Plant Materials, Growth Conditions, and Treatment

Arabidopsis thaliana ecotype Columbia (Col-0), efr-2 (SALK_068675), fls2 (SALK_062054), med19a-1 (SALK_037435), med19a-2 (SALK_034955), and med26b-1 (SALK_020870) mutant plants were used. T-DNA insertion lines were obtained from the SALK collection. Homozygous plants for the T-DNA insertion were selected by genotyping progeny plants according to Alonso et al. (2003). Absence of target gene expression in homozygous plants was further confirmed by RT-PCR. Plants of all genotypes were grown on 0.6% agar media containing 0.5× Murashige and Skoog (MS) salts (MP Biomedicals), 1% sucrose (Fisher), and 0.5 g/L MES hydrate (Sigma-Aldrich) in a growth room at 22°C under 16 h light/8 h dark with white fluorescent light (∼100 μmol m⁻² s⁻¹). For elf18 (EZBiolab) or flg22 (EZBiolab) treatment, 10-d-old seedlings grown on MS solid medium were transferred to MS liquid medium (pH 5.7) with 1% sucrose and 5 μM elf18 or flg22 and incubated under the same condition and then seedlings were harvested at each time point after peptide application.