Paolo Pesaresi

Photosystem II core phosphorylation and photosynthetic acclimation require two different protein kinases

Illumination changes elicit modifications of thylakoid proteins and reorganization of the photosynthetic machinery. This involves, in the short term, phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. PSII phosphorylation is thought to be relevant for PSII turnover, whereas LHCII phosphorylation is associated with the relocation of LHCII and the redistribution of excitation energy (state transitions) between photosystems. In the long term, imbalances in energy distribution between photosystems are counteracted by adjusting photosystem stoichiometry. In the green alga Chlamydomonas and the plant Arabidopsis, state transitions require the orthologous protein kinases STT7 and STN7, respectively. Here we show that in Arabidopsis a second protein kinase, STN8, is required for the quantitative phosphorylation of PSII core proteins. However, PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild type, implying that reversible protein phosphorylation is not essential for PSII repair. Acclimation to changes in light quality is defective in stn7 but not in stn8 mutants, indicating that short-term and long-term photosynthetic adaptations are coupled. Therefore the phosphorylation of LHCII, or of an unknown substrate of STN7, is also crucial for the control of photosynthetic gene expression.

A complex containing PGRL1 and PGR5 is involved in the switch between linear and cyclic electron flow in Arabidopsis.

During photosynthesis, two photoreaction centers located in the thylakoid membranes of the chloroplast, photosystems I and II (PSI and PSII), use light energy to mobilize electrons to generate ATP and NADPH. Different modes of electron flow exist, of which the linear electron flow is driven by PSI and PSII, generating ATP and NADPH, whereas the cyclic electron flow (CEF) only generates ATP and is driven by the PSI alone. Different environmental and metabolic conditions require the adjustment of ATP/NADPH ratios and a switch of electron distribution between the two photosystems. With the exception of PGR5, other components facilitating CEF are unknown. Here, we report the identification of PGRL1, a transmembrane protein present in thylakoids of Arabidopsis thaliana. Plants lacking PGRL1 show perturbation of CEF, similar to PGR5-deficient plants. We find that PGRL1 and PGR5 interact physically and associate with PSI. We therefore propose that the PGRL1-PGR5 complex facilitates CEF in eukaryotes.

The protein kinase Pstol1 from traditional rice confers tolerance of phosphorus deficiency

As an essential macroelement for all living cells, phosphorus is indispensable in agricultural production systems. Natural phosphorus reserves are limited, and it is therefore important to develop phosphorus-efficient crops. A major quantitative trait locus for phosphorus-deficiency tolerance, Pup1, was identified in the traditional aus-type rice variety Kasalath about a decade ago. However, its functional mechanism remained elusive until the locus was sequenced, showing the presence of a Pup1-specific protein kinase gene, which we have named phosphorus-starvation tolerance 1 (PSTOL1). This gene is absent from the rice reference genome and other phosphorus-starvation-intolerant modern varieties. Here we show that overexpression of PSTOL1 in such varieties significantly enhances grain yield in phosphorus-deficient soil. Further analyses show that PSTOL1 acts as an enhancer of early root growth, thereby enabling plants to acquire more phosphorus and other nutrients. The absence of PSTOL1 and other genes—for example, the submergence-tolerance gene SUB1A—from modern rice varieties underlines the importance of conserving and exploring traditional germplasm. Introgression of this quantitative trait locus into locally adapted rice varieties in Asia and Africa is expected to considerably enhance productivity under low phosphorus conditions.

Arabidopsis STN7 Kinase Provides a Link between Short- and Long-Term Photosynthetic Acclimation

Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

Role of Plastid Protein Phosphatase TAP38 in LHCII Dephosphorylation and Thylakoid Electron Flow

Regulation of photosynthesis efficiency involves reversible phosphorylation of the light-harvesting complex through the activity of the newly identified phosphatase TAP38.

PGRL1 Is the Elusive Ferredoxin-Plastoquinone Reductase in Photosynthetic Cyclic Electron Flow

During plant photosynthesis, photosystems I (PSI) and II (PSII), located in the thylakoid membranes of the chloroplast, use light energy to mobilize electron transport. Different modes of electron flow exist. Linear electron flow is driven by both photosystems and generates ATP and NADPH, whereas cyclic electron flow (CEF) is driven by PSI alone and generates ATP only. Two variants of CEF exist in flowering plants, of which one is sensitive to antimycin A (AA) and involves the two thylakoid proteins, PGR5 and PGRL1. However, neither the mechanism nor the site of reinjection of electrons from ferredoxin into the thylakoid electron transport chain during AA-sensitive CEF is known. Here, we show that PGRL1 accepts electrons from ferredoxin in a PGR5-dependent manner and reduces quinones in an AA-sensitive fashion. PGRL1 activity itself requires several redox-active cysteine residues and a Fe-containing cofactor. We therefore propose that PGRL1 is the elusive ferredoxin-plastoquinone reductase (FQR).

SPL8, an SBP-Box Gene That Affects Pollen Sac Development in Arabidopsis

SQUAMOSA PROMOTER BINDING PROTEIN–box genes (SBP-box genes) encode plant-specific proteins that share a highly conserved DNA binding domain, the SBP domain. Although likely to represent transcription factors, little is known about their role in development. In Arabidopsis, SBP-box genes constitute a structurally heterogeneous family of 16 members known as SPL genes. For one of these genes, SPL8, we isolated three independent transposon-tagged mutants, all of which exhibited a strong reduction in fertility. Microscopic analysis revealed that this reduced fertility is attributable primarily to abnormally developed microsporangia, which exhibit premeiotic abortion of the sporocytes. In addition to its role in microsporogenesis, the SPL8 knockout also seems to affect megasporogenesis, trichome formation on sepals, and stamen filament elongation. The SPL8 mutants described help to uncover the roles of SBP-box genes in plant development.

Nuclear Photosynthetic Gene Expression Is Synergistically Modulated by Rates of Protein Synthesis in Chloroplasts and Mitochondria

Arabidopsis thaliana mutants prors1-1 and -2 were identified on the basis of a decrease in effective photosystem II quantum yield. Mutations were localized to the 5′-untranslated region of the nuclear gene PROLYL-tRNA SYNTHETASE1 (PRORS1), which acts in both plastids and mitochondria. In prors1-1 and -2, PRORS1 expression is reduced, along with protein synthesis in both organelles. PRORS1 null alleles (prors1-3 and -4) result in embryo sac and embryo development arrest. In mutants with the leaky prors1-1 and -2 alleles, transcription of nuclear genes for proteins involved in photosynthetic light reactions is downregulated, whereas genes for other chloroplast proteins are upregulated. Downregulation of nuclear photosynthetic genes is not associated with a marked increase in the level of reactive oxygen species in leaves and persists in the dark, suggesting that the transcriptional response is light and photooxidative stress independent. The mrpl11 and prpl11 mutants are impaired in the mitochondrial and plastid ribosomal L11 proteins, respectively. The prpl11 mrpl11 double mutant, but neither of the single mutants, resulted in strong downregulation of nuclear photosynthetic genes, like that seen in leaky mutants for PRORS1, implying that, when organellar translation is perturbed, signals derived from both types of organelles cooperate in the regulation of nuclear photosynthetic gene expression.

Dynamics of reversible protein phosphorylation in thylakoids of flowering plants: The roles of STN7, STN8 and TAP38☆

Phosphorylation is the most common post-translational modification found in thylakoid membrane proteins of flowering plants, targeting more than two dozen subunits of all multiprotein complexes, including some light-harvesting proteins. Recent progress in mass spectrometry-based technologies has led to the detection of novel low-abundance thylakoid phosphoproteins and localised their phosphorylation sites. Three of the enzymes involved in phosphorylation/dephosphorylation cycles in thylakoids, the protein kinases STN7 and STN8 and the phosphatase TAP38/PPH1, have been characterised in the model species Arabidopsis thaliana. Differential protein phosphorylation is associated with changes in illumination and various other environmental parameters, and has been implicated in several acclimation responses, the molecular mechanisms of which are only partly understood. The phenomenon of State Transitions, which enables rapid adaptation to short-term changes in illumination, has recently been shown to depend on reversible phosphorylation of LHCII by STN7-TAP38/PPH1. STN7 is also necessary for long-term acclimation responses that counteract imbalances in energy distribution between PSII and PSI by changing the rates of accumulation of their reaction-centre and light-harvesting proteins. Another aspect of photosynthetic acclimation, the modulation of thylakoid ultrastructure, depends on phosphorylation of PSII core proteins, mainly executed by STN8. Here we review recent advances in the characterisation of STN7, STN8 and TAP38/PPH1, and discuss their physiological significance within the overall network of thylakoid protein phosphorylation. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Cytoplasmic N-terminal protein acetylation is required for efficient photosynthesis in Arabidopsis

The Arabidopsis atmak3-1 mutant was identified on the basis of a decreased effective quantum yield of photosystem II. In atmak3-1, the synthesis of the plastome-encoded photosystem II core proteins D1 and CP47 is affected, resulting in a decrease in the abundance of thylakoid multiprotein complexes. DNA array–based mRNA analysis indicated that extraplastid functions also are altered. The mutation responsible was localized to AtMAK3, which encodes a homolog of the yeast protein Mak3p. In yeast, Mak3p, together with Mak10p and Mak31p, forms the N-terminal acetyltransferase complex C (NatC). The cytoplasmic AtMAK3 protein can functionally replace Mak3p, Mak10p, and Mak31p in acetylating N termini of endogenous proteins and the L-A virus Gag protein. This result, together with the finding that knockout of the Arabidopsis MAK10 homolog does not result in obvious physiological effects, indicates that AtMAK3 function does not require NatC complex formation, as it does in yeast. We suggest that N-acetylation of certain chloroplast precursor protein(s) is necessary for the efficient accumulation of the mature protein(s) in chloroplasts.