The impact of higher energy output from chloroplasts or mitochondria in plant physiology

Abstract

Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2) is a protein dually anchored on the outer membranes of chloroplasts and mitochondria. Overexpression of AtPAP2 simultaneously modulates the physiology of mitochondria and chloroplasts, resulting in faster growth and higher seed yield. Based on this, we generated 2 transgenic lines that overexpress AtPAP2 either in mitochondria (P2TOM lines) or chloroplasts (P2TOC lines). Compared to wild type (WT), the P2TOM lines exhibited early senescence, lower seed yield, lower sucrose level but higher ATP content. While overexpression of AtPAP2 in mitochondria increased mitochondrial activity via modulating the tricarboxylic acid (TCA) cycle pathway and the mitochondrial electron transport chain (ETC), the P2TOM line also exhibited a lower photosynthetic electron transport rate (ETR) and decreased activity of key enzymes in the Calvin–Benson–Bassham (CBB) cycle, implying a lower capacity for carbon fixation. In addition, metabolomic studies implied that P2TOM mitochondria consumed more citrate synthesized in the dark as a substrate for amino acid synthesis in the light. Thus, the early senescence, low seed yield and low sucrose phenotypes of the P2TOM lines could be explained by its higher mitochondrial activity with limited carbon supply from the chloroplasts. By contrast, the P2TOC lines exhibited higher ETR and increased capacities of some key enzymes in the CBB cycle. Interestingly, the its respiration rate was higher and the capacities of some TCA enzymes were altered compared to WT. Mitochondria play a role in dissipating reducing equivalents from the chloroplasts, consequently regenerate the electron acceptor nicotinamide adenine dinucleotide phosphate (NADP+) for the linear electron flow (LEF) in chloroplasts. This could explain why the growth rate and seed yield of the P2TOC lines are higher than the WT but lower than the OE lines, in which the mitochondria may regenerate NADP+ more efficiently. For the OE7 line, its chloroplasts and mitochondria were both more active than WT, with higher capacities of several key enzymes in the CBB and TCA cycles in addition to higher LEF activity. These lead to higher chloroplastic output of reducing equivalents and fixed carbon. Meanwhile, the hyperactive mitochondria could efficiently consume the excess reducing equivalents to generate more ATP and indirectly recycle NADP+ to chloroplasts. Together, the combination of a higher chloroplastic output of fixed carbon and a higher mitochondrial output of ATP boosts the production of sucrose in the OE7 line, thereby promoting its growth and seed yield. The energy metabolism in plant cells requires a balance between the activities of chloroplasts and mitochondria, as they are the producers and consumers of carbons and reducing equivalents respectively. This study presents 3 scenarios of the energy flow between chloroplasts and mitochondria. In the model of OE, there is an efficient energy flow between the two organelles. However, the presence of only one hyperactive organelle leads to suboptimal energy production in P2TOC lines and even becomes metabolically detrimental in P2TOM lines. These two unique transgenic lines allow us to study how hyperactive chloroplasts or mitochondria affect the physiology of its counterpart, modify cellular metabolism, and influence plant physiology.