PADI3 and infection: To combat pathogen infection, the AtWRKY33 protein interact with MPK4 indirectly to regulate the expression of defense gene PAD3 (phytoalexin deficient 3) by the activated MEKK1–MKK1/2–MPK4 module after bacterial pathogen attack (Qiu et al. 2008), while MPK3/MPK6 could direct phosphorylate AtWRKY25/33 in PAD3 regulation (Mao et al. 2011).