Plant responses to abiotic stress: the chromatin context of transcriptional regulation

The ability of plants to cope with abiotic environmental stresses such as drought, salinity, heat, cold or flooding relies on flexible mechanisms for re-programming gene expression. Over recent years it has become apparent that transcriptional regulation needs to be understood within its structural context. Chromatin, the assembly of DNA with histone proteins, generates a local higher-order structure that impacts on the accessibility and effectiveness of the transcriptional machinery, as well as providing a hub for multiple protein interactions. Several studies have shown that chromatin features such as histone variants and post-translational histone modifications are altered by environmental stress, and they could therefore be primary stress targets that initiate transcriptional stress responses. Alternatively, they could act downstream of stress-induced transcription factors as an integral part of transcriptional activity. A few experimental studies have addressed this ‘chicken-and-egg’ problem in plants and other systems, but to date the causal relationship between dynamic chromatin changes and transcriptional responses under stress is still unclear. In this review we have collated the existing information on concurrent epigenetic and transcriptional responses of plants to abiotic stress, and we have assessed the evidence using a simple theoretical framework of causality scenarios

Efectos del ozono troposférico sobre el cultivo de patata en Carcaixent (Valencia).

RESUMEN El ozono troposférico es el contaminante atmosférico más fitotóxico conocido. El ozono provoca estrés oxidativo en las plantas y sus concentraciones actuales causan importantes disminuciones del rendimiento agrícola en cultivos sensibles. El objetivo general de esta tesis doctoral es el estudio de los efectos de una exposición crónica a ozono durante todo el período de cultivo en plantas de patata (Solanum tuberosum L. cv. Agria). La metodología de cultivo empleada han sido las cámaras de techo abierto (OTC), realizando tres tratamientos: aire filtrado de ozono (CF), aire con concentraciones ambientales de ozono (NF) y aire ambiental enriquecido con ozono (NF+). El primer objetivo ha sido el estudio del efecto del ozono sobre el rendimiento agrícola: se ha encontrado que el tratamiento NF provoca una disminución de un 9-24 %, y el tratamiento NF+ una disminución de un 25-73 %; además existe una alta correlación entre la disminución del rendimiento agrícola y los índices de exposición acumulada a ozono AOT40 y AOT00. En segundo lugar se ha aplicado difenilamina y citrulina a cultivos en campo para evaluar su acción protectora frente al ozono ambiental. Con ambos compuestos químicos, especialmente con la citrulina, se ha encontrado un aumento en el rendimiento agrícola de las patatas comerciales; el mecanismo de acción de ambos agentes parece estar basado en la disminución del estrés oxidativo. El tercer objetivo ha consistido en el estudio de la respuesta del antioxidante glutatión a la exposición crónica a ozono. Como resultado, la concentración de GSSG y, en mayor proporción, la de GSH, han aumentado, de manera que las plantas aumentan la capacidad amortiguadora del glutatión frente al estrés oxidativo; una de las causas del aumento de la concentración de GSH es una mayor síntesis regulada por la expresión de la enzima γ-GCS. El cuarto objetivo ha sido la cuantificación de radicales libres formados por la exposición a ozono mediante espectroscopía ESR en banda X y banda Q. Se ha detectado una señal perteneciente a uno o varios radicales libres permanentes. Además, se ha desarrollado un método de análisis de imagen para cuantificar el área de hoja con síntomas visibles inducidos por ozono. Las técnicas empleadas han permitido encontrar una estrecha relación entre la intensidad de la señal del(de los) radical(es) libre(s) y el porcentaje de superficie foliar con síntomas visibles por ozono. Por último, se ha realizado el estudio de las alteraciones histológicas producidas por la exposición crónica a ozono, así como la detección histoquímica de las especies reactivas del oxígeno radical superóxido y peróxido de hidrógeno. El ozono provoca una degeneración de la lámina media y de la pared celular, además de una pérdida de turgencia de la vacuola, dando como resultado una pérdida de contacto entre las células y finalmente el colapso celular; las células más afectadas han sido las del parénquima lagunar próximas a los estomas. Se ha encontrado un taponamiento del floema debido a la acumulación de calosa, y una acumulación de almidón en las células dañadas del parénquima, fenómenos que podrían ser la principal causa de la disminución del rendimiento agrícola. El radical superóxido se ha detectado principalmente en el parénquima lagunar, concretamente en la pared y/o membrana celular y en membranas intracelulares como el tonoplasto, membrana de los cloroplastos y grana. El peróxido de hidrógeno se ha detectado principalmente en la pared celular, y en los peroxisomas, que se revelan como una fuente importante de esta especie reactiva del oxígeno en condiciones de estrés por ozono; se ha encontrado peróxido de hidrógeno de manera abundante en las paredes del floema, y se discute una posible relación con la acumulación de calosa. __________________________________________________________________________________________________Tropospheric ozone is considered the most phytotoxic air pollutant. The general aim of this thesis is to study the effects of a chronic ozone exposure during the entire growing season on potato plants (Solanum tuberosum L. cv. Agria). Plants were grown in open-top chambers (OTC), and treatments consisted of charcoal-filtered air (CF), ambient air (NF) and ambient air enriched with ozone (NF+). The first aim was to study the effects of ozone on crop yield: the result was a 9-24 % decrease, and a 25-73 % decrease in NF and NF+ treatments, respectively. A strong correlation was found between yield decrease and accumulated ozone exposure indexes AOT40 and AOT00. Secondly, diphenylamine and citrulline were applied to crops in field conditions to test their protective role against ambient ozone. Both chemical compounds, specially citrulline, caused a yield increase in commercial tubers; these compounds seemed to alleviate oxidative stress. Furthermore, glutathione response to chronic ozone exposure was studied. There was an increase in GSSG and GSH concentrations. An enhanced γ-GCS enzyme expression was observed, being a cause for the increase in GSH concentration. Another aim was the quantification of free radicals generated by ozone exposure employing X- and Q-band ESR spectroscopy. A signal belonging to one or several permanent free radicals was detected. Also, an image analysis method was developed to quantify leaf area exhibiting visible ozone symptoms. A strong relationship between free radical(s) signal intensity and percentage of visible ozone-induced damage was found. Finally, research into ozone stress effects on foliar histology was conducted. Ozone caused cell wall degeneration and loss of vacuolar turgence, resulting in the loss of contact between cells and cell collapse. Phloem was blocked by callose, and starch accumulated in injured cells of parenchyma. Superoxide radical was mainly detected in the spongy parenchyma, specifically in the cell wall and/or plasmalemma, tonoplast, chloroplast membrane and grana. Hydrogen peroxide was principally observed in the cell wall, and in peroxisomes, which became an important source of this reactive oxygen species under ozone stress; hydrogen peroxide was abundant in phloem walls, and a possible relationship with callose accumulation is discussed

Ozone-induced reductions in below-ground biomass: an anatomical approach in potato

[EN] Potato plants were grown in open-top chambers under three ozone concentrations during two complete cropping seasons (93 and 77 d in 2004 and 2005, respectively). The effects of chronic exposure to ozone on leaf anatomy, cell ultrastructure and crop yield were studied. Severe cell damage was found, even at ambient ozone levels, mainly affecting the spongy parenchyma and areas near the stomata. Damage to the cell wall caused loss of cell contact, and loss of turgor pressure due to tonoplast disintegration, contributed to cell collapse. Phloem sieve plates were obstructed by callose accumulation, and damaged mesophyll cells increased their starch stores. Tuber yield fell sharply (24–44%), due to the biggest tubers becoming smaller, which affected commercial yield. These anatomical findings show the mechanisms of ozone effect on assimilate partitioning, and thus crop yield decrease, in potato. Further implications of ozone causing reductions in belowground biomass are also discussed.The authors thank Prof Secundino del Valle (Valencia University, Spain) for his helpful comments. We are also grateful to Mr Duncan Gates for revising the English style of the text. AAF was supported by a grant from the Generalitat Valenciana's FPI programme (Government of Valencia, Spain).Asensi-Fabado, A.; García-Breijo, F.; Reig Armiñana, J. (2010). Ozone-induced reductions in below-ground biomass: an anatomical approach in potato. Plant, Cell and Environment. 33(7):1070-1083. doi:10.1111/j.1365-3040.2010.02128.xS1070108333

Tocopherol deficiency reduces sucrose export from salt-stressed potato leaves independently of oxidative stress and symplastic obstruction by callose

Tocopherol cyclase, encoded by the gene SUCROSE EXPORT DEFECTIVE1, catalyses the second step in the synthesis of the antioxidant tocopherol. Depletion of SXD1 activity in maize and potato leaves leads to tocopherol deficiency and a ‘sugar export block’ phenotype that comprises massive starch accumulation and obstruction of plasmodesmata in paraveinal tissue by callose. We grew two transgenic StSXD1:RNAi potato lines with severe tocopherol deficiency under moderate light conditions and subjected them to salt stress. After three weeks of salt exposure, we observed a strongly reduced sugar exudation rate and a lack of starch mobilization in leaves of salt-stressed transgenic plants, but not in wild-type plants. However, callose accumulation in the vasculature declined upon salt stress in all genotypes, indicating that callose plugging of plasmodesmata was not the sole cause of the sugar export block phenotype in tocopherol-deficient leaves. Based on comprehensive gene expression analyses, we propose that enhanced responsiveness of SnRK1 target genes in mesophyll cells and altered redox regulation of phloem loading by SUT1 contribute to the attenuation of sucrose export from salt-stressed SXD:RNAi source leaves. Furthermore, we could not find any indication that elevated oxidative stress may have served as a trigger for the salt-induced carbohydrate phenotype of SXD1:RNAi transgenic plants. In leaves of the SXD1:RNAi plants, sodium accumulation was diminished, while proline accumulation and pools of soluble antioxidants were increased. As supported by phytohormone contents, these differences seem to increase longevity and prevent senescence of SXD:RNAi leaves under salt stress

JUNGBRUNNEN1, a Reactive Oxygen Species–Responsive NAC Transcription Factor, Regulates Longevity in Arabidopsis[W][OA]

Aging in plants is an intricate process that balances vegetative growth with flowering and reproductive success. This work describes the identification of JUNGBRUNNEN1, a NAC transcription factor that regulates this process in Arabidopsis thaliana and additionally affects abiotic stress tolerance by activating expression of the DREB2A transcription factor