Antagonism between SUMO1/2 and SUMO3 regulates SUMO conjugate levels and fine-tunes immunity

Arabidopsis-expressing lysine-null SUMO1 reveals a non-essential role for secondary SUMO modifications in plants

The reversible conjugation of small ubiquitin-like modifier (SUMO) to other proteins has pervasive roles in various aspects of plant development and stress defense through its selective attachment to numerous intracellular substrates. An intriguing aspect of SUMO is that it can be further modified by SUMOylation and ubiquitylation, which isopeptide-link either or both polypeptides to internal lysines within previously bound SUMOs. Although detectable by mass spectrometry, the functions of these secondary modifications remain obscure. Here, we generated transgenic Arabidopsis that replaced the two related and essential SUMO isoforms (SUMO1 and SUMO2) with a lysine-null SUMO1 variant (K0) immune to further SUMOylation/ubiquitylation at these residues. Remarkably, homozygous SUMO1(K0) sumo1 sumo2 plants developed normally, were not hypersensitive to heat stress, and have nearly unaltered SUMOylation profiles during heat shock. However, subtle changes in tolerance to salt, paraquat, and the DNA-damaging agents bleomycin and methane methylsulfonate were evident, as were increased sensitivities to ABA and the gibberellic acid biosynthesis inhibitor paclobutrazol, suggesting roles for these secondary modifications in stress defense, DNA repair, and hormone signaling. We also generated viable sumo1 sumo2 lines expressing a SUMO1(K0) variant specifically designed to help isolate SUMO conjugates and map SUMOylation sites, thus offering a new tool for investigating SUMO in planta.

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.

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.

Arabidopsis inositol polyphosphate kinases IPK1 and ITPK1 modulate crosstalk between SA-dependent immunity and phosphate-starvation responses

KEY MESSAGE

Selective Arabidopsis thaliana inositol phosphate kinase functions modulate response amplitudes in innate immunity by balancing signalling adjustments with phosphate homeostasis networks. Pyrophosphorylation of InsP6 generates InsP7 and/or InsP8 containing high-energy phosphoanhydride bonds that are harnessed during energy requirements of a cell. As bona fide co-factors for several phytohormone networks, InsP7/InsP8 modulate key developmental processes. With requirements in transducing jasmonic acid (JA) and phosphate-starvation responses (PSR), InsP8 exemplifies a versatile metabolite for crosstalks between different cellular pathways during diverse stress exposures. Here we show that Arabidopsis thaliana INOSITOL PENTAKISPHOSPHATE 2-KINASE 1 (IPK1), INOSITOL 1,3,4-TRISPHOSPHATE 5/6-KINASE 1 (ITPK1), and DIPHOSPHOINOSITOL PENTAKISPHOSPHATE KINASE 2 (VIH2) implicated in InsP8 biosynthesis, suppress salicylic acid (SA)-dependent immunity. In ipk1, itpk1 or vih2 mutants, constitutive activation of defenses lead to enhanced resistance against the Pseudomonas syringae pv tomato DC3000 (PstDC3000) strain. Our data reveal that upregulated SA-signaling sectors potentiate increased expression of several phosphate-starvation inducible (PSI)-genes, previously known in these mutants. In reciprocation, upregulated PSI-genes moderate expression amplitudes of defense-associated markers. We demonstrate that SA is induced in phosphate-deprived plants, however its defense-promoting functions are likely diverted to PSR-supportive roles. Overall, our investigations reveal selective InsPs as crosstalk mediators in defense-phosphate homeostasis and in reprogramming stress-appropriate response intensities.

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.

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.