Evidence for a Block between Plastoquinone and Cytochrome f in a Photosynthetic Mutant of Lemna with Abnormal Flowering Behavior

Abstract

Mutant strain 1073 of Lemna perpusilla is conduded to be blocked between plastoquinone and cytochromef in the photosynthetic electron transport system. The location of the block is based on the following observations of activities in chloroplasts isolated from the mutant and wild-type plants. (a) Relative to wild type, electron flow rates from water to ferricyanide, 2,6-dichlorophenol indophenol or NADP were very low in the mutant, but rates of photosystem I-dependent electron flow and cydic phosphorylation were high. Chloroplasts were prepared from Lemna and lettuce as previously described (2), except for the addition of 0.5 mg/ml of BSA to the grinding and washing media and 10 mg/ml to the resuspension medium (7). Chl concentration was determined according to Arnon (1). The reduction of ferricyanide, DCPIP, or NADP was measured in a Cary 14 spectrophotometer with an attachment to reduce scattering effects. Oxygen uptake was measured with a Yellow Springs Instrument Clark type 02 electrode. ATP formation was assayed as described (2). Chl fluorescence induction and ATP-driven reverse electron flow (18) RESULTSThe ability of mutant 1073 to photoreduce several electron acceptors in reactions requiring excitation of photosystem II, photosystem I, or both is given in Table I. Electron transport from water to ferricyanide, DCPIP, or NADP was very poor in the mutant chloroplasts relative to the WT or to lettuce. The rates of ferricyanide reduction in the WT or lettuce chloroplasts were stimulated 2-to 3-fold by the addition of the uncoupler methylamine, whereas the rate in the mutant chloroplasts was unaffected, indicating that the block was not located in the energy-converting system. In the photosystem I-sensitized electron transport from reduced DAD to 02 (9), the mutant was about as effective as the WT. Consistent with the above results, mutant chloroplasts were inactive in photophosphorylation coupled to electron flow from water to ferricyanide but were active in PMS-catalyzed cyclic photophosphorylation. The major block was therefore in the electron transport system close to photosystem II. To determine whether the block was located before, at, or after photosystem II, Chl fluorescence induction was followed. Figure 1 shows that the WT and mutant had essentially identical fluorescence induction curves, which were similarly affected by the addition of DCMU. The dark recovery of the variable fluorescence yield was also similar in the wild type and mutant (not shown). Since the rise in the variable fluorescence yield upon illumination is thought to be due to the electron transport system from water to and including the "pool" of electron acceptors (14), mainly plastoquinone, it is clear from the latter observa-577 www.plantphysiol.org on May 8, 2018 -Published by Downloaded from

Effects of Light and 3-(3,4-Dichlorophenyl)-1,1-Dimethylurea on Levels of ATP in Lemna paucicostata 6746 and a Photosynthetic Mutant with Abnormal Flowering Responses

“…Studies on chloroplast suspensions (20) showed that the photosynthetic electron transport chain of the mutant was blocked between PQ3 and Cyt f, but was capable of PMS-catalyzed PSI cyclicphosphorylation, results consistent with the ATP hypothesis described above. Illumination with white light for short term exposures was from a 300-w tungsten filament projector bulb filtered through 5 cm of water.…”

“…The results are consistent with the scheme in which the electron transport pathway for endogenous cyclic photophosphorylation, in contrast to the artificial pathway, involves PQ (22). The endogenous cyclic photophosphorylation in wild type fronds was inhibited by DCMU, whereas the artificial PMScatalyzed system in chloroplasts (20) or in intact fronds (Table I) Earlier results also showed that under certain conditions, DCMU promoted flowering of wild type, whereas flowering of the photosynthetic mutant occurred in the absence of DCMU (16,18). It has been hypothesized (10) that DCMU promotion of flowering resulted from increased levels of ATP caused by cyclic photophosphorylation in the absence of ATP-consuming CO2 reduction reactions.…”

“…The next expenment was designed to resolve the apparent discrepancies between these in vitro results and the in vivo results described in the previous sections. The effect of light on levels of ATP was measured in intact fronds treated with PMS, the artificial cofactor that had been used in the in vitro experiments with chloroplast suspensions (20). Table I shows that wild type and mutant fronds had comparable increases in ATP levels when illuminated in the presence of PMS.…”

“…that in nitrogen, fermentation Endogenous versus PMS-catalyzed Photophosphorylation. In vitro studies (20) had shown that wild type and mutant chloroplast suspensions were capable of appreciable rates of DCMU-resistant cyclic photophosphorylation. The next expenment was designed to resolve the apparent discrepancies between these in vitro results and the in vivo results described in the previous sections.…”

“…In contrast to wild type, mutant fronds did not show increased pool sizes of ATP during exposures to light in either air or nitrogen, indicating that the mutant was defective in both cyclic and noncycic photophosphorylation. Earlier work with chloroplast suspensions had shown that the mutant was blocked between PQ and Cytf, but was capable of PMS-atalyzed cyclic photophosphorylation (20 (Table I). The results are consistent with the scheme in which the electron transport pathway for endogenous cyclic photophosphorylation, in contrast to the artificial pathway, involves PQ (22).…”