Plants employ diverse strategies to cope with different types of heat stress. The response to short-term acute heat stress differs significantly from that to moderate heat stress followed by severe stress events. After experiencing moderate heat stress, plants exhibit a more robust response to subsequent severe stress, a phenomenon known as thermopriming or acquired thermotolerance. Thermopriming creates a memory by maintaining the heat stress (HS) memory-related genes in an alert state. In this work, we investigated the role of Arabidopsis Universal Stress Protein 1 (USP1) in plant heat stress responses. CRISPR-Cas9 generated knockout usp1 mutant lines showed no morphological changes during development and normal growth conditions. However, usp1 mutant plants showed enhanced levels of apoplast hydrogen peroxide and superoxide reactive oxygen species accumulation upon heat stress. Transcriptome analyses revealed that genes related to protein folding, electron transport, and oxidative phosphorylation are strongly upregulated in usp1 mutant plants. USP1 is essential for acquired thermotolerance, as usp1 mutants are compromised in heat stress memory but show normal responses to acute heat stress similar to hsfa2 mutants. Biochemical assays showed that USP1 functions as a molecular chaperone, protecting the transcription factor HSFA2 from heat-induced denaturation. Moreover, usp1 mutant plants show decreased transcript levels of heat stress response genes and reduced H3K4me3 enrichment at memory gene loci. These data show that USP1 plays an important role as a chaperone of HSFA2 in mediating plant heat stress memory.

The activation of immune Mitogen Activated Protein Kinases (MAPKs) is one of the earliest cellular responses following pathogen attack leading to the phosphorylation of appropriate cytosolic or nuclear targets to regulate cellular processes. In this work, we show that the Arabidopsis nuclear protein DEK2 is a target of MPK6 and regulator of plant defense against bacterial and fungal plant pathogens. Transcriptomic analysis unveils that AtDEK2 modulates transcription of defense-related genes, as well as genes associated with hormone synthesis and signaling. Chromatin immunoprecipitation sequencing (ChIP-Seq) analysis reveals that DEK2 potentially binds to TGGGC(C/T) motifs of ZNF, bZIP, WRKY and TCP transcription factors. DEK2 orchestrates diverse chromatin-related processes. Our findings suggest that DEK2 is recruited to specific chromatin regions by transcription factors and functions as a reader of the bivalent histone mark H3K4me3K27me3.

Non-host resistance refers to the immunity of plant species to virtually all isolates of a potential pathogen and represents an underexplored avenue for breeding and engineering disease resistance. In domesticated and wild barley, cell surface-localized lectin receptor kinases (LecRKs) contribute to determining the host status to leaf rust fungi, which pose a major threat to global cereal production. Here, we identify a conserved family of leaf rust glycoside hydrolases as ligands for these barley LecRKs and show that direct ligand-receptor binding triggers immune responses. This mechanism of pathogen perception is conserved across multiple cereal species and can be functionally transferred between them. We also uncover previously uncharacterized recognition specificities among distinct LecRK variants, expanding the repertoire of LecRK-mediated rust pathogen detection. Our findings define a molecular mechanism underlying non-host resistance in cereals and provide a basis for harnessing non-host rust resistance across diverse crop-pathogen systems.

A central regulator of condensate formation in mammals is the Ras GTPase-activating protein SH3 domain-binding protein (G3BP) family of RNA-binding proteins. In Arabidopsis, G3BP homologs can also form condensates and exhibit diverse expression patterns and subcellular localization. Previously, we identified G3BP1 as a negative regulator of plant immunity that is phosphorylated at Ser257 in vivo. Here, we generated phospho-mimic (G3BP1D) and phospho-dead (G3BP1A) variants and expressed them in Arabidopsis, revealing that the phosphorylation state of G3BP1 affects susceptibility to bacterial infection by influencing ROS production and salicylic acid (SA) accumulation. G3BP1 phosphorylation also influences stomatal immunity by maintaining stomatal opening, thereby modulating pre-invasive defense mechanisms. Furthermore, we show that phosphorylation at Ser257 contributes to the stabilization of G3BP1 by limiting its degradation. Collectively, these findings identify G3BP1 phosphorylation as an important regulatory mechanism in plant immunity and provide new insights into the role of RNA-binding proteins in plant defense responses.

Arak (Salvadora persica L.) is known as a toothbrush tree for its medicinal benefits for oral health and its antioxidant, analgesic, and anti-inflammatory properties. The plant has a remarkable ability to tolerate abiotic stress, especially drought and high salinity. The molecular mechanisms underlying this tolerance are yet to be determined. In this study, we show that salinity tolerance in the Arak plant is mediated by the ability of its roots to maintain a Na+/K+ balance when subjected to high salinity. Our proteome analysis of Arak leaves found an accumulation of proteins involved in energy metabolism pathways, indicating that Arak leaves maintain their energy-driving mechanisms under salinity stress conditions. While in roots the proteins involved in ROS scavenging and stress-related pathways were significantly differentially expressed. This suggests that the roots act as a first barrier to alleviating salinity-induced oxidative stress. Our study identifies key proteins and pathways that could have biotechnological importance and could be translated to crop species to improve their abiotic stress tolerance capacities.

Mitogen-activated protein kinase (MAPK) cascades play critical roles in plant immunity by phosphorylating downstream effectors that regulate stress responses. While MAPK-mediated transcriptional regulation has been well examined, the involvement of MAPKs in post-transcriptional and post-translational regulation is still poorly understood. In this study, we identify the RNA binding-protein AtG3BP1 as a phosphorylation target of MPK3, MPK4, and MPK6 and demonstrate that phosphorylation of AtG3BP1 at Ser257 modulates key aspects of Arabidopsis immunity. Using phospho-mimic (G3BP1D) and phospho-dead (G3BP1A) mutants, we investigated the functional consequences of AtG3BP1 phosphorylation. Our data indicate that phosphorylation of AtG3BP1 promotes susceptibility to bacterial infection, suppresses accumulation of reactive oxygen species (ROS), and downregulates salicylic acid (SA) biosynthesis. Furthermore, we demonstrate that AtG3BP1 phosphorylation influences stomatal immunity by maintaining stomatal opening, thereby regulating pre-invasive defense mechanisms. Additionally, we provide evidence that phosphorylation stabilizes AtG3BP1 and prevents its degradation via the proteasome, thus leading to sustained immune signaling. These findings validate AtG3BP1 as a central integrator of MAPK signaling during plant immunity and reveal a new level of post-translational control. This study enhances our understanding of plant defense mechanisms and provides potential targets for engineering disease-resistant crops. ### Competing Interest Statement The authors have declared no competing interest.

The secretory pathway is essential for plant immunity, delivering diverse antimicrobial molecules into the extracellular space. Arabidopsis thaliana soluble N-ethylmaleimide-sensitive-factor attachment protein receptor SNAP33 is a key actor of this process. The snap33 mutant displays dwarfism and necrotic lesions, however the molecular determinants of its macroscopic phenotypes remain elusive. Here, we isolated several new snap33 mutants that exhibited constitutive cell death and H2O2 accumulation, further defining snap33 as an autoimmune mutant. We then carried out quantitative transcriptomic and proteomic analyses showing that numerous defense transcripts and proteins were up-regulated in the snap33 mutant, among which genes/proteins involved in defense hormone, pattern-triggered immunity, and nucleotide-binding domain leucine-rich-repeat receptor signaling. qRT-PCR analyses and hormone dosages supported these results. Furthermore, genetic analyses elucidated the diverse contributions of the main defense hormones and some nucleotide-binding domain leucine-rich-repeat receptor signaling actors in the establishment of the snap33 phenotype, emphasizing the preponderant role of salicylic acid over other defense phytohormones. Moreover, the accumulation of pattern-triggered immunity and nucleotide-binding domain leucine-rich-repeat receptor signaling proteins in the snap33 mutant was confirmed by immunoblotting analyses and further shown to be salicylic acid-dependent. Collectively, this study unveiled molecular determinants underlying the Arabidopsis snap33 mutant phenotype and brought new insights into autoimmunity signaling.

Abstract In Arabidopsis thaliana, the nuclear protein DEK2 orchestrates diverse chromatin-related processes and exhibits phosphorylation in response to flagellin22 treatment, implicating its involvement in plant immunity against bacterial pathogens. Loss-of-function mutants of dek2 have their immunity compromised to both bacterial and fungal pathogens. Transcriptomic analysis of the dek2-1 mutant unveils AtDEK2 as a transcriptional repressor of defense-related genes, as well as genes associated with hormone synthesis and signaling. Chromatin immunoprecipitation sequencing (ChIP-Seq) analysis reveals that DEK2 binds to motifs of various transcription factor families, with a notable enrichment in class I TCP binding motif regions. Our findings indicate that DEK2 is recruited to specific chromatin regions by transcription factors and functions as a reader of the bivalent histone mark H3K4me3K27me3. Consequently, we propose a hypothetical working model wherein DEK2 acts as a transcriptional repressor targeting regions marked by H3K4me3K27me3, shedding light on its role in plant immunity.

High-light stress strongly limits agricultural production in subtropical and tropical regions owing to photooxidative damage, decreased growth, and decreased yield. Here, we investigated whether beneficial microbes can protect plants under high-light stress. We found that Enterobacter sp. SA187 (SA187) supports the growth of Arabidopsis thaliana under high-light stress by reducing the accumulation of reactive oxygen species and maintaining photosynthesis. Under high-light stress, SA187 triggers dynamic changes in the expression of Arabidopsis genes related to fortified iron metabolism and redox regulation, thereby enhancing the antioxidative glutathione/glutaredoxin redox system of the plant. Genetic analysis showed that the enhancement of iron and sulfur metabolism by SA187 is coordinated by ethylene signaling. In summary, beneficial microbes could be an effective and inexpensive means of enhancing high-light-stress tolerance in plants.

Arabidopsis LIM proteins are named after the initials of three proteins Lin-11, Isl-1, and MEC-3, which belong to a class of transcription factors that play an important role in the developmental regulation of eukaryotes and are also involved in a variety of life processes, including gene transcription, the construction of the cytoskeleton, signal transduction, and metabolic regulation. Plant LIM proteins have been shown to regulate actin bundling in different cells, but their role in immunity remains elusive. Mitogen-activated protein kinases (MAPKs) are a family of conserved serine/threonine protein kinases that link upstream receptors to their downstream targets. Pathogens produce pathogen-associated molecular patterns (PAMPs) that trigger the activation of MAPK cascades in plants. Recently, we conducted a large-scale phosphoproteomic analysis of PAMP-induced Arabidopsis plants to identify putative MAPK targets. One of the identified phospho-proteins was WLIM2A, an Arabidopsis LIM protein. In this study, we investigated the role of WLIM2A in plant immunity. We employed a reverse-genetics approach and generated wlim2a knockout lines using CRISPR-Cas9 technology. We also generated complementation and phosphosite-mutated WLIM2A expression lines in the wlim2a background. The wlim2a lines were compromised in their response to Pseudomonas syringae Pst DC3000 but showed enhanced resistance to the necrotrophic fungus Botrytis cinereae. Transcriptome analyses of wlim2a mutants revealed the deregulation of immune hormone biosynthesis and signaling of salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) pathways. The wlim2a mutants also exhibited altered stomatal phenotypes. Analysis of plants expressing WLIM2A variants of the phospho-dead or phospho-mimicking MAPK phosphorylation site showed opposing stomatal behavior and resistance phenotypes in response to Pst DC3000 infection, proving that phosphorylation of WLIM2A plays a crucial role in plant immunity. Overall, these data demonstrate that phosphorylation of WLIM2A by MAPKs regulates Arabidopsis responses to plant pathogens.

Expression of OXIDATIVE SIGNAL-INDUCIBLE1 (OXI1) is induced by a number of stress conditions and regulates the interaction of plants with pathogenic and beneficial microbes. In this work, we generated Arabidopsis OXI1 knockout and genomic OXI1 overexpression lines and show by transcriptome, proteome, and metabolome analysis that OXI1 triggers ALD1, SARD4, and FMO1 expressions to promote the biosynthesis of pipecolic acid (Pip) and N-hydroxypipecolic acid (NHP). OXI1 contributes to enhanced immunity by induced SA biosynthesis via CBP60g-induced expression of SID2 and camalexin accumulation via WRKY33-targeted transcription of PAD3. OXI1 regulates genes involved in reactive oxygen species (ROS) generation such as RbohD and RbohF. OXI1 knock out plants show enhanced expression of nuclear and chloroplast genes of photosynthesis and enhanced growth under ambient conditions, while OXI1 overexpressing plants accumulate NHP, SA, camalexin, and ROS and show a gain-of-function (GOF) cell death phenotype and enhanced pathogen resistance. The OXI1 GOF phenotypes are completely suppressed when compromising N-hydroxypipecolic acid (NHP) synthesis in the fmo1 or ald1 background, showing that OXI1 regulation of immunity is mediated via the NHP pathway. Overall, these results show that OXI1 plays a key role in basal and effector-triggered plant immunity by regulating defense and programmed cell death via biosynthesis of salicylic acid, N-hydroxypipecolic acid, and camalexin.

MAPKs are a family of highly conserved serine/threonine protein kinases that link upstream receptors to their downstream targets which can be localized in the cytoplasm or the nucleus. Pathogens produce pathogen-associated molecular patterns (PAMPs) that trigger the activation of MAPK cascades in plants. Phosphoproteomic analysis of PAMP-induced Arabidopsis plants led to the identification of several putative MAPK targets, WLIM2A. Here, we investigated the role of WLIM2A in plant immunity via a reverse-genetics approach generating wlim2a knockout lines using CRISPR-Cas9, as well as complementation and phosphosite mutated WLIM2A expression lines in the wlim2a background. The wlim2a lines were compromised in their response to Pst DC3000 but showed enhanced resistance to fungal infection by Botrytis cinereae . Transcriptome analyses revealed that immune hormone signaling and biosynthesis genes of salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are differentially regulated in the wlim2a knockout lines. Pathogen assays with Pst DC3000 showed altered stomatal phenotypes in wlim2a mutants. Importantly, WLIM2A phosphomutants had opposing stomatal behaviour and resistance phenotypes in response to Pst DC3000 infection. Overall, these data show that phosphorylation of WLIM2A by MAPKs regulates Arabidopsis stomatal immunity.

To safeguard bread wheat against pests and diseases, breeders have introduced over 200 resistance genes into its genome, thus nearly doubling the number of designated resistance genes in the wheat gene pool 1 . Isolating these genes facilitates their fast-tracking in breeding programs and incorporation into polygene stacks for more durable resistance. We cloned the stem rust resistance gene Sr43 , which was crossed into bread wheat from the wild grass Thinopyrum elongatum 2 , 3 . Sr43 encodes an active protein kinase fused to two domains of unknown function. The gene, which is unique to the Triticeae, appears to have arisen through a gene fusion event 6.7 to 11.6 million years ago. Transgenic expression of Sr43 in wheat conferred high levels of resistance to a wide range of isolates of the pathogen causing stem rust, highlighting the potential value of Sr43 in resistance breeding and engineering.

Linker H1 histones play an important role in animal and human pathogenesis, but their function in plant immunity is poorly understood. Here, we analyzed mutants of the three canonical variants of Arabidopsis H1 histones, namely H1.1, H1.2 and H1.3. We observed that double h1.1h1.2 and triple h1.1h1.2h1.3 (3h1) mutants were resistant to Pseudomonas syringae and Botrytis cinerea infections. Transcriptome analysis of 3h1 mutant plants showed H1s play a key role in regulating the expression of early and late defense genes upon pathogen challenge. Moreover, 3h1 mutant plants showed enhanced production of reactive oxygen species and activation of mitogen activated protein kinases upon pathogen-associated molecular pattern (PAMP) treatment. However, 3h1 mutant plants were insensitive to priming with flg22, a well-known bacterial PAMP which induces enhanced resistance in WT plants. The defective defense response in 3h1 upon priming was correlated with altered DNA methylation and reduced global H3K56ac levels. Our data place H1 as a molecular gatekeeper in governing dynamic changes in the chromatin landscape of defense genes during plant pathogen interaction.

In plants, the detection of microbe-associated molecular patterns (MAMPs) induces primary innate immunity by the activation of mitogen-activated protein kinases (MAPKs). We show here that the MAMP-activated MAPK MPK6 not only modulates defense through transcriptional regulation but also via the ribosomal protein translation machinery. To understand the effects of MPK6 on ribosomes and their constituent ribosomal proteins (RPs), polysomes, monosomes and the phosphorylation status of the RPs, MAMP-treated WT and mpk6 mutant plants were analysed. MAMP-activation induced rapid changes in RP composition of monosomes, polysomes and in the 60S ribosomal subunit in an MPK6-specific manner. Phosphoproteome analysis showed that MAMP-activation of MPK6 regulates the phosphorylation status of the P-stalk ribosomal proteins by phosphorylation of RPP0 and the concomitant dephosphorylation of RPP1 and RPP2. These events coincide with a significant decrease in the abundance of ribosome-bound RPP0s, RPP1s and RPP3s in polysomes. The P-stalk is essential in regulating protein translation by recruiting elongation factors. Accordingly, we found that RPP0C mutant plants are compromised in basal resistance to Pseudomonas syringae infection. These data suggest that MAMP-induced defense also involves MPK6-induced regulation of P-stalk proteins, highlighting a new role of ribosomal regulation in plant innate immunity.

The INDETERMINATE DOMAIN (IDD) family belongs to a group of plant-specific transcription factors that coordinates plant growth/development and immunity. However, the function and mode of action of IDDs during abiotic stress, such as salt, are poorly understood. We used idd4 transgenic lines and screened them under salt stress to find the involvement of IDD4 in salinity stress tolerance The genetic disruption of IDD4 increases salt-tolerance, characterized by sustained plant growth, improved Na+/K+ ratio, and decreased stomatal density/aperture. Yet, IDD4 overexpressing plants were hypersensitive to salt-stress with an increase in stomatal density and pore size. Transcriptomic and ChIP-seq analyses revealed that IDD4 directly controls an important set of genes involved in abiotic stress/salinity responses. Interestingly, using anti-IDD4-pS73 antibody we discovered that IDD4 is specifically phosphorylated at serine-73 by MPK6 in vivo under salinity stress. Analysis of plants expressing the phospho-dead and phospho-mimicking IDD4 versions proved that phosphorylation of IDD4 plays a crucial role in plant transcriptional reprogramming of salt-stress genes. Altogether, we show that salt stress adaption involves MPK6 phosphorylation of IDD4 thereby regulating IDD4 DNA-binding and expression of target genes.

Biogenesis of ribonucleoproteins occurs in dynamic subnuclear compartments called Cajal bodies (CBs). COILIN is a critical scaffolding component essential for CB formation, composition, and activity. We recently showed that Arabidopsis (Arabidopsis thaliana) AtCOILIN is phosphorylated in response to bacterial elicitor treatment. Here, we further investigated the role of AtCOILIN in plant innate immunity. Atcoilin mutants are compromised in defense responses to bacterial pathogens. Besides confirming a role of AtCOILIN in alternative splicing (AS), Atcoilin showed differential expression of genes that are distinct from those of AS, including factors involved in RNA biogenesis, metabolism, plant immunity, and phytohormones. Atcoilin mutant plants have reduced levels of defense phytohormones. As expected, the mutant plants were more sensitive to the necrotrophic fungal pathogen Botrytis cinerea. Our findings reveal an important role for AtCOILIN in innate plant immunity.

Stress induces extensive reprogramming of mRNA metabolism, which includes the transcription and translation of stress-related genes and the formation of stress granules. RasGAP SH3 domain-binding proteins (G3BPs, also called Rasputins) form a highly conserved family of proteins found throughout eukaryotic evolution, which coordinate signal transduction and posttranscriptional gene regulation and play a key role in the formation of stress granules. G3BPs play a role in osmotic, oxidative, and biotic stress in mammals, and recent results revealed that they play similar functions in higher plants. Although simple eukaryotes such as yeast have only one G3BP gene, higher plants show a massive expansion of their G3BP genes into distinct subfamilies. However, because this family of genes has not been well-characterized in plants, functions that have evolved during this expansion remain unidentified. Therefore, we carried out a phylogenetic analysis of G3BPs in different eukaryotes, particularly focusing on the green lineage. On the basis of this evolutionary analysis of G3BPs in eukaryotes, we propose a uniform nomenclature for plant G3BPs that should help predict the evolutionary and functional diversification in this family.

EDITORIAL article Front. Plant Sci., 14 June 2022Sec. Plant Abiotic Stress https://doi.org/10.3389/fpls.2022.955159