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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., (© 2022 The Author(s).)

Ubiquitin is well established as a major modifier of signalling in eukaryotes. However, the extent to which plants rely on ubiquitin for regulating their lifecycle is only recently becoming apparent. This is underlined by the over-representation of genes encoding ubiquitin-metabolizing enzymes in Arabidopsis when compared with other model eukaryotes. The main characteristic of ubiquitination is the conjugation of ubiquitin onto lysine residues of acceptor proteins. In most cases the targeted protein is rapidly degraded by the 26S proteasome, the major proteolysis machinery in eukaryotic cells. The ubiquitin-proteasome system is responsible for removing most abnormal peptides and short-lived cellular regulators, which, in turn, control many processes. This allows cells to respond rapidly to intracellular signals and changing environmental conditions. This review maps out the roles of the components of the ubiquitin-proteasome system with emphasis on areas where future research is urgently needed. We provide a flavour of the diverse aspects of plant lifecycle where the ubiquitin-proteasome system is implicated. We aim to highlight common themes using key examples that reiterate the importance of the ubiquitin-proteasome system to plants. The future challenge in plant biology is to define the targets for ubiquitination, their interactors and their molecular function within the regulatory context., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

In this issue of Molecular Cell , Kong et al. (2021) report that in Arabidopsis , immune elicitation promotes mono(ADP-ribosyl)ation (MARylation) of immune regulators SZP1 and SZP2 by a noncanonical ADP-ribosyltransferase, SRO2. MARylation results in stabilization of SZF1 by antagonizing its ubiquitin mediated proteasomal degradation. Consequently, these MARylation events ensure appropriate immune responses. [ABSTRACT FROM AUTHOR]

Hypersensitive response programmed cell death (HR-PCD) is a critical feature in plant immunity required for pathogen restriction and prevention of disease development. The precise control of this process is paramount to cell survival and an effective immune response. The discovery of new components that function to suppress HR-PCD will be instrumental in understanding the regulation of this fundamental mechanism. Here we report the identification and characterisation of a BTB domain E3 ligase protein, POB1, that functions to suppress HR-PCD triggered by evolutionarily diverse pathogens. Nicotiana benthamiana and tobacco plants with reduced POB1 activity show accelerated HR-PCD whilst those with increased POB1 levels show attenuated HR-PCD. We demonstrate that POB1 dimerization and nuclear localization are vital for its function in HR-PCD suppression. Using protein-protein interaction assays, we identify the Plant U-Box E3 ligase PUB17, a well established positive regulator of plant innate immunity, as a target for POB1-mediated proteasomal degradation. Using confocal imaging and in planta immunoprecipitation assays we show that POB1 interacts with PUB17 in the nucleus and stimulates its degradation. Mutated versions of POB1 that show reduced interaction with PUB17 fail to suppress HR-PCD, indicating that POB1-mediated degradation of PUB17 U-box E3 ligase is an important step for negative regulation of specific immune pathways in plants. Our data reveals a new mechanism for BTB domain proteins in suppressing HR-PCD in plant innate immune responses. [ABSTRACT FROM AUTHOR]

The SnRK1 protein kinase balances cellular energy levels in accordance with extracellular conditions and is thereby key for plant stress tolerance. In addition, SnRK1 has been implicated in numerous growth and developmental processes from seed filling and maturation to flowering and senescence. Despite its importance, the mechanisms that regulate SnRK1 activity are poorly understood. Here, we demonstrate that the SnRK1 complex is SUMOylated on multiple subunits and identify SIZ1 as the E3 Small Ubiquitin-like Modifier (SUMO) ligase responsible for this modification. We further show that SnRK1 is ubiquitinated in a SIZ1-dependent manner, causing its degradation through the proteasome. In consequence, SnRK1 degradation is deficient in siz1-2 mutants, leading to its accumulation and hyperactivation of SnRK1 signaling. Finally, SnRK1 degradation is strictly dependent on its activity, as inactive SnRK1 variants are aberrantly stable but recover normal degradation when expressed as SUMO mimetics. Altogether, our data suggest that active SnRK1 triggers its own SUMOylation and degradation, establishing a negative feedback loop that attenuates SnRK1 signaling and prevents detrimental hyperactivation of stress responses. [ABSTRACT FROM AUTHOR]

Ubiquitin is a peptide modifier able to form polymers of varying length and linkage as part of a powerful signaling system. Perhaps the best-known aspect of this protein's function is as the driver of targeted protein degradation through the Ubiquitin Proteasome System (UPS). Through the formation of lysine 48-linked polyubiquitin chains, it is able to direct the degradation of tagged proteins by the 26S proteasome, indirectly controlling many processes within the cell. However, recent research has indicated that ubiquitin performs a multitude of other roles within the cell beyond protein degradation. It is able to form 6 other "atypical" linkages though lysine residues at positions 6, 11, 27, 29, 33, and 63. These atypical chains perform a range of diverse functions, including the regulation of iron uptake in response to perceived deficiency, repair of double stranded breaks in the DNA, and regulation of the auxin response through the non-proteasomal degradation of auxin efflux carrier protein PIN1. This review explores the role ubiquitin chain topology plays in plant cellular function. We aim to highlight the importance of these varying functions and the future challenges to be encountered within this field. [ABSTRACT FROM AUTHOR]

Ubiquitin (Ub) is well-established as a major modifier of signaling in eukaryotes. However, the extent to which plants rely on Ub for regulating nutrient uptake is still in its infancy. The main characteristic of ubiquitination is the conjugation of Ub onto lysine residues of acceptor proteins. In most cases the targeted protein is rapidly degraded by the 26S proteasome, the major proteolysis machinery in eukaryotic cells. The Ub-proteasome system is responsible for removing most abnormal peptides and short-lived cellular regulators, which, in turn, control many processes. This allows cells to respond rapidly to intracellular signals and changing environmental conditions. This perspective will discuss how plants utilize Ub conjugation for sensing environmental nutrient levels. We will highlight recent advances in understanding how Ub aids nutrient homeostasis by affecting the trafficking of membrane bound transporters. Given the overrepresentation of genes encoding Ub-metabolizing enzymes in plants, intracellular signaling events regulated by Ub that lead to transcriptional responses due to nutrient starvation is an under explored area ripe for new discoveries.We provide new insight into how Ub based biochemical tools can be exploited to reveal new molecular components that affect nutrient signaling.The mechanistic nature of Ub signaling indicates that dominant form of any new molecular components can be readily generated and are likely shed new light on how plants cope with nutrient limiting conditions. Finally as part of future challenges in this research area we introduce the newly discovered roles of Ub-like proteins in nutrient homeostasis. [ABSTRACT FROM AUTHOR]

Plant pathogens establish infection by secretion of effector proteins that may be delivered inside host cells to manipulate innate immunity. It is increasingly apparent that the ubiquitin proteasome system (UPS) contributes significantly to the regulation of plant defences and, as such, is a target for pathogen effectors. Bacterial effectors delivered by the type III and IV secretion systems have been shown to interact with components of the host UPS. Some of these effectors possess functional domains that are conserved in UPS enzymes, whilst others contain novel domains with ubiquitination activities. Relatively little is known about effector activities in eukaryotic microbial plant pathogens. Nevertheless, effectors from oomycetes that contain an RXLR motif for translocation to the inside of plant cells have been shown to suppress host defences. Annotation of the genome of one such oomycete, the potato late blight pathogen Phytophthora infestans, and protein–protein interaction assays to discover host proteins targeted by the RXLR effector AVR3a, have revealed that this eukaryotic plant pathogen also has the potential to manipulate host plant UPS functions. [ABSTRACT FROM PUBLISHER]

In yeast and in animals the ubiquitin–proteasome system (UPS) is responsible for removing or modifying most abnormal peptides and also short-lived cellular regulators. The UPS therefore influences many processes such as the cell cycle, signal transduction, transcription, and stress responses including defence. In recent years, similar regulatory roles have been identified in plants. In Arabidopsis, mutations in the ubiquitin–proteasome pathway block development, circadian rhythms, photomorphogenesis, floral homeosis, hormone responses, senescence, and pathogen invasion. Plants have evolved an armoury of defence mechanisms that allow them to counter infection. These encompass both basal responses, triggered by recognition of conserved pathogen-associated molecular patterns, and pathogen-specific responses, mediated via pathogen- and plant-specific gene-for-gene recognition events. The role of E3 ubiquitin ligases in mediating plant defence signalling is reviewed and examples where pathogens impinge on the host's ubiquitination machinery acting as molecular mimics to undermine defence are also highlighted. [ABSTRACT FROM PUBLISHER]

Understanding salt stress signaling is key to producing salt-tolerant crops. The small ubiquitin-like modifier (SUMO) is a crucial regulator of signaling proteins in eukaryotes. Attachment of SUMO onto substrates is reversible, and SUMO proteases, which specifically cleave the SUMO-substrate linkages, play a vital regulatory role during SUMOylation. We have identified two SUMO proteases, OVERLY TOLERANT TO SALT1 (OTS1) and OTS2, which are localized in the nucleus and act redundantly to regulate salt stress responses in Arabidopsis thaliana, ots1 ots2 double mutants show extreme sensitivity to salt. However, under low-salt conditions, otsl ots2 double mutants are phenotypically similar to wild-type plants. We demonstrate that salt stress induces a dose-dependent accumulation of SUMO1/2-conjugated proteins in Arabidopsis. ots1 ots2 double mutants constitutively accumulate high levels of SUMO1/2-conjugated proteins even under nonstress conditions and show a further dramatic increase in SUMO1/2-conjugated proteins in response to salt stress. Transgenic lines overexpressing OTS1 have increased salt tolerance and a concomitant reduction in the levels of SUMOylated proteins. Conversely, the ectopic expression of the mutant ots1(C526S) protein lacking SUMO protease activity fails to produce a salt-tolerant phenotype. We show that salt directly affects OTS1-dependent signaling by inducing OTS1 protein degradation. Our results indicate a requirement for OTS1 deSUMOylation activity in plant salt tolerance responses. [ABSTRACT FROM AUTHOR]

Previous analysis of transcriptional changes after elicitation of Cf-9 transgenic tobacco (Nicotiana tabacum) by Avr9 peptide revealed a rapidly upregulated gene, ACRE276. We show that ACRE276 is transiently induced in wounded leaves within 15 min, but upon Avr9 elicitor treatment, this upregulation is enhanced and maintained until cell death onset in Cf-9 tobacco. ACRE276 RNA interference (RNAi) silencing in tobacco results in loss of hypersensitive response (HR) specified by Cf resistance genes. ACRE276 RNAi plants are also compromised for HR mediated by the tobacco mosaic virus defense elicitor p50. Silencing tomato (Lycopersicon esculentum) ACRE276 leads to breakdown of Cf-9-specified resistance against Cladosporium fulvum leaf mold. We confirmed that tobacco ACRE276 is an E3 ubiquitin ligase requiring an intact U-box domain. Bioinformatic analyses revealed Arabidopsis thaliana PLANT U-BOX17 (PUB17) and Brassica napus ARC1 as the closest homologs of tobacco ACRE276. Transiently expressing PUB17 in Cf-9 tobacco silenced for ACRE276 restores HR, while mutant PUB17 lacking E3 ligase activity fails to do so, demonstrating that PUB17 ligase activity is crucial for defense signaling. Arabidopsis PUB17 knockout plants are compromised in RPM1- and RPS4-mediated resistance against Pseudomonas syringae pv tomato containing avirulence genes AvrB and AvrRPS4, respectively. We identify a conserved class of U-box ARMADILLO repeat E3 ligases that are positive regulators of cell death and defense across the Solanaceae and Brassicaceae. [ABSTRACT FROM AUTHOR]

Examines the use of virus-induced gene silencing in determining the functions of ubiquitin ligase-associated protein SGT1 in Nicotiana benthamiana. Recognition of pathogen avirulence; General factors of disease resistance; Mechanisms of R protein-mediated and non host resistance.

The article present a research experiment the enzymes of ubiquitin metabolism in Arabidopsis plants, including the identification of deubiquitinating enzymes (DUBs) AtUBP12 and AtUBP13 and their Solanaceous ortholoque as negative regulators of plant immunity. Overexpression of proteins in tobacco are also analyzed, including the proteins AtUBP12 and NtUBP12.