The Arabidopsis extracellular UNUSUAL SERINE PROTEASE INHIBITOR functions in resistance to necrotrophic fungi and insect herbivory

<jats:title>Summary</jats:title><jats:p>Protease inhibitors (PIs) function in the precise regulation of proteases, and are thus involved in diverse biological processes in many organisms. Here, we studied the functions of the Arabidopsis <jats:italic>UNUSUAL SERINE PROTEASE INHIBITOR</jats:italic> (<jats:italic>UPI</jats:italic>) gene, which encodes an 8.8 kDa protein of atypical sequence relative to other PIs. Plants harboring a loss‐of‐function <jats:italic>UPI</jats:italic> allele displayed enhanced susceptibility to the necrotrophic fungi <jats:italic>Botrytis cinerea</jats:italic> and <jats:italic>Alternaria brassicicola</jats:italic> as well as the generalist herbivore <jats:italic>Trichoplusia ni</jats:italic>. Further, ectopic expression conferred increased resistance to <jats:italic>B. cinerea</jats:italic> and <jats:italic>T. ni.</jats:italic> In contrast, the mutant has wild‐type responses to virulent, avirulent and non‐pathogenic strains of <jats:italic>Pseudomonas syringae</jats:italic>, thus limiting the defense function of <jats:italic>UPI</jats:italic> to necrotrophic fungal infection and insect herbivory. Expression of <jats:italic>UPI</jats:italic> is significantly induced by jasmonate, salicylic acid and abscisic acid, but is repressed by ethylene, indicating complex phytohormone regulation of <jats:italic>UPI</jats:italic> expression. The <jats:italic>upi</jats:italic> mutant also shows significantly delayed flowering, associated with decreased <jats:italic>SOC1</jats:italic> expression and elevated levels of MAF1, two regulators of floral transition. Recombinant UPI strongly inhibits the serine protease chymotrypsin but also weakly blocks the cysteine protease papain. Interestingly, jasmonate induces intra‐ and extracellular UPI accumulation, suggesting a possible role in intercellular or extracellular functions. Overall, our results show that UPI is a dual‐specificity PI that functions in plant growth and defense, probably through the regulation of endogenous proteases and/or those of biotic invaders.</jats:p>