Environmental cues influence the development of stomata on the leaf epidermis, and allow plants to exert plasticity in leaf stomatal abundance in response to the prevailing growing conditions. It is reported that Arabidopsis thaliana ‘Landsberg erecta’ plants grown under low relative humidity have a reduced stomatal index and that two genes in the stomatal development pathway, SPEECHLESS and FAMA, become de novo cytosine methylated and transcriptionally repressed. These environmentally-induced epigenetic responses were abolished in mutants lacking the capacity for de novo DNA methylation, for the maintenance of CG methylation, and in mutants for the production of short-interfering non-coding RNAs (siRNAs) in the RNA-directed DNA methylation pathway. Induction of methylation was quantitatively related to the induction of local siRNAs under low relative humidity. Our results indicate the involvement of both transcriptional and post-transcriptional gene suppression at these loci in response to environmental stress. Thus, in a physiologically important pathway, a targeted epigenetic response to a specific environmental stress is reported and several of its molecular, mechanistic components are described, providing a tractable platform for future epigenetics experiments. Our findings suggest epigenetic regulation of stomatal development that allows for anatomical and phenotypic plasticity, and may help to explain at least some of the plant’s resilience to fluctuating relative humidity.

Summary• The consequences of increasing atmospheric carbon dioxide for long-term adaptation of forest ecosystems remain uncertain, with virtually no studies undertaken at the genetic level. A global analysis using cDNA microarrays was conducted following 6 yr exposure of Populus × euramericana (clone I-214) to elevated [CO 2 ] in a FACE (free-air CO 2 enrichment) experiment.• Gene expression was sensitive to elevated [CO 2 ] but the response depended on the developmental age of the leaves, and < 50 transcripts differed significantly between different CO 2 environments. For young leaves most differentially expressed genes were upregulated in elevated [CO 2 ], while in semimature leaves most were downregulated in elevated [CO 2 ].• For transcripts related only to the small subunit of Rubisco, upregulation in LPI 3 and downregulation in LPI 6 leaves in elevated CO 2 was confirmed by ANOVA . Similar patterns of gene expression for young leaves were also confirmed independently across year 3 and year 6 microarray data, and using real-time RT-PCR.• This study provides the first clues to the long-term genetic expression changes that may occur during long-term plant response to elevated CO 2 .

Drought and heat stress cause losses in wheat productivity in major growing regions worldwide, and both the occurrence and the severity of these events are likely to increase with global climate change. Water deficits and high temperatures frequently occur simultaneously at sensitive growth stages, reducing wheat yields by reducing grain number or weight. Although genetic variation and underlying quantitative trait loci for either individual stress are known, the combination of the two stresses has rarely been studied. Complex and often antagonistic physiology means that genetic loci underlying tolerance to the combined stress are likely to differ from those for drought or heat stress tolerance alone. Here, we review what is known of the physiological traits and genetic control of drought and heat tolerance in wheat and discuss potential physiological traits to study for combined tolerance. We further place this knowledge in the context of breeding for new, more tolerant varieties and discuss opportunities and constraints. We conclude that a fine control of water relations across the growing cycle will be beneficial for combined tolerance and might be achieved through fine management of spatial and temporal gas exchange.

Traits such as transpiration efficiency (TE) that are influenced by plant water use can be used to characterise the adaptability of crops to specific growth environments. TE is defined as the amount of biomass produced per unit of water used, and can ensure continued crop production in drought-prone regions. Where TE is associated with reduced use of soil water during the vegetative growth phase, water availability during grain filling may be greater, which can delay the onset of drought stress and increase grain yield under water-limited conditions. This may become even more pertinent with predicted increases in severity and frequency of droughts with climate change. The aims of this study were to firstly dissect TE into its leaf-level physiological components to better understand the effects of genetic variation in these components on TE in sorghum. Secondly, to examine whether TE responses observed under well-watered conditions were preserved under drought, and whether transpiration response was an adaptive response to drought. Twenty-seven genotypes were screened for TE under well-watered conditions using a fully automated lysimetry platform to obtain accurate plant water use data. To determine whether variation in TE among these genotypes was associated with differences in maximum photosynthesis (Amax) or leaf conductance (g) we measured the net carbon assimilation rate of the second last fully expanded leaf at high light intensity, using an infrared gas analyser, and leaf water flux was measured using a porometer, as a proxy for conductance. Genotypic variation in TE among the sorghum germplasm used was mainly associated with differences in the response of transpiration rates to vapour pressure deficit (VPD). Genotypes with low transpiration rates per unit green leaf area (T/GLA) tended to have high TE. Variation in Amax explained some of the differences in TE that could not be explained by T/GLA and may have been a result of mechanisms associated with differences in biochemical pathways that affect the efficiency of conversion of CO2 into photosynthate. While drought tended to increase TE, genotypic variation in TE was largely conserved. However, the response of transpiration rates to drought stress differed across genotypes, with some genotypes showing reduced T/GLA under drought when VPD was high, whereas others did not. These contrasting responses were associated with differences in stomatal responses to drought stress, such that some genotypes were better able to conserve water under drought stress than others. This adaptive response was not related to TE per se and may have important implications for adaptation to drought stress. Hence, the phenotyping of sorghum lines using the associated physiological traits underpinning TE differences is beneficial in identifying traits that may support growth in certain environments and can optimise grain yield production under water-limited conditions. III

BackgroundThe plant cuticle is the outermost layer covering aerial tissues and is composed of cutin and waxes. The cuticle plays an important role in protection from environmental stresses and glaucousness, the bluish-white colouration of plant surfaces associated with cuticular waxes, has been suggested as a contributing factor in crop drought tolerance. However, the cuticle structure and composition is complex and it is not clear which aspects are important in determining a role in drought tolerance. Therefore, we analysed residual transpiration rates, cuticle structure and epicuticular wax composition under well-watered conditions and drought in five Australian bread wheat genotypes, Kukri, Excalibur, Drysdale, RAC875 and Gladius, with contrasting glaucousness and drought tolerance.ResultsSignificant differences were detected in residual transpiration rates between non-glaucous and drought-sensitive Kukri and four glaucous and drought-tolerant lines. No simple correlation was found between residual transpiration rates and the level of glaucousness among glaucous lines. Modest differences in the thickness of cuticle existed between the examined genotypes, while drought significantly increased thickness in Drysdale and RAC875. Wax composition analyses showed various amounts of C31 β-diketone among genotypes and increases in the content of alkanes under drought in all examined wheat lines.ConclusionsThe results provide new insights into the relationship between drought stress and the properties and structure of the wheat leaf cuticle. In particular, the data highlight the importance of the cuticle’s biochemical makeup, rather than a simple correlation with glaucousness or stomatal density, for water loss under limited water conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-017-1033-3) contains supplementary material, which is available to authorized users.

Leaf expansion in the fast-growing tree, Populus ϫ euramericana was stimulated by elevated [CO 2 ] in a closed-canopy forest plantation, exposed using a free air CO 2 enrichment technique enabling long-term experimentation in field conditions. The effects of elevated [CO 2 ] over time were characterized and related to the leaf plastochron index (LPI), and showed that leaf expansion was stimulated at very early (LPI, 0-3) and late (LPI, 6-8) stages in development. Early and late effects of elevated [CO 2 ] were largely the result of increased cell expansion and increased cell production, respectively. Spatial effects of elevated [CO 2 ] were also marked and increased final leaf size resulted from an effect on leaf area, but not leaf length, demonstrating changed leaf shape in response to [CO 2 ]. Leaves exhibited a basipetal gradient of leaf development, investigated by defining seven interveinal areas, with growth ceasing first at the leaf tip. Interestingly, and in contrast to other reports, no spatial differences in epidermal cell size were apparent across the lamina, whereas a clear basipetal gradient in cell production rate was found. These data suggest that the rate and timing of cell production was more important in determining leaf shape, given the constant cell size across the leaf lamina. The effect of elevated [CO 2 ] imposed on this developmental gradient suggested that leaf cell production continued longer in elevated [CO 2 ] and that basal increases in cell production rate were also more important than altered cell expansion for increased final leaf size and altered leaf shape in elevated [CO 2 ].Given the importance of forests for global bioproductivity, the consequences of increased atmospheric [CO 2 ] for the global carbon cycle are potentially extremely large (Malhi et al., 1999). Despite this, there are still relatively few large-scale, long-term experiments from which predictions about likely forest responses can be made. Few studies have been completed where trees are allowed to develop to canopy closure and where a "stable" response to [CO 2 ] is likely. Determining the response of leaf area development to elevated [CO 2 ] is important. It is still unknown whether forests of the future will maintain a higher leaf area index (LAI), as implied from smalltree studies (Ceulemans et al., 1997) or whether the long-term (decades) responses will be reduced allocation to foliage and lower LAI, as suggested by some modeling approaches (Medlyn and Dewar, 1996) or involve acclimation to limited nitrogen (Oren et al., 2001).Leaf growth is often stimulated in short-term response to elevated [CO 2 ] (Taylor et al., 1994;Pritchard et al., 1999), and both leaf cell expansion and cell production are sensitive to [CO 2 ] (Taylor et al., 1994). It is likely that these processes respond to additional carbohydrate from photosynthesis and, as such, altered atmospheric [CO 2 ] provides a critical insight into how carbon regulates plant development and growth (Masle, 2000). The importance of leaf develo...

Epigenetics has progressed rapidly from an obscure quirk of heredity into a data-heavy 'omic' science. Our understanding of the molecular mechanisms of epigenomic regulation, and the extent of its importance in nature, are far from complete, but in spite of such drawbacks, population-level studies are extremely valuable: epigenomic regulation is involved in several processes central to evolutionary biology including phenotypic plasticity, evolvability and the mediation of intragenomic conflicts. The first studies of epigenomic variation within populations suggest high levels of phenotypically relevant variation, with the patterns of epigenetic regulation varying between individuals and genome regions as well as with environment. Epigenetic mechanisms appear to function primarily as genome defences, but result in the maintenance of plasticity together with a degree of buffering of developmental programmes; periodic breakdown of epigenetic buffering could potentially cause variation in rates of phenotypic evolution.

Using a free-air CO2 enrichment (FACE) experiment, poplar trees (Populus x euramericana clone I214) were exposed to either ambient or elevated [CO2] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm(-2)) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (gs, micromol m(-2) s(-1)), photosynthetic CO2 fixation (A, mmol m(-2) s(-1)), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO2 (Ci:Ca). Elevated [CO2] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO2] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO2], gs was reduced, A/E was stimulated, and Ci:Ca was reduced relative to ambient [CO2]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO2]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO2], although the mechanism for this remains obscure.