhibition and degradation of D1 protein has been described in the chloroplasts of C3 plants but so far, the PSII turnover has not been characterised in any C4 plant, which contains two types of chloroplasts differing biochemically and structurally. In maize (Zea mays L. Oleńka), chloroplasts located in mesophyll (M) develop grana, while bundle sheath (BS) chloroplasts are agranal. In this paper, we report the D1 protein phosphorylation, damage and proteolysis in mesophyll as well as in agranal bundle sheath thylakoids of maize plants. Photoinhibitory treatment (1800 μmol photons m–2 s–1) of isolated thylakoids led to donor side inhibition of PSII electron transport and then to damage of reaction centre in both M and BS thylakoids. Rate of D1 degradation rate was faster in BS than in M thylakoids, and the addition of ATP to incubation medium delayed D1 degradation in both types of thylakoids. Furthermore, we demonstrated that the proteases belonging to FtsH and Deg families were present but their amounts significantly differed in M and BS thylakoids. Protease inhibitor studies revealed that serine- and metallo-proteases were involved in degradation of D1 protein. Apparent existence of D1 degradation cycle and the presence of proteolytic enzymes responsible for this process in BS thylakoids confirm that PSII plays an important role in agranal membranes, and when damaged, D1 can be rapidly degraded to enable PSII repair and restoration in these membranes.

Structural Organization of Photosynthetic Apparatus in Agranal Chloroplasts of Maize

“…Interestingly, DDM caused more intensive detachment of CP43 compared with digitonin (data tot shown), in agreement with Heinemeyer et al (35). It cannot be excluded that a subpool of the PSII devoid of CP43 may be a product of the PSII repair cycle, since this pathway has been shown to exist in the BS agranal membranes (22). Additional experiments are required to test this possibility.…”

“…In addition, it was demonstrated that the level of PSII activity in BS chloroplasts, as well as the presence of the oxygen-evolving complex subunits, depends on the differences in the age of the source leaf tissue, contamination with MS chloroplasts during isolation procedures, and illumination conditions (14,20,21). The apparent existence of the D1 degradation/repair cycle in BS chloroplasts and the identification of proteolytic enzymes responsible for these processes in these organelles confirm that PSII indeed plays an important function in the BS agranal membranes (22). Indeed, the active dimeric PSII complexes present in the stroma lamellae of C 3 chloroplasts have been shown to undergo the repair cycle (23,24), whereby PSII with a damaged D1 reaction center subunit migrates from the grana to the stroma and subsequently undergoes disassembly, repair, and refolding (25).…”

“…In particular, the supramolecular organization of the dimeric LHCII⅐PSII (Figs. 1 and 6) and the existence of the PSII repair cycle (22) in the BS thylakoids strongly suggest that PSII in the BS agranal membranes may donate electrons from the substrate water molecules to a cyclic electron transport around PSI. In C 4 plants, ATP is used to drive the C 4 cycle, and the requirement for ATP is cell-specific and varies between MS and BS cells.…”

Differential Mobility of Pigment-Protein Complexes in Granal and Agranal Thylakoid Membranes of C3 and C4 Plants

“…The chlorophyll a/b ratio of maize and sorghum is significantly higher in agranal BS tissues compared with granacontaining M cells, as reported earlier (Ku et al, 1974;Pokorska and Romanowska, 2007). The higher chlorophyll a/b ratio, and the higher ratios of chlorophyll fluorescence under liquid nitrogen temperature at 730/685 nm for thylakoids of M than for BS thylakoids (5-to 6-fold), are indicative of a much higher abundance of LHCI-PSI complexes in BS thylakoids of Fluorescence spectra of chloroplasts of M and BS cells from maize and sorghum and of isolated grana core and stroma lamellae from Arabidopsis.…”

“…Evidently, this structural separation is not present in agranal BS cells of C 4 plants. Interestingly, the degradation of photodamaged PSII in maize is significantly faster in agranal BS chloroplasts than in M cells (Pokorska and Romanowska, 2007). This could be due to faster migration of damaged components of PSII in agranal membranes.…”