Investigations into the »inverse contrast« of chloroplast thylakoids

We detected an »inverse contrast« (i. e. dense lumen and lightly staining membrane) of thylakoids in some species or organs where it has not previously been described. We probed the chemical character of the dense lumenal substance by in situ extraction on ultrathin sections, and found that it was less hydrophilic in Perilla frutescens, than in Helianthus annuus, where it could be neither lipid nor protein. Its disappearance from the lumen upon prolonged illumination was accompanied by a drop in the plastidial polyphenol oxidase activity, so it may be a phenolic compound. Growing Helianthus seedlings at 30 °C (rather than at 25 °C) resulted in the density appearing in the stroma (rather than in the lumen), but still preventing the strong staining of the thylakoid membrane. Upon the removal of the dense substance from the stroma by plastid isolation the staining pattern reverted to its usual appearance in the plastids. This excludes the explanation that light membrane staining is produced by some compositional deficiency. This conclusion was supported also by gel electrophoretograms of polypeptides from such membranes. We think that the dense substance on either side of the membrane protects lipid molecules from a conformational change during solvent dehydration, which normally leads to the strong staining of the membrane

Kis-dózisú stresszorok stimuláló hatásának jelátviteli és hormonális háttere növényi sejtekben = Signal transduction and hormonal effects in plant cells stimulated by low-dose stressors

Vizsgálat tárgyává tettük a kis-koncentrációjú kémiai stresszorok által kiváltott serkentő hatás (hardening, eustressz) mechanizmusát az alábbi kísérleti rendszerekben : 1. levágott, nem gyökeresedő levél (árpa), 2. levágott, gyökeresedő levél (bab), 3. csíranövény, gyökéren át kezelve (árpa), 4. csíranövény, levélen át kezelve (bab). Azt találtuk, hogy a hatás nem ágens-specifikus, tehát szubmikromólos koncentrációban fémion vagy herbicid ugyanúgy hat. Nevezetesen a PIP2-IP3/DAG jelpálya DAG-ágán, majd a PKC-től egy MAPK útvonalon küld szignált a sejtmembrántól a sejtmagig. A válasz viszont specifikus arra a sejttípusra, amely kontaktusba került az ágenssel. Gyökér esetében a citokinin-szint emelkedik, a levél klorofill-szintézissel és fokozott CO2-beépítéssel válaszol. | The mechanism of stimulation by low-concentration chemical stressors (hardening, eustress) was investigated in the following experimental systems : 1.detached non-rooting leaf (barley), 2. detached rooting leaf (bean), 3. seedling treated via the root (barley), 4. seedling treated via the leaf (bean). It was found that the effect was not agent-specific, i.e. submicromolar metal ion or herbicide acted the same way. Namely, a signal was sent from the plasma membrane via the DAG branch of the PIP2-IP3/DAG pathway, then PKC activated a MAPK signaling pathway leading to the nucleus. The response, however, was specific to the cell type coming into contact with the agent. In case of root, the cytokinin level increased, while the leaf responded by increased chlorophyll synthesis and CO2 incorporation

On the mechanism of stimulation by low-concentration stressors in barley seedlings

Beneficial effect of low-concentration chemical stressors like Cd salt and a herbicide DCMU have been investigated previously in different plant model systems. The symptoms of stimulation are known from earlier studies, but information about the mechanism is limited. In this work, the mechanism of stimulation of low-concentration Cd (5.10-8 M) and 3-(3,4-di-chlorophenyl)-1,1-dimethylurea (DCMU, 10-7 M) was investigated in barley seedlings. In treated plants, the amount of cytokinins increased in roots and, after being transported to the leaves, they caused stimulation of chlorophyll accumulation, photosynthetic activity, and delayed senescence. To identify the signal transduction pathway(s) involved in the primary stimulation of cytokinin synthesis in roots, specific phosphatidylinositol-4,5-bisphosphate – inositol-1,4,5-tri-phosphate/diacylglycerol (PIP2-IP3/DAG) and mitogen activated protein kinase (MAPK) signaling pathway inhibitors were added to the nutrient solutions. Both of them proved to be effective, eliminating the stimulation of the stressors

Effect of low-concentration stressors on the senescence of detached barley leaves

Effect of low (sub-micromolar) concentrations of some stress-inducing heavy metals like Cd, Pb, Ni, and Ti salts and the herbicide DCMU on the senescence of chloroplasts was investigated in detached leaves of barley. These agents delayed the loss of chlorophylls and photosynthetic activity (14CO2 fixation). Decrease of the number and size of plastoglobuli in treated chloroplasts also indicated the anti-senescence effect of low-concentration stressors. The active cytokinin content of barley leaves tested by Amaranthus betacyanin bioassay did not show any changes. It is assumed that these stressors may activate a cytokinin-independent signal transduction pathway in the cells

Light sensitivity of Haberlea rhodopensis shade adapted phenotype under drought stress

Haberlea rhodopensis belongs to the group of homoiochlorophyllous desiccation-tolerant plants which preserve their chlorophyll content during dehydration. It is a typical shade adapted plant and it is proved to be very sensitive to light intensity higher than the natural during drought stress. To reveal the reasons of their light sensitivity, we compared the damages and protective mechanisms of shade plants during desiccation either simulating their natural light conditions (30 lmol photons m-2 s-1, LL) or at a moderately higher light intensity (100 lmol photons m-2 s-1, ML). In the desiccated stage, no damage could be discovered in terms of thylakoid membrane quantity or integrity either at LL or ML. Nevertheless, the altered structure and localization of chloroplasts did not restore in plants desiccated and rehydrated at ML, where no starch could be re-synthesized, but a number of plastoglobuli appeared. The PSII activity and the amount of b-carotene and lutein decreased more strongly in ML leaves in agreement with their higher MDA production. Lack of recovery of ML plants may be connected with the very high number of damaged PSII reaction centres caused by the loss of the subtle balance between ROS production and scavenging. In addition, because of the impaired starch re-synthesis, there is no sink for the water-replacing sugars and water cannot be taken up which proved to be lethal to ML plants

Plasztisz-differenciálódás: a szerkezet és a működés összefüggései = Plastid differentiation: connection between structure and function

A plasztisz-differenciálódás folyamatában a struktúra és a funkció közötti összefüggéseket vizsgáltuk különböző szerveződési szinteken. Sötétben nevelt csíranövényeket, szerv- és szövettenyészetet, szövet-homogenátumokat, membrán-preparátumokat, intakt növényeket és a természetből begyűjtött rügyeket és egyéb szerveket használtunk. A folyamat indikátoraként a NADPH:protoklorofillid oxidoreduktáz (POR) enzim, illetve a pigmentek komplexeit használtuk. Különböző fluoreszcencia spektroszkópiai módszereket és elektronmikroszkópos vizsgálatokat végeztünk. Megállapítottuk, hogy a POR monomer, dimer és oligomer komplexei is flash fotoaktívak, és dinamikusan egymásba alakulnak. A nem levél eredetű szervekben proplasztisz jellegű etioplasztiszok vannak, amelyekben a POR- és nem kötött pigment-monomerek dominálnak, valamint NADPH hiány jellemző. Emiatt a monomer dominanciájú szervek természetes fényen fotodegradálódnak. A makrodomén szerveződés együtt jár a prolamelláris test kialakulásával, fejlett etioplasztiszok képződésével, növeli a fotoredukció hatékonyságát, és véd a fotodegradáció ellen. A plasztisz-differenciálódást a citokinin szint és a N-ellátottság szabályozza. Proplasztiszok, etioplasztiszok és átmeneti állapotú plasztiszok természetes körülmények között is kialakulnak, egyes kiválasztásra differenciálódott szövetekben a kloroplasztisz speciálisan módosul. Eredményeink alapján a klorofill bioszintézis befejező lépését és a plasztisz-differenciálódást leíró reakciósémákat módosítani kell. | Relationships between the structure and function were studied in the process of plastid differentiation, on different organizational levels. Dark-grown seedlings, organ- and tissue cultures, tissue homogenates, membrane preparations, intact plants, buds and other organs collected from the nature were used. Different complexes of the NADPH:protochlorophyllide oxidoreductase (POR) enzyme and of pigments were used as indicators of the differentiation process. Various fluorescence spectroscopic measurements and electron microscopy studies were done. We have proved that the monomer, dimer and oligomer complexes of POR are flash-photoactive and they transform into each other dinamically. In non-leaf organs, proplastid type etioplasts can be found in which monomers of POR and of non-bound pigments are dominant and NADPH shortage is characteristic. Therefore, these organs undergo photodegradation at natural light conditions. The macrodomain organization proceeds together with the formation of prolamellar bodies, i.e. of developed etioplasts, it increases the efficiency of photoreduction and provides protection against photodegradation. Plastid differentiation is regulated by citokinin concentrations and nitrogen supply. Proplastids, etioplasts and plastids of transitional developmental stages can be found in the nature. In certain secretion tissues, the chloroplasts are specifically modified. On the basis of our results, the reaction schemes describing the terminal steps of chlorophyll biosynthesis and plastid differentiation should be modified

Hazai fafajok fotoszintézisválaszai és fotoprotektív mechanizmusai stresszkörülmények közt = Photosynthetic responses and photoprotective mechanisms of tree species under stress conditions

Erdőalkotó fafajainkon laboratóriumi és szabadföldi körülmények közt azt vizsgáltuk, hogyan változtatja meg a stressz minősége, erőssége, illetve a növény fejlettségi állapota a fotoszintézissel kapcsolatos stresszvédő mechanizmusokat. A kis dózisú nehézfémstressz a citokinin szint növelésével stimulálta a fotoszintézist. A közepes nehézfém (Cd) dózis hatásait kiváltó legfontosabb tényezők a Fe hiány (a tünetek a vastáplálás emelésével megszüntethetők) és a fotoszintézisgátlás okozta relatív fényfelesleg. A fényfelesleg miatt megjelenő oxidatív stressz a kezelés folyamán mérséklődött. A kifejlett levelekben a CO2-megkötés sztómazáródás miatti gátlását elsősorban a fotorespiráció erősödése védte ki, míg a fejlődő levelekben fotokémiai (víz-víz ciklus) és nem-fotokémiai kioltó folyamatok, amihez a violaxantin ciklus aktivitás és az antenna (Lhc) összetétel transzkripciós szinten szabályozott változásai is hozzájárulhattak, egyaránt szerepet játszottak. Termőhelyi/szabadföldi körülmények közt a tölgy és bükk a levélnövekedés kezdeti időszakában volt a legérzékenyebb a fotoinhibícióra. A megemelt UV-B sugárzás fokozta a fotoinhibiciós érzékenységet. A védelemben a nem-fotokémiai disszipáló mechanizmusok voltak a legfontosabbak, szoros összefüggésben a xantofill-, illetve a tölgyfajoknál újonnan kimutatott lutein-epoxid ciklus működésével. Emellett, a Cd stesszt is beleértve, fontos szerepe volt a különleges fényvédő vegyületek (flavonoidok és még nem azonosított vegyületek) felhalmozódásának. | In laboratory and field experiments, effects of stresses (heavy metals, UV-B) of different strength were studied on photosynthesis and related protective mechanisms in leaves of forest-tree species being in different developmental stage. Photosynthesis was stimulated by low-dose heavy metal treatments due to the elevated level of cytokinins in leaves. The most important impacts of moderate heavy metal (Cd) doses on photosynthesis were the Cd-induced iron deficiency (symptoms could be eliminated by increased Fe supply) and the relative light excess due to the inhibition of photosynthesis. Light excess induced oxidative stress was diminished during the treatment. Inhibition of CO2 fixation due to Cd-induced decrease in stomatal conductance was counterbalanced by the increased activity of photorespiration in leaves treated in photosynthetically competent stage, while both photochemical (water-water cycle) and non-photochemical mechanisms, which included violaxanthin cycle activity and changes in the antenna composition regulated at transcriptional level, contributed to the protection of newly emerging leaves. In field experiments, oak and beech species showed the highest sensitivity to photoinhibition at the beginning of leaf development, and the sensitivity was enhanced under elevated UV-B. Non-photochemical quenching connected with the activities of xanthophyll cycle and the newly discovered lutein-epoxid cycle was the protective mechanisms of utmost importance. Accumulation of light-protective substances such as flavonoids and yet unidentified ones (Cd stress) was also important

Reactivation of the Photosynthetic Apparatus of Resurrection Plant Haberlea rhodopensis during the Early Phase of Recovery from Drought- and Freezing-Induced Desiccation

Haberlea rhodopensis is a unique desiccation-tolerant angiosperm that also survives winter frost. As, upon freezing temperatures, H. rhodopensis desiccates, the taxon is proposed to survive low temperature stress using its desiccation tolerance mechanisms. To reveal the validity of this hypothesis, we analyzed the structural alterations and organization of photosynthetic apparatus during the first hours of recovery after drought- and freezing-induced desiccation. The dynamics of the ultrastructure remodeling in the mesophyll cells and the restoration of the thylakoid membranes shared similarities independent of the reason for desiccation. Among the most obvious changes in thylakoid complexes, the proportion of the PSI-LHCII complex strongly increased around 70% relative water content (RWC), whereas the proportion of Lhc monomers decreased from the beginning of rehydration. We identified enhanced levels of cyt b6f complex proteins that contributed to the enhanced electron flow. The high abundance of proteins related to excitation energy dissipation, PsbS, Lhcb5, Lhcb6 and ELIPs, together with the increased content of dehydrins contributed to the preservation of cellular integrity. ELIP expression was maintained at high levels up to 9 h into recovery. Although the recovery processes from drought- and freezing-induced desiccation were found to be similar in progress and time scale, slight variations indicate that they are not identical