Limitation of nocturnal import of ATP into Arabidopsis chloroplasts leads to photooxidative damage^†

Abstract

SummaryWhen grown in short day conditions and at low light, leaves of Arabidopsis plants with mutations in the genes encoding two plastidial ATP/ADP transporters (so-called null mutants) spontaneously develop necrotic lesions. Under these conditions, the mutants also display light-induced accumulation of H 2 O 2 and constitutive expression of genes for copper/zinc superoxide dismutase 2 and ascorbate peroxidase 1. In the light phase, null mutants accumulate high levels of phototoxic protoporphyrin IX but have only slightly reduced levels of Mg protoporphyrin IX. The physiological changes are associated with reduced magnesium-chelatase activity. Since the expression of genes encoding any of the three subunits of magnesium-chelatase is similar in wild type and null mutants, decreased enzyme activity is probably due to post-translational modification which might be due to limited availability of ATP in plastids during the night. Surprisingly, the formation of necrotic lesions was absent when null mutants were grown either in long days and low light intensity or in short days and high light intensity. We ascribe the lack of lesion phenotype to increased nocturnal ATP supply due to glycolytic degradation of starch which may lead to additional substrate-level phosphorylation in the stroma. Thus, nocturnal import of ATP into chloroplasts represents a crucial, previously unknown process that is required for controlled chlorophyll biosynthesis and for preventing photooxidative damage.

The Sink-Specific Plastidic Phosphate Transporter PHT4;2 Influences Starch Accumulation and Leaf Size in Arabidopsis

“…6A). The proposed functions of GPT and NTT are to provide nonphotosynthetic plastids with the carbon and energy, respectively, needed for starch synthesis and the oxidative pentose phosphate pathway, although mutant analyses indicate that these proteins also have roles in photosynthetic tissues (Niewiadomski et al, 2005;Reinhold et al, 2007). In addition, expression of the AGPase small subunit gene, APS1, was induced only in roots of pht4;2 plants, whereas AGPase large subunit genes, APL1 and APL3, were induced only in leaves (Fig.…”

“…Metabolism in such plastids, therefore, is dependent on the import of ATP from the cytosol. Nucleotide transporters (NTTs) located in the plastid inner envelope fulfill this energy requirement by catalyzing the stoichiometric exchange of cytosolic ATP for stromal ADP (Reiser et al, 2004;Reinhold et al, 2007). A consequence of the unbalanced phosphate moieties associated with this exchange is that inorganic phosphate (Pi) would accumulate to deleterious levels within the stroma if not countered by Pi export.…”

The Import and Export Business in Plastids: Transport Processes across the Inner Envelope Membrane

“…Therefore, it has been proposed that the function of ATP transport is to energize unknown processes in the stroma during the night, rather than to export ATP during photosynthesis (Heldt, 1969). Only very recently, this hypothesis could be validated through the analysis of Arabidopsis lines lacking both NTTs (Reinhold et al, 2007). These mutants showed a dwarf phenotype with necrotic lesions on the leaves under short-day conditions at low-light intensities, a phenotype that is caused by photooxidation due to the accumulation of protoporphyrin IX.…”

“…The increased protoporphyrin IX levels are most likely due to a reduced activity of magnesium-chelatase, which requires ATP for catalysis and also for stabilization of the holoenzyme. Thus, one important ATP-dependent process during the night is magnesiumchelatase assembly (Reinhold et al, 2007). Whether the inhibition other energy-dependent processes in chloroplasts in the dark are also impaired in the mutant is not known so far.…”

Understanding metabolite transport and metabolism in C4 plants through RNA-seq

“…The action of PEP-CK requires one ATP per reaction which is likely supplied by the mitochondria as the plastidic ATP exchanger is not relevant during day-time metabolism [43]. Export of ATP is mediated by AAC [44], a gene transcriptionally abundant in many species (Table 1) but specifically upregulated in the C 4 species Megathyrsus maximus which relies to a large degree on PEP-CK [8].…”