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Histone methylation catalyzed by histone methyltransferase is essential in transcriptional regulation of gene expression. Histone methyltransferases are known to play crucial roles in multiple cellular processes in plants. My PhD work investigated the biological function of two histone methyltransferases in controlling plant responses to various environmental stimuli in Arabidopsis thaliana. In the first part, my results demonstrated that the H3K36-methyltransferase SDG8 transcriptionally regulates NPR1, a central player in salicylic acid-mediated immunity and co-acts with the RNAPII to enable the efficient transcriptional induction of several defense genes upon stimulation. In the second part, my work unraveled that SDG26, another ortholog of the animal H3K36-methyltransferase, plays an important role in plant response to abiotic stresses. By focusing on cold stress, SDG26 was shown to regulate the cold stress response by directly activating the transcription of SOC1 and CBF genes through binding their chromatin and depositing H3K36me3. Interestingly, SDG26 mastered the accumulation of ABA by regulating the expression of ABA homeostasis-related genes, suggesting an involvement of ABA pathway in the cold response. In the last part, using a genetic approach my work established SDG26 as an autonomous flowering pathway component. Accordingly, SDG26 was found in a multiple-protein complex comprising the histone demethylase FLD, the homeobox-domain transcription factor LD, as well as a putative COMPASS component APRF1. This multiple-protein complex was found in controlling the repression of the major flowering repressor FLC as well as the activation of the flowering activator SOC1 to precisely regulate the floral transition.; La méthylation des histones par des histone-méthyltransférases est essentielle à la régulation de la transcription. Chez les végétaux, plusieurs méthyltransférases jouent un rôle central dans différents processus cellulaires. Lors de ma thèse, j'ai étudié la fonction biologique de deux d’entre elles dans le contrôle des réponses à plusieurs stimuli chez Arabidopsis thaliana. Premièrement, mes résultats ont permis de démontrer que SDG8, une méthyltransférase spécifique de H3K36, contrôlait la transcription de NPR1, un acteur central de l’immunité induite par l’acide salicylique, et en lien avec la RNAPII permettait l’induction transcriptionnelle efficace de plusieurs gènes de défense suite à leur stimulation. Deuxièmement, j'ai démontré que SDG26, une méthyltransférase proche de SDG8, jouait un rôle important dans la réponse aux stress abiotiques. J'ai découvert ainsi que SDG26 régulait la réponse au froid en activant directement la transcription des gènes SOC1 et CBF en se liant à leur chromatine et en déposant H3K36me3. Sachant que l’acide abscissique (ABA) joue un rôle important dans la réponse aux stress abiotiques, j'ai pu constater que SDG26 contrôlait l'accumulation d'ABA en régulant l'expression de gènes liés à son homéostasie. Enfin, j'ai confirmé par une approche génétique que SDG26 était un élément appartenant à la voie de floraison dite autonome. Au sein d'un complexe multi-protéique comprenant l'histone déméthylase FLD, le facteur de transcription homéobox LD, ainsi qu'un composant de COMPASS, APRF1, SDG26 semble contrôler la transcription du répresseur de floraison, FLC, ainsi que de sa cible, l'activateur de floraison SOC1, afin de réguler précisément la transition florale.

Rice spikelet sterility caused by chilling during microspore stage of panicle development is a major cause of the rareness of the indica sub-species in cool environments. A diversity panel of 200 indica accessions including traditional and improved accessions forming 4 genetic sub-groups (I1 to I4), along with 22 accessions representing other genetic groups of Oryza sativa L., was field phenotyped for spikelet sterility under flooded management. Environments were six seasonal climatic situations (sowing dates) in Senegal, ranging from cool to hot; and at two altitudes (857 and 1497 m asl) and two years in Madagascar. The apex temperature during the critical phenological stage was estimated from weather records and an empirical model estimating the daily minimum water temperature. Sterility incidence was regressed for each accession against apex temperature across all environments after discarding observations potentially involving heat induced sterility. From the correlations, the slope of the temperature response and sterility at 20 °C (no stress) and 15 °C (chilling stress) were derived and used to evaluate the accessions and establish clusters. Indicas were on average more sensitive than temperate and tropical japonicas. Among indicas, large diversity in cold sterility was observed that formed 6 clusters, which represented the different genetic sub-groups at a distinct frequency. Sterility was greater in Senegal than in Madagascar for all clusters. Sub-group I3 indigenous to Madagascar (traditional varieties) was nearly exclusively present in the most cold tolerant cluster C1. Some improved indicas were also in C1 but belonged to sub-group I2. The phenomics resource will be used in a genome wide association (GWA) study to identify putative candidate genes.

BACKGROUND: The effect of temperature on chilling injury during fruit growth was studied in a new banana hybrid CIRAD925 in which seasonal variability in chilling susceptibility was observed when fruits were stored at 13 ∘ C. RESULTS: The relationship between the response to chilling (presence/absence) and the temperature during banana fruit growth was examined with a logistic regression model. An explanatory variable X N,P was defined as the mean temperature during a period, expressed in weeks, which began N week(s) after flowering and lasted P week(s). The model was calibrated with 143 bunches with a green life of 30 ± 5 days and validated with 156 bunches grown in six plots under different growing conditions. Chilling injury was best predicted by the mean temperature during the period beginning 1 week after flowering and lasting 5 weeks (X 1,5 ). Above a mean temperature of 24.1 ∘ C in the period concerned, banana fruits had a 95% probability of chilling injury at 13 ∘ C. Below a temperature of 23.4 ∘ C, banana fruits only had a 5% probability of chilling injury. CONCLUSION: The results provide a tool to predict chilling susceptibility in banana fruit whatever the thermal conditions in tropical regions.

International audience; Grain yield in any given environment is determined by yield components developed at different phenophases. Yield components are influenced by the environmental conditions the plant experiences during the respective phases. The final yield of a given cultivar depends on the interaction between geno-type and its responses to environmental conditions. Hence, it is necessary to evaluate the plasticity in yield components formation while selecting genotype for a given environment. For this, we conducted field trials comprising 10 upland rice genotypes representing a large share of genetic variation, with two sowing dates in two consecutive years in three altitudinal locations creating 12 environments in Madagascar. Crop duration, grain yield and yield components (tillers per hill, panicles per tiller, grains per panicle, sterility, grain weight) were strongly affected by sowing dates, location, year and genotypes. Sowing date and years resulted in comparatively more variable environments in high and low altitudes than in mid altitude. Yield stability across environments reflected the target environments the geno-types were originally selected for. Variation in grain yield among planting dates within altitudes was not mainly due to temperature but rather to the combinations of abiotic factors the genotypes experi-enced during the different phenological stages duringwhich the different yield componentswere formed. Yield components and their contribution to environmentally induced yield penalties were analyzed in detail. The contribution of individual yield components to final yield changed with the environmen-tal conditions the rice experienced during the development stages. This effect may have a stronger influence on final yield than the genetic control of the individual yield components. New combina-tions of traits are required to better exploit the environmental potential which may only be possible via advanced crop models simulating the environmental effects on yield components and their inter-dependencies to develop ideotypes for the target environments thus guiding breeding and selection efforts.

In lowland rice production, water-saving irrigation technologies have been developed, but it has rarely been considered that the absence of a ponded water layer could change the field's microclimate due to the different thermal characteristics of water compared to air. At a site in the Senegal River valley, canopy and soil temperature as well as temperature at meristem level and relative humidity inside the canopy were observed in the presence and absence of a ponded water layer in an irrigated rice field. Gas-exchange measurements were conducted at different development stages of three varieties (IR4630, IR64, and Sahel108) sown in bi-monthly intervals and the effects of climatic and microclimatic parameters on stomatal conductance, assimilation rate, and intrinsic water use efficiency were investigated. Minimum soil ( T s min) and meristem temperature ( T M min) were usually lower in the absence of a ponded water layer. Stomatal conductance depended mainly on T s min, T M min, and minimum relative humidity inside the canopy. Assimilation rate was positively correlated with solar radiation, T s min and T M min, but depended mainly on stomatal conductance. Without standing water, stomatal conductance was significantly lower, but reductions could be explained with lower T s min and/or T M min. Nevertheless, T s min and/or T M min were the major determinants of stomatal conductance and assimilation rate, which suggests a pivotal role of root zone temperature on plant growth probably via water uptake and, thus, overall plant water status. Varietal differences were found, with assimilation rate in IR4630 and Sahel108 having been less affected by low temperature than in IR64. When water-saving irrigation measures are applied in irrigated rice, the negative effects of lower soil and meristem temperature in the absence of a ponded water layer in the field on the productivity of rice need to be considered. In regions where night temperatures below 20 °C occur, varieties should be used that are less temperature-responsive, if the effect of cool nights on meristem temperature cannot be mitigated by a ponded water layer.

We aimed to evaluate the importance of modulations of within-tree carbon (C) allocation by water and low-temperature stress for the prediction of annual forest growth with a process-based model. A new C allocation scheme was implemented in the CASTANEA model that accounts for lagged and direct environmental controls of C allocation. Different approaches (static vs dynamic) to modelling C allocation were then compared in a model–data fusion procedure, using satellite-derived leaf production estimates and biometric measurements at c. 104 sites. The modelling of the environmental control of C allocation significantly improved the ability of CASTANEA to predict the spatial and year-to-year variability of aboveground forest growth along regional gradients. A significant effect of the previous year's water stress on the C allocation to leaves and wood was reported. Our results also are consistent with a prominent role of the environmental modulation of sink demand in the wood growth of the studied species. Data available at large scales can inform forest models about the processes driving annual and seasonal C allocation. Our results call for a greater consideration of C allocation drivers, especially sink–demand fluctuations, for the simulations of current and future forest productivity with process-based models.