SUMO conjugation to the pattern recognition receptor FLS2 triggers intracellular signalling in plant innate immunity

Heat-induced SUMOylation differentially affects bacterial effectors in plant cells

Bacterial pathogens deliver effectors into host cells to suppress immunity. How host cells target these effectors is critical in pathogen-host interactions. SUMOylation, an important type of posttranslational modification in eukaryotic cells, plays a critical role in immunity, but its effect on bacterial effectors remains unclear in plant cells. In this study, using bioinformatic and biochemical approaches, we found that at least 16 effectors from the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 are SUMOylated by the enzyme cascade from Arabidopsis thaliana. Mutation of SUMOylation sites on the effector HopB1 enhances its function in the induction of plant cell death via stability attenuation of a plant receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1)-ASSOCIATED RECEPTOR KINASE 1. By contrast, SUMOylation is essential for the function of another effector, HopG1, in the inhibition of mitochondria activity and jasmonic acid signaling. SUMOylation of both HopB1 and HopG1 is increased by heat treatment, and this modification modulates the functions of these 2 effectors in different ways in the regulation of plant survival rates, gene expression, and bacterial infection under high temperatures. Therefore, the current work on the SUMOylation of effectors in plant cells improves our understanding of the function of dynamic protein modifications in plant-pathogen interactions in response to environmental conditions.

SUMO protease FUG1, histone reader AL3 and chromodomain protein LHP1 are integral to repeat expansion-induced gene silencing in Arabidopsis thaliana

Epigenetic gene silencing induced by expanded repeats can cause diverse phenotypes ranging from severe growth defects in plants to genetic diseases such as Friedreich's ataxia in humans. The molecular mechanisms underlying repeat expansion-induced epigenetic silencing remain largely unknown. Using a plant model with a temperature-sensitive phenotype, we have previously shown that expanded repeats can induce small RNAs, which in turn can lead to epigenetic silencing through the RNA-dependent DNA methylation pathway. Here, using a genetic suppressor screen and yeast two-hybrid assays, we identified novel components required for epigenetic silencing caused by expanded repeats. We show that FOURTH ULP GENE CLASS 1 (FUG1)-an uncharacterized SUMO protease with no known role in gene silencing-is required for epigenetic silencing caused by expanded repeats. In addition, we demonstrate that FUG1 physically interacts with ALFIN-LIKE 3 (AL3)-a histone reader that is known to bind to active histone mark H3K4me2/3. Loss of function of AL3 abolishes epigenetic silencing caused by expanded repeats. AL3 physically interacts with the chromodomain protein LIKE HETEROCHROMATIN 1 (LHP1)-known to be associated with the spread of the repressive histone mark H3K27me3 to cause repeat expansion-induced epigenetic silencing. Loss of any of these components suppresses repeat expansion-associated phenotypes coupled with an increase in IIL1 expression with the reversal of gene silencing and associated change in epigenetic marks. Our findings suggest that the FUG1-AL3-LHP1 module is essential to confer repeat expansion-associated epigenetic silencing and highlight the importance of post-translational modifiers and histone readers in epigenetic silencing.

Temperature-dependent action of pepper mildew resistance locus O 1 in inducing pathogen immunity and thermotolerance

Plant diseases tend to be more serious under conditions of high-temperature/high-humidity (HTHH) than under moderate conditions, and hence disease resistance under HTHH is an important determinant for plant survival. However, how plants cope with diseases under HTHH remains poorly understood. In this study, we used the pathosystem consisting of pepper (Capsicum annuum) and Ralstonia solanacearum (bacterial wilt) as a model to examine the functions of the protein mildew resistance locus O 1 (CaMLO1) and U-box domain-containing protein 21 (CaPUB21) under conditions of 80% humidity and either 28 °C or 37 °C. Expression profiling, loss- and gain-of-function assays involving virus-induced gene-silencing and overexpression in pepper plants, and protein-protein interaction assays were conducted, and the results showed that CaMLO1 acted negatively in pepper immunity against R. solanacearum at 28 °C but positively at 37 °C. In contrast, CaPUB21 acted positively in immunity at 28 °C but negatively at 37 °C. Importantly, CaPUB21 interacted with CaMLO1 under all of the tested conditions, but only the interaction in response to R. solanacearum at 37 °C or to exposure to 37 °C alone led to CaMLO1 degradation, thereby turning off defence responses against R. solanacearum at 37 °C and under high-temperature stress to conserve resources. Thus, we show that CaMLO1 and CaPUB21 interact with each other and function distinctly in pepper immunity against R. solanacearum in an environment-dependent manner.

The cell surface is the place to be for brassinosteroid perception and responses

Adjusting the microenvironment around the cell surface is critical to responding to external cues or endogenous signals and to maintaining cell activities. In plant cells, the plasma membrane is covered by the cell wall and scaffolded with cytoskeletal networks, which altogether compose the cell surface. It has long been known that these structures mutually interact, but the mechanisms that integrate the whole system are still obscure. Here we spotlight the brassinosteroid (BR) plant hormone receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) since it represents an outstanding model for understanding cell surface signalling and regulation. We summarize how BRI1 activity and dynamics are controlled by plasma membrane components and their associated factors to fine-tune signalling. The downstream signals, in turn, manipulate cell surface structures by transcriptional and post-translational mechanisms. Moreover, the changes in these architectures impact BR signalling, resulting in a feedback loop formation. This Review discusses how BRI1 and BR signalling function as central hubs to integrate cell surface regulation.

NUA positively regulates plant immunity by coordination with ESD4 to deSUMOylate TPR1 in Arabidopsis

Nuclear pore complex (NPC) is composed of multiple nucleoporins (Nups). A plethora of studies have highlighted the significance of NPC in plant immunity. However, the specific roles of individual Nups are poorly understood. NUCLEAR PORE ANCHOR (NUA) is a component of NPC. Loss of NUA leads to an increase in SUMO conjugates and pleiotropic developmental defects in Arabidopsis thaliana. Herein, we revealed that NUA is required for plant defense against multiple pathogens. NUCLEAR PORE ANCHOR associates with the transcriptional corepressor TOPLESS-RELATED1 (TPR1) and contributes to TPR1 deSUMOylation. Significantly, NUA-interacting protein EARLY IN SHORT DAYS 4 (ESD4), a SUMO protease, specifically deSUMOylates TPR1. It has been previously established that the SUMO E3 ligase SAP AND MIZ1 DOMAIN-CONTAINING LIGASE 1 (SIZ1)-mediated SUMOylation of TPR1 represses the immune-related function of TPR1. Consistent with this notion, the hyper-SUMOylated TPR1 in nua-3 leads to upregulated expression of TPR1 target genes and compromised TPR1-mediated disease resistance. Taken together, our work uncovers a mechanism by which NUA positively regulates plant defense responses by coordination with ESD4 to deSUMOylate TPR1. Our findings, together with previous studies, reveal a regulatory module in which SIZ1 and NUA/ESD4 control the homeostasis of TPR1 SUMOylation to maintain proper immune output.

Shoot Maturation Strengthens FLS2-Mediated Resistance to Pseudomonas syringae

Temporospatial regulation of immunity components is essential for properly activating plant defense response. Flagellin-sensing 2 (FLS2) is a surface-localized receptor that recognizes bacterial flagellin. The immune function of FLS2 is compromised in early stages of shoot development. However, the underlying mechanism for the age-dependent FLS2 signaling is not clear. Here, we show that the reduced basal immunity of juvenile leaves against Pseudomonas syringae pv. tomato DC3000 is independent of FLS2. The flg22-induced marker gene expression and reactive oxygen species activation were comparable in juvenile and adult stages, but callose deposition was more evident in the adult stage than the juvenile stage. We further demonstrated that microRNA156, a master regulator of plant aging, does not influence the expression of FLS2 and FRK1 (Flg22-induced receptor-like kinase 1) but mildly suppresses callose deposition in juvenile leaves. Our experiments revealed an intrinsic mechanism that regulates the amplitude of FLS2-mediated resistance during aging. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Paradigms of receptor kinase signaling in plants

Plant receptor kinases (RKs) function as key plasma-membrane localized receptors in the perception of molecular ligands regulating development and environmental response. Through the perception of diverse ligands, RKs regulate various aspects throughout the plant life cycle from fertilization to seed set. Thirty years of research on plant RKs has generated a wealth of knowledge on how RKs perceive ligands and activate downstream signaling. In the present review, we synthesize this body of knowledge into five central paradigms of plant RK signaling: (1) RKs are encoded by expanded gene families, largely conserved throughout land plant evolution; (2) RKs perceive many different kinds of ligands through a range of ectodomain architectures; (3) RK complexes are typically activated by co-receptor recruitment; (4) post-translational modifications fulfill central roles in both the activation and attenuation of RK-mediated signaling; and, (5) RKs activate a common set of downstream signaling processes through receptor-like cytoplasmic kinases (RLCKs). For each of these paradigms, we discuss key illustrative examples and also highlight known exceptions. We conclude by presenting five critical gaps in our understanding of RK function.

S-acylation stabilizes ligand-induced receptor kinase complex formation during plant pattern-triggered immune signaling

Plant receptor kinases are key transducers of extracellular stimuli, such as the presence of beneficial or pathogenic microbes or secreted signaling molecules. Receptor kinases are regulated by numerous post-translational modifications.1,2,3 Here, using the immune receptor kinases FLS24 and EFR,5 we show that S-acylation at a cysteine conserved in all plant receptor kinases is crucial for function. S-acylation involves the addition of long-chain fatty acids to cysteine residues within proteins, altering their biochemical properties and behavior within the membrane environment.6 We observe S-acylation of FLS2 at C-terminal kinase domain cysteine residues within minutes following the perception of its ligand, flg22, in a BAK1 co-receptor and PUB12/13 ubiquitin ligase-dependent manner. We demonstrate that S-acylation is essential for FLS2-mediated immune signaling and resistance to bacterial infection. Similarly, mutating the corresponding conserved cysteine residue in EFR suppressed elf18-triggered signaling. Analysis of unstimulated and activated FLS2-containing complexes using microscopy, detergents, and native membrane DIBMA nanodiscs indicates that S-acylation stabilizes, and promotes retention of, activated receptor kinase complexes at the plasma membrane to increase signaling efficiency.

Shoot maturation strengthens FLS2-mediated resistance to Pseudomonas syringae

A temporal-spatial regulation of immunity components is essential for properly activating plant defense response. Flagellin-sensing 2 (FLS2) is a surface-localized receptor that recognizes bacterial flagellin. The immune function of FLS2 is compromised in early stages of shoot development. However, the underlying mechanism for the age-dependent FLS2 signaling is not clear. Here, we show that the reduced basal immunity of juvenile leaves against Pseudomonas syringae pv. tomato DC3000 is independent of FLS2. The flg22-induced marker gene expression and ROS activation were comparable in juvenile and adult stage, but callose deposition was more evident in the adult stage than that of juvenile stage. We further demonstrated that microRNA156, a master regulator of plant aging, suppressed callose deposition in juvenile leaves in response to flg22 but not the expression of FLS2 and FRK1 (Flg22-induced receptor-like kinase 1) . Altogether, we revealed an intrinsic mechanism that regulates the amplitude of FLS2-mediated resistance during aging.

SUMO/deSUMOylation of the BRI1 brassinosteroid receptor modulates plant growth responses to temperature

Brassinosteroids (BRs) are a class of steroid molecules perceived at the cell surface that act as plant hormones. The BR receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) offers a model to understand receptor-mediated signaling in plants and the role of post-translational modifications. Here we identify SUMOylation as a new modification targeting BRI1 to regulate its activity. BRI1 is SUMOylated in planta on two lysine residues, and the levels of BRI1 SUMO conjugates are controlled by the Desi3a SUMO protease. Loss of Desi3a leads to hypersensitivity to BRs, indicating that Desi3a acts as a negative regulator of BR signaling. Besides, we demonstrate that BRI1 is deSUMOylated at elevated temperature by Desi3a, leading to increased BRI1 interaction with the negative regulator of BR signaling BIK1 and to enhanced BRI1 endocytosis. Loss of Desi3a or BIK1 results in increased response to temperature elevation, indicating that BRI1 deSUMOylation acts as a safety mechanism necessary to keep temperature responses in check. Altogether, our work establishes BRI1 deSUMOylation as a molecular crosstalk mechanism between temperature and BR signaling, allowing plants to translate environmental inputs into growth response.

Pepper SUMO E3 ligase CaDSIZ1 enhances drought tolerance by stabilizing the transcription factor CaDRHB1

Small ubiquitin-like modifier (SUMO) conjugation (SUMOylation) is a reversible post-translational modification associated with protein stability and activity, and modulates hormone signaling and stress responses in plants. Previously, we reported that the pepper dehydration-responsive homeobox domain transcription factor CaDRHB1 acts as a positive modulator of drought response. Here, we show that CaDRHB1 protein stability is enhanced by SUMO E3 ligase Capsicum annuum DRHB1-interacting SAP and Miz domain (SIZ1) (CaDSIZ1)-mediated SUMOylation in response to drought, thereby positively modulating abscisic acid (ABA) signaling and drought responses. Substituting lysine (K) 138 of CaDRHB1 with arginine reduced CaDSIZ1-mediated SUMOylation, indicating that K138 is the principal site for SUMO conjugation. Virus-induced silencing of CaDSIZ1 promoted CaDRHB1 degradation, suggesting that CaDSIZ1 is involved in drought-induced SUMOylation of CaDRHB1. CaDSIZ1 interacted with and facilitated SUMO conjugation of CaDRHB1. CaDRHB1, mainly localized in the nucleus, but also in the cytoplasm in the SUMOylation mimic state, suggesting that SUMOylation of CaDRHB1 promotes its nuclear export, leading to cytoplasmic accumulation. Moreover, CaDSIZ1-silenced pepper plants were less sensitive to ABA and considerably sensitive to drought stress, whereas CaDSIZ1-overexpressing plants displayed ABA-hypersensitive and drought-tolerant phenotypes. Collectively, our data indicate that CaDSIZ1-mediated SUMOylation of CaDRHB1 functions in ABA-mediated drought tolerance.

Understanding SUMO-mediated adaptive responses in plants to improve crop productivity

The response to abiotic and biotic stresses in plants and crops is considered a multifaceted process. Due to their sessile nature, plants have evolved unique mechanisms to ensure that developmental plasticity remains during their life cycle. Among these mechanisms, post-translational modifications (PTMs) are crucial components of adaptive responses in plants and transduce environmental stimuli into cellular signalling through the modulation of proteins. SUMOylation is an emerging PTM that has received recent attention due to its dynamic role in protein modification and has quickly been considered a significant component of adaptive mechanisms in plants during stress with great potential for agricultural improvement programs. In the present review, we outline the concept that small ubiquitin-like modifier (SUMO)-mediated response in plants and crops to abiotic and biotic stresses is a multifaceted process with each component of the SUMO cycle facilitating tolerance to several different environmental stresses. We also highlight the clear increase in SUMO genes in crops when compared with Arabidopsis thaliana. The SUMO system is understudied in crops, given the importance of SUMO for stress responses, and for some SUMO genes, the apparent expansion provides new avenues to discover SUMO-conjugated targets that could regulate beneficial agronomical traits.

The conjugation of SUMO to the transcription factor MYC2 functions in blue light-mediated seedling development in Arabidopsis

A key function of photoreceptor signaling is the coordinated regulation of a large number of genes to optimize plant growth and development. The basic helix loop helix (bHLH) transcription factor MYC2 is crucial for regulating gene expression in Arabidopsis thaliana during development in blue light. Here we demonstrate that blue light induces the SUMOylation of MYC2. Non-SUMOylatable MYC2 is less effective in suppressing blue light-mediated photomorphogenesis than wild-type (WT) MYC2. MYC2 interacts physically with the SUMO proteases SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2. Blue light exposure promotes the degradation of SPF1 and SPF2 and enhances the SUMOylation of MYC2. Phenotypic analysis revealed that SPF1/SPF2 function redundantly as positive regulators of blue light-mediated photomorphogenesis. Our data demonstrate that SUMO conjugation does not affect the dimerization of MYC transcription factors but modulates the interaction of MYC2 with its cognate DNA cis-element and with the ubiquitin ligase Plant U-box 10 (PUB10). Finally, we show that non-SUMOylatable MYC2 is less stable and interacts more strongly with PUB10 than the WT. Taken together, we conclude that SUMO functions as a counterpoint to the ubiquitin-mediated degradation of MYC2, thereby enhancing its function in blue light signaling.

Organization and regulation of the apple SUMOylation system under salt and ABA

Small ubiquitin-related modifier (SUMO)-mediated post-translational protein modification is widely conserved among eukaryotes. SUMOylation refers to the covalent attachment of SUMO to target proteins that alters their function, location, and protein-protein interactions when plants are under abiotic stress. We identified 37 genes in the apple genome that encoded members of the SUMOylation pathway. In addition, RNA-Seq data shows their expression levels between different tissues. We can find that there are mainly expressed genes between each component to ensure that the entire pathway works in the plant. We found that the expression levels of 12 genes were significantly changed under NaCl and ABA treatment through qRT-PCR. MdSIZ1a strongly expression responded to NaCl and ABA treatment. Subsequently, MdSIZ1a was cloned and transformed into apple callus, further verifying the important role of the SUMOylation pathway under stress conditions. The interaction between MdSIZ1a and MdSCEa was verified by yeast two-hybrid, confirming that MdSIZ1a acts as bridge enzyme on MdSCEa and target substrates. Finally, we predicted and analyzed the functional interaction network of E3 ligase to shed light on protein interactions and gene regulatory networks associated with DNA damage repair under abiotic stress in apples.

Review: Exploring possible approaches using ubiquitylation and sumoylation pathways in modifying plant stress tolerance

Ubiquitin and similar proteins, such as SUMO, are utilized by plants to modify target proteins to rapidly change their stability and activity in cells. This review will provide an overview of these crucial protein interactions with a focus on ubiquitylation and sumoylation in plants and how they contribute to stress tolerance. The work will also explore possibilities to use these highly conserved pathways for novel approaches to generate more robust crop plants better fit to cope with abiotic and biotic stress situations.

Activation of NLR-Mediated Autoimmunity in Arabidopsis Early in Short Days 4 Mutant

From a reverse genetic screen using CRISPR/Cas9 gene editing tool, we unintentionally identified an autoimmune mutant. Map-based cloning and whole-genome sequencing revealed that it contains a deletion in SMALL UBIQUITIN-RELATED MODIFIER (SUMO) protease encoding gene EARLY IN SHORT DAYS 4 (ESD4). Previous studies reported that esd4 mutants accumulate elevated levels of plant defense hormone salicylic acid (SA). However, upregulated PATHOGENESIS-RELATED GENE 1 (PR1) expression in esd4 only partly relies on SA level. In this study, we show that plant metabolite N-hydroxypipecolic acid (NHP) biosynthetic genes are upregulated in esd4, and NHP biosynthesis mutant flavin-dependent-monooxygenase 1 (fmo1) partially suppresses the autoimmune phenotypes of esd4, suggestive of a requirement of NHP signaling for the autoimmunity in esd4. As activation of nucleotide-binding leucine-rich repeat immune receptors (NLRs) are associates with the biosynthesis of SA and NHP and lipase-like protein ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) is a key component downstream of many NLRs, we examined the relationship between EDS1 and ESD4 by analyzing the eds1 esd4 double mutant. We found that eds1 largely suppresses esd4 autoimmunity and blocks the elevated expressions of SA and NHP biosynthesis-related genes in esd4. Overall, our study provides evidence supporting the hypothesis that SUMO protease ESD4 likely targets a yet to be identified guardee of NLR by removing its SUMO modification to avoid recognition by the cognate NLR. Loss of ESD4 results in activation of NLR-mediated autoimmunity.

Full-Length Transcriptome Sequencing-Based Analysis of Pinus sylvestris var. mongolica in Response to Sirex noctilio Venom

Simple Summary

Sirex noctilio, as a devastating international forestry quarantine pest whose venom can cause a series of physiological changes in the host plants, such as needle wilting, yellowing, decreased transpiration rate and increased respiration rate, etc. In this study, a full-length reference transcript of Pinus sylvestris var. mongolica was constructed by combining second- and third-generation transcriptome sequencing technologies. We also identified the specific expression genes and transcription factors of P. sylvestris var. mongolica under S. noctilio venom and wounding stress. S. noctilio venom mainly induced the expression of genes related to ROS, GAPDH and GPX, and mechanical damage mainly induced the photosynthesis−related genes. The results provide a better understanding of the molecular regulation of pine trees in response to S. noctilio venom.

Abstract

Sirex noctilio is a major international quarantine pest that recently emerged in northeast China to specifically invade conifers. During female oviposition, venom is injected into the host together with its symbiotic fungus to alter the normal Pinus physiology and weaken or even kill the tree. In China, the Mongolian pine (Pinus sylvestris var. mongolica), an important wind-proof and sand-fixing species, is the unique host of S. noctilio. To explore the interplay between S. noctilio venom and Mongolian pine, we performed a transcriptome comparative analysis of a 10-year-old Mongolian pine after wounding and inoculation with S. noctilio venom. The analysis was performed at 12 h, 24 h and 72 h. PacBio ISO-seq was used and integrated with RNA-seq to construct an accurate full-length transcriptomic database. We obtained 52,963 high-precision unigenes, consisting of 48,654 (91.86%) unigenes that were BLASTed to known sequences in the public database and 4309 unigenes without any annotation information, which were presumed to be new genes. The number of differentially expressed genes (DEGs) increased with the treatment time, and the DEGs were most abundant at 72 h. A total of 706 inoculation-specific DEGs (475 upregulated and 231 downregulated) and 387 wounding-specific DEGs (183 upregulated and 204 downregulated) were identified compared with the control. Under venom stress, we identified 6 DEGs associated with reactive oxygen species (ROS) and 20 resistance genes in Mongolian pine. Overall, 52 transcription factors (TFs) were found under venom stress, 45 of which belonged to the AP2/ERF TF family and were upregulated. A total of 13 genes related to the photosystem, 3 genes related photo-regulation, and 9 TFs were identified under wounding stress. In conclusion, several novel putative genes were found in Mongolian pine by PacBio ISO seq. Meanwhile, we also identified various genes that were resistant to S. noctilio venom, such as GAPDH, GPX, CAT, FL2, CERK1, and HSP83A, etc.

The converging path of protein SUMOylation in phytohormone signalling: highlights and new frontiers

The intersection of phytohormone signalling pathways with SUMOylation, a key post-translational modification, offers an additional layer of control to the phytohormone signalling for sophisticated regulation of plant development. Plants live in a constantly changing environment that are often challenging for the growth and development of plants. Phytohormones play a critical role in modulating molecular-level changes for enabling plants to resist climatic aberrations. The orchestration of such effective molecular responses entails rapid regulation of phytohormone signalling at transcriptional, translational and post-translational levels. Post-translational modifications have emerged as a key player in modulating hormonal pathways. The current review lays emphasis on the role of SUMOylation, a key post-translational modification, in manipulating individual hormone signalling pathways for better plant adaptability. Here, we discuss the recent advancement in the field and highlights how SUMO targets key signalling intermediates including transcription factors to provide a quick response to different biotic or abiotic stresses, sometimes even prior to changes in hormone levels. The understanding of the convergence of SUMOylation and hormonal pathways will offer an additional layer of control to the phytohormone signalling for an intricate and sophisticated regulation of plant development and can be utilised as a tool to generate climate-resilient crops.

Global SUMOylome Adjustments in Basal Defenses of Arabidopsis thaliana Involve Complex Interplay Between SMALL-UBIQUITIN LIKE MODIFIERs and the Negative Immune Regulator SUPPRESSOR OF rps4-RLD1

Steady-state SUMOylome of a plant is adjusted locally during developmental transitions and more globally during stress exposures. We recently reported that basal immunity in Arabidopsis thaliana against Pseudomonas syringae pv tomato strain DC3000 (PstDC3000) is associated with strong enhancements in the net SUMOylome. Transcriptional upregulations of SUMO conjugases, suppression of protease, and increased SUMO translations accounted for this enhanced SUMOylation. Antagonistic roles of SUMO1/2 and SUMO3 isoforms further fine-tuned the SUMOylome adjustments, thus impacting defense amplitudes and immune outcomes. Loss of function of SUPPRESSOR OF rps4-RLD1 (SRFR1), a previously reported negative regulator of basal defenses, also caused constitutive increments in global SUMO-conjugates through similar modes. These suggest that SRFR1 plays a pivotal role in maintenance of SUMOylation homeostasis and its dynamic changes during immune elicitations. Here, we demonstrate that SRFR1 degradation kinetically precedes and likely provides the salicylic acid (SA) elevations necessary for the SUMOylome increments in basal defenses. We show that SRFR1 not only is a SUMOylation substrate but also interacts in planta with both SUMO1 and SUMO3. In sum1 or sum3 mutants, SRFR1 stabilities are reduced albeit by different modes. Whereas a srfr1 sum1 combination is lethal, the srfr1 sum3 plants retain developmental defects and enhanced immunity of the srfr1 parent. Together with increasing evidence of SUMOs self-regulating biochemical efficiencies of SUMOylation-machinery, we present their impositions on SRFR1 expression that in turn counter-modulates the SUMOylome. Overall, our investigations reveal multifaceted dynamics of regulated SUMOylome changes via SRFR1 in defense-developmental balance.

More than an on-and-off switch: Post-translational modifications of plant pattern recognition receptor complexes

Sensing microbe-associated molecular patterns (MAMPs) by cell surface-resident pattern recognition receptors (PRRs) constitutes a core process in launching a successful immune response. Over the last decade, remarkable progress has been made in delineating the mechanisms of PRR-mediated plant immunity. As the frontline of defense, the homeostasis, activities, and subcellular dynamics of PRR and associated regulators are subjected to tight regulations. The layered protein post-translational modifications, particularly the intertwined phosphorylation and ubiquitylation of PRR complexes, play a central role in regulating PRR signaling outputs and plant immune responses. This review provides an update about the PRR complex regulation by various post-translational modifications and discusses how protein phosphorylation and ubiquitylation act in concert to ensure a rapid, proper, and robust immune response.

SUMOylation in Phytopathogen Interactions: Balancing Invasion and Resistance

Plants are constantly confronted by a multitude of biotic stresses involving a myriad of pathogens. In crops, pathogen infections result in significant agronomical losses worldwide posing a threat to food security. In order to enter plant tissues and establish a successful infection, phytopathogens have to surpass several physical, and chemical defense barriers. In recent years, post-translational modification (PTM) mechanisms have emerged as key players in plant defense against pathogens. PTMs allow a highly dynamic and rapid response in front of external challenges, increasing the complexity and precision of cellular responses. In this review, we focus on the role of SUMO conjugation (SUMOylation) in plant immunity against fungi, bacteria, and viruses. In plants, SUMO regulates multiple biological processes, ranging from development to responses arising from environmental challenges. During pathogen attack, SUMO not only modulates the activity of plant defense components, but also serves as a target of pathogen effectors, highlighting its broad role in plant immunity. Here, we summarize known pathogenic strategies targeting plant SUMOylation and, the plant SUMO conjugates involved in host-pathogen interactions. We also provide a catalog of candidate SUMO conjugates according to their role in defense responses. Finally, we discuss the complex role of SUMO in plant defense, focusing on key biological and experimental aspects that contribute to some controversial conclusions, and the opportunities for improving agricultural productivity by engineering SUMOylation in crop species.

SUMO enables substrate selectivity by mitogen-activated protein kinases to regulate immunity in plants

The versatility of mitogen-activated protein kinases (MAPKs) in translating exogenous and endogenous stimuli into appropriate cellular responses depends on its substrate specificity. In animals, several mechanisms have been proposed about how MAPKs maintain specificity to regulate distinct functional pathways. However, little is known of mechanisms that enable substrate selectivity in plant MAPKs. Small ubiquitin-like modifier (SUMO), a posttranslational modification system, plays an important role in plant development and defense by rapid reprogramming of cellular events. In this study we identified a functional SUMO interaction motif (SIM) in Arabidopsis MPK3 and MPK6 that reveals a mechanism for selective interaction of MPK3/6 with SUMO-conjugated WRKY33, during defense. We show that WRKY33 is rapidly SUMOylated in response to Botrytis cinerea infection and flg22 elicitor treatment. SUMOylation mediates WRKY33 phosphorylation by MPKs and consequent transcription factor activity. Disruption of either WRKY33 SUMO or MPK3/6 SIM sites attenuates their interaction and inactivates WRKY33-mediated defense. However, MPK3/6 SIM mutants show normal interaction with a non-SUMOylated form of another transcription factor, SPEECHLESS, unraveling a role for SUMOylation in differential substrate selectivity by MPKs. We reveal that the SUMO proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2 control WRKY33 SUMOylation and demonstrate a role for these SUMO proteases in defense. Our data reveal a mechanism by which MPK3/6 prioritize molecular pathways by differentially selecting substrates using the SUMO-SIM module during defense responses.

Understanding and Exploiting Post-Translational Modifications for Plant Disease Resistance

Plants are constantly threatened by pathogens, so have evolved complex defence signalling networks to overcome pathogen attacks. Post-translational modifications (PTMs) are fundamental to plant immunity, allowing rapid and dynamic responses at the appropriate time. PTM regulation is essential; pathogen effectors often disrupt PTMs in an attempt to evade immune responses. Here, we cover the mechanisms of disease resistance to pathogens, and how growth is balanced with defence, with a focus on the essential roles of PTMs. Alteration of defence-related PTMs has the potential to fine-tune molecular interactions to produce disease-resistant crops, without trade-offs in growth and fitness.

The ubiquitin E3 ligase SR1 modulates the submergence response by degrading phosphorylated WRKY33 in Arabidopsis

Oxygen deprivation caused by flooding activates acclimation responses to stress and restricts plant growth. After experiencing flooding stress, plants must restore normal growth; however, which genes are dynamically and precisely controlled by flooding stress remains largely unknown. Here, we show that the Arabidopsis thaliana ubiquitin E3 ligase SUBMERGENCE RESISTANT1 (SR1) regulates the stability of the transcription factor WRKY33 to modulate the submergence response. SR1 physically interacts with WRKY33 in vivo and in vitro and controls its ubiquitination and proteasomal degradation. Both the sr1 mutant and WRKY33 overexpressors exhibited enhanced submergence tolerance and enhanced expression of hypoxia-responsive genes. Genetic experiments showed that WRKY33 functions downstream of SR1 during the submergence response. Submergence induced the phosphorylation of WRKY33, which enhanced the activation of RAP2.2, a positive regulator of hypoxia-response genes. Phosphorylated WRKY33 and RAP2.2 were degraded by SR1 and the N-degron pathway during reoxygenation, respectively. Taken together, our findings reveal that the on-and-off module SR1-WRKY33-RAP2.2 is connected to the well-known N-degron pathway to regulate acclimation to submergence in Arabidopsis. These two different but related modulation cascades precisely balance submergence acclimation with normal plant growth.

Ubiquitin signalling: controlling the message of surface immune receptors

Microbial attack is first detected by immune receptors located at the plasma membrane. Their activation triggers a plethora of signalling cascades that culminate in the immune response. Ubiquitin and ubiquitin-like protein modifiers play key roles in controlling signalling amplitude and intensity, as well as in buffering proteome imbalances caused by pathogen attack. Here I highlight some of the important advances in the field, which are starting to reveal an intertwined and complex signalling circuitry, which regulates cellular dynamics and protein degradation to maintain homeostasis.

Prunus persica plant endogenous peptides PpPep1 and PpPep2 cause PTI-like transcriptome reprogramming in peach and enhance resistance to Xanthomonas arboricola pv. pruni

BACKGROUND

Rosaceae species are economically highly relevant crops. Their cultivation systems are constrained by phytopathogens causing severe losses. Plants respond to invading pathogens through signaling mechanisms, a component of which are of them being plant elicitor peptides (Peps). Exogenous application of Peps activates defense mechanisms and reduces the symptoms of pathogen infection in various pathosystems. We have previously identified the Rosaceae Peps and showed, in an ex vivo system, that their topical application efficiently enhanced resistance to the bacterial pathogen Xanthomonas arboricola pv. pruni (Xap).

RESULTS

Here we demonstrate the effectiveness of Prunus persica peptides PpPep1 and PpPep2 in protecting peach plants in vivo at nanomolar doses, with 40% reduction of the symptoms following Xap massive infection. We used deep sequencing to characterize the transcriptomic response of peach plants to preventive treatment with PpPep1 and PpPep2. The two peptides induced highly similar massive transcriptomic reprogramming in the plant. One hour, 1 day and 2 days after peptide application there were changes in expression in up to 8% of peach genes. We visualized the transcriptomics dynamics in a background knowledge network and detected the minor variations between plant responses to PpPep1 and PpPep2, which might explain their slightly different protective effects. By designing a P. persica Pep background knowledge network, comparison of our data and previously published immune response datasets was possible.

CONCLUSIONS

Topical application of P. persica Peps mimics the PTI natural response and protects plants against massive Xap infection. This makes them good candidates for deployment of natural, targeted and environmental-friendly strategies to enhance resistance in Prunus species and prevent important biotic diseases.

Lessons from Comparison of Hypoxia Signaling in Plants and Mammals

Hypoxia is an important stress for organisms, including plants and mammals. In plants, hypoxia can be the consequence of flooding and causes important crop losses worldwide. In mammals, hypoxia stress may be the result of pathological conditions. Understanding the regulation of responses to hypoxia offers insights into novel approaches for crop improvement, particularly for the development of flooding-tolerant crops and for producing better therapeutics for hypoxia-related diseases such as inflammation and cancer. Despite their evolutionary distance, plants and mammals deploy strikingly similar mechanisms to sense and respond to the different aspects of hypoxia-related stress, including low oxygen levels and the resulting energy crisis, nutrient depletion, and oxidative stress. Over the last two decades, the ubiquitin/proteasome system and the ubiquitin-like protein SUMO have been identified as key regulators that act in concert to regulate core aspects of responses to hypoxia in plants and mammals. Here, we review ubiquitin and SUMO-dependent mechanisms underlying the regulation of hypoxia response in plants and mammals. By comparing and contrasting these mechanisms in plants and mammals, this review seeks to pinpoint conceptually similar mechanisms but also highlight future avenues of research at the junction between different fields of research.

Non-TZF Protein AtC3H59/ZFWD3 Is Involved in Seed Germination, Seedling Development, and Seed Development, Interacting with PPPDE Family Protein Desi1 in Arabidopsis

Despite increasing reports on the function of CCCH zinc finger proteins in plant development and stress response, the functions and molecular aspects of many non-tandem CCCH zinc finger (non-TZF) proteins remain uncharacterized. AtC3H59/ZFWD3 is an Arabidopsis non-TZF protein and belongs to the ZFWD subfamily harboring a CCCH zinc finger motif and a WD40 domain. In this study, we characterized the biological and molecular functions of AtC3H59, which is subcellularly localized in the nucleus. The seeds of AtC3H59-overexpressing transgenic plants (OXs) germinated faster than those of wild type (WT), whereas atc3h59 mutant seeds germinated slower than WT seeds. AtC3H59 OX seedlings were larger and heavier than WT seedlings, whereas atc3h59 mutant seedlings were smaller and lighter than WT seedlings. Moreover, AtC3H59 OX seedlings had longer primary root length than WT seedlings, whereas atc3h59 mutant seedlings had shorter primary root length than WT seedlings, owing to altered cell division activity in the root meristem. During seed development, AtC3H59 OXs formed larger and heavier seeds than WT. Using yeast two-hybrid screening, we isolated Desi1, a PPPDE family protein, as an interacting partner of AtC3H59. AtC3H59 and Desi1 interacted via their WD40 domain and C-terminal region, respectively, in the nucleus. Taken together, our results indicate that AtC3H59 has pleiotropic effects on seed germination, seedling development, and seed development, and interacts with Desi1 in the nucleus via its entire WD40 domain. To our knowledge, this is the first report to describe the biological functions of the ZFWD protein and Desi1 in Arabidopsis.

DYNAMIN-RELATED PROTEIN DRP1A functions with DRP2B in plant growth, flg22-immune responses, and endocytosis

Ligand-induced endocytosis of the immune receptor FLAGELLIN SENSING2 (FLS2) is critical for maintaining its proper abundance in the plasma membrane (PM) to initiate and subsequently down regulate cellular immune responses to bacterial flagellin or flg22-peptide. The molecular components governing PM abundance of FLS2, however, remain mostly unknown. Here, we identified Arabidopsis (Arabidopsis thaliana) DYNAMIN-RELATED PROTEIN1A (DRP1A), a member of a plant-specific family of large dynamin GTPases, as a critical contributor to ligand-induced endocytosis of FLS2 and its physiological roles in flg22-signaling and immunity against Pseudomonas syringae pv. tomato DC3000 bacteria in leaves. Notably, drp1a single mutants displayed similar flg22-defects as those previously reported for mutants in another dynamin-related protein, DRP2B, that was previously shown to colocalize with DRP1A. Our study also uncovered synergistic roles of DRP1A and DRP2B in plant growth and development as drp1a drp2b double mutants exhibited severely stunted roots and cotyledons, as well as defective cell shape, cytokinesis, and seedling lethality. Furthermore, drp1a drp2b double mutants hyperaccumulated FLS2 in the PM prior to flg22-treatment and exhibited a block in ligand-induced endocytosis of FLS2, indicating combinatorial roles for DRP1A and DRP1B in governing PM abundance of FLS2. However, the increased steady-state PM accumulation of FLS2 in drp1a drp2b double mutants did not result in increased flg22 responses. We propose that DRP1A and DRP2B are important for the regulation of PM-associated levels of FLS2 necessary to attain signaling competency to initiate distinct flg22 responses, potentially through modulating the lipid environment in defined PM domains.

Post-translational Modifications of bZIP Transcription Factors in Abscisic Acid Signaling and Drought Responses

Under drought stress, plants have developed various mechanisms to survive in the reduced water supply, of which the regulation of stress-related gene expression is responsible for several transcription factors. The basic leucine zippers (bZIPs) are one of the largest and most diverse transcription factor families in plants. Among the 10 Arabidopsis bZIP groups, group A bZIP transcription factors function as a positive or negative regulator in ABA signal transduction and drought stress response. These bZIP transcription factors, which are involved in the drought response, have also been isolated in various plant species such as rice, pepper, potato, and maize. Recent studies have provided substantial evidence that many bZIP transcription factors undergo the post-translational modifications, through which the regulation of their activity or stability affects plant responses to various intracellular or extracellular stimuli. This review aims to address the modulation of the bZIP proteins in ABA signaling and drought responses through phosphorylation, ubiquitination and sumoylation.

SUMO mediated regulation of transcription factors as a mechanism for transducing environmental cues into cellular signaling in plants

Across all species, transcription factors (TFs) are the most frequent targets of SUMOylation. The effect of SUMO conjugation on the functions of transcription factors has been extensively studied in animal systems, with over 200 transcription factors being documented to be modulated by SUMOylation. This has resulted in the establishment of a number of paradigms that seek to explain the mechanisms by which SUMO regulates transcription factor functions. For instance, SUMO has been shown to modulate TF DNA binding activity; regulate both localization as well as the abundance of TFs and also influence the association of TFs with chromatin. With transcription factors being implicated as master regulators of the cellular signalling pathways that maintain phenotypic plasticity in all organisms, in this review, we will discuss how SUMO mediated regulation of transcription factor activity facilitates molecular pathways to mount an appropriate and coherent biological response to environmental cues.

Towards understanding the multifaceted role of SUMOylation in plant growth and development

Post-translational modifications (PTMs) play a critical role in regulating plant growth and development through the modulation of protein functionality and its interaction with its partners. Analysis of the functional implication of PTMs on plant cellular signalling presents grand challenges in understanding their significance. Proteins decorated or modified with another chemical group or polypeptide play a significant role in regulating physiological processes as compared with non-decorated or non-modified proteins. In the past decade, SUMOylation has been emerging as a potent PTM influencing the adaptability of plants to growth, in response to various environmental cues. Deciphering the SUMO-mediated regulation of plant stress responses and its consequences is required to understand the mechanism underneath. Here, we will discuss the recent advances in the role and significance of SUMOylation in plant growth, development and stress response.

An Insight into the Factors Influencing Specificity of the SUMO System in Plants

Due to their sessile nature, plants are constantly subjected to various environmental stresses such as drought, salinity, and pathogen infections. Post-translational modifications (PTMs), like SUMOylation, play a vital role in the regulation of plant responses to their environment. The process of SUMOylation typically involves an enzymatic cascade containing the activation, (E1), conjugation (E2), and ligation (E3) of SUMO to a target protein. Additionally, it also requires a class of SUMO proteases that generate mature SUMO from its precursor and cleave it off the target protein, a process termed deSUMOylation. It is now clear that SUMOylation in plants is key to a plethora of adaptive responses. How this is achieved with an extremely limited set of machinery components is still unclear. One possibility is that novel SUMO components are yet to be discovered. However, current knowledge indicates that only a small set of enzymes seem to be responsible for the modification of a large number of SUMO substrates. It is yet unknown where the specificity lies within the SUMO system. Although this seems to be a crucial question in the field of SUMOylation studies, not much is known about the factors that provide specificity. In this review, we highlight the role of the localisation of SUMO components as an important factor that can play a vital role in contributing to the specificity within the process. This will introduce a new facet to our understanding of the mechanisms underlying such a dynamic process.

Global transcriptome analyses reveal the molecular signatures in the early response of potato (Solanum tuberosum L.) to Phytophthora infestans, Ralstonia solanacearum, and Potato virus Y infection

Specific and common genes including transcription factors, resistance genes and pathways were significantly induced in potato by Phytophthora infestans, Ralstonia solanacearum, and Potato virus Y infection. The three major pathogens, namely, Phytophthora infestans, Ralstonia solanacearum, and Potato virus Y, can cause late blight, bacterial wilt, and necrotic ringspot, respectively, and thus severely reduce the yield and quality of potatoes (Solanum tuberosum L.). This study was the first to systematically analyze the relationship between transcriptome alterations in potato infected by these pathogens at the early stages. A total of 75,500 unigenes were identified, and 44,008 were annotated into 5 databases, namely, non-redundant (NR), Swiss-Prot protein, clusters of orthologous groups for eukaryotic complete genomes (KOG), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. A total of 6945 resistance genes and 11,878 transcription factors (TFs) were identified from all transcriptome data. Differential expression analysis revealed that 13,032 (9490 specifics), 9877 (6423 specifics), and 6661 (4144 specifics) differentially expressed genes (DEGs) were generated from comparisons of the P. infestans/control (Pi vs. Pi-CK), R. solanacearum/control (Rs vs. Rs-CK), and PVY/control (PVY vs. PVY-CK) treatments, respectively. The specific DEGs from the 3 comparisons were assigned to 13 common pathways, such as biosynthesis of amino acids, plant hormone signal transduction, carbon metabolism, and starch and sucrose metabolism. Weighted Gene Co-Expression Network Analysis (WGCNA) identified many hub unigenes, of which several unigenes were reported to regulate plant immune responses, such as FLAGELLIN-SENSITIVE 2 and chitinases. The present study provide crucial systems-level insights into the relationship between transcriptome changes in potato infected with the three pathogens. Moreover, this study presents a theoretical basis for breeding broad-spectrum and specific pathogen-resistant cultivars.

SIZ1-Mediated SUMO Modification of SEUSS Regulates Photomorphogenesis in Arabidopsis

Small ubiquitin-like modifier (SUMO) post-translational modification (SUMOylation) plays essential roles in regulating various biological processes; however, its function and regulation in the plant light signaling pathway are largely unknown. SEUSS (SEU) is a transcriptional co-regulator that integrates light and temperature signaling pathways, thereby regulating plant growth and development in Arabidopsis thaliana. Here, we show that SEU is a substrate of SUMO1, and that substitution of four conserved lysine residues disrupts the SUMOylation of SEU, impairs its function in photo- and thermomorphogenesis, and enhances its interaction with PHYTOCHROME-INTERACTING FACTOR 4 transcription factors. Furthermore, the SUMO E3 ligase SIZ1 interacts with SEU and regulates its SUMOylation. Moreover, SEU directly interacts with phytochrome B photoreceptors, and the SUMOylation and stability of SEU are activated by light. Our study reveals a novel post-translational modification mechanism of SEU in which light regulates plant growth and development through SUMOylation-mediated protein stability.

Revised annotation and extended characterizations of components of the Chlamydomonas reinhardtii SUMOylation system

Small ubiquitin-like modifier (SUMO) conjugation, or SUMOylation, is a reversible post-translational modification that is important for regulation of many cellular processes including cell division cycle in the eukaryotic kingdom. However, only a portion of the components of the Chlamydomonas SUMOylation system are known and their functions and regulation investigated. The present studies are aimed at extending discovery and characterization of new components and improving the annotation and nomenclature of all known proteins and genes involved in the system. Even though only one copy of the heterodimerized SUMO-activating enzyme, SAE1 and SAE2, was identified, the number of SUMO-conjugating enzymes (SCEs) and SUMO proteases/isopeptidase was expanded in Chlamydomonas. Using the reconstituted SUMOylation system, we showed that SCE1, SCE2, and SCE3 have SUMO-conjugating activity. In addition to SUMOylation, components required for other post-translational modifications such as NEDDylation, URMylation, and UFMylation, were confirmed to be present in Chlamydomonas. Our data also showed that besides isopeptidase activity, the SUMO protease domain of SUPPRESSOR OF MAT3 7/SENTRIN-SPECIFIC PROTEASE 1 (SMT7/SENP1) has endopeptidase activity that is capable of processing SUMO precursors. Moreover, the key cell cycle regulators of Chlamydomonas E2F1, DP1, CDKG1, CYCD2, and CYCD3 were SUMOylated in vitro, suggesting SUMOylation may be part of regulatory pathway modulating cell cycle regulators.

Ubiquitin-Like protein 5 interacts with the silencing suppressor p3 of rice stripe virus and mediates its degradation through the 26S proteasome pathway

Ubiquitin like protein 5 (UBL5) interacts with other proteins to regulate their function but differs from ubiquitin and other UBLs because it does not form covalent conjugates. Ubiquitin and most UBLs mediate the degradation of target proteins through the 26S proteasome but it is not known if UBL5 can also do that. Here we found that the UBL5s of rice and Nicotiana benthamiana interacted with rice stripe virus (RSV) p3 protein. Silencing of NbUBL5s in N. benthamiana facilitated RSV infection, while UBL5 overexpression conferred resistance to RSV in both N. benthamiana and rice. Further analysis showed that NbUBL5.1 impaired the function of p3 as a suppressor of silencing by degrading it through the 26S proteasome. NbUBL5.1 and OsUBL5 interacted with RPN10 and RPN13, the receptors of ubiquitin in the 26S proteasome. Furthermore, silencing of NbRPN10 or NbRPN13 compromised the degradation of p3 mediated by NbUBL5.1. Together, the results suggest that UBL5 mediates the degradation of RSV p3 protein through the 26S proteasome, a previously unreported plant defense strategy against RSV infection.

Crosstalk between Ubiquitination and Other Post-translational Protein Modifications in Plant Immunity

Post-translational modifications (PTMs) are central to the modulation of protein activity, stability, subcellular localization, and interaction with partners. They greatly expand the diversity and functionality of the proteome and have taken the center stage as key players in regulating numerous cellular and physiological processes. Increasing evidence indicates that in addition to a single regulatory PTM, many proteins are modified by multiple different types of PTMs in an orchestrated manner to collectively modulate the biological outcome. Such PTM crosstalk creates a combinatorial explosion in the number of proteoforms in a cell and greatly improves the ability of plants to rapidly mount and fine-tune responses to different external and internal cues. While PTM crosstalk has been investigated in depth in humans, animals, and yeast, the study of interplay between different PTMs in plants is still at its infant stage. In the past decade, investigations showed that PTMs are widely involved and play critical roles in the regulation of interactions between plants and pathogens. In particular, ubiquitination has emerged as a key regulator of plant immunity. This review discusses recent studies of the crosstalk between ubiquitination and six other PTMs, i.e., phosphorylation, SUMOylation, poly(ADP-ribosyl)ation, acetylation, redox modification, and glycosylation, in the regulation of plant immunity. The two basic ways by which PTMs communicate as well as the underlying mechanisms and diverse outcomes of the PTM crosstalk in plant immunity are highlighted.

Reversible SUMOylation of FHY1 Regulates Phytochrome A Signaling in Arabidopsis

In response to far-red light (FR), FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) transports the photoactivated phytochrome A (phyA), the primary FR photoreceptor, into the nucleus, where it initiates FR signaling in plants. Light promotes the 26S proteasome-mediated degradation of FHY1, which desensitizes FR signaling, but the underlying regulatory mechanism remains largely unknown. Here, we show that reversible SUMOylation of FHY1 tightly regulates this process. Lysine K32 (K32) and K103 are major SUMOylation sites of FHY1. We found that FR exposure promotes the SUMOylation of FHY1, which accelerates its degradation. Furthermore, we discovered that ARABIDOPSIS SUMO PROTEASE 1 (ASP1) interacts with FHY1 in the nucleus under FR and facilitates its deSUMOylation. FHY1 was strongly SUMOylated and its protein level was decreased in the asp1-1 loss-of-function mutant compared with that in the wild type under FR. Consistently, asp1-1 seedlings exhibited a decreased sensitivity to FR, suggesting that ASP1 plays an important role in the maintenance of proper FHY1 levels under FR. Genetic analysis further revealed that ASP1 regulates FR signaling through an FHY1- and phyA-dependent pathway. Interestingly, We found that continuous FR inhibits ASP1 accumulation, perhaps contributing to the desensitization of FR signaling. Taken together, these results indicate that FR-induced SUMOylation and ASP1-dependent deSUMOylation of FHY1 represent a key regulatory mechanism that fine-tunes FR signaling.

Diversity, Function and Regulation of Cell Surface and Intracellular Immune Receptors in Solanaceae

The first layer of the plant immune system comprises plasma membrane-localized receptor proteins and intracellular receptors of the nucleotide-binding leucine-rich repeat protein superfamily. Together, these immune receptors act as a network of surveillance machines in recognizing extracellular and intracellular pathogen invasion-derived molecules, ranging from conserved structural epitopes to virulence-promoting effectors. Successful pathogen recognition leads to physiological and molecular changes in the host plants, which are critical for counteracting and defending against biotic attack. A breadth of significant insights and conceptual advances have been derived from decades of research in various model plant species regarding the structural complexity, functional diversity, and regulatory mechanisms of these plant immune receptors. In this article, we review the current state-of-the-art of how these host surveillance proteins function and how they are regulated. We will focus on the latest progress made in plant species belonging to the Solanaceae family, because of their tremendous importance as model organisms and agriculturally valuable crops.

Surface Sensor Systems in Plant Immunity

Protein complexes at the cell surface facilitate the detection of danger signals from diverse pathogens and initiate a series of complex intracellular signaling events that result in various immune responses.

The Role of Proteases in Determining Stomatal Development and Tuning Pore Aperture: A Review

Plant proteases, the proteolytic enzymes that catalyze protein breakdown and recycling, play an essential role in a variety of biological processes including stomatal development and distribution, as well as, systemic stress responses. In this review, we summarize what is known about the participation of proteases in both stomatal organogenesis and on the stomatal pore aperture tuning, with particular emphasis on their involvement in numerous signaling pathways triggered by abiotic and biotic stressors. There is a compelling body of evidence demonstrating that several proteases are directly or indirectly implicated in the process of stomatal development, affecting stomatal index, density, spacing, as well as, size. In addition, proteases are reported to be involved in a transient adjustment of stomatal aperture, thus orchestrating gas exchange. Consequently, the proteases-mediated regulation of stomatal movements considerably affects plants' ability to cope not only with abiotic stressors, but also to perceive and respond to biotic stimuli. Even though the determining role of proteases on stomatal development and functioning is just beginning to unfold, our understanding of the underlying processes and cellular mechanisms still remains far from being completed.

SUMO Conjugation to BZR1 Enables Brassinosteroid Signaling to Integrate Environmental Cues to Shape Plant Growth

Brassinosteroids (BRs) play crucial roles in plant development, but little is known of mechanisms that integrate environmental cues into BR signaling. Conjugation to the small ubiquitin-like modifier (SUMO) is emerging as an important mechanism to transduce environmental cues into cellular signaling. In this study, we show that SUMOylation of BZR1, a key transcription factor of BR signaling, provides a conduit for environmental influence to modulate growth during stress. SUMOylation stabilizes BZR1 in the nucleus by inhibiting its interaction with BIN2 kinase. During salt stress, Arabidopsis plants arrest growth through deSUMOylation of BZR1 in the cytoplasm by promoting the accumulation of the BZR1 targeting SUMO protease, ULP1a. ULP1a mutants are salt tolerant and insensitive to the BR inhibitor, brassinazole. BR treatment stimulates ULP1a degradation, allowing SUMOylated BZR1 to accumulate and promote growth. This study uncovers a mechanism for integrating environmental cues into BR signaling to shape growth.

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BZR1 SUMOylation allows brassinosteroids to shape plant growth to its environment

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SUMOylation stabilizes BZR1 by inhibiting BIN2 interaction, promoting plant growth

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Salinity stimulates BZR1 deSUMOylation via ULP1a SUMO protease to suppress growth

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BRs destabilize ULP1a, allowing SUMOylated BZR1 to accumulate and promote growth

Plant immune signaling: Advancing on two frontiers

Plants have evolved multiple defense strategies to cope with pathogens, among which plant immune signaling that relies on cell-surface localized and intracellular receptors takes fundamental roles. Exciting breakthroughs were made recently on the signaling mechanisms of pattern recognition receptors (PRRs) and intracellular nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domain receptors (NLRs). This review summarizes the current view of PRRs activation, emphasizing the most recent discoveries about PRRs' dynamic regulation and signaling mechanisms directly leading to downstream molecular events including mitogen-activated protein kinase (MAPK) activation and calcium (Ca²⁺ ) burst. Plants also have evolved intracellular NLRs to perceive the presence of specific pathogen effectors and trigger more robust immune responses. We also discuss the current understanding of the mechanisms of NLR activation, which has been greatly advanced by recent breakthroughs including structures of the first full-length plant NLR complex, findings of NLR sensor-helper pairs and novel biochemical activity of Toll/interleukin-1 receptor (TIR) domain.

Common and unique Arabidopsis proteins involved in stomatal susceptibility to Salmonella enterica and Pseudomonas syringae

Salmonella enterica is one of the most common pathogens associated with produce outbreaks worldwide; nonetheless, the mechanisms uncovering their interaction with plants are elusive. Previous reports demonstrate that S. enterica ser. Typhimurium (STm), similar to the phytopathogen Pseudomonas syringae pv. tomato (Pst) DC3000, triggers a transient stomatal closure suggesting its ability to overcome this plant defense and colonize the leaf apoplast. In order to discover new molecular players that function in the stomatal reopening by STm and Pst DC3000, we performed an Arabidopsis mutant screening using thermal imaging. Further stomatal bioassay confirmed that the mutant plants exo70h4-3, sce1-3, bbe8, stp1, and lsu2 have smaller stomatal aperture widths than the wild type Col-0 in response to STm 14028s. The mutants bbe8, stp1 and lsu2 have impaired stomatal movement in response to Pst DC3000. These findings indicate that EXO70H4 and SCE1 are involved in bacterial-specific responses, while BBE8, STP1, and LSU2 may be required for stomatal response to a broad range of bacteria. The identification of new molecular components of the guard cell movement induced by bacteria will enable a better understanding of the initial stages of plant colonization and facilitate targeted prevention of leaf contamination with harmful pathogens.

Exploring the regulatory levels of SUMOylation to increase crop productivity

SUMOylation is an essential post-translational modification that affects several cellular processes, from gene replication to stress response. Studies using the SUMO (de)conjugation machinery have provided evidence regarding its potential to improve crop performance and productivity under normal and adverse conditions. However, the pleiotropic effect of SUMOylation can be a disadvantage in both situations, especially when considering unpredictable environmental conditions caused by climate changes. Here, we discuss the pleiotropic effects caused by disrupting the SUMOylation machinery, and new strategies that may help to overcome pleiotropy. We propose exploring the several regulatory levels of SUMOylation recently revealed, including transcriptional, post-transcriptional regulation by alternative splicing, and post-translational modifications. These new findings may provide valuable tools to increase crop productivity.

Dealing With Stress: A Review of Plant SUMO Proteases

The SUMO system is a rapid dynamic post-translational mechanism employed by eukaryotic cells to respond to stress. Plant cells experience hyperSUMOylation of substrates in response to stresses such as heat, ethanol, and drought. Many SUMOylated proteins are located in the nucleus, SUMOylation altering many nuclear processes. The SUMO proteases play two key functions in the SUMO cycle by generating free SUMO; they have an important role in regulating the SUMO cycle, and by cleaving SUMO off SUMOylated proteins, they provide specificity to which proteins become SUMOylated. This review summarizes the broad literature of plant SUMO proteases describing their catalytic activity, domains and structure, evolution, localization, and response to stress and highlighting potential new areas of research in the future.