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

Protein function is regulated at multiple levels. One prevalent mode is to form covalent adducts between small proteins and specific amino acid residues on targets thereby affecting their fate and/or function. One such protein modification is SUMOylation and it involves attachment of SUMO (Small ubiquitin-like modifiers) on lysine residues of targets. Plants maintain apparently strict control over their pool of SUMO-modified proteins, which increases massively but transiently in response to (a)biotic stress conditions and then returns back to steady-state levels. Mutants of the model plant Arabidopsis thaliana with abnormal SUMO1/2-conjugate levels, exhibit growth and developmental defects. These mutants also display a constitutive immune response evidenced by enhanced resistance to the bacterial pathogen Pseudomonas syringae pv tomato strain DC3000 (PstDC3000) infections. This study first reports the induction of SUMO1-conjugates during auto-immunity (in a constitutively defensive Arabidopsis mutant plant) or during immune responses elicited in wild-type plants in response to PstDC3000 infections. We demonstrate that the net enhanced SUMOylation outcome is likely because of efficient translation of SUMO1 transcripts, simultaneous to upregulation of SUMO-conjugation, and down-regulation of SUMO proteases gene expressions. Using unbiased enrichment methodologies and LC-MS/MS proteomics approach, we identify differential ‘SUMOylome’ of 261 proteins during autoimmunity and PstDC3000 infections. Of these, 29 SUMO1-substrates proteins have previously implicated roles in basal defense responses whereas others expand to diverse cell signalling pathways including normal growth or developmental processes. Our results provide the wide diversity of SUMOylation candidates differentially adjusted during defences and places SUMOylation as a key adaptation in plant immune responses.

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