N-degron pathways of ubiquitin-mediated proteolysis (formerly known as the N-end rule pathway) control the stability of substrate proteins dependent on the amino-terminal (Nt) residue. Unlike yeast or mammalian N-recognin E3 ligases, which each recognize several different classes of Nt residues, in Arabidopsis thaliana, N-recognin functions of different N-degron pathways are carried out independently by PROTEOLYSIS (PRT)1, PRT6, and other unknown proteins. PRT1 recognizes type 2 aromatic Nt-destabilizing residues and PRT6 recognizes type 1 basic residues. These two N-recognin functions diverged as separate proteins early in the evolution of plants, before the conquest of the land. We demonstrate that loss of PRT1 function promotes the plant immune system, as mutant prt1-1 plants showed greater apoplastic resistance than WT to infection by the bacterial hemi-biotroph Pseudomonas syringae pv tomato (Pst) DC3000. Quantitative proteomics revealed increased accumulation of proteins associated with specific components of plant defense in the prt1-1 mutant, concomitant with increased accumulation of salicylic acid. The effects of the prt1 mutation were additional to known effects of prt6 in influencing the immune system, in particular, an observed over-accumulation of pipecolic acid (Pip) in the double-mutant prt1-1 prt6-1. These results demonstrate a potential role for PRT1 in controlling aspects of the plant immune system and suggest that PRT1 limits the onset of the defense response via degradation of substrates with type 2 Nt-destabilizing residues.
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