Apoplast, the diffusional space between plant cell plasma membranes, is an important medium for signaling within and between the cells. Apoplastic reactive oxygen species （ROS） are crucial signaling ...molecules in various biological processes. ROS signaling is interconnected with the response to several hormones, including jasmonic acid （JA）, salicylic acid （SA） and ethylene. Using ozone （03） to activate apo- plastic ROS signaling, we performed global and targeted analysis of transcriptional changes and cell death assays to dissect the contribution of hormone signaling and various transcription factors （TFs） in the regulation of gene expression and cell death. The contributions of SA, JA, and ethylene were assessed through analysis of single, double, and triple mutants deficient in biosynthesis or signaling for all three hormones. Even in the triple mutant, the global gene expression responses to 03 were mostly similar to the wild-type. Cell death in the JA receptor mutant coil-16 was suppressed by impairment of the NADPH oxidase RBOHF, suggesting a role for a ROS signal in limiting the spread of cell death. In response to apoplastic ROS, there is not a single signaling pathway that regulates gene expression or cell death. Instead, several pathways regulate the apoplastic ROS response via combinatorial or overlapping mechanisms.
Trans-methylation reactions are intrinsic to cellular metabolism in all living organisms. In land plants, a range of substrate-specific methyltransferases catalyze the methylation of DNA, RNA, ...proteins, cell wall components and numerous species-specific metabolites, thereby providing means for growth and acclimation in various terrestrial habitats. Trans-methylation reactions consume vast amounts of S-adenosyl-L-methionine (SAM) as a methyl donor in several cellular compartments. The inhibitory reaction by-product, S-adenosyl-L-homocysteine (SAH), is continuously removed by SAH hydrolase (SAHH), which essentially maintains trans-methylation reactions in all living cells. Here we report on the evolutionary conservation and post-translational control of SAHH in land plants. We provide evidence suggesting that SAHH forms oligomeric protein complexes in phylogenetically divergent land plants and that the predominant protein complex is composed by a tetramer of the enzyme. Analysis of light-stress-induced adjustments of SAHH in Arabidopsis thaliana and Physcomitrella patens further suggests that regulatory actions may take place on the levels of protein complex formation and phosphorylation of this metabolically central enzyme. Collectively, these data suggest that plant adaptation to terrestrial environments involved evolution of regulatory mechanisms that adjust the trans-methylation machinery in response to environmental cues.
•ROS production is strictly regulated and integrated with other signaling networks in plants.•Calcium has a critical role in ROS signaling and the control of ROS production.•Extracellular ROS ...production originates from different sources that have non-overlapping roles.•Intracellular ROS perception is provided by parallel systems of oxidative protein modification and redox status monitoring.•NPR1 serves as a central intracellular integrator of ROS, nitric oxide and salicylic acid signaling pathways.
Reactive oxygen species are recognized as important signaling components in a wide range of processes in plants and most other organisms. Reactive oxygen species are produced in different subcellular compartments in response to specific stimuli and the production is under tight control in order to avoid detrimental side-effects. Calcium signaling, protein phosphorylation and other signaling pathways are intimately involved in the control and coordination of reactive oxygen production. Any signal that should result in a specific response must eventually be perceived. Direct redox modification of transcription factors and other proteins are critical for the perception of intracellular reactive oxygen species; however, sensing of their extracellular counterparts awaits elucidation.
Cysteine-rich receptor-like kinases (CRKs) are transmembrane proteins characterized by the presence of two domains of unknown function 26 (DUF26) in their ectodomain. The CRKs form one of the largest ...groups of receptor-like protein kinases in plants, but their biological functions have so far remained largely uncharacterized. We conducted a large-scale phenotyping approach of a nearly complete crk T-DNA insertion line collection showing that CRKs control important aspects of plant development and stress adaptation in response to biotic and abiotic stimuli in a non-redundant fashion. In particular, the analysis of reactive oxygen species (ROS)-related stress responses, such as regulation of the stomatal aperture, suggests that CRKs participate in ROS/redox signalling and sensing. CRKs play general and fine-tuning roles in the regulation of stomatal closure induced by microbial and abiotic cues. Despite their great number and high similarity, large-scale phenotyping identified specific functions in diverse processes for many CRKs and indicated that CRK2 and CRK5 play predominant roles in growth regulation and stress adaptation, respectively. As a whole, the CRKs contribute to specificity in ROS signalling. Individual CRKs control distinct responses in an antagonistic fashion suggesting future potential for using CRKs in genetic approaches to improve plant performance and stress tolerance.
Summary
Respiration in leaves and the continued elevation in the atmospheric CO2 concentration cause CO2‐mediated reduction in stomatal pore apertures. Several mutants have been isolated for which ...stomatal responses to both abscisic acid (ABA) and CO2 are simultaneously defective. However, there are only few mutations that impair the stomatal response to elevated CO2, but not to ABA. Such mutants are invaluable in unraveling the molecular mechanisms of early CO2 signal transduction in guard cells. Recently, mutations in the mitogen‐activated protein (MAP) kinase, MPK12, have been shown to partially impair CO2‐induced stomatal closure. Here, we show that mpk12 plants, in which MPK4 is stably silenced specifically in guard cells (mpk12 mpk4GC homozygous double‐mutants), completely lack CO2‐induced stomatal responses and have impaired activation of guard cell S‐type anion channels in response to elevated CO2/bicarbonate. However, ABA‐induced stomatal closure, S‐type anion channel activation and ABA‐induced marker gene expression remain intact in the mpk12 mpk4GC double‐mutants. These findings suggest that MPK12 and MPK4 act very early in CO2 signaling, upstream of, or parallel to the convergence of CO2 and ABA signal transduction. The activities of MPK4 and MPK12 protein kinases were not directly modulated by CO2/bicarbonate in vitro, suggesting that they are not direct CO2/bicarbonate sensors. Further data indicate that MPK4 and MPK12 have distinguishable roles in Arabidopsis and that the previously suggested role of RHC1 in stomatal CO2 signaling is minor, whereas MPK4 and MPK12 act as key components of early stomatal CO2 signal transduction.
Significance statement
We show that MPK4 and MPK12 are required for CO2‐triggered stomatal responses but not for ABA‐induced stomatal closure. These results suggest that these two MAP kinases function upstream of the convergence of CO2 and ABA signaling. However, activities of MPK4 and MPK12 were not modulated by CO2/bicarbonate and, thus, direct HCO3−/CO2 sensing mechanisms remain to be identified.
Genetic variation between naturally occurring populations provides a unique source to unravel the complex mechanisms of stress tolerance. Here, we have analysed O₃ sensitivity of 93 natural ...Arabidopsis thaliana accessions together with five O₃-sensitive mutants to acute O₃ exposure. The variation in O₃ sensitivity among the natural accessions was much higher than among the O₃-sensitive mutants and corresponding wild types. A subset of nine accessions with major variation in their O₃ responses was studied in more detail. Among the traits assayed, stomatal conductance (gst) was an important factor determining O₃ sensitivity of the selected accessions. The most O₃-sensitive accession, Cvi-0, had constitutively high gst, leading to high initial O₃ uptake rate and dose received during the first 30 min of exposure. Analyzing O₃-induced changes in stress hormone concentrations indicated that jasmonate (JA) concentration was also positively correlated with leaf damage. Quantitative trait loci (QTL) mapping in a Col-0 x Cvi-0 recombinant inbred line (RIL) population identified three QTLs for O₃ sensitivity, and one for high water loss of Cvi-0. The major O₃ QTL mapped to the same position as the water loss QTL further supporting the role of stomata in regulating O₃ entry and damage.
High levels of phenotypic variation in resistance appears to be nearly ubiquitous across natural host populations. Molecular processes contributing to this variation in nature are still poorly known, ...although theory predicts resistance to evolve at specific loci driven by pathogen-imposed selection. Nucleotide-binding leucine-rich repeat (NLR) genes play an important role in pathogen recognition, downstream defense responses and defense signaling. Identifying the natural variation in NLRs has the potential to increase our understanding of how NLR diversity is generated and maintained, and how to manage disease resistance. Here, we sequenced the transcriptomes of five different
genotypes when inoculated by the same strain of obligate fungal pathogen
. A
transcriptome assembly of RNA-sequencing data yielded 24,332 gene models with N50 value of 1,329 base pairs and gene space completeness of 66.5%. The gene expression data showed highly varying responses where each plant genotype demonstrated a unique expression profile in response to the pathogen, regardless of the resistance phenotype. Analysis on the conserved NB-ARC domain demonstrated a diverse NLR repertoire in
consistent with the high phenotypic resistance diversity in this species. We find evidence of selection generating diversity at some of the NLR loci. Jointly, our results demonstrate that phenotypic resistance diversity results from a crosstalk between different defense mechanisms. In conclusion, characterizing the architecture of resistance in natural host populations may shed unprecedented light on the potential of evolution to generate variation.
Reactive oxygen species (ROS), important signaling molecules in plants, are involved in developmental control and stress adaptation. ROS production can trigger broad transcriptional changes; however, ...it is not clear how specificity in transcriptional regulation is achieved.
A large collection of public transcriptome data from the model plant Arabidopsis thaliana is available for analysis. These data can be used for the analysis of biological processes that are associated with ROS signaling and for the identification of suitable transcriptional indicators. Several online tools, such as Genevestigator and Expression Angler, have simplified the task to analyze, interpret, and visualize this wealth of data.
The analysis of the exact transcriptional responses to ROS requires the production of specific ROS in distinct subcellular compartments with precise timing, which is experimentally difficult. Analyses are further complicated by the effect of ROS production in one subcellular location on the ROS accumulation in other compartments. In addition, even subtle differences in the method of ROS production or treatment can lead to significantly different outcomes when various stimuli are compared.
Due to the difficulty of inducing ROS production specifically with regard to ROS type, subcellular localization, and timing, we propose that the concept of a "ROS marker gene" should be re-evaluated. We suggest guidelines for the analysis of transcriptional data in ROS signaling. The use of "ROS signatures," which consist of a set of genes that together can show characteristic and indicative responses, should be preferred over the use of individual marker genes.
The Arabidopsis guard cell anion channel SLAC1 is essential for stomatal closure in response to various endogenous and environmental stimuli. Interestingly, here we reveal an unexpected impairment of ...slac1 alleles on stomatal opening.
We report that mutations in SLAC1 unexpectedly slow stomatal opening induced by light, low CO2 and elevated air humidity in intact plants and that this is caused by the severely reduced activity of inward K+ (K+
in) channels in slac1 guard cells.
Expression of channels and transporters involved in stomatal opening showed small but significant reductions in transcript levels in slac1 guard cells; however, this was deemed insufficient to explain the severely impaired K+
in channel activity in slac1. We further examined resting cytosolic Ca2+ concentration (Ca2+cyt) and K+
in channel sensitivity to Ca2+cyt in slac1. These experiments showed higher resting Ca2+cyt in slac1 guard cells and that reducing Ca2+cyt to < 10 nM rapidly restored the activity of K+
in channels in slac1 closer to wild-type levels.
These findings demonstrate an unanticipated compensatory feedback control in plant stomatal regulation, which counteracts the impaired stomatal closing response of slac1, by down-regulating stomatal opening mechanisms and implicates enhanced Ca2+cyt sensitivity priming as a mechanistic basis for the down-regulated K+
in channel activity.
SUMMARY
Jasmonic acid (JA) and salicylic acid (SA) regulate stomatal closure, preventing pathogen invasion into plants. However, to what extent abscisic acid (ABA), SA and JA interact, and what the ...roles of SA and JA are in stomatal responses to environmental cues, remains unclear. Here, by using intact plant gas‐exchange measurements in JA and SA single and double mutants, we show that stomatal responsiveness to CO2, light intensity, ABA, high vapor pressure deficit and ozone either did not or, for some stimuli only, very slightly depended upon JA and SA biosynthesis and signaling mutants, including dde2, sid2, coi1, jai1, myc2 and npr1 alleles. Although the stomata in the mutants studied clearly responded to ABA, CO2, light and ozone, ABA‐triggered stomatal closure in npr1‐1 was slightly accelerated compared with the wild type. Stomatal reopening after ozone pulses was quicker in the coi1‐16 mutant than in the wild type. In intact Arabidopsis plants, spraying with methyl‐JA led to only a modest reduction in stomatal conductance 80 min after treatment, whereas ABA and CO2 induced pronounced stomatal closure within minutes. We could not document a reduction of stomatal conductance after spraying with SA. Coronatine‐induced stomatal opening was initiated slowly after 1.5–2.0 h, and reached a maximum by 3 h after spraying intact plants. Our results suggest that ABA, CO2 and light are major regulators of rapid guard cell signaling, whereas JA and SA could play only minor roles in the whole‐plant stomatal response to environmental cues in Arabidopsis and Solanum lycopersicum (tomato).
Significance Statement
Jasmonic acid (JA) and salicylic acid (SA) can promote stomatal closure; however, the roles of these hormones in stomatal responses to environmental cues remain unclear. Here, we show that stomatal responsiveness to CO2, light intensity, abscisic acid, high vapor pressure deficit and ozone either did not or, for some stimuli only, very slightly depended on JA and SA.
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