PDF2.2 links constitutive defensin expression to disease resistance in Arabidopsis

Background: Plant defensins are small cysteine-rich antimicrobial peptides. In Arabidopsis thaliana, the inducible Class I gene AtPDF1.2a is widely used as a transcriptional marker of jasmonate/ethylene immunity, whereas the constitutively expressed Class II members have received less functional attention. AtPDF2.2 was compared with AtPDF1.2a during infection of Arabidopsis by the vascular fungal pathogen Verticillium longisporum, the necrotrophic fungus Sclerotinia sclerotiorum and the bacterium Pseudomonas syringae pv. tomato DC3000. Results: Profiling of the AtPDF family showed that AtPDF2.2, together with the Class II genes AtPDF2.3 and AtPDF2.5, was significantly reduced during V. longisporum infection, whereas AtPDF1.2a was induced. A promoter-reporter line showed a corresponding decrease in AtPDF2.2 promoter activity during V. longisporum infection, strongest in infected roots. KD-pdf2.2 plants contained more V. longisporum DNA, whereas OE-PDF2.2 plants contained less. The same reciprocal disease pattern was observed during S. sclerotiorum and Pst DC3000 infection. Histochemical Prom::AtPDF2.2::GUS reporter signal decreased during S. sclerotiorum infection but increased following Pst DC3000 infection. Basal AtPDF2.2 expression was lower in JA and ET-signaling mutants and higher in the SA-deficient sid2 background, while the analyzed promoter lacked the GCC-box that mediates inducible AtPDF1.2a regulation. AtPDF1.2a overexpression reduced symptoms across the three pathosystems, but loss of AtPDF1.2a increased disease only during V. longisporum infection under the conditions tested and did not alter Pst DC3000 DNA abundance. AtPDF2.2 overexpression was not associated with reduced vegetative growth under the conditions tested, whereas AtPDF1.2a overexpression reduced rosette size and leaf number. Conclusions: AtPDF2.2 is a constitutively expressed Class II defensin whose expression level is associated with disease outcome across two fungal pathogens and a bacterium. Its promoter shows tissue-dependent regulation during V. longisporum infection, while histochemical reporter signal decreases during S. sclerotiorum infection and increases during Pst DC3000 infection. This regulatory behavior differs from that of the inducible reference gene AtPDF1.2a. The reciprocal disease phenotypes of the AtPDF2.2 transgenic backgrounds support a protective association that may involve direct antimicrobial activity, modulation of host responses or both. The absence of reduced vegetative growth in OE-PDF2.2 under the conditions tested supports further evaluation of PDF2-type genes in Brassica.