Botryticides affect grapevine leaf photosynthesis without inducing defense mechanisms

The effects of the two botryticides, fludioxonil (fdx) and fenhexamid (fhd), were investigated on grapevine leaves (Vitis vinifera L. cv. Pinot noir) following photosynthesis and defense mechanisms. Treatments were carried out in vineyard at the end of flowering. Phytotoxicity of both fungicides was evaluated by measuring variations of leaf photosynthetic parameters and correlated expression of photosynthesis-related genes. Results demonstrated that similar decrease in photosynthesis was caused by fdx and fhd applications. Moreover, the mechanism leading to photosynthesis alteration seems to be the same for both fungicides. Stomatal limitation to photosynthetic gas exchange did not change following treatments indicating that inhibition of photosynthesis was mostly attributed to non-stomatal factors. Nevertheless, fungicides-induced depression of photosynthesis was related neither to a decrease in Rubisco carboxylation efficiency and in the capacity for regeneration of ribulose 1,5-bisphosphate nor to loss in PSII activity. However, fdx and fhd treatments generated repression of genes encoding proteins involved in the photosynthetic process. Indeed, decreased photosynthesis was coupled with repression of PsbP subunit of photosystem II (psbP1), chlorophyll a/b binding protein of photosystem I (cab) and Rubisco small subunit (rbcS) genes. A repression of these genes may participate in the photosynthesis alteration. To our knowledge, this is the first study of photosynthesis-related gene expression following fungicide stress. In the meantime, defense responses were followed by measuring chitinase activity and expression of varied defense-related genes encoding proteins involved in phenylpropanoid synthesis (PAL) or octadecanoid synthesis (LOX), as well as pathogenesis-related protein (Chi4C). No induction of defense was observed in botryticides-treated leaves. To conclude, the photosynthesis is affected without any triggering of plant defense responses.     

Identification of novel genes putatively involved in the photosystem synthesis of <Emphasis Type="Italic">Bradyrhizobium</Emphasis> sp. ORS 278

In aerobic anoxygenic phototrophs, oxygen is required for both the formation of the photosynthetic apparatus and an efficient cyclic electron transfer. Mutants of Bradyrhizobium sp. ORS278 affected in photosystem synthesis were selected by a bacteriochlorophyll fluorescence-based screening. Out of the 9,600 mutants of a random Tn5 insertion library, 50 clones, corresponding to insertions in 28 different genes, present a difference in fluorescence intensity compared to the WT. Besides enzymes and regulators known to be involved in photosystem synthesis, 14 novel components of the photosynthesis control are identified. Among them, two genes, hsIU and hsIV, encode components of a protein degradation complex, probably linked to the renewal of photosystem, an important issue in Bradyrhizobia which have to deal with harmful reactive oxygen species. The presence of homologs in non-photosynthetic bacteria for most of the regulatory genes identified during study suggests that they could be global regulators, as the RegA–RegB system. Eric Giraud and André Verméglio should be considered co-senior authors.     

Transcriptome profiling of genes involved in photosynthesis in <Emphasis Type="Italic">Elaeagnus angustifolia</Emphasis> L. under salt stress

High salt concentration is a major abiotic stress limiting plant growth and productivity in many areas of the world. Elaeagnus angustifolia L. adapts to adverse environments and is widely planted in the western region of China as a windbreaker and for landscape and soil stabilization. High salt concentrations inhibited photosynthesis of E. angustifolia, but the mechanism is not known. In this paper, RNA-sequencing was used to investigate effects of salt stress on the photosynthetic characteristics of the species. In total, 584 genes were identified and involved in photosynthetic pathways. The downregulation of genes that encode key enzymes involved in photosynthesis and genes correlated to important structures in photosystem and light-harvesting complexes might be the main reason, particularly, the downregulation of the gene that encodes magnesium chelatase. This would decrease the activity of enzymes involved in chlorophyll synthesis and the downregulation of the key gene that encodes Rubisco, and thereby decreases enzyme activity and the protein content of Rubisco.     

PpsR: a multifaceted regulator of photosynthesis gene expression in purple bacteria

Purple bacteria control the level of expression and the composition of their photosystem according to light and redox conditions. This control involves several regulatory systems that have been now well characterized. Among them, the PpsR regulator plays a central role, because it directly or indirectly controls the synthesis of all of the different components of the photosystem. In this review, we report our knowledge of the PpsR protein, highlighting the diversity of its mode of action and focusing on the proteins identified in four model purple bacteria (Rhodobacter capsulatus, Rhodobacter sphaeroides, Rubrivivax gelatinosus, Bradyrhizobium ORS278). This regulator exhibits unique regulatory features in each bacterium: it can activate and/or repress the expression of photosynthesis genes, its activity can be modulated or not by the redox conditions, it can interact with other specific regulators and therefore be involved differently in light and/or redox regulatory circuits.     

Redox and light regulation of gene expression in photosynthetic prokaryotes

All photosynthetic organisms control expression of photosynthesis genes in response to alterations in light intensity as well as to changes in cellular redox potential. Light regulation in plants involves a well-defined set of red- and blue-light absorbing photoreceptors called phytochrome and cryptochrome. Less understood are the factors that control synthesis of the plant photosystem in response to changes in cellular redox. Among a diverse set of photosynthetic bacteria the best understood regulatory systems are those synthesized by the photosynthetic bacterium Rhodobacter capsulatus. This species uses the global two-component signal transduction cascade, RegB and RegA, to anaerobically de-repress anaerobic gene expression. Under reducing conditions, the phosphate on RegB is transferred to RegA, which then activates genes involved in photosynthesis, nitrogen fixation, carbon fixation, respiration and electron transport. In the presence of oxygen, there is a second regulator known as CrtJ, which is responsible for repressing photosynthesis gene expression. CrtJ responds to redox by forming an intramolecular disulphide bond under oxidizing, but not reducing, growth conditions. The presence of the disulphide bond stimulates DNA binding activity of the repressor. There is also a flavoprotein that functions as a blue-light absorbing anti-repressor of CrtJ in the related bacterial species Rhodobacter sphaeroides called AppA. AppA exhibits a novel long-lived photocycle that is initiated by blue-light absorption by the flavin. Once excited, AppA binds to CrtJ thereby inhibiting the repressor activity of CrtJ. Various mechanistic aspects of this photocycle will be discussed.     

Characterization of a light-responding trans-activator responsible for differentially controlling reaction center and light-harvesting-I gene expression in Rhodobacter capsulatus.

The purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus regulates synthesis of its photosystem in response to two environmental stimuli, oxygen tension and light intensity. Here we describe the identification and characterization of the trans-acting regulatory gene hvrA, which we show is involved in differentially controlling reaction center and light-harvesting gene expression in response to alterations in light intensity. An hvrA mutant strain is shown to lack the capability to trans-activate light-harvesting-I and reaction center gene expression but retain normal light-harvesting-II and photopigment regulation, in response to a reduction in light intensity. As a consequence of altered expression, hvrA mutant strains exhibit reduced photosynthetic growth capabilities under dim-light conditions. The results of this study and additional studies indicate that regulated synthesis of the photosystem involves complex sets of overlapping regulatory circuits that differentially control photosystem gene expression in response to environmental stimuli such as oxygen tension and light intensity.     

Proteomic Analysis of Horseweed (Conyza canadensis) Subjected to Caprylic Acid Stress

Caprylic acid (CAP) is anticipated to be a potential biocontrol herbicide in the control of weeds, however the molecular mechanism of how CAP affects weeds is poorly understood. Here, the physiological and biochemical (protein‐level) changes in horseweed (Conyza canadensis L.) are studied under CAP treatment, with infrared gas analyzer and label‐free quantitative proteomics methods. In total, 112 differentially‐accumulated proteins (DAPs) (>1.5 fold change, p < 0.05) are present between treated horseweed and control samples, with 46 up‐regulated and 66 down‐regulated proteins. These DAPs are involved in 28 biochemical pathways, including photosynthesis pathways. In particular, six photosynthesis proteins show significant abundance changes in the CAP‐treated horseweed. The qRT‐PCR results confirm three of the six genes involved in photosynthesis. Moreover, by measuring photosynthesis characteristics, CAP was shown to decrease photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and the transpiration rate of horseweed. These results suggest that photosystem I is one of the main biological processes involved in the response of horseweed to CAP.     

Iron nutrition-mediated chloroplast development

Membrane development in chloroplasts was explored by resupplying iron to iron-deficient sugar beet (Beta vulgaris L. cv F58-554H1) and monitoring changes in lamellar components during regreening. The synthesis of chlorophyll a, chlorophyll b, and Q, the first stable electron acceptor of photosystem II, exhibited a lag phase during the first 24 to 48 hours of resupply. In contrast, the per area amounts of P₇₀₀ and cytochrome f increased linearly over the first 48 hours. During the early regreening period, the Q to P₇₀₀ ratio was 2.6 and decreased to 0.7 after 96 hours of regreening. The rate of photosynthesis (net CO₂ uptake) per chlorophyll increased during the first 48 hours of resupply, then by 96 hours decreased to values typical of control plants. The results suggest that there was preferential synthesis of the measured photosystem I components during the first 24 to 48 hours, while from 48 to 96 hours there was rapid synthesis of all components. The iron nutrition-mediated chloroplast development system provides a useful experimental approach for studying biomembrane synthesis and structural-functional relations of the photosynthetic apparatus.     

集胞藻PCC6803 EGY同源基因破坏突变体的构建及表型分析

摘要：拟南芥中近来发现的定位于叶绿体的膜嵌合金属蛋白酶EGY1影响叶绿体发育与脂肪酸合成,经生物信息学分析,集胞藻PCC6803 (Synechocystis sp. PCC6803)中slr0643、sll0862基因编码同源蛋白。【目的】为了鉴定这两个基因的功能,【方法】本文通过同源重组插入卡那霉素抗性基因、切断目的基因,分别构建了slr0643::km和sll0862::km两种突变体,检测突变体的生理生化表型。【结果】在30℃,20 μE/m2s自养培养下,slr0643::km与野生型相比,早期     

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.