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In natural environments, long-lived and stationary organisms such as trees are subjected to a wide variety of environmental cues from both biotic and abiotic origins. With on-going climate change, it is expected to have more severe and recurrent abiotic stresses such as drought and ozone. All these combined abiotic stresses negatively impact trees productivity, e.g. by decreasing carbon assimilation and allocation to stems and roots. Still, only poor data exist regarding how these abiotic stresses can impact the functioning (bi-directional exchanges of nutrients) of ectomycorrhizal associations and the long- lasting positive effect of this symbiosis on tree productivity. It is thus crucial to address the challenging question of how trees interact with and respond to their biotic and abiotic environments at the same time. We aim at better understanding the molecular plasticity in the above and below part of mycorrhized poplar (Populus trichocarpa) trees under different environmental cues: drought and ozone. In this context, the specific objectives are to decipher the impact of these abiotic stresses on (i) metabolites, (ii) hormonal and (iii) transcript signatures of Populus leaves and ECM or no-ECM roots as well as the impact of ectomycorrhization on tree responses to stress. The first results of this project will be presented.

The presence of zones of high mechanical resistance is one of the most common physical limitations to soil exploration by roots, which has direct impacts on yield crops. In order to decipher the elementary mechanisms involved in the mechanical feedback between the plant root growth and the granular structure of the soil [1], we investigated the mechanical properties of the root as well as the root trajectory in different impeding substrates. First, we studied the longitudinal Young modulus of non-lignified plant roots tested in compression for different external environments. Control experiments on non-drying roots placed in isotonic osmotic solutions showed no evolution of the root’s Young’s modulus whose value was around 2 MPa. On the contrary, the experiments performed in air exhibited a dramatic increase of the root’s Young’s modulus with the drying time, sometimes by a factor of 35. Moreover, the Young’s modulus was observed to scale as a decaying power-law with the root’s cross-section measured at different times of drying. We interpreted our results in the framework of the mechanics of cellular foams [2]. This explains why the rigidity of thinner roots whose drying process is more efficient, increases faster with time. Second, we began to characterize the root growth, trajectory or branching pattern, in different model substrates exerting either a localized constriction on the root axis or repeated mechanical stresses. We focused on the elementary event of a root growing in a short elastic tube mimicking the passage of a root in the pore between the movable grains of a granular soil. We progressively complicate the interaction between roots and the mechanical obstacles by observing via time-lapse photography the root growth in arrays of fixed posts of different rigidities, or even in assembly of grains.

Root growth and development are highly modulated by soil conditions which influence its mechanical impedance i.e. the soil resistance to root penetration. Root cap cells surrounded by the mucilage they secrete are the firsts to encounter obstacles during soil exploration, and are known to sense and integrate environmental cues, making the root cap a relevant candidate as mechanoperception site. We thus investigated in this study a putative implication of the root cap in the sensing and in the response to mechanical impedance. We developed two-Layer-Phytagel media in which an increase in mechanical impedance was generated by a lower harder layer covered by an upper softer layer. Root growth and trajectory of primary roots of Arabidopsis thaliana seedlings were investigated using image analysis. In response to the contact with the harder layer, the roots rapidly bent allowing the reorientation of root primary growth. The root bending was associated with an inhibition of root growth rate, suggesting both mechanical and active root responses to increase in mechanical impedance. To investigate the role of root cap integrity, we examined in the two-layer media the behavior of the primary root of mutants affected either in root cap establishment, stem cell differentiation or border cell sloughing. Interestingly, the primary roots of mutant seedlings showed impaired mechanical responses, suggesting a function of the root cap in the mecanosensing.

Oaks are an important part of our natural and cultural heritage. Not only are they ubiquitous in our most common landscapes1 but they have also supplied human societies with invaluable services, including food and shelter, since prehistoric times2. With 450 species spread throughout Asia, Europe and America3, oaks constitute a critical global renewable resource. The longevity of oaks (several hundred years) probably underlies their emblematic cultural and historical importance. Such long-lived sessile organisms must persist in the face of a wide range of abiotic and biotic threats over their lifespans. We investigated the genomic features associated with such a long lifespan by sequencing, assembling and annotating the oak genome. We then used the growing number of whole-genome sequences for plants (including tree and herbaceous species) to investigate the parallel evolution of genomic characteristics potentially underpinning tree longevity. A further consequence of the long lifespan of trees is their accumulation of somatic mutations during mitotic divisions of stem cells present in the shoot apical meristems. Empirical4 and modelling5 approaches have shown that intra-organismal genetic heterogeneity can be selected for6 and provides direct fitness benefits in the arms race with short-lived pests and pathogens through a patchwork of intra-organismal phenotypes7. However, there is no clear proof that large-statured trees consist of a genetic mosaic of clonally distinct cell lineages within and between branches. Through this case study of oak, we demonstrate the accumulation and transmission of somatic mutations and the expansion of disease-resistance gene families in trees.

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International audience

International audience; The aim of this study was to investigate ecophysiological responses of young beech seedlings to doubling of atmospheric CO2 concentration ([CO2]: 350 and 700 μmol.mol-1 air) combined with a moderate drought. Beech seedlings were grown for one year in semi controlled greenhouses under controlled [CO2] and non limiting nutrient fertilization. Under doubled [CO2], net CO2 assimilation (A) was stimulated while stomatal conductance (gw) was not affected. 15Nitrogen uptake was increased as well as the water use efficiency. Biomass accumulation of shoots and roots was significantly stimulated without a change in root:shoot ratio. The specific hydraulic conductance of shoots and hydraulic conductivity of roots was not affected by [CO2]. After 20 days of moderate drought, C assimilation was still increased in response to doubled [CO2] and stomatal conductance (gw) was not affected, while 15N uptake showed no change. Under doubled [CO2], growth of water stressed seedlings was increased more than control one (350 μmol.mol-1, well irrigated). However, the root:shoot ratio was decreased by 20% at both [CO2]. Instantaneous water use efficiency of water stressed seedlings was increased. However, WUE was unchanged when it was calculated using 13C of leaves, a second measure of water-use efficiency. The specific hydraulic conductance of shoots did not change but hydraulic conductivity of roots was decreased by drought under both CO2 treatments. [CO2] had no direct effect on stomatal functioning of beech leaves of seedlings, in contrast to results on other species such as oaks. In reforestation context, this lack of antiperspirant effect of CO2 could have major implications for survival and adaptation to drought of beech seedlings during the next decades.

National audience

International audience; In temperate zones the global change will include a decrease of summer precipitations inducing longer and more severe drought episodes. Transcriptional regulation plays a major role in regulating many biological processes, we looked for gene expression networks related to drought stress. The final aim is to identify candidate genes for water deficit acclimation and/or adaptation in poplar, a model tree species. The transcriptome of two Populus deltoides x nigra genotypes of contrasted tolerance to water deficit (Carpaccio and Soligo) were analysed in young mature leaves, at various stages and intensities of stress. Four watering treatments were applied: control (CTL) watered to field capacity six-times a day; early response to water deficit (EAR): watering was stopped 30h before harvest; and acclimated response to mild (AMI) and to moderate (AMO) water deficit. The latter two stress levels were low enough so primary and secondary growth were reduced but not stopped. Mature leaves were harvested and their transcriptome analysed using wholegenome microarray (GeneChip Poplar Genome Array, Affymetrix). The physiological responses to water deficit confirmed the higher tolerance of the Carpaccio as compared to the Soligo genotype. On the other hand, the transcriptomic results show strong cultivar specificity in optimal watering conditions as well as in water deficit treatments. Analyses of the responses allowed us to confirm potential drought markers and to identify candidate genes for drought tolerance.

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