Illuminated leaves assimilate CO 2 via gross photosynthesis and liberate CO 2 via photorespiration and ‘day respiration’, often denoted as R d. Day respiration is a minor CO 2-exchange component of net photosynthesis but is important for carbon use efficiency, computations of internal conductance, or the interpretation of net photosynthetic 12C/ 13C fractionation. Unfortunately, there is no simple method to measure R d and tracing the origin of C-atoms found in day-respired CO 2 is difficult. As a result, a common misconception is that day respiration is simply a catabolic, CO 2-producing flux through ordinary catabolism (glycolysis and Krebs ‘cycle’). In the past few years, considerable progress has been made in our understanding of day respiration. It appears that R d is a net flux resulting from several CO 2-generating and CO 2-fixing reactions, not only related to catabolism but also to anabolism (biosyntheses). In addition, there is now direct evidence that decarboxylating reactions are partly fed by carbon sources disconnected from current photosynthesis and this effect has consequences for isotopic mass-balance. Therefore, leaf day ‘respiration’ is much more than just CO 2 production by respiratory catabolism. Rather, it reflects whole metabolic orchestration of leaves from N fixation to secondary metabolism and it perhaps deserves another name, such as “day decarboxylations”.

While dynamic regulation of photosynthesis in fluctuating light is increasingly recognized as an important driver of carbon uptake, acclimation to realistic patterns of light fluctuations is still largely unexplored. We subjected Arabidopsis thaliana (L.) plants to light fluctuations with distinct amplitudes and average irradiance. Using gas-exchange and thermal imaging, we examined how light fluctuations affected leaf structure, photosynthetic capacity, and water relations. We found a wider amplitude of fluctuations produced a stronger acclimation response. Large reductions of leaf mass per area under fluctuating light framed our interpretation of changes in pigment content and photosynthetic capacity, in that photosynthetic investment increased markedly on a mass basis, but only a little on an area basis. Moreover, leaf transpiration rose sharply, being nearly four times under fluctuating light. Our findings indicate that leaves growing under fluctuating light, although thinner, maintained their photosynthetic capacity; suggesting their photosynthesis may be more cost-efficient than those under steady light, but overall may incur increased maintenance costs. This is especially relevant for plant performance globally, because naturally fluctuating light created conflicting acclimation cues for photosynthesis and transpiration that may hinder progress towards ensuring food security under climate-related extremes of water stress.

Singlet Oxygen (SO) is among the most potent reactive oxygen species, and readily oxidizes proteins, lipids, and DNA. It can be generated at the plant surface by phototoxins in the epidermis, acting as a direct defense against pathogens and herbivores (including humans). SO can also accumulate within mitochondria, peroxisomes, cytosol, and the nucleus through multiple enzymatic and non-enzymatic processes. However, the primary location of SO in plants is in the chloroplast, where it results from transfer of light energy from PhotosystemII to triplet oxygen. SO accumulates in response to diverse stresses that perturb chloroplast metabolism, and while its short half-life precludes exiting the chloroplast, it participates in retrograde signaling through the EXECUTER1 sensor, generation of carotenoid metabolites, and possibly other unknown pathways. SO thereby reprograms nuclear gene expression and modulates hormone signaling and programmed cell death. While SO signaling has long been known to regulate plant responses to high-light stress, recent literature also suggests a role in plant interactions with insects, bacteria, and fungi. The goals of this review are to provide a brief overview of SO, summarize evidence for its involvement in biotic stress responses, and discuss future directions for the study of SO in signaling and defense.

Drought threatens plant growth and related ecosystem services. The emergence of plant drought stress under edaphic drought is well studied, whilst the importance of atmospheric drought only recently gained momentum. Yet, little is known about the interaction and relative contribution of edaphic and atmospheric drought on the emergence of plant drought stress. We conducted a gradient experiment, fully crossing gravimetric water content (GWC: field capacity-permanent wilting point) and vapour pressure deficit (VPD: 1-2.25kPa) using five wheat varieties from three species ( Triticum monococcum, T. durum & T. aestivum). We quantified the emergence of plant drought stress on molecular (ABA), cellular (stomatal conductance), organ (leaf water potential) and stand level (evapotranspiration). Plant drought stress increased with decreasing GWC across all organisational levels. This effect was magnified non-linearly by VPD after passing a critical threshold of soil water availability. At around 20% GWC plants lost their ability to regulate leaf water potential via stomata regulation, followed by the emergence of hydraulic dysfunction. The emergence of plant drought stress is characterized by changing relative contributions of soil vs. atmosphere and their non-linear interaction. This highly non-linear response, consequently, is likely to abruptly alter plant-related ecosystem services in a drying world.

Due to their stationery nature, plants are exposed to a diverse range of biotic and abiotic stresses, of which heavy metals stress poses as one of the most detrimental abiotic stresses, targeting crucial and vital processes. Heavy metals instigate the over-production of reactive oxygen species (ROS), and in order to mitigate the adverse effects of ROS, plants induce multiple defence mechanisms. Besides the negative implications of overproduction of ROS, these molecules play a multitude of signaling roles in plants, acting as a central player in the complex signaling network of cells. One of the signaling mechanisms it is involved in is the mitogen-activated protein kinase (MAPK) cascade, a signaling pathway used to transduce extracellular stimuli into intracellular responses. Plant MAPKs have been implicated in signaling of stresses, phytohormones and cell cycle cues. However, the influence of various heavy metals on MAPKs activation has not been well documented. In this review, we will attempt to address and summarize several aspects related to various heavy metal-induced ROS signaling, how these signals activate the MAPK cascade and the downstream transcription factors that instigates the plants response to these heavy metals. Moreover, we will highlight a modern research methodology that could characterize the novel genes associated with MAPKs and their roles in heavy metal stress.

Summary: The hindrance of kernel development caused by waterlogging stress (WS) is the direct reason for the peanut yield reduction. Currently, the mechanism of kernel filling responding to WS remains unknown. The waterlogging-sensitive variety Huayu 39 was subjected to WS for 3 days after 7 days after gynophores touched the ground (DAG), and the key stage of WS affecting kernel filling is 14, 21, and 28 DAG. WS decreased the average filling rate and kernel dry weight. Therefore, transcriptome sequencing and widely-targeted metabolomic analysis were conducted on kernel to elucidate the mechanism for the decrease in average filling rate under WS, revealing that overexpression of the gene encoding tryptophan decarboxylase ( AhTDC), which caused the accumulation of melatonin, reduced kernel weight. The sucrose transformation rate was limited by the crosstalk between melatonin and ethylene, thereby reducing the kernel filling rate and hindering kernel development. Our results are crucial for formulating measures to alleviate the negative impact of WS on peanut yield and quality, providing a basis for exploring high-yield and high-quality cultivation, molecular-assisted breeding, and waterlogging prevention.

With continued global warming, plants are forecast to increasingly experience abiotic stress(es). Stomata on leaf surfaces are the gatekeepers to plant interiors, regulating gaseous exchanges that are crucial for both photosynthesis and outward water release. To optimise future productivity, accurate modelling of how stomata govern plant-environment interactions will be crucial. Here, we synergise optical and thermal imaging data to enhance transpiration modelling during water and/or nitrogen stress. By utilising hyperspectral data and partial least squares regression analysis of six plant traits and fluxes in wheat ( Triticum aestivum), we have developed a new spectral vegetation index; the combined nitrogen and drought index (CNDI), which can be used to detect both water stress and/or nitrogen deficiency. Upon full stomatal closure during drought, CNDI reduces as leaf biochemistry changes unfold, and during a combined stress experiment (drought and nitrogen deficiency), this is reflected in CNDI showing a strong relationship with leaf relative water content ( r2 = 0.70). By incorporating CNDI transformed with a sigmoid function into thermal-based transpiration modelling, we have increased the accuracy of modelling water fluxes during abiotic stress. If employed using future remote sensing technologies, our findings have the potential to markedly improve agricultural water usage and yields.

In plants, salicylic acid (SA) hydroxylation regulates SA homoeostasis, playing an essential role during plant development and response to pathogens. This reaction is catalyzed by SA hydroxylase enzymes, which hydroxylate SA producing 2,3- dihydroxybenzoic acid (2,3-DHBA) and/or 2,5-dihydroxybenzoic acid (2,5-DHBA). Several SA hydroxylases have been recently identified and characterized from different plant species; however, no such activity has been previously reported in maize. In this work, we describe the identification and characterization of a new SA hydroxylase in maize plants. This enzyme, with high sequence similarity to previously described SA hydroxylases from Arabidopsis and rice, converts SA into 2,5-DHBA; however, it shows different kinetics properties to those from previously characterized enzymes, and it also catalyzes the conversion of the flavonoid dihydroquercetin into quercetin in in vitro activity assays, suggesting that the maize enzyme may have different roles in vivo as those previously reported from other species. Despite this, ZmS5H can complement the resistance to pathogen and early senescence phenotypes of Arabidopsis s3h mutant plants. Finally, we characterized a maize mutant in the S5H gene ( s5hMu) that has altered growth, senescence and increased resistance against Colletotrichum graminicola infection, showing not only changes in SA and 2,5-DHBA but also variations in flavonol levels. Together, the results presented here provide evidence that SA hydroxylases in different plant species have evolved to show differences in catalytic properties that may be important to fine tune SA levels and other phenolic compounds such as flavonols to regulate different aspects of plant development and defense against pathogens.

Day respiration ( R d) is the metabolic, non-photorespiratory process by which illuminated leaves liberate CO 2 during photosynthesis. R d is used routinely in photosynthetic models and is thus critical for calculations. However, metabolic details associated with R d are poorly known, and this can be problematic to predict how R d changes with environmental conditions and relates to night respiration. It is often assumed that day respiratory CO 2 release just reflects ‘ordinary’ catabolism (glycolysis and Krebs ‘cycle’). Here, we carried out a pulse-chase experiment, whereby a 13CO 2 pulse in the light was followed by a chase period in darkness and then in the light. We took advantage of non-targeted, isotope-assisted metabolomics to determine non-‘ordinary’ metabolism, detect carbon remobilisation, and compare light and dark 13C utilisation. We found that several concurrent metabolic pathways (‘ordinary’ catabolism, oxidative pentose phosphates pathway, amino acid production, nucleotide biosynthesis, and secondary metabolism) took place in the light and participate in net CO 2 efflux associated with day respiration. Flux reconstruction from metabolomics leads to an underestimation of R d, further suggesting the contribution of a variety of CO 2-evolving processes. Also, the cornerstone of the Krebs ‘cycle’, citrate, is synthetised de novo from photosynthates mostly in darkness, and remobilised or synthesised from stored material in the light. Collectively, our data provides direct evidence that leaf day respiration ( i) involves several CO 2-producing reactions and ( ii) is fed by different carbon sources, including stored carbon disconnected from current photosynthates.

Wood serves crucial functions in plants, yet our understanding of the molecular regulation governing the composition, arrangement, and dimensions of its cells remains limited. The abrupt change in wood anatomy of lianas represents an excellent model to address the underlying mechanism, although consistent triggering factors for this process remain uncertain. In this study we examined how physical support attachment impacts the development of lianescent xylem anatomy in Bignonia magnifica (Bignoniaceae), employing a comprehensive approach integrating detailed anatomical analysis with gene expression profiling of cambium and differentiating xylem. Our findings demonstrate that attachment to physical supports triggers the formation of lianescent xylem, leading to increased vessel size, range of vessel sizes, broader vessel distribution, reduced fiber content, and higher potential specific water conductivity. These shifts in wood anatomy coincide with the downregulation of genes associated with cell division and cell wall biosynthesis, and the upregulation of transcription factors (TFs), defense/cell death, and hormone-responsive genes in the lianescent xylem. Based on our results, we propose a model delineating the molecular control underlying the formation of lianescent xylem, revealing how the increased complexity of lianescent anatomy reflects a more intricate transcriptional regulatory network encompassing a more diverse repertoire of TFs and hormone-responsive genes.

RNA editing is a tightly controlled process by which cytidines are converted to uridines in RNAs transcribed from the chloroplast and mitochondrial genomes in flowering plants. Multiple organellar RNA editing factor (MORF) complex was recently shown to be highly associated with C-to-U RNA editing activity of vascular plant editosome. However, mechanisms by which MORF9 mediates plastid RNA editing to control plant development in response to environmental cues remains obscure. In this study, we found that loss of MORF9 function impaired PSII efficiency, NDH activity, and carbohydrate production, rapidly promoted nuclear gene expression including sucrose transporter and sugar/energy responsive genes, and attenuated seedling development under sugar starvation conditions. Sugar repletion increased MORF9 and MORF2 expression in wild-type seedlings and promoted inefficiency of matK-706C, accD-794C, ndhD-383C and ndhF-290C RNA editing in morf9 mutant. This RNA editing inefficiency was associated with altered cell division in root meristem zone and nuclear gene expression in the morf9 mutant. Using gin2, snrk1, morf9 single and double mutants and overexpression of SnRK1 (KIN10) or HXK1 in the morf9 mutant background demonstrated that RNA editing efficiency of ndhD-383C and ndhF-290C sites was diminished in the gin2/morf9 double mutants, and editing efficiency of matK-706C, accD-794C, ndhD-383C and ndhF-290C sites was significantly diminished in the snrk1/morf9 double mutants. Overexpressing HXK1 or SnRK1 promoted RNA editing rate of matK, accD, ndhD, and ndhF in leaves of morf9 mutants，indicating that HXK1 might be required for MORF9 mediated ndhD-383C and ndhF-290C editing, while SnRK1 may only be required for MORF9 mediated ndhF-290C site editing. Collectively these findings suggest that sugar and/or its intermediary metabolites impair MORF9 mediated plastid RNA editing resulting in derangements of plant root development.

Carbonate-rich soils limit plant performance and crop production. Previously, local adaptation to carbonated soils was detected in wild Arabidopsis thaliana accessions, allowing the selection of two demes with contrasting phenotypes: A1 (carbonate tolerant, c+) and T6 (carbonate sensitive, c-). Here, A1 (c+) and T6 (c-) seedlings were grown hydroponically under control (pH 5.9) and bicarbonate conditions (10 mM NaHCO 3, pH 8.3) to obtain ionomic profiles and conduct transcriptomic analysis. In parallel, A1 (c+) and T6 (c-) parental lines and their progeny were cultivated on carbonated soil to evaluate fitness and segregation patterns. To understand the genetic architecture beyond the contrasted phenotypes a bulk segregant analysis sequencing (BSA-Seq) was performed. Transcriptomics revealed 208 root and 2503 leaf differentially expressed genes (DEGs) in A1 (c+) vs T6 (c-) comparison under bicarbonate stress, mainly involved in iron, nitrogen and carbon metabolism, hormones, and glycosylates biosynthesis. Based on A1 (c+) and T6 (c-) genome contrasts and BSA-Seq analysis, 69 genes were associated with carbonate tolerance. Comparative analysis of genomics and transcriptomics discovered a final set of 18 genes involved in bicarbonate stress responses that may have relevant roles in soil carbonate tolerance.

The hypersensitive response (HR) characterizes monogenic qualitative resistance traits in several pathosystems. Knowledge on its role in plant resistance to insects is so far limited to a few resistance ( R) gene-based resistances against piercing-sucking insects. Egg deposition by cabbage white butterflies ( Pieris spp.), pests of cabbage crops ( Brassica spp.), can trigger an HR-like cell death, which reduces egg survival and represents an effective plant resistance trait before feeding damage occurs. Here, we identified natural variation of HR-like to Pieris egg deposition in the black mustard ( B. nigra L.) and performed genetic mapping. HR-like segregated as a Mendelian trait and a single dominant locus on chromosome B3, named PEK ( P ieris e gg- k illing) was identified. In the ~50 kb region, eleven candidate genes, are located, including a cluster of genes encoding intracellular receptor proteins, TIR-NBS-LRR (TNLs). The PEK locus was found to be highly polymorphic between the parental accessions of our mapping populations and among B. nigra reference genomes. Our study is the first that identifies a single locus potentially involved in HR-like cell death induced by insect eggs. Further fine-mapping, comparative genomics and validation of the PEK locus will shed light on the role of TNL receptors in egg-induced HR-like cell death.

Climate change is often associated with increasing vapor pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil‒atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (g br), transpiration rate (E) and net photosynthesis (A net) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g br to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the g br and E response to VPD. These results demonstrate that avoiding drought on the supply side (soil moisture) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.

A stoma forms by a series of asymmetric divisions of a stomatal lineage precursor cell and the terminal division of a guard mother cell (GMC). The symmetric division of the GMC is rigidly restricted to only once through complex genetic regulation mechanisms. Here, we show that nitric oxide (NO) is involved in the regulation of the GMC terminal division. NO donor treatment results in the formation of single guard cells (SGCs). SGCs are also produced in plants that accumulate high NO, whereas clustered guard cells (GCs) appear in plants with low NO accumulation. NO treatment promotes the formation of SGCs in the stomatal cell signaling mutants sdd1, epf1 epf2, tmm1, erl2 and yda-1, reduces the cell number per stomatal cluster in the fama-1 and flp-1 myb88, but has no effect on stomatal cells of cdkb1;1 cyca2;2 cyca2;3 cyca2;4 quadruple mutants. Aminocyclopropane-1-carboxylic acid (ACC), a positive regulator of GMC division, reduces the NO-induced SGC formation. Further investigation found that NO inhibits ACC synthesis by repressing the expression of several ACC SYNTHASE ( ACS) genes, and in turn ACC represses NO accumulation by promoting the expression of HEMOGLOBIN 1 ( HB1) which encodes a NO scavenger. This work shows that NO plays a role in the regulation of the GMC terminal division by modulating ACC accumulation in the Arabidopsis cotyledon.

Examining how plant traits respond to and affect herbivory is a common approach to exploring plant-herbivore interactions and their impact on ecosystem processes and functioning. Despite plants being potentially exposed to both vertebrate and invertebrate herbivores simultaneously, fundamental differences in the ecology and evolution of these two herbivore guilds results in them often being studied separately. A synthesis of the literature is needed to understand the types of plant traits examined and their response to, and effect on (in terms of forage selection) vertebrate and invertebrate herbivory, and to identify associated knowledge gaps. Focusing on grassland systems and species, we found 139 articles that met our criteria: 40 invertebrate, 97 vertebrate and 2 focussed on both vertebrate and invertebrate herbivores. Invertebrate focussed research, research conducted in the Southern Hemisphere and research on non-domesticated herbivores was significantly underrepresented based on our search. Differences in study focus (trait response or trait affect), along with considerable differences in the types of traits examined, led to limited capacity for comparison between the two herbivore guilds. For both invertebrates and vertebrates however, plant traits related to growth, such as leaf nitrogen and photosynthetic capacity, were often positively associated with herbivory. Future research should prioritise understanding how invertebrates, and the combined impact of both invertebrates and vertebrates’ respond to and affect plant traits. This review can be used as a guide for future research to select plant traits which are commonly measured either within one, or across both guild/s, as to help improve comparability and the broader significance of results, while also extending research breadth and knowledge.

Hydathodes are usually associated with water exudation in plants. However, foliar water uptake (FWU) through the hydathodes has long been suspected in the leaf-succulent genus Crassula (Crassulaceae), a highly diverse group in southern Africa, and, to our knowledge, no empirical observations exist in the literature that unequivocally link FWU to hydathodes in this genus. FWU is expected to be particularly beneficial on the arid western side of southern Africa, where up to 50% of Crassula species occur and where periodically high air humidity leads to fog and/or dew formation. To investigate if FWU is operational in different Crassula species we used the apoplastic fluorescent tracer Lucifer Yellow in combination with different imaging techniques. Our images of dye-treated leaves confirm that hydathode-mediated FWU does indeed occur in Crassula and that it is probably widespread across the genus. Hydathodes in Crassula have been repurposed as moisture-harvesting structures, besides their more common purpose of guttation, an adaptation that has likely played an important role in the evolutionary history of the genus. Our observations suggest that FWU ability is independent of geographical distribution and its associated environmental conditions, as FWU is possible in species occurring within the fog belt of western southern Africa but also in those from the rather humid eastern side. We did not find a strong apparent link between FWU ability and leaf surface wettability. Instead, the hierarchically sculptured leaf surfaces of several Crassula species may facilitate FWU due to hydrophilic leaf surface microdomains, even in seemingly hydrophobic species. Overall, these results confirm the ecophysiological relevance of FWU in Crassula and reassert the importance of atmospheric humidity for some arid-adapted plant groups.

For many fruit crops, the colour of the fruit outwardly defines its eating quality. Fruit pigments provide reproductive advantage for the plant as well as providing protection against unfavourable environmental conditions and pathogens. For consumers these colours are considered attractive and provide many of the dietary benefits derived from fruits. In the majority of species, the main pigments are either carotenoids and/or anthocyanins. They are produced in the fruit as part of the ripening process, orchestrated by phytohormones and an ensuing transcriptional cascade, culminating in pigment biosynthesis. Whilst this is a controlled developmental process, the production of pigments is also attuned to environmental conditions such as light quantity and quality, availability of water and ambient temperature. If these factors intensify to stress levels, fruit tissues respond by increasing (or ceasing) pigment production. In many cases, if the stress is not severe, this can have a positive outcome for fruit quality. Here, we focus on the principal environmental factors (light, temperature and water) that can influence fruit colour.