997 resultados para Photosynthetic Response


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Under ocean acidification (OA), the 200 % increase in CO2(aq) and the reduction of pH by 0.3-0.4 units are predicted to affect the carbon physiology and growth of macroalgae. Here we examined how the physiology of the giant kelp Macrocystis pyrifera is affected by elevated pCO2/low pH. Growth and photosynthetic rates, external and internal carbonic anhydrase (CA) activity, HCO3 (-) versus CO2 use were determined over a 7-day incubation at ambient pCO2 400 µatm/pH 8.00 and a future OA treatment of pCO2 1200 µatm/pH 7.59. Neither the photosynthetic nor growth rates were changed by elevated CO2 supply in the OA treatment. These results were explained by the greater use of HCO3 (-) compared to CO2 as an inorganic carbon (Ci) source to support photosynthesis. Macrocystis is a mixed HCO3 (-) and CO2 user that exhibits two effective mechanisms for HCO3 (-) utilization; as predicted for species that possess carbon-concentrating mechanisms (CCMs), photosynthesis was not substantially affected by elevated pCO2. The internal CA activity was also unaffected by OA, and it remained high and active throughout the experiment; this suggests that HCO3 (-) uptake via an anion exchange protein was not affected by OA. Our results suggest that photosynthetic Ci uptake and growth of Macrocystis will not be affected by elevated pCO2/low pH predicted for the future, but the combined effects with other environmental factors like temperature and nutrient availability could change the physiological response of Macrocystis to OA. Therefore, further studies will be important to elucidate how this species might respond to the global environmental change predicted for the ocean.

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We examined the long-term effect of naturally acidified water on a Cymodocea nodosa meadow growing at a shallow volcanic CO2 vent in Vulcano Island (Italy). Seagrass and adjacent unvegetated habitats growing at a low pH station (pH = 7.65 ± 0.02) were compared with corresponding habitats at a control station (pH = 8.01 ± 0.01). Density and biomass showed a clear decreasing trend at the low pH station and the below- to above-ground biomass ratio was more than 10 times lower compared to the control. C content and delta 13C of leaves and epiphytes were significantly lower at the low pH station. Photosynthetic activity of C. nodosa was stimulated by low pH as seen by the significant increase in Chla content of leaves, maximum electron transport rate and compensation irradiance. Seagrass community metabolism was intense at the low pH station, with significantly higher net community production, respiration and gross primary production than the control community, whereas metabolism of the unvegetated community did not differ between stations. Productivity was promoted by the low pH, but this was not translated into biomass, probably due to nutrient limitation, grazing or poor environmental conditions. The results indicate that seagrass response in naturally acidified conditions is dependable upon species and geochemical characteristics of the site and highlight the need for a better understanding of complex interactions in these environments.

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The response of respiration, photosynthesis, and calcification to elevated pCO2 and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 °C), and at ambient (ca. 400 µatm) or elevated pCO2 (ca. 700 µatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO2 but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO2. Conversely, photosynthesis was strongly affected by pCO2 with a decline under elevated pCO2 in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO2. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO2 but decreased with rising temperature under elevated pCO2. Surprisingly, the highest rates in summer were found under elevated pCO2 and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO2. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO2. However, there is likely a trade-off with other physiological processes. For example, photosynthesis declines in response to increased pCO2 under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.

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El tomate (Solanum lycopersicum L.) es considerado uno de los cultivos hortícolas de mayor importancia económica en el territorio Español. Sin embargo, su producción está seriamente afectada por condiciones ambientales adversas como, salinidad, sequía y temperaturas extremas. Para resolver los problemas que se presentan en condiciones de estrés, se han empleado una serie de técnicas culturales que disminuyen sus efectos negativos, siendo de gran interés el desarrollo de variedades tolerantes. En este sentido la obtención y análisis de plantas transgénicas, ha supuesto un avance tecnológico, que ha facilitado el estudio y la evaluación de genes seleccionados en relación con la tolerancia al estrés. Estudios recientes han mostrado que el uso de genes reguladores como factores de transcripción (FTs) es una gran herramienta para obtener nuevas variedades de tomate con mayor tolerancia a estreses abióticos. Las proteínas DOF (DNA binding with One Finger) son una familia de FTs específica de plantas (Yangisawa, 2002), que están involucrados en procesos fisiológicos exclusivos de plantas como: asimilación del nitrógeno y fijación del carbono fotosintético, germinación de semilla, metabolismo secundario y respuesta al fotoperiodo pero su preciso rol en la tolerancia a estrés abiótico se desconoce en gran parte. El trabajo descrito en esta tesis tiene como objetivo estudiar genes reguladores tipo DOF para incrementar la tolerancia a estrés abiotico tanto en especies modelo como en tomate. En el primer capítulo de esta tesis se muestra la caracterización funcional del gen CDF3 de Arabidopsis, así como su papel en la respuesta a estrés abiótico y otros procesos del desarrollo. La expresión del gen AtCDF3 es altamente inducido por sequía, temperaturas extremas, salinidad y tratamientos con ácido abscísico (ABA). La línea de inserción T-DNA cdf3-1 es más sensible al estrés por sequía y bajas temperaturas, mientras que líneas transgénicas de Arabidopsis 35S::AtCDF3 aumentan la tolerancia al estrés por sequía, osmótico y bajas temperaturas en comparación con plantas wild-type (WT). Además, estas plantas presentan un incremento en la tasa fotosintética y apertura estomática. El gen AtCDF3 se localiza en el núcleo y que muestran una unión específica al ADN con diferente afinidad a secuencias diana y presentan diversas capacidades de activación transcripcional en ensayos de protoplastos de Arabidopsis. El dominio C-terminal de AtCDF3 es esencial para esta localización y su capacidad activación, la delección de este dominio reduce la tolerancia a sequía en plantas transgénicas 35S::AtCDF3. Análisis por microarray revelan que el AtCDF3 regula un set de genes involucrados en el metabolismo del carbono y nitrógeno. Nuestros resultados demuestran que el gen AtCDF3 juega un doble papel en la regulación de la respuesta a estrés por sequía y bajas temperaturas y en el control del tiempo de floración. En el segundo capítulo de este trabajo se lleva a cabo la identificación de 34 genes Dof en tomate que se pueden clasificar en base a homología de secuencia en cuatro grupos A-D, similares a los descritos en Arabidopsis. Dentro del grupo D se han identificado cinco genes DOF que presentan características similares a los Cycling Dof Factors (CDFs) de Arabidopsis. Estos genes son considerados ortólogos de Arabidopsis CDF1-5, y han sido nombrados como Solanum lycopersicum CDFs o SlCDFs. Los SlCDF1-5 son proteínas nucleares que muestran una unión específica al ADN con diferente afinidad a secuencias diana y presentan diversas capacidades de activación transcripcional in vivo. Análisis de expresión de los genes SlCDF1-5 muestran diferentes patrones de expresión durante el día y son inducidos de forma diferente en respuesta a estrés osmótico, salino, y de altas y bajas temperaturas. Plantas de Arabidopsis que sobre-expresan SlCDF1 y SlCDF3 muestran un incremento de la tolerancia a la sequía y salinidad. Además, de la expresión de varios genes de respuesta estrés como AtCOR15, AtRD29A y AtERD10, son expresados de forma diferente en estas líneas. La sobre-expresión de SlCDF3 en Arabidopsis promueve un retardo en el tiempo de floración a través de la modulación de la expresión de genes que controlan la floración como CONSTANS (CO) y FLOWERING LOCUS T (FT). En general, nuestros datos demuestran que los SlCDFs están asociados a funciones aun no descritas, relacionadas con la tolerancia a estrés abiótico y el control del tiempo de floración a través de la regulación de genes específicos y a un aumento de metabolitos particulares. ABSTRACT Tomato (Solanum lycopersicum L.) is one of the horticultural crops of major economic importance in the Spanish territory. However, its production is being affected by adverse environmental conditions such as salinity, drought and extreme temperatures. To resolve the problems triggered by stress conditions, a number of agricultural techniques that reduce the negative effects of stress are being frequently applied. However, the development of stress tolerant varieties is of a great interest. In this direction, the technological progress in obtaining and analysis of transgenic plants facilitated the study and evaluation of selected genes in relation to stress tolerance. Recent studies have shown that a use of regulatory genes such as transcription factors (TFs) is a great tool to obtain new tomato varieties with greater tolerance to abiotic stresses. The DOF (DNA binding with One Finger) proteins form a family of plant-specific TFs (Yangisawa, 2002) that are involved in the regulation of particular plant processes such as nitrogen assimilation, photosynthetic carbon fixation, seed germination, secondary metabolism and flowering time bur their precise roles in abiotic stress tolerance are largely unknown. The work described in this thesis aims at the study of the DOF type regulatory genes to increase tolerance to abiotic stress in both model species and the tomato. In the first chapter of this thesis, we present molecular characterization of the Arabidopsis CDF3 gene as well as its role in the response to abiotic stress and in other developmental processes. AtCDF3 is highly induced by drought, extreme temperatures, salt and abscisic acid (ABA) treatments. The cdf3-1 T-DNA insertion mutant was more sensitive to drought and low temperature stresses, whereas the AtCDF3 overexpression enhanced the tolerance of transgenic plants to drought, cold and osmotic stress comparing to the wild-type (WT) plants. In addition, these plants exhibit increased photosynthesis rates and stomatal aperture. AtCDF3 is localized in the nuclear region, displays specific binding to the canonical DNA target sequences and has a transcriptional activation activity in Arabidopsis protoplast assays. In addition, the C-terminal domain of AtCDF3 is essential for its localization and activation capabilities and the deletion of this domain significantly reduces the tolerance to drought in transgenic 35S::AtCDF3 overexpressing plants. Microarray analysis revealed that AtCDF3 regulated a set of genes involved in nitrogen and carbon metabolism. Our results demonstrate that AtCDF3 plays dual roles in regulating plant responses to drought and low temperature stress and in control of flowering time in vegetative tissues. In the second chapter this work, we carried out to identification of 34 tomato DOF genes that were classified by sequence similarity into four groups A-D, similar to the situation in Arabidopsis. In the D group we have identified five DOF genes that show similar characteristics to the Cycling Dof Factors (CDFs) of Arabidopsis. These genes were considered orthologous to the Arabidopsis CDF1 - 5 and were named Solanum lycopersicum CDFs or SlCDFs. SlCDF1-5 are nuclear proteins that display specific binding to canonical DNA target sequences and have transcriptional activation capacities in vivo. Expression analysis of SlCDF1-5 genes showed distinct diurnal expression patterns and were differentially induced in response to osmotic, salt and low and high temperature stresses. Arabidopsis plants overexpressing SlCDF1 and SlCDF3 showed increased drought and salt tolerance. In addition, various stress-responsive genes, such as AtCOR15, AtRD29A and AtERD10, were expressed differently in these lines. The overexpression of SlCDF3 in Arabidopsis also results in the late flowering phenotype through the modulation of the expression of flowering control genes such CONSTANS (CO) and FLOWERING LOCUS T (FT). Overall, our data connet SlCDFs to undescribed functions related to abiotic stress tolerance and flowering time through the regulation of specific target genes and an increase in particular metabolites.

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Ulmus minor es una especie arbórea originaria de Europa cuyas poblaciones han sido diezmadas por el hongo patógeno causante de la enfermedad de la grafiosis. La conservación de los olmos exige plantearse su propagación a través de plantaciones y conocer mejor su ecología y biología. Ulmus minor es un árbol de ribera, pero frecuentemente se encuentra alejado del cauce de arroyos y ríos, donde la capa freática sufre fuertes oscilaciones. Por ello, nuestra hipótesis general es que esta especie es moderadamente resistente tanto a la inundación como a la sequía. El principal objetivo de esta tesis doctoral es entender desde un punto de vista funcional la respuesta de U. minor a la inundación, la sequía y la infección por O. novo-ulmi; los factores que posiblemente más influyen en la distribución actual de U. minor. Con este objetivo se persigue dar continuidad a los esfuerzos de conservación de esta especie que desde hace años se dedican en varios centros de investigación a nivel mundial, ya que, entender mejor los mecanismos que contribuyen a la resistencia de U. minor ante la inoculación con O. novo-ulmi y factores de estrés abiótico ayudará en la selección y propagación de genotipos resistentes a la grafiosis. Se han planteado tres experimentos en este sentido. Primero, se ha comparado la tolerancia de brinzales de U. minor y U. laevis – otro olmo ibérico – a una inmersión controlada con el fin de evaluar su tolerancia a la inundación y comprender los mecanismos de aclimatación. Segundo, se ha comparado la tolerancia de brinzales de U. minor y Quercus ilex – una especie típica de ambientes Mediterránea secos – a la falta de agua en el suelo con el fin de evaluar el grado de tolerancia y los mecanismos de aclimatación a la sequía. El hecho de comparar dos especies contrastadas responde al interés en entender mejor cuales son los procesos que conducen a la muerte de una planta en condiciones de sequía – asunto sobre el que hay una interesante discusión desde hace algunos años. En tercer lugar, con el fin de entender mejor la resistencia de algunos genotipos de U. minor a la grafiosis, se han estudiado las diferencias fisiológicas y químicas constitutivas e inducidas por O. novo-ulmi entre clones de U. minor seleccionados a priori por su variable grado de resistencia a esta enfermedad. En el primer experimento se observó que los brinzales de U. minor sobrevivieron 60 días inmersos en una piscina con agua no estancada hasta una altura de 2-3 cm por encima del cuello de la raíz. A los 60 días, los brinzales de U. laevis se sacaron de la piscina y, a lo largo de las siguientes semanas, fueron capaces de recuperar las funciones fisiológicas que habían sido alteradas anteriormente. La conductividad hidráulica de las raíces y la tasa de asimilación de CO2 neta disminuyeron en ambas especies. Por el contrario, la tasa de respiración de hojas, tallos y raíces aumentó en las primeras semanas de la inundación, posiblemente en relación al aumento de energía necesario para desarrollar mecanismos de aclimatación a la inundación, como la hipertrofia de las lenticelas que se observó en ambas especies. Por ello, el desequilibrio del balance de carbono de la planta podría ser un factor relevante en la mortalidad de las plantas ante inundaciones prolongadas. Las plantas de U. minor (cultivadas en envases de 16 litros a media sombra) sobrevivieron por un prolongado periodo de tiempo en verano sin riego; la mitad de las plantas murieron tras 90 días sin riego. El cierre de los estomas y la pérdida de hojas contribuyeron a ralentizar las pérdidas de agua y tolerar la sequía en U. minor. Las obvias diferencias en tolerancia a la sequía con respecto a Q. ilex se reflejaron en la distinta capacidad para ralentizar la aparición del estrés hídrico tras dejar de regar y para transportar agua en condiciones de elevada tensión en el xilema. Más relevante es que las plantas con evidentes síntomas de decaimiento previo a su muerte exhibieron pérdidas de conductividad hidráulica en las raíces del 80% en ambas especies, mientras que las reservas de carbohidratos apenas variaron y lo hicieron de forma desigual en ambas especies. Árboles de U. minor de 5 y 6 años de edad (plantados en eras con riego mantenido) exhibieron una respuesta a la inoculación con O. novo-ulmi consistente con ensayos previos de resistencia. La conductividad hidráulica del tallo, el potencial hídrico foliar y la tasa de asimilación de CO2 neta disminuyeron significativamente en relación a árboles inoculados con agua, pero solo en los clones susceptibles. Este hecho enlaza con el perfil químico “más defensivo” de los clones resistentes, es decir, con los mayores niveles de suberina, ácidos grasos y compuestos fenólicos en estos clones que en los susceptibles. Ello podría restringir la propagación del hongo en el árbol y preservar el comportamiento fisiológico de los clones resistentes al inocularlos con el patógeno. Los datos indican una respuesta fisiológica común de U. minor a la inundación, la sequía y la infección por O. novo-ulmi: pérdida de conductividad hidráulica, estrés hídrico y pérdida de ganancia neta de carbono. Pese a ello, U. minor desarrolla varios mecanismos que le confieren una capacidad moderada para vivir en suelos temporalmente anegados o secos. Por otro lado, el perfil químico es un factor relevante en la resistencia de ciertos genotipos a la grafiosis. Futuros estudios deberían examinar como este perfil químico y la resistencia a la grafiosis se ven alteradas por el estrés abiótico. ABSTRACT Ulmus minor is a native European elm species whose populations have been decimated by the Dutch elm disease (DED). An active conservation of this species requires large-scale plantations and a better understanding of its biology and ecology. U. minor generally grows close to water channels. However, of the Iberian riparian tree species, U. minor is the one that spread farther away from rivers and streams. For these reasons, we hypothesize that this species is moderately tolerant to both flooding and drought stresses. The main aim of the present PhD thesis is to better understand the functional response of U. minor to the abiotic stresses – flooding and drought – and the biotic stress – DED – that can be most influential on its distribution. The overarching goal is to aid in the conservation of this emblematic species through a better understanding of the mechanisms that contribute to resistance to abiotic and biotic stresses; an information that can help in the selection of resistant genotypes and their expansion in large-scale plantations. To this end, three experiments were set up. First, we compared the tolerance to experimental immersion between seedlings of U. minor and U. laevis – another European riparian elm species – in order to assess their degree of tolerance and understand the mechanisms of acclimation to this stress. Second, we investigated the tolerance to drought of U. minor seedlings in comparison with Quercus ilex (an oak species typical of dry Mediterranean habitats). Besides assessing and understanding U. minor tolerance to drought at the seedling stage, the aim was to shed light into the functional alterations that trigger drought-induced plant mortality – a matter of controversy in the last years. Third, we studied constitutive and induced physiological and biochemical differences among clones of variable DED resistance, before and following inoculation with Ophiostoma novo-ulmi. The goal is to shed light into the factors of DED resistance that is evident in some genotypes of U. minor, but not others. Potted seedlings of U. minor survived for 60 days immersed in a pool with running water to approximately 2-3 cm above the stem collar. By this time, U. minor seedlings died, whereas U. laevis seedlings moved out of the pool were able to recover most physiological functions that had been altered by flooding. For example, root hydraulic conductivity and leaf photosynthetic CO2 uptake decreased in both species; while respiration initially increased with flooding in leaves, stems and roots possibly to respond to energy demands associated to mechanisms of acclimation to soil oxygen deficiency; as example, a remarkable hypertrophy of lenticels was soon observed in flooded seedlings of both species. Therefore, the inability to maintain a positive carbon balance somehow compromises seedling survival under flooding, earlier in U. minor than U. laevis, partly explaining their differential habitats. Potted seedlings of U. minor survived for a remarkable long time without irrigation – half of plants dying only after 90 days of no irrigation in conditions of high vapour pressure deficit typical of summer. Some mechanisms that contributed to tolerate drought were leaf shedding and stomata closure, which reduced water loss and the risk of xylem cavitation. Obviously, U. minor was less tolerant to drought than Q. ilex, differences in drought tolerance resulting mostly from the distinct capacity to postpone water stress and conduct water under high xylem tension among species. More relevant was that plants of both species exhibited similar symptoms of root hydraulic failure (i.e. approximately 80% loss of hydraulic conductivity), but a slight and variable depletion of non-structural carbohydrate reserves preceding dieback. Five- and six-year-old trees of U. minor (planted in the field with supplementary watering) belonging to clones of contrasted susceptibility to DED exhibited a different physiological response to inoculation with O. novo-ulmi. Stem hydraulic conductivity, leaf water potential and photosynthetic CO2 uptake decreased significantly relative to control trees inoculated with water only in DED susceptible clones. This is consistent with the “more defensive” chemical profile observed in resistant clones, i.e. with higher levels of saturated hydrocarbons (suberin and fatty acids) and phenolic compounds than in susceptible clones. These compounds could restrict the spread of O. novo-ulmi and contribute to preserving the near-normal physiological function of resistant trees when exposed to the pathogen. These results evidence common physiological responses of U. minor to flooding, drought and pathogen infection leading to xylem water disruption, leaf water stress and reduced net carbon gain. Still, seedlings of U. minor develop various mechanisms of acclimation to abiotic stresses that can play a role in surviving moderate periods of flood and drought. The chemical profile appears to be an important factor for the resistance of some genotypes of U. minor to DED. How abiotic stresses such as flooding and drought affect the capacity of resistant U. minor clones to face O. novo-ulmi is a key question that must be contemplated in future research.

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Net photosynthesis (Pn) is inhibited by moderate heat stress. To elucidate the mechanism of inhibition, we examined the effects of temperature on gas exchange and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation in cotton and tobacco leaves and compared the responses to those of the isolated enzymes. Depending on the CO2 concentration, Pn decreased when temperatures exceeded 35–40°C. This response was inconsistent with the response predicted from the properties of fully activated Rubisco. Rubisco deactivated in leaves when temperature was increased and also in response to high CO2 or low O2. The decrease in Rubisco activation occurred when leaf temperatures exceeded 35°C, whereas the activities of isolated activase and Rubisco were highest at 42°C and >50°C, respectively. In the absence of activase, isolated Rubisco deactivated under catalytic conditions and the rate of deactivation increased with temperature but not with CO2. The ability of activase to maintain or promote Rubisco activation in vitro also decreased with temperature but was not affected by CO2. Increasing the activase/Rubisco ratio reduced Rubisco deactivation at higher temperatures. The results indicate that, as temperature increases, the rate of Rubisco deactivation exceeds the capacity of activase to promote activation. The decrease in Rubisco activation that occurred in leaves at high CO2 was not caused by a faster rate of deactivation, but by reduced activase activity possibly in response to unfavorable ATP/ADP ratios. When adjustments were made for changes in activation state, the kinetic properties of Rubisco predicted the response of Pn at high temperature and CO2.

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Microorganisms must sense their environment and rapidly tune their metabolism to ambient conditions to efficiently use available resources. We have identified a gene encoding a response regulator, NblR, that complements a cyanobacterial mutant unable to degrade its light-harvesting complex (phycobilisome), in response to nutrient deprivation. Cells of the nblR mutant (i) have more phycobilisomes than wild-type cells during nutrient-replete growth, (ii) do not degrade phycobilisomes during sulfur, nitrogen, or phosphorus limitation, (iii) cannot properly modulate the phycobilisome level during exposure to high light, and (iv) die rapidly when starved for either sulfur or nitrogen, or when exposed to high light. Apart from regulation of phycobilisome degradation, NblR modulates additional functions critical for cell survival during nutrient-limited and high-light conditions. NblR does not appear to be involved in acclimation responses that occur only during a specific nutrient limitation. In contrast, it controls at least some of the general acclimation responses; those that occur during any of a number of different stress conditions. NblR plays a pivotal role in integrating different environmental signals that link the metabolism of the cell to light harvesting capabilities and the activities of the photosynthetic apparatus; this modulation is critical for cell survival.

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Feedback regulation of photosynthesis by carbon metabolites has long been recognized, but the underlying cellular mechanisms that control this process remain unclear. By using an Arabidopsis cell culture, we show that a block in photosynthetic electron flux prevents the increase in transcript levels of chlorophyll a/b-binding protein and the small subunit of Rubisco that typically occurs when intracellular sugar levels are depleted. In contrast, the expression of the nitrate reductase gene, which is induced by sugars, is not affected. These findings were confirmed in planta by using Arabidopsis carrying the firefly luciferase reporter gene fused to the plastocyanin and chlorophyll a/b-binding protein 2 gene promoters. Transcription from both promoters increases on carbohydrate depletion. Blocking photosynthetic electron transport with 3-(3′, 4′-dichlorophenyl)-1,1′-dimethylurea prevents this increase in transcription. We conclude that plastid-derived redox signaling can override the sugar-regulated expression of nuclear-encoded photosynthetic genes. In the sugar-response mutant, sucrose uncoupled 6 (sun6), plastocyanin-firefly luciferase transcription actually increases in response to exogenous sucrose rather than decreasing as in the wild type. Interestingly, plastid-derived redox signals do not influence this defective pattern of sugar-regulated gene expression in the sun6 mutant. A model, which invokes a positive inducer originating from the photosynthetic electron transport chain, is proposed to explain the nature of the plastid-derived signal.

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The G-protein activator mastoparan (MP) was found to elicit the hypersensitive response (HR) in isolated Asparagus sprengeri mesophyll cells at micromolar concentrations. The HR was characterized by cell death, extracellular alkalinization, and an oxidative burst, indicated by the reduction of molecular O2 to O2⋅−. To our knowledge, this study was the first to monitor photosynthesis during the HR. MP had rapid and dramatic effects on photosynthetic electron transport and excitation energy transfer as determined by variable chlorophyll a fluorescence measurements. A large increase in nonphotochemical quenching of chlorophyll a fluorescence accompanied the initial stages of the oxidative burst. The minimal level of fluorescence was also quenched, which suggests the origin of this nonphotochemical quenching to be a decrease in the antenna size of photosystem II. In contrast, photochemical quenching of fluorescence decreased dramatically during the latter stages of the oxidative burst, indicating a somewhat slower inhibition of photosystem II electron transport. The net consumption of O2 and the initial rate of O2 uptake, elicited by MP, were higher in the light than in the dark. These data indicate that light enhances the oxidative burst and suggest a complex relationship between photosynthesis and the HR.