Molecular plasticity to soil water deficit differs between sessile oak (Quercus Petraea (Matt.) Liebl.) high- and low-water use efficiency genotypes.

Water use efficiency (WUE) is an important adaptive trait for soil water deficit. The molecular and physiological bases of WUE regulation in crops have been studied in detail in the context of plant breeding. Knowledge for most forest tree species lags behind, despite the need to identify populations or genotypes able to cope with the longer, more intense drought periods likely to result from climate warming. We aimed to bridge this gap in knowledge for sessile oak (Quercus petraea (Matt.) Liebl.), one of the most ecologically and economically important tree species in Europe, using a factorial design including trees with contrasted phenotypic values (low and high WUE) and two watering regimes (control and drought). By monitoring the ecophysiological response, we first qualified genotypes for their WUE (by using instantaneous and long-term measures). We then performed RNA-seq to quantify gene expression for the three most extreme genotypes exposed to the two watering regimes. By analyzing the interaction term, we were able to capture the molecular strategy of each group of plants for coping with drought. We identified putative candidate genes potentially involved in the regulation of transpiration rate in high-WUE phenotypes. Regardless of water availability, trees from the high-WUE phenotypic class overexpressed genes associated with drought responses, and in the control of stomatal density and distribution, and displayed a downregulation of genes associated with early stomatal closure and high transpiration rate. Fine physiological screening of sessile oaks with contrasting WUE, and their molecular characterization (i) highlighted subtle differences in transcription between low- and high-WUE genotypes, identifying key molecular players in the genetic control of this trait and (ii) revealed the genes underlying the molecular strategy that evolved in each group to potentially cope with water deficit, providing new insight into the within-species diversity in drought adaptation strategies.

Recovery of silver fir (Abies alba Mill.) seedlings from ungulate browsing mirrors soil nitrogen availability.

Abies alba (Mill.) has a high potential for mitigating climate change in European mountain forests; yet, its natural regeneration is severely limited by ungulate browsing. Here, we simulated browsing in a common garden experiment to study growth and physiological traits, measured from bulk needles, using a randomized block design with two levels of browsing severity and seedlings originating from 19 populations across Switzerland. Genetic factors explained most variation in growth (on average, 51.5%) and physiological traits (10.2%) under control conditions, while heavy browsing considerably reduced the genetic effects on growth (to 30%), but doubled those on physiological traits related to carbon storage. While browsing reduced seedling height, it also lowered seedling water-use efficiency (decreased $\delta ^{13}$C) and increased their $\delta ^{15}$N. Different populations reacted differently to browsing stress, and for seedling height, starch concentration and $\delta ^{15}$N, population differences appeared to be the result of natural selection. First, we found that populations originating from the warmest regions recovered the fastest from browsing stress, and they did so by mobilizing starch from their needles, which suggests a genetic underpinning for a growth-storage trade-off across populations. Second, we found that seedlings originating from mountain populations growing on steep slopes had a higher $\delta ^{15}$N in the common garden than those originating from flat areas, indicating that they have been selected to grow on N-poor, potentially drained, soils. This finding was corroborated by the fact that nitrogen concentration in adult needles was lower on steep slopes than on flat ground, strongly indicating that steep slopes are the most N-poor environments. These results suggest that adaptation to climate and soil nitrogen availability, as well as ungulate browsing pressure, co-determine the regeneration and range limit of silver fir.

Physiological and anatomical responses to drought stress differ between two larch species and their hybrid.

KEY MESSAGE: Hybrid saplings were more reactive to soil water deficit than Japanese and European larch. European larch had hydraulically safer wood and anisohydric behavior, Japanese and hybrid larch showed isohydric strategy. ABSTRACT: Deciduous larch species could be an alternative to evergreen conifers in reforestation, but little is known about drought sensitivity of their saplings. The effect of an experimental drought on hydraulics and quantitative wood anatomy was tested on saplings of European larch (EL, Larix decidua), Japanese larch (JL, Larix kaempferi) and their hybrid (HL). Across species, biomass, transpiration rate and relative water content were higher in controls than in drought stressed trees, but transpiration efficiency was lower. JL had the highest transpiration efficiency under drought, and EL the lowest, coinciding with slower growth of EL. Wood of EL formed before drought was hydraulically safer as shown by higher wall/lumen ratio and lower pit cavity area. EL neither had a significant increase in transpiration efficiency nor a reduction in transpiration rate under drought, suggesting that the stomata remained open under soil water deficit. HL saplings were the most reactive to water shortage, indicated by intra-annual density fluctuations and a decrease in relative water content of the sapwood. Significant reduction in transpiration by HL suggested a higher stomatal sensitivity, while the same leaf surface area was maintained and radial growth was still similar to its best parent, the JL. The latter showed a significantly lower leaf surface area under drought than controls. EL, with its hydraulically safer wood, followed an anisohydric behavior, while JL and HL revealed an isohydric strategy. Altogether, our results suggest species dependent acclimations to drought stress, whereby HL followed the strategy of JL rather than that of EL. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00468-021-02129-4.

Changes in irradiance and vapour pressure deficit under drought induce distinct stomatal dynamics between glasshouse and field-grown poplars.

Recent research has shown that plant acclimation to diverse patterns of light intensity modifies the dynamics of their stomatal response. Therefore, whether plants are grown in controlled conditions or in the field may impact their stomatal dynamics. We analysed the stomatal dynamics of two Populus euramericana and two Populus nigra genotypes grown in the field under contrasting water availability. By comparing their stomatal dynamics with that of the same genotypes grown in a glasshouse, we were able to test whether differences between these growing conditions interacted with genotypic differences in affecting stomatal dynamics and responses to soil water deficit. We found that, despite higher stomatal density and smaller size, in the field stomatal dynamics were much slower than in the glasshouse. Overall, differences among genotypes and their response to soil water deficit were much less pronounced in the field compared with the glasshouse. These results indicate that stomatal dynamics are regulated by both genotype-specific and environmental factors. Moreover, having slower stomata may be advantageous under some conditions. While stomatal dynamics were linked with whole-plant transpiration per leaf area in both experiments, the contribution of stomatal morphology varies dependent on the environmental conditions.

Microgeographic adaptation and the effect of pollen flow on the adaptive potential of a temperate tree species.

In species with long-distance dispersal capacities and inhabiting a large ecological niche, local selection and gene flow are expected to be major evolutionary forces affecting the genetic adaptation of natural populations. Yet, in species such as trees, evidence of microgeographic adaptation and the quantitative assessment of the impact of gene flow on adaptive genetic variation are still limited. Here, we used extensive genetic and phenotypic data from European beech seedlings collected along an elevation gradient, and grown in a common garden, to study the signature of selection on the divergence of eleven potentially adaptive traits, and to assess the role of gene flow in resupplying adaptive genetic variation. We found a significant signal of adaptive differentiation among plots separated by < 1 km, with selection acting on growth and phenological traits. Consistent with theoretical expectations, our results suggest that pollen dispersal contributes to increase genetic diversity for these locally differentiated traits. Our results thus highlight that local selection is an important evolutionary force in natural tree populations and suggest that management interventions to facilitate movement of gametes along short ecological gradients would boost genetic diversity of individual tree populations, and enhance their adaptive potential to rapidly changing environments.

Element content and expression of genes of interest in guard cells are connected to spatiotemporal variations in stomatal conductance.

Element content and expression of genes of interest on single cell types, such as stomata, provide valuable insights into their specific physiology, improving our understanding of leaf gas exchange regulation. We investigated how far differences in stomatal conductance (gs ) can be ascribed to changes in guard cells functioning in amphistomateous leaves. gs was measured during the day on both leaf sides, on well-watered and drought-stressed trees (two Populus euramericana Moench and two Populus nigra L. genotypes). In parallel, guard cells were dissected for element content and gene expressions analyses. Both were strongly arranged according to genotype, and drought had the lowest impact overall. Normalizing the data by genotype highlighted a structure on the basis of leaf sides and time of day both for element content and gene expression. Guard cells magnesium, phosphorus, and chlorine were the most abundant on the abaxial side in the morning, where gs was at the highest. In contrast, genes encoding H+ -ATPase and aquaporins were usually more abundant in the afternoon, whereas genes encoding Ca2+ -vacuolar antiporters, K+ channels, and ABA-related genes were in general more abundant on the adaxial side. Our work highlights the unique physiology of each leaf side and their analogous rhythmicity through the day.

Robust Response of Terrestrial Plants to Rising CO2.

Human-caused CO2 emissions over the past century have caused the climate of the Earth to warm and have directly impacted on the functioning of terrestrial plants. We examine the global response of terrestrial gross primary production (GPP) to the historic change in atmospheric CO2. The GPP of the terrestrial biosphere has increased steadily, keeping pace remarkably in proportion to the rise in atmospheric CO2. Water-use efficiency, namely the ratio of CO2 uptake by photosynthesis to water loss by transpiration, has increased as a direct leaf-level effect of rising CO2. This has allowed an increase in global leaf area, which has conspired with stimulation of photosynthesis per unit leaf area to produce a maximal response of the terrestrial biosphere to rising atmospheric CO2 and contemporary climate change.

Accelerated flowering time reduces lifetime water use without penalizing reproductive performance in Arabidopsis.

Natural selection driven by water availability has resulted in considerable variation for traits associated with drought tolerance and leaf-level water-use efficiency (WUE). In Arabidopsis, little is known about the variation of whole-plant water use (PWU) and whole-plant WUE (transpiration efficiency). To investigate the genetic basis of PWU, we developed a novel proxy trait by combining flowering time and rosette water use to estimate lifetime PWU. We validated its usefulness for large-scale screening of mapping populations in a subset of ecotypes. This parameter subsequently facilitated the screening of water use and drought tolerance traits in a recombinant inbred line population derived from two Arabidopsis accessions with distinct water-use strategies, namely, C24 (low PWU) and Col-0 (high PWU). Subsequent quantitative trait loci mapping and validation through near-isogenic lines identified two causal quantitative trait loci, which showed that a combination of weak and nonfunctional alleles of the FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) genes substantially reduced plant water use due to their control of flowering time. Crucially, we observed that reducing flowering time and consequently water use did not penalize reproductive performance, as such water productivity (seed produced per unit of water transpired) improved. Natural polymorphisms of FRI and FLC have previously been elucidated as key determinants of natural variation in intrinsic WUE (δ13 C). However, in the genetic backgrounds tested here, drought tolerance traits, stomatal conductance, δ13 C. and rosette water use were independent of allelic variation at FRI and FLC, suggesting that flowering is critical in determining lifetime PWU but not always leaf-level traits.

Altered stomatal dynamics induced by changes in irradiance and vapour-pressure deficit under drought: impacts on the whole-plant transpiration efficiency of poplar genotypes.

Recent findings were able to show significant variability of stomatal dynamics between species, but not much is known about factors influencing stomatal dynamics and its consequences on biomass production, transpiration and water-use efficiency (WUE). We assessed the dynamics of stomatal conductance (gs ) to a change of irradiance or vapour-pressure deficit (VPD) in two Populus euramericana and two Populus nigra genotypes grown under control and drought conditions. Our objectives were to determine the diversity of stomatal dynamics among poplar genotypes, and if soil water deficit can alter it. Physiological and morphological factors were investigated to find their potential links with stomatal morphology, WUE and its components at the whole-plant level. We found significant genotypic variability of gs dynamics to both irradiance and VPD. Genotypes with faster stomatal dynamics were correlated with higher stomatal density and smaller stomata, and the implications of these correlations are discussed. Drought slowed gs dynamics, depending on genotype and especially during stomatal closing. This finding is contrary to previous research on more drought-tolerant species. Independently of the treatment, faster stomatal dynamics were negatively correlated with daily whole-plant transpiration, presenting new evidence of a previously hypothesized contribution of stomatal dynamics to whole-plant water use.

Natural variation of life-history traits, water use, and drought responses in Arabidopsis.

The ability of plants to acquire and use water is critical in determining life-history traits such as growth, flowering, and allocation of biomass into reproduction. In this context, a combination of functionally linked traits is essential for plants to respond to environmental changes in a coordinated fashion to maximize resource use efficiency. We analyzed different water-use traits in Arabidopsis ecotypes to identify functionally linked traits that determine water use and plant growth performance. Water-use traits measured were (i) leaf-level water-use efficiency (WUE i ) to evaluate the amount of CO 2 fixed relative to water loss per leaf area and (ii) short-term plant water use at the vegetative stage (VWU) as a measure of whole-plant transpiration. Previously observed phenotypic variance in VWU, WUE i and life-history parameters, highlighted C24 as a valuable ecotype that combined drought tolerance, preferential reproductive biomass allocation, high WUE i , and reduced water use. We therefore screened 35 Arabidopsis ecotypes for these parameters, in order to assess whether the phenotypic combinations observed in C24 existed more widely within Arabidopsis ecotypes. All parameters were measured on a short dehydration cycle. A segmented regression analysis was carried out to evaluate the plasticity of the drought response and identified the breakpoint as a reliable measure of drought sensitivity. VWU was largely dependent on rosette area, but importantly the drought sensitivity and plasticity measures were independent of the transpiring leaf surface. A breakpoint at high rSWC indicated a more drought-sensitive plant that closed stomata early during the dehydration cycle and consequently showed stronger plasticity in leaf-level WUE i parameters. None of the sensitivity, plasticity, or water-use measurements were able to predict the overall growth performance; however, there was a general trade-off between vegetative and reproductive biomass. PCA and hierarchical clustering revealed that C24 was unique among the 35 ecotypes in uniting all the beneficial water use and stress tolerance traits, while also maintaining above average plant growth. We propose that a short dehydration cycle, measuring drought sensitivity and VWU is a fast and reliable screen for plant water use and drought response strategies.

The expression of VE-cadherin in breast cancer cells modulates cell dynamics as a function of tumor differentiation and promotes tumor-endothelial cell interactions.

The cadherin switch has profound consequences on cancer invasion and metastasis. The endothelial-specific vascular endothelial cadherin (VE-cadherin) has been demonstrated in diverse cancer types including breast cancer and is supposed to modulate tumor progression and metastasis, but underlying mechanisms need to be better understood. First, we evaluated VE-cadherin expression by tissue microarray in 392 cases of breast cancer tumors and found a diverse expression and distribution of VE-cadherin. Experimental expression of fluorescence-tagged VE-cadherin (VE-EGFP) in undifferentiated, fibroblastoid and E-cadherin-negative MDA-231 (MDA-VE-EGFP) as well as in differentiated E-cadherin-positive MCF-7 human breast cancer cell lines (MCF-VE-EGFP), respectively, displayed differentiation-dependent functional differences. VE-EGFP expression reversed the fibroblastoid MDA-231 cells to an epithelial-like phenotype accompanied by increased ß-catenin expression, actin and vimentin remodeling, increased cell spreading and barrier function and a reduced migration ability due to formation of VE-cadherin-mediated cell junctions. The effects were largely absent in both MDA-VE-EGFP and in control MCF-EGFP cell lines. However, MCF-7 cells displayed a VE-cadherin-independent planar cell polarity and directed cell migration that both developed in MDA-231 only after VE-EGFP expression. Furthermore, VE-cadherin expression had no effect on tumor cell proliferation in monocultures while co-culturing with endothelial cells enhanced tumor cell proliferation due to integration of the tumor cells into monolayer where they form VE-cadherin-mediated cell contacts with the endothelium. We propose an interactive VE-cadherin-based crosstalk that might activate proliferation-promoting signals. Together, our study shows a VE-cadherin-mediated cell dynamics and an endothelial-dependent proliferation in a differentiation-dependent manner.

Genes and gene clusters related to genotype and drought-induced variation in saccharification potential, lignin content and wood anatomical traits in Populus nigra.

Wood is a renewable resource that can be employed for the production of second generation biofuels by enzymatic saccharification and subsequent fermentation. Knowledge on how the saccharification potential is affected by genotype-related variation of wood traits and drought is scarce. Here, we used three Populus nigra L. genotypes from habitats differing in water availability to (i) investigate the relationships between wood anatomy, lignin content and saccharification and (ii) identify genes and co-expressed gene clusters related to genotype and drought-induced variation in wood traits and saccharification potential. The three poplar genotypes differed in wood anatomy, lignin content and saccharification potential. Drought resulted in reduced cambial activity, decreased vessel and fiber lumina, and increased the saccharification potential. The saccharification potential was unrelated to lignin content as well as to most wood anatomical traits. RNA sequencing of the developing xylem revealed that 1.5% of the analyzed genes were differentially expressed in response to drought, while 67% differed among the genotypes. Weighted gene correlation network analysis identified modules of co-expressed genes correlated with saccharification potential. These modules were enriched in gene ontology terms related to cell wall polysaccharide biosynthesis and modification and vesicle transport, but not to lignin biosynthesis. Among the most strongly saccharification-correlated genes, those with regulatory functions, especially kinases, were prominent. We further identified transcription factors whose transcript abundances differed among genotypes, and which were co-regulated with genes for biosynthesis and modifications of hemicelluloses and pectin. Overall, our study suggests that the regulation of pectin and hemicellulose metabolism is a promising target for improving wood quality of second generation bioenergy crops. The causal relationship of the identified genes and pathways with saccharification potential needs to be validated in further experiments.

Effects of kinetics of light-induced stomatal responses on photosynthesis and water-use efficiency.

Both photosynthesis (A) and stomatal conductance (gs ) respond to changing irradiance, yet stomatal responses are an order of magnitude slower than photosynthesis, resulting in noncoordination between A and gs in dynamic light environments. Infrared gas exchange analysis was used to examine the temporal responses and coordination of A and gs to a step increase and decrease in light in a range of different species, and the impact on intrinsic water use efficiency was evaluated. The temporal responses revealed a large range of strategies to save water or maximize photosynthesis in the different species used in this study but also displayed an uncoupling of A and gs in most of the species. The shape of the guard cells influenced the rapidity of response and the overall gs values achieved, with different impacts on A and Wi . The rapidity of gs in dumbbell-shaped guard cells could be attributed to size, whilst in elliptical-shaped guard cells features other than anatomy were more important for kinetics. Our findings suggest significant variation in the rapidity of stomatal responses amongst species, providing a novel target for improving photosynthesis and water use.

Genetic architecture of carbon isotope composition and growth in Eucalyptus across multiple environments.

In the context of climate change, the water-use efficiency (WUE) of highly productive tree varieties, such as eucalypts, has become a major issue for breeding programmes. This study set out to dissect the genetic architecture of carbon isotope composition (δ(13) C), a proxy of WUE, across several environments. A family of Eucalyptus urophylla × E. grandis was planted in three trials and phenotyped for δ(13) C and growth traits. High-resolution genetic maps enabled us to target genomic regions underlying δ(13) C quantitative trait loci (QTLs) on the E. grandis genome. Of the 15 QTLs identified for δ(13) C, nine were stable across the environments and three displayed significant QTL-by-environment interaction, suggesting medium to high genetic determinism for this trait. Only one colocalization was found between growth and δ(13) C. Gene ontology (GO) term enrichment analysis suggested candidate genes related to foliar δ(13) C, including two involved in the regulation of stomatal movements. This study provides the first report of the genetic architecture of δ(13) C and its relation to growth in Eucalyptus. The low correlations found between the two traits at phenotypic and genetic levels suggest the possibility of improving the WUE of Eucalyptus varieties without having an impact on breeding for growth.

Vapour pressure deficit during growth has little impact on genotypic differences of transpiration efficiency at leaf and whole-plant level: an example from Populus nigra†L.

Poplar genotypes differ in transpiration efficiency (TE) at leaf and whole-plant level under similar conditions. We tested whether atmospheric vapour pressure deficit (VPD) affected TE to the same extent across genotypes. Six Populus nigra genotypes were grown under two VPD. We recorded (1) (13)C content in soluble sugars; (2) (18)O enrichment in leaf water; (3) leaf-level gas exchange; and (4) whole-plant biomass accumulation and water use. Whole-plant and intrinsic leaf TE and (13)C content in soluble sugars differed significantly among genotypes. Stomatal conductance contributed more to these differences than net CO2 assimilation rate. VPD increased water use and reduced whole-plant TE. It increased intrinsic leaf-level TE due to a decline in stomatal conductance. It also promoted higher (18)O enrichment in leaf water. VPD had no genotype-specific effect. We detected a deviation in the relationship between (13)C in leaf sugars and (13)C predicted from gas exchange and the standard discrimination model. This may be partly due to genotypic differences in mesophyll conductance, and to its lack of sensitivity to VPD. Leaf-level (13)C discrimination was a powerful predictor of the genetic variability of whole-plant TE irrespective of VPD during growth.

The genetics of water-use efficiency and its relation to growth in maritime pine.

To meet the increasing demand of wood biomass worldwide in the context of climate change, developing improved forest tree varieties for high productivity in water-limited conditions is becoming a major issue. This involves breeding for genotypes combining high growth and moderate water loss and thus high water-use efficiency (WUE). The present work provides original data about the genetics of intrinsic WUE (the ratio between net CO2 assimilation rate and stomatal conductance, also estimated by carbon isotope composition of plant material; δ(13)C) and its relation to growth in Pinus pinaster Ait. First, heritability for δ(13)C was estimated (0.29) using a 15-year-old progeny trial (Landes provenance), with no significant differences among three sites contrasting in water availability. High intersite correlations (0.63-0.91) and significant but low genotype-environment interactions were detected. Secondly, the genetic architectures of δ(13)C and growth were studied in a three-generation inbred pedigree, introducing the genetic background of a more-drought-adapted parent (Corsican provenance), at ages of 2 years (greenhouse) and 9 years (plantation). One of the quantitative trait loci (QTLs) identified in the field experiment, explaining 67% of the phenotypic variance, was also found among the QTLs detected in the greenhouse experiment, where it colocalized with QTLs for intrinsic WUE and stomatal conductance. This work was able to show that higher WUE was not genetically linked to less growth, allowing thus genetic improvement of water use. As far as is known, the heritability and QTL effects estimated here are based on the highest number of genotypes measured to date.

Is the coefficient of variation a valid measure for variability of stable isotope abundances in biological materials?

RATIONALE: The isotopic compositions of low-mass elements such as carbon are commonly expressed on the δ scale, relative to the isotopic ratio of an international standard (RST). Carbon stable isotope measurements of plant material are interpreted with an apparently biological meaning as an estimator of water use efficiency, and this has been used widely in recent years to screen plant material for variation. To compare the observed variability with other traits, the coefficient of variation (CV) is often used as an intrinsic variability measure. METHODS: Theoretical considerations as well as an example dataset were used to test the independence of the CV from the RST on which the values of isotopic composition on the δ scale are based. RESULTS: It can be shown mathematically that the CV of a dataset of δ(13)C values is directly dependent on RST. The exploration of the example dataset confirmed this but also showed that the conversion of absolute isotopic ratios into atom fraction does not change the resulting CV. Similarly, a discrimination calculated between two δ(13)C values is independent of RST, but depends on accurate knowledge of both δ(13)C values. It was also shown that results of statistics (e.g. ANOVA) are robust among different units of isotopic composition. CONCLUSIONS: As CVs estimated from isotopic compositions expressed on the δ scale depend on the underlying standard, they should not be compared with those of other traits, and even comparisons of CVs among isotopic datasets should be interpreted with care.

Performance of a new dynamic model for predicting diurnal time courses of stomatal conductance at the leaf level.

Under natural conditions, plants are subjected to continuous changes of irradiance that drive variations of stomatal conductance to water vapour (g(s)). We propose a dynamic model to predict the temporal response of g(s) at the leaf level using an asymmetric sigmoid function with a unique parameter describing time constants for increasing and decreasing g(s). The model parameters were adjusted to observed data using Approximate Bayesian Computation. We tested the model performance for (1) instant changes of irradiance; or (2) continuous and controlled variations of irradiance simulating diurnal time courses. Compared with the two mostly used steady-state models, our dynamic model described daily time courses of g(s) with a higher accuracy. In particular, it was able to describe the hysteresis of g(s) responses to increasing/decreasing irradiance and the resulting rapid variations of intrinsic water-use efficiency. Compared to the mechanistic model of temporal responses of g(s) by Kirschbaum, Gross & Pearcy, for which time constants were estimated with a large variance, our model estimated time constants with a higher precision. It is expected to improve predictions of water loss and water-use efficiency in higher scale models by using a small number of parameters.