Shu Yuan

Effects of light on cyanide-resistant respiration and alternative oxidase function in Arabidopsis seedlings.

Mitochondrial alternative oxidase (AOX), the unique respiratory terminal oxidase in plants, catalyzes the energy wasteful cyanide (CN)-resistant respiration and plays a role in optimizing photosynthesis. Although it has been demonstrated that leaf AOX is upregulated after illumination, the in vivo mechanism of AOX upregulation by light and its physiological significance are still unknown. In this report, red light and blue light-induced AOX (especially AOX1a) expressions were characterized. Phytochromes, phototropins and cryptochromes, all these photoreceptors mediate the light-response of AOX1a gene. When aox1a mutant seedlings were grown under a high-light (HL) condition, photobleaching was more evident in the mutant than the wild-type plants. More reactive oxygen species (ROS) accumulation and inefficient dissipation of chloroplast reducing-equivalents in aox1a mutant may account for its worse adaptation to HL stress. When etiolated seedlings were exposed to illumination for 4 h, chlorophyll accumulation was largely delayed in aox1a plants. We first suggest that more reduction of the photosynthetic electron transport chain and more accumulation of reducing-equivalents in the mutant during de-etiolation might be the main reasons.

Salicylic Acid and Jasmonic Acid Are Essential for Systemic Resistance Against Tobacco mosaic virus in Nicotiana benthamiana

Systemic resistance is induced by pathogens and confers protection against a broad range of pathogens. Recent studies have indicated that salicylic acid (SA) derivative methyl salicylate (MeSA) serves as a long-distance phloem-mobile systemic resistance signal in tobacco, Arabidopsis, and potato. However, other experiments indicate that jasmonic acid (JA) is a critical mobile signal. Here, we present evidence suggesting both MeSA and methyl jasmonate (MeJA) are essential for systemic resistance against Tobacco mosaic virus (TMV), possibly acting as the initiating signals for systemic resistance. Foliar application of JA followed by SA triggered the strongest systemic resistance against TMV. Furthermore, we use a virus-induced gene-silencing-based genetics approach to investigate the function of JA and SA biosynthesis or signaling genes in systemic response against TMV infection. Silencing of SA or JA biosynthetic and signaling genes in Nicotiana benthamiana plants increased susceptibility to TMV. Genetic experiments also proved the irreplaceable roles of MeSA and MeJA in systemic resistance response. Systemic resistance was compromised when SA methyl transferase or JA carboxyl methyltransferase, which are required for MeSA and MeJA formation, respectively, were silenced. Moreover, high-performance liquid chromatography-mass spectrometry analysis indicated that JA and MeJA accumulated in phloem exudates of leaves at early stages and SA and MeSA accumulated at later stages, after TMV infection. Our data also indicated that JA and MeJA could regulate MeSA and SA production. Taken together, our results demonstrate that (Me)JA and (Me)SA are required for systemic resistance response against TMV.

Effect of water stress on photosystem 2 in two wheat cultivars

The effects of osmotic dehydration on the photochemical activity, gene transcription, and protein content of photosystem 2 (PS 2) in leaves of two wheat (Triticum aestivum L.) cultivars Miannnong No. 4 and No. 5 were studied. Roots of both cultivars were submerged into polyethylene glycol (PEG) solutions with an osmotic potential of −0.5 MPa for 0, 24, 48, and 72 h. Relative water content (RWC) decreased markedly after 48 and 72 h. Simultaneously, marked increase in electrolyte leakage, decrease in contents of chlorophylls (Chl) a and b, and inhibition in PS 2 activity were observed. Northern hybridization indicated that progressive water stress remarkably reduced contents of the chloroplast gene psbA and psbD and nuclear gene cab transcripts. Urea-SDS-PAGE and Western blotting analysis showed that the contents of major PS 2 proteins, including the D1 and D2 proteins in the PS 2 reaction centre (RC) and the light-harvesting Chl a/b-protein complex (LHC 2) in periphery, declined with increasing water stress. Miannong No. 5 had less destroyed plasma membranes and higher RWC, Chl contents, and PS 2 activity during water stress than Miannong No. 4, which suggested its better drought resistance. The significant difference in steady state contents of LHC 2 proteins of two cultivars can be mainly attributed to the marked difference in transcript level of cab gene, which indicated that LHC 2 proteins protect PS 2 RC.

Transient accumulation of Mg-protoporphyrin IX regulates expression of PhANGs ― New evidence for the signaling role of tetrapyrroles in mature Arabidopsis plants

Genetic and physiological studies have revealed evidence for multiple signaling pathways by which the plastid exerts retrograde control over photosynthesis associated nuclear genes (PhANGs). It has been proposed that the tetrapyrrole pathway intermediate Mg-protoporphyrin IX (Mg-proto IX) acts as the signaling molecule in the pathways and accumulates in the chloroplasts and cytosol of the cell after treatment with the herbicide Norflurazon (NF). However, the role of Mg-Proto IX in plastid signaling has been challenged by two recent reports. In this paper, new evidence is presented supporting Mg-Proto IX as a plastid-signaling molecule in mature Arabidopsis seedlings. Fluorescence HPLC and confocal microscope observation verified that a short-term (<96h) NF treatment resulted in a large accumulation of Mg-Proto IX accompanying with Lhcb repression, whereas the long-term NF treatments caused marked changes of tetrapyrrole pools, while Lhcb expression was continuously repressed. These results may explain the discrepancies among different reports. Reactive oxygen species (ROS) eliminator treatments only partly reversed the NF-induced repression of Lhcb. Therefore, the NF generates both ROS signals and Mg-Proto IX signals. Furthermore, our data suggested that plastid signal transduction through plastid GUN1 protein is independent of tetrapyrrole export from the plastid.

Dephosphorylation of photosystem II proteins and phosphorylation of CP29 in barley photosynthetic membranes as a response to water stress

Kinetic studies of protein dephosphorylation in barley thylakoid membranes revealed accelerated dephosphorylation of photosystem II (PSII) proteins, and meanwhile rapidly induced phosphorylation of a light-harvesting complex (LHCII) b4, CP29 under water stress. Inhibition of dephosphorylation aggravates stress damages and hampers photosystem recovery after rewatering. This increased dephosphorylation is catalyzed by both intrinsic and extrinsic membrane protein phosphatase. Water stress did not cause any thylakoid destacking, and the lateral migration from granum membranes to stroma-exposed lamellae was only found to CP29, but not other PSII proteins. Activation of plastid proteases and release of TLP40, an inhibitor of the membrane phosphatases, were also enhanced during water stress. Phosphorylation of CP29 may facilitate disassociation of LHCII from PSII complex, disassembly of the LHCII trimer and its subsequent degradation, while general dephosphorylation of PSII proteins may be involved in repair cycle of PSII proteins and stress-response-signaling.

The plastid hexokinase pHXK: A node of convergence for sugar and plastid signals in Arabidopsis

The inhibitors to plastid gene expression (PGE) were effective in preventing nuclear photosynthetic gene expression only if applied within the first 2–3 days of Arabidopsis seedling development. However, the signal transduction processes are still unknown. In this investigation, we found 3% glucose with 1 mM chloramphenicol co‐treatment repressed LHCB transcript significantly in mature Arabidopsis seedlings, while effective solo glucose treatment needed a concentration of 7%. The repressive effects of glucose and chloramphenicol on LHCB expression were inhibited in phxk (plastid hexokinase) mutant. pHXK enzyme activities, location, function in signal transduction, and cross talk to plastid GUN1 protein (a key signaling factor) were also investigated. The data suggest that pHXK may be a node of convergence for sugar‐mediated and PGE‐derived signals in Arabidopsis.

Lack of Salicylic Acid in Arabidopsis Protects Plants against Moderate Salt Stress

Previous studies showed that salicylic acid (SA)-deficient transgenic Arabidopsis expressing the salicylate hydroxylase gene NahG had a higher tolerance to moderate salt stress. SA may potentiate the stress response of germination and growth of Arabidopsis seedlings by inducing reactive oxygen species (ROS). However, the detailed mechanism for a better adaption of NahG plants to moderate salt stress is largely unknown. In the present study we found that a higher GSH/GSSG (glutathione/oxidized glutathione) ratio and ASA/DHA (ascorbic acid/dehydroascorbate) ratio in NahG plants during the stress may be the key reason for their stress-tolerance advantage. NahG plants actually could not produce more active antioxidant enzymes than the wild-type ones under natural conditions, but maintain higher activities of glutathione reductase (GR) and dehydroascorbate reductase (DHAR) during the stress. Hereby, the reduced glutathione and reduced ascorbic acid contents are higher in NahG plants under salt stress. However, NahG plants do not adapt better under severe salt stress. All antioxidant enzyme activities, GSH/GSSG ratio and ASA/DHA ratio declined substantively at 400 mM NaCl stress in both NahG and wild-type seedlings.

Phosphorylation of Photosynthetic Antenna Protein CP29 and Photosystem II Structure Changes in Monocotyledonous Plants under Environmental Stresses

Kinetic studies of protein dephosphorylation in thylakoid membranes showed that the minor light-harvesting antenna protein CP29 could be phosphorylated in barley (C3) and maize (C4) seedlings, but not in spinach under water [Liu, W. J., et al. (2009) Biochim. Biophys. Acta 1787, 1238-1245], salt, or cold stress [Pursiheimo, S., et al. (2003) Plant Cell Environ. 26, 1995-2003], suggesting that phosphorylation of CP29 is a general phenomenon in monocots, but not in dicots under environmental stresses. Abscisic acid (ABA), reactive oxygen species (ROS), salicylic acid (SA), jasmonic acid (JA), ethylene (ET), NO, and the scavenger of H(2)O(2) had weak effects on CP29 phosphorylation. However, three protein kinase inhibitors, U0126, W7, and K252a (for mitogen-activated protein kinase, Ca(2+)-dependent protein kinase, and Ser/Thr protein kinases, respectively), decrease the level of CP29 phosphorylation in barley apparently under environmental stresses. Therefore, these three protein kinases are involved in CP29 phosphorylation. We also found that most CP29 phosphorylation was accompanied by its lateral migration from granum membranes to stroma-exposed thylakoid regions, and the instability of PSII supercomplexes and LHCII trimers under environmental stresses.

Red blood cell extrudes nucleus and mitochondria against oxidative stress

Mammal red blood cells (erythrocytes) contain neither nucleus nor mitochondria. Traditional theory suggests that the presence of a nucleus would prevent big nucleated erythrocytes to squeeze through these small capillaries. However, nucleus is too small to hinder erythrocyte deformation. And, there is no sound reason to abandon mitochondria for the living cells. Here, we found that mammal erythrocyte reactive oxygen species (ROS) levels kept stable under diabetes, ischemia reperfusion, and malaria conditions or in vitro sugar/heme treatments, whereas bird erythrocyte ROS levels increased dramatically in these circumstances. Nuclear and mitochondrial extrusion may help mammal erythrocytes to better adapt to high‐sugar and high‐heme conditions by limiting ROS generation.

The significance of CP29 reversible phosphorylation in thylakoids of higher plants under environmental stresses

Reversible phosphorylation of proteins is a key event in many fundamental cellular processes. Under stressful conditions, many thylakoid membrane proteins in photosynthetic apparatus of higher plants undergo rapid phosphorylation and dephosphorylation in response to environmental changes. CP29 is the most frequently phosphorylated protein among three minor antennae complexes in higher plants. CP29 phosphorylation in dicotyledons has been known for several decades and is well characterized. However, CP29 phosphorylation in monocotyledons is less studied and appears to have a different phosphorylation pattern. In this review, we discuss recent advancements in CP29 phosphorylation and dephosphorylation studies and its physiological significance under environmental stresses in higher plants, especially in the monocotyledonous crops. Physiologically, the phosphorylation of CP29 is likely to be a prerequisite for state transitions and the disassembly of photosystem II supercomplexes, but not involved in non-photochemical quenching (NPQ). CP29 is phosphorylated in monocots exposed to environmental cues, with its subsequent lateral migration from grana stacks to stroma lamellae. However, neither CP29 phosphorylation nor its lateral migration occurs in dicotyledonous plants after drought, cold, or salt stress. Since the molecular mechanisms of differential CP29 phosphorylation under stresses are not fully understood, this review provides insights for future studies regarding the physiological function of CP29 reversible phosphorylation.