Crucial role of the PTOX and CET pathways in optimizing ATP synthesis in mesophyll chloroplasts of C3 and C4 plants

Abstract

Besides the linear electron flow (LET), several regulatory and alternative electron transfer pathways exist in chloroplasts, among them the cyclic electron transport around photosystem I (CET) and PTOX enzyme which oxidizes the plastoquinone pool. Until now has not been investigated the influence of PTOX activity on the regulation of CET pathways and hence the regulation of ATP production. The present study reveals differential responses of CET and PTOX regulation to environmental changes and how these pathways can adjust the ATP content to C3 and C4 metabolism. Here, we conducted a comparative study in maize (C4) and pea (C3) chloroplasts by measuring the activity of PSI in the presence of the inhibitors antimycin A and propyl gallate, estimating the content of thylakoid proteins that participate in CET, and determining ATP content and ATP synthase activity, and fluorescence analysis. We found that NDH-CET pathway plays an important role in maize chloroplasts, whereas PGR5/PGRL1 proteins play a dominant role in pea chloroplasts. The obtained results indicate a substantial involvement of PTOX activity in regulating electron flow in response to ATP demand. We also found that C3 and C4 chloroplasts contained quantitatively different CET membrane components. Based on the study findings, we propose a model of regulation of the linear electron transport (LET) and CET pathways in mesophyll chloroplasts of C4 and C3 plants under low light intensity based on ATP level and PTOX activity. • The NDH complex and PGR5/PGRL1 play a main role in CET in maize and pea mesophyll chloroplasts, respectively. • High PTOX activity in maize seems to be the key for producing ATP required for C4 metabolism. • PTOX is localized mainly in grana in maize and in stroma membranes in pea chloroplasts. • Antenna aggregation in maize indicates a high reduction, which may regulate ATP/NADPH demand.