Dealing With Stress: A Review of Plant SUMO Proteases

SUMOylation contributes to proteostasis of the chloroplast protein import receptor TOC159 during early development

Chloroplast biogenesis describes the transition of non-photosynthetic proplastids to photosynthetically active chloroplasts in the cells of germinating seeds. Chloroplast biogenesis requires the import of thousands of nuclear-encoded preproteins by essential import receptor TOC159. We demonstrate that the small ubiquitin-related modifier (SUMO) pathway crosstalks with the ubiquitin-proteasome pathway to affect TOC159 stability during early plant development. We identified a SUMO3-interacting motif (SIM) in the TOC159 GTPase domain and a SUMO3 covalent SUMOylation site in the membrane domain. A single K to R substitution (K1370R) in the M-domain disables SUMOylation. Compared to wild-type TOC159, TOC159K1370R was destabilized under UPS-inducing stress conditions. However, TOC159K1370R recovered to same protein level as wild-type TOC159 in the presence of a proteasome inhibitor. Thus, SUMOylation partially stabilizes TOC159 against UPS-dependent degradation under stress conditions. Our data contribute to the evolving model of tightly controlled proteostasis of the TOC159 import receptor during proplastid to chloroplast transition.

Proteomic analysis of SUMO1-SUMOylome changes during defense elicitation in Arabidopsis

Rapid adaptation of plants to developmental or physiological cues is facilitated by specific receptors that transduce the signals mostly via post-translational modification (PTM) cascades of downstream partners. Reversible covalent attachment of SMALL UBIQUITIN-LIKE MODIFIER (SUMO), a process termed as SUMOylation, influence growth, development and adaptation of plants to various stresses. Strong regulatory mechanisms maintain the steady-state SUMOylome and mutants with SUMOylation disturbances display mis-primed immunity often with growth consequences. Identity of the SUMO-substrates undergoing SUMOylation changes during defenses however remain largely unknown. Here we exploit either the auto-immune property of an Arabidopsis mutant or defense responses induced in wild-type plants against Pseudomonas syringae pv tomato (PstDC3000) to enrich and identify SUMO1-substrates. Our results demonstrate massive enhancement of SUMO1-conjugates due to increased SUMOylation efficiencies during defense responses. Of the 261 proteins we identify, 29 have been previously implicated in immune-associated processes. Role of others expand to diverse cellular roles indicating massive readjustments the SUMOylome alterations may cause during induction of immunity. Overall, our study highlights the complexities of a plant immune network and identifies multiple SUMO-substrates that may orchestrate the signaling. SIGNIFICANCE: In all eukaryotes, covalent linkage of the SMALL UBIQUITIN-LIKE MODIFIER (SUMOs), a process termed as SUMOylation, on target proteins affect their fate and function. Plants display reversible readjustments in the pool of SUMOylated proteins during biotic and abiotic stress responses. Here, we demonstrate net increase in global SUMO1/2-SUMOylome of Arabidopsis thaliana at induction of immunity. We enrich and identify 261 SUMO1-substrates enhanced in defenses that categorize to diverse cellular processes and include novel candidates with uncharacterized immune-associated roles. Overall, our results highlight intricacies of SUMO1-orchestration in defense signaling networks.

Effects of lead, cadmium and zinc on protein changes in Silene vulgaris shoots cultured in vitro

In the present research, Silene vulgaris as a representative species growing on both unpolluted and heavy metal (HM) polluted terrains were used to identify ecotype-specific responses to metallic stress. Growth, cell ultrastructure and element accumulations were compared between non-metallicolous (NM), calamine (CAL) and serpentine (SER) specimens untreated with HMs and treated with Pb, Cd and Zn ions under in vitro conditions. Moreover, proteins' modifications related to their level, carbonylation and degradations via vacuolar proteases were verified and linked with potential mechanisms to cope with ions toxicity. Our experiment revealed diversified strategy of HM uptake in NM and both metallicolous ecotypes, in which antagonistic relationship of Zn and Pb/Cd ions provided survival benefits for the whole organism. Despite this similarity, growth rate and metabolic pathways induced in CAL and SER shoots varied significantly. Exposition to HMs in CAL culture led to drop in protein level by approximately 16% compared to the control. This parameter nearly correlated with the enhanced activity of proteases at pH 5.2 as well as possible glutamate changes to proline and reduced glutathione, resulting in intensified growth and first signs of cell senescence. In turn, SER shoots were characterized by growth retardation (to 53% of the control), although protein level and carbonylation were not modified, while a deeper insight into protein network showed its remodeling towards production of polyamines and 2-oxoglutarate delivered to the Krebs cycle. Contrary, an uncontrolled HM influx in NM shoots contributed to morpho-structural disorders accompanied by an increase activity of proteases involved in the degradation of oxidized proteins, what pointed to metal-induced autophagy. Taken together, S. vulgaris ecotypes respond to stress by triggering various mechanisms engaged their survival and/or death under HM treatment.

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.

Wheat Line "RYNO3936" Is Associated With Delayed Water Stress-Induced Leaf Senescence and Rapid Water-Deficit Stress Recovery

Random mutagenesis was applied to produce a new wheat mutant (RYNO3926) with superior characteristics regarding tolerance to water deficit stress induced at late booting stage. The mutant also displays rapid recovery from water stress conditions. Under water stress conditions mutant plants reached maturity faster and produced more seeds than its wild type wheat progenitor. Wild-type Tugela DN plants died within 7 days after induction of water stress induced at late booting stage, while mutant plants survived by maintaining a higher relative moisture content (RMC), increased total chlorophyll, and a higher photosynthesis rate and stomatal conductance. Analysis of the proteome of mutant plants revealed that they better regulate post-translational modification (SUMOylation) and have increased expression of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) proteins. Mutant plants also expressed unique proteins associated with dehydration tolerance including abscisic stress-ripening protein, cold induced protein, cold-responsive protein, dehydrin, Group 3 late embryogenesis, and a lipoprotein (LAlv9) belonging to the family of lipocalins. Overall, our results suggest that our new mutant RYNO3936 has a potential for inclusion in future breeding programs to improve drought tolerance under dryland conditions.

Updates on BES1/BZR1 Regulatory Networks Coordinating Plant Growth and Stress Responses

Brassinosteroids (BRs) play pivotal roles in the regulation of many dimensions of a plant's life. Hence, through extensive efforts from many research groups, BR signaling has emerged as one of the best-characterized plant signaling pathways. The key molecular players of BR signaling from the cell surface to the nucleus important for the regulation of plant growth and development are well-established. Recent data show that BRs also modulate plant responses to environmental stresses such as drought and pathogen infection. In this mini review, we present the recent progress in BR signaling specifically in the post-translational SUMO modification of BR's master regulators, BES1/BZR1. We also discuss recent findings on the crosstalk between BR, UV light, and jasmonic acid signaling pathways to balance growth during light stress and pathogen infections. Finally, we describe the current update on the molecular link between BR signaling and intracellular auxin transport that essential for plant development.