The stress-induced attachment of small ubiquitin-like modifier (SUMO) to a diverse collection of nuclear proteins regulating chromatin architecture, transcription, and RNA biology has been implicated in protecting plants and animals against numerous environmental challenges. In order to better understand stress-induced SUMOylation, we combined stringent purification of SUMO conjugates with isobaric tag for relative and absolute quantification mass spectrometry and an advanced method to adjust for sample-to-sample variation so as to study quantitatively the SUMOylation dynamics of intact Arabidopsis seedlings subjected to stress. Inspection of 172 SUMO substrates during and after heat shock (37°C) revealed that stress mostly increases the abundance of existing conjugates, as opposed to modifying new targets. Some of the most robustly up-regulated targets participate in RNA processing and turnover and RNA-directed DNA modification, thus implicating SUMO as a regulator of the transcriptome during stress. Many of these targets were also strongly SUMOylated during ethanol and oxidative stress, suggesting that their modification is crucial for general stress tolerance. Collectively, our quantitative data emphasize the importance of SUMO to RNA-related processes protecting plants from adverse environments. The ability of cellular organisms to cope with environmental challenges requires the detection and immediate initiation of defense responses designed to mitigate the damage inflicted and enhance the organism's ability to tolerate future insults. For sessile organisms such as plants, robust stress responses are fundamental to their survival in a wide range of adverse environments (1, 2). Genetic and biochemical studies have identified a plethora of input pathways and output responses for stress protection in both prokaryotes and eukaryotes. Universally important is the synthesis of heat shock proteins that minimize protein aggregation and stimulate protein refolding through intrinsic chaperone activities (3).The stress-induced modification of intracellular proteins by small ubiquitin-like modifier (SUMO) 1 has recently emerged as an additional line of defense in eukaryotes (4 -6). Attachment of the ϳ100-amino-acid SUMO protein is driven by an ATPdependent, three-step enzyme cascade, which in Arabidopsis thaliana involves the E1 heterodimer (SAE2 together with either of two SAE1 isoforms), a single E2 SCE1, and at least two E3s (SAP, MIZ1 (SIZ1), and MMS21/HYP2) (7-11). The end result is the isopeptide linkage of one or more SUMO moieties to accessible target lysine(s). Most commonly, a consensus ⌿KxE SUMO-binding motif is modified, where ⌿ represents a bulky hydrophobic residue (12, 13). In some cases, the bound SUMOs themselves are also substrates, which results in poly-SUMO chains decorating the target (14, 15). Once generated, SUMO conjugates can be disassembled by a family of deSUMOylating proteases that specifically cleave these isopeptide bonds, thus allowing SUMO to act reversibly (e.g. Refs. 7,[16][17][18].Imp...

The posttranslational addition of small ubiquitin-like modifier (SUMO) is an essential protein modification in plants that provides protection against numerous environmental challenges. Ligation is accomplished by a small set of SUMO ligases, with the SAP-MIZ domain-containing SIZ1 and METHYL METHANESULFONATE-SENSITIVE21 (MMS21) ligases having critical roles in stress protection and DNA endoreduplication/repair, respectively. To help identify their corresponding targets in , we used and mutants for proteomic analyses of SUMOylated proteins enriched via an engineered SUMO1 isoform suitable for mass spectrometric studies. Through multiple data sets from seedlings grown at normal temperatures or exposed to heat stress, we identified over 1000 SUMO targets, most of which are nuclear localized. Whereas no targets could be assigned to MMS21, suggesting that it modifies only a few low abundance proteins, numerous targets could be assigned to SIZ1, including major transcription factors, coactivators/repressors, and chromatin modifiers connected to abiotic and biotic stress defense, some of which associate into multisubunit regulatory complexes. SIZ1 itself is also a target, but studies with mutants protected from SUMOylation failed to uncover a regulatory role. The catalog of SIZ1 substrates indicates that SUMOylation by this ligase provides stress protection by modifying a large array of key nuclear regulators.

In response to abiotic and biotic challenges, plants rapidly attach small ubiquitin-related modifier (SUMO) to a large collection of nuclear proteins, with studies in Arabidopsis (Arabidopsis thaliana) linking SUMOylation to stress tolerance via its modification of factors involved in chromatin and RNA dynamics. Despite this importance, little is known about SUMOylation in crop species. Here, we describe the plant SUMO system at the phylogenetic, biochemical, and transcriptional levels with a focus on maize (Zea mays). In addition to canonical SUMOs, land plants encode a loosely constrained noncanonical isoform and a variant containing a long extension upstream of the signature b-grasp fold, with cereals also expressing a novel diSUMO polypeptide bearing two SUMO b-grasp domains in tandem. Maize and other cereals also synthesize a unique SUMO-conjugating enzyme variant with more restricted expression patterns that is enzymatically active despite a distinct electrostatic surface. Maize SUMOylation primarily impacts nuclear substrates, is strongly induced by high temperatures, and displays a memory that suppresses subsequent conjugation. Both in-depth transcript and conjugate profiles in various maize organs point to tissue/cell-specific functions for SUMOylation, with potentially significant roles during embryo and endosperm maturation. Collectively, these studies define the organization of the maize SUMO system and imply important functions during seed development and stress defense.