Arabidopsis mutants that are impaired in SA responsiveness, such as npr1 (nonexpressor of PR[9]–[10]), or are defective in pathogen-induced SA accumulation, such as eds1 (enhanced disease susceptibility 1[11]), eds5[12], sid2 (isochorishmate synthase [13]) and pad4 (phytoalexin deficient 4[14]), exhibit enhanced susceptibility to pathogen infection and show impaired PR gene expression. The gene discussed is PADI4; the disease is infection.