A suppressor screen uncovers flu as a weak mutant and chloroplastic 1O2 triggers EX1-independent stress responses in Arabidopsis

Studies using the Arabidopsis conditional flu (flu1-1) mutant have shown that chloroplastic 1O2 acts as a signal, activating retrograde signaling that eventually leads to photoinhibition or cell death. • Through a forward genetic screen for suppressors using EMS-mutagenized Arabidopsis flu1-1/ex1 mutants, we isolated two suppressor mutants (mut1 and mut2) that restored 1O2-induced stress responses.

Using combined methods of map-based cloning and high-throughput sequencing, we mapped the causal mutations in both mut1 and mut2 to the FLU gene again. • We verified that the widely used flu1-1 is not the long-recognized null mutant, but rather a weak allele, encoding a variant that binds weakly to GluTR1 and leads to partial shutdown of tetrapyrrole biosynthesis in darkness. We further found that the FLU-knockout mutants produce more 1O2, activating transcriptional changes, enzymatic lipid peroxidation, and PCD even in the absence of EX1.