Isochorismate synthase involved in the synthesis of salicylic acid (SA) required for both local and systemic acquired resistance (LAR and SAR) while SA synthesized through the phenylalanine ammonium lyase (PAL) pathway seems to potentiate plant cell death. Also involved in phylloquinone (vitamin K1) synthesis. Has no isochorismate pyruvate lyase (IPL) activity. chorismate = isochorismate Optimum pH is 7.5-8. Optimum temperature is 23 degrees Celsius. 90% of maximal activity from 4 to 37 degrees Celsius. Siderophore biosynthesis; salicylate biosynthesis. Monomer. Leaves. By pathogen infection. Systemic induction during systemic acquired resistance (SAR). Not induced by light. No visible phenotype; due to the redundancy with ICS2. Nevertheless salicylic acid accumulation upon induction is severely impaired. Ics1 and ics2 double mutant is seedling lethal due to photosynthetic lesions induced by the lack of phylloquinone. Belongs to the isochorismate synthase family. Truncated N-terminus.