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.

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.

Brassica crops include various edible vegetable and plant oil crops, and their production is limited by low temperature beyond their tolerant capability. The key regulators of low-temperature resistance in Brassica remain largely unexplored. To identify post-transcriptional regulators of plant response to low temperature, we performed small RNA profiling, and found that 16 known miRNAs were responsive to cold treatment in Brassica rapa. The cold response of seven of those miRNAs were further confirmed by qRT-PCR and/or northern blotting analyses. In parallel, a genome-wide association study of 220 accessions of Brassica napus identified four candidate MIRNA genes, all of which were cold-responsive, at the loci associated with low temperature resistance. Specifically, these large-scale data analyses revealed a link between miR1885 and the plant response to low temperature in both B. rapa and B. napus. Using 5′ rapid amplification of cDNA ends approach, we validated that miR1885 can cleave its putative target transcripts, Bn.TIR.A09 and Bn.TNL.A03, in B. napus. Furthermore, overexpression of miR1885 in Semi-winter type B. napus decreased the mRNA abundance of Bn.TIR.A09 and Bn.TNL.A03, resulting in increased sensitivity to low temperature. Knocking down of miR1885 in Spring type B. napus led to increased mRNA abundance of its targets and improved rapeseed tolerance to low temperature. Together, our results suggested that the loci of miR1885 and its targets could be potential candidates for the molecular breeding of low temperature-tolerant Spring type Brassica crops.

In cellular circumstances where carbohydrates are scarce, plants can use alternative substrates for cellular energetic maintenance. In plants, the main protein reserve is present in the chloroplast, which contains most of the total leaf proteins and represents a rich source of nitrogen and amino acids. Autophagy plays a key role in chloroplast breakdown, a well-recognized symptom of both natural and stress-induced plant senescence. Remarkably, an autophagic-independent route of chloroplast degradation associated with Chloroplast Vesiculation (CV) gene was recently demonstrated. During extended darkness, CV is highly induced in the absence of autophagy, contributing to the early senescence phenotype of atg mutants. To further investigate the role of CV under dark-induced senescence conditions, mutants with low expression of CV ( amircv) and double mutants amircv1xatg5 were characterized. Following darkness treatment, no aberrant phenotypes were observed in amircv single mutants; however, amircv1xatg5 double mutants displayed early senescence and enhanced dismantling of chloroplast and membrane structures under these conditions. Metabolic characterization revealed that the functional lack of both CV and autophagy leads to higher impairment of amino acid release and differential organic acid accumulation during starvation conditions. The data obtained are discussed in the context of the role of CV and autophagy, both in terms of cellular metabolism and the regulation of chloroplast degradation.

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.

Stomata are the key nodes linking photosynthesis and transpiration. By regulating the opening degree of stomata, plants successively achieve the balance between water loss and carbon dioxide acquisition. The dynamic behavior of stomata is an important cornerstone of plant adaptability. Though there have been miscellaneous experimental results on stomata and their constituent cells, the guard cells and the subsidiary cells, current theory of stomata regulation is far from clear and unified. In this work, we develop an integrated model to describe the stomatal dynamics of seed plants based on existing experimental results. The model includes three parts: 1) a passive mechanical model of the stomatal aperture as a bivariate function of the guard-cell and the subsidiary-cell turgor pressures; 2) an active regulation model with a targeted ion-content in guard cells as a function of their water potential; and 3) a dynamical model for the movement of potassium ions and water content. Our model has been used to reproduce different experimental phenomena semi and stomatal responses to environment 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.

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.

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.

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.

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.

Gummy stem blight (GSB), a severe and widespread disease causing great losses to cucurbit production, is a major threat to melon production. However, the melon-GSB interaction remains largely unknown, which significantly impedes the genetic improvement of melon for GSB resistance. Here, full-length transcriptome and widely targeted metabolome were used to reveal the early defense responses of resistant (PI511890) and susceptible (Payzawat) melon to GSB. Differentially expressed genes were specifically enriched in the secondary metabolite biosynthesis and MAPK signaling pathway in PI511890, while in carbohydrate metabolism and amino acid metabolism in Payzawat. More than 1000 novel genes were identified in PI511890, which were enriched in the MAPK signaling pathway. There were 11,793 alternative splicing events identified and involved the defense response to GSB. A total of 910 metabolites were identified, with flavonoids as the dominant metabolites. Integrated full-length transcriptome and metabolome analysis showed that eriodictyol and oxalic acid may be used as marker metabolites for GSB resistance in melon. Moreover, post-transcription regulation was widely involved in the defense response of melon to GSB. These results improve our understanding of the interaction between melon and GSB and may facilitate the genetic improvement of GSB resistance of melon.

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.

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.

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”.

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapor gradient (Δ w) increases with temperature if ambient vapor pressure is held constant, and stomata respond to changes in Δ w. However, the direct response of stomata to temperature (DRST; the response when Δ w is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meager available data suggest the response is usually positive, results differ widely and defy broad generalization. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesized biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

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.

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.