Photophosphorylation in Isolated Maize Bundle Sheath Chloroplasts and Cells

Abstract

Isolated maize bundle sheath chloroplasts showed substantial rates of noncycic photophosphorylation. A typical rate of phosphorylation coupled to whole-chain electron transport (methylviologen or ferricyanide as acceptor) was 60 pmol per hour per milligram chlorophyll) with a coupling efficiency (P/e2) of 0.6. Partial electron transport reactions driven by photosystem I or II supported phosphorylation with P/e2 values of 0.2 to 03. Thus, two sites of phosphorylation seem to be associated with the photosynthetic chain in much the same way as in spinach chloroplasts.Isolated bundle sheath cells were capable of photosynthetic electron transport with membrane permeant electron carriers (but not with ferfcyanide) at rates which were similar to those found in isolated chloroplasts. ATP formation also occurred during electron transport when ADP and phosphate were present in the cell suspension. The rates of photophosphorylation reactions in cells were about 30 to 40% of those found in isolated chloroplasts (maximum rate in cells 80 imoles ATP per hour per milligram chlorophyll with diaminodurene as electron carrier), with the exception of endogenous photophosphorylation (photophosphorylation without added electron carriers) whose rate was three to four times higher in cells than in chloroplasts. The endogenous photophosphorylation in cells appeared to be coupled to pseudo-cyclic, rather than cyclic, electron transport. It was accompanied by 02 uptake (when the catalase inhibitor KCN was present), was sensitive to dichlorophenyldimethylurea and methylamine, but was totally insensitive to 20 micromolar antimycin which completely inhibited succinate-supported oxidative phosphorylation in the cells. The implications of these and other phenomena associated with photophosphorylation in bundle sheath cells are discussed.It has been assumed that the primary function of maize BS3 chloroplasts is to produce ATP through cyclic photophosphorylation. This concept stems from early observations that "PSIIdeficient" BS chloroplasts were capable of substantial rates of cyclic photophosphorylation (e.g. pyocyanine-mediated) ( (9) and was only partially inhibited by DCMU when exogenous NADPH was present (7).There seems no doubt that BS chloroplasts in vivo have some device with which to meet the high ATP demand (per NADPH) of photosynthesis in BS cells (10). Further characterization of photophosphorylation reactions in BS chloroplasts is necessary before any definitive conclusions can be achieved about the ATP producing reactions in vivo. The importance of such a study is emphasized by the recent finding of Hardt and Kok (8) that the PSII and the whole chain electron transport activities of isolated maize BS chloroplasts can be at least half as high as those of isolated spinach chloroplasts. Our experiments using isolated maize BS cells produced similar results (28). In 02 and proton change experiments with cell suspensions, we also detected a relatively high rate of endogenous pseudocyclic electron flow and an energy-coupling a...

Photosynthetic electron transport in the bundle sheath of maize

“…The marked stimulation of photosynthesis which malate causes in isolated bundle sheath tissue is consistent with this view [3,5]. Although the extent of the deficiency in photosystem II has since been questioned [6,7], recent work with bundle sheath strands having photosynthesis rates equal to those of the parent tissue implied that photosynthetic Oa evolution (and uptake) with COz as the acceptor is limited and that the high ATP demand for COa fucation is largely met by cyclic photophosphorylation mediated by photosystem I rather than via noncyclic or pseudocyclic photophosphorylation [5]. Cyclic photophosphorylation requires activation by electron flow from a suitable reductanr, which establishes appropriate redox poise in the electron carriers.…”