Decoding plant defense signaling using the defenseless mutant.

Can plants live without defenses? Mutant analysis in Arabidopsis thaliana has identified numerous regulators of biotic, abiotic, and hormone-based defenses, but the redundancy among separate defense pathways remains unexplored. We constructed an Arabidopsis mutant, defenseless, lacking six canonical defense pathways using abi1-1 (abscisic acid), coi1 (jasmonic acid), sid2 (salicylic acid), ein2 (ethylene), eds1 (biotic defense signaling), and rbohD (apoplastic reactive oxygen species production), enabling dissection of defense network resilience. In optimal growth conditions, defenseless exhibited no stress phenotypes, demonstrating that plant defenses are dispensable under favorable environments. Stress assays revealed paradoxical responses: some defenses remained functional in defenseless, while others were severely compromised. Notably, ozone-triggered apoplastic ROS signaling was largely preserved, uncovering alternative and redundant defense mechanisms. Transcriptome profiling identified a core set of immune-related genes consistently downregulated in defenseless, yet pathogen susceptibility was not elevated beyond known immunity-deficient mutants, underscoring extensive redundancy and positioning defenseless as a platform to probe several layers of plant defenses.