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International audience; Environmental heat stress impacts on the physiology and viability of microbial cells with concomitant implications for microbial activity and diversity. Previously, it has been demonstrated that gradual heating of Saccharomyces cerevisiae induces a degree of thermal resistance, whereas a heat shock results in a high level of cell death. Here, we show that the impact of exogenous nutrients on acquisition of thermal resistance differs between strains.Using single‐cell methods, we demonstrate the extent of heterogeneity of the heat‐stress response within populations of yeast cells and the presence of subpopulations that are reversibly damaged by heat stress. Such cells represent potential for recovery of entire populations once stresses are removed. The results show that plasma membrane permeability and potential are key factors involved in cell survival, but thermal resistance is not related to homeoviscous adaptation of the plasma membrane. These results have implications for growth and regrowth of populations experiencing environmental heat stress and our understanding of impacts at the level of the single cell. Given the important role of microbes in biofuel production and bioremediation, a thorough understanding of the impact of stress responses of populations and individuals is highly desirable.

Tropospheric ozone acts as a phytotoxin which produces an oxidative stress in plants. Two ways of defense are used, either by preventing ozone input through the regulation of stomatal conductance, or by detoxifying ozone and ROS in cells. It is known that stomatal movements are alt ered by ozone. We performed fumigation experiment on three euramerican poplar genotypes ( Po pulus deltoides x Populus Nigra : ‘Carpaccio’, ‘Cima’ and ‘Robusta’ ), cultivated in pots in phytotronic chambers submitted to 120 ppb ozone or filtered air. We explor ed the effects of ozone on stomatal responses to four environmental parameters (blue light, red light, CO 2 and VPD). We also find out using a porometer that the decrease of stomatal conductance due to ozone is earlier on the adaxial face than on abaxial fa ce. Finally, to better understand these impacts, we studied ultrastructure of guard cells by TEM, stomatal density and size of stomata by SEM, and we performed X - ray microanalysis of guard cells mineral content. These approaches are coupled with the study on microdissected stomata of expression of genes involved in regulation of stomatal movements

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Ozone induces stomatal sluggishness, which impacts photosynthesis and transpiration. Stomatal responses to variation of environmental parameters are slowed and reduced by ozone and may be linked to difference of ozone sensitivity. Here we determine the ozone effects on stomatal conductance of each leaf surface. Potential causes of this sluggish movement, such as ultrastructural or ionic fluxes modification, were studied independently on both leaf surfaces of three Euramerican poplar genotypes differing in ozone sensitivity and in stomatal behaviour. The element contents in guard cells were linked to the gene expression of ion channels and transporters involved in stomatal movements, directly in microdissected stomata. In response to ozone, we found a decrease in the stomatal conductance of the leaf adaxial surface correlated with high calcium content in guard cells compared with a slight decrease on the abaxial surface. No ultrastructural modifications of stomata were shown except an increase in the number of mitochondria. The expression of vacuolar H(+) /Ca(2+) -antiports (CAX1 and CAX3 homologs), β-carbonic anhydrases (βCA1 and βCA4) and proton H(+) -ATPase (AHA11) genes was strongly decreased under ozone treatment. The sensitive genotype characterized by constitutive slow stomatal response was also characterized by constitutive low expression of genes encoding vacuolar H(+) /Ca(2+) -antiports.

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Tropospheric ozone is a major air pollutant that acts as a phytotoxin. It enters the leaf through the stomata before being dissolved in the apoplast by generating reactive oxygen species (ROS) causing oxidative stress. Two defenses exist to restrict the effects of ozone: (i) the stomata which can limit ozone uptake by regulating stomatal conductance and (ii) the detoxification processes of ROS generated by ozone.We studied the effects of ozone (120 ppb) on these two mechanisms of defense in three euramerican poplar genotypes (Populus deltoides x Populus nigra) under controlled conditions in phytotronic chambers. A direct effect of ozone on photosynthesis and stomatal movements in response to changes in environmental factors (by slowing the stomatal opening and closure) has been highlighted. Models of stomatal conductance, on which indicators of critical level of ozone for vegetation are based, must take them into account. In addition, these studies have highlighted the role of constitutive concentrations of antioxidants in tolerance to ozone as well as the complexity of these detoxification mechanisms. The notion of effective ozone flux must consider these two aspects to better characterize the intra-and inter-specific differences in sensitivity to ozone; L'ozone troposphérique est un polluant atmosphérique majeur qui agit comme une phytotoxine. Il pénètre dans les feuilles par les stomates avant d'être dissout dans l'apoplaste en générant des radicaux libres oxygénés (ROS) provoquant ainsi un stress oxydatif. Deux barrières existent pour restreindre les effets de l'ozone : (i) les stomates qui peuvent limiter les flux entrants par contrôle de la conductance stomatique et (ii) le système de détoxication des ROS issus de la dégradation de l'ozone. Nous avons étudié les effets de l'ozone (120 ppb) sur ces deux moyens de défense chez trois génotypes de peuplier euraméricain (Populus deltoides x Populus nigra) placés en conditions contrôlées dans des chambres phytotroniques. Un effet direct de l'ozone sur la photosynthèse et sur les mouvements stomatiques en réponse à des variations de facteurs environnementaux (ralentissement des phénomènes d'ouverture et de fermeture) a été mis en évidence. Les modèles de calcul de la conductance stomatique, sur lesquels se basent les indicateurs de seuil de risque à l'ozone pour la végétation, doivent donc les prendre en compte. De plus, ces travaux ont mis en évidence le rôle prépondérant des concentrations constitutives en antioxidants dans la tolérance à l'ozone ainsi que la complexité de ces mécanismes de détoxication. La notion de flux effectif d'ozone doit prendre en considération ces deux aspects afin de caractériser au mieux les différences de sensibilité à l'ozone intra et inter spécifique

Tropospheric ozone is a major air pollutant that acts as a phytotoxin. It enters the leaf through the stomata before being dissolved in the apoplast by generating reactive oxygen species (ROS) causing oxidative stress. Two defenses exist to restrict the effects of ozone: (i) the stomata which can limit ozone uptake by regulating stomatal conductance and (ii) the detoxification processes of ROS generated by ozone.We studied the effects of ozone (120 ppb) on these two mechanisms of defense in three euramerican poplar genotypes (Populus deltoides x Populus nigra) under controlled conditions in phytotronic chambers. A direct effect of ozone on photosynthesis and stomatal movements in response to changes in environmental factors (by slowing the stomatal opening and closure) has been highlighted. Models of stomatal conductance, on which indicators of critical level of ozone for vegetation are based, must take them into account. In addition, these studies have highlighted the role of constitutive concentrations of antioxidants in tolerance to ozone as well as the complexity of these detoxification mechanisms. The notion of effective ozone flux must consider these two aspects to better characterize the intra-and inter-specific differences in sensitivity to ozone, L'ozone troposphérique est un polluant atmosphérique majeur qui agit comme une phytotoxine. Il pénètre dans les feuilles par les stomates avant d'être dissout dans l'apoplaste en générant des radicaux libres oxygénés (ROS) provoquant ainsi un stress oxydatif. Deux barrières existent pour restreindre les effets de l'ozone : (i) les stomates qui peuvent limiter les flux entrants par contrôle de la conductance stomatique et (ii) le système de détoxication des ROS issus de la dégradation de l'ozone. Nous avons étudié les effets de l'ozone (120 ppb) sur ces deux moyens de défense chez trois génotypes de peuplier euraméricain (Populus deltoides x Populus nigra) placés en conditions contrôlées dans des chambres phytotroniques. Un effet direct de l'ozone sur la photosynthèse et sur les mouvements stomatiques en réponse à des variations de facteurs environnementaux (ralentissement des phénomènes d'ouverture et de fermeture) a été mis en évidence. Les modèles de calcul de la conductance stomatique, sur lesquels se basent les indicateurs de seuil de risque à l'ozone pour la végétation, doivent donc les prendre en compte. De plus, ces travaux ont mis en évidence le rôle prépondérant des concentrations constitutives en antioxidants dans la tolérance à l'ozone ainsi que la complexité de ces mécanismes de détoxication. La notion de flux effectif d'ozone doit prendre en considération ces deux aspects afin de caractériser au mieux les différences de sensibilité à l'ozone intra et inter spécifique