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Plant defense and development are intricately interconnected. The putative RNA binding protein FLOWERING LOCUS K HOMOLOGY DOMAIN (FLK) was previously shown to play a critical role in regulating pathogen defense and flowering time in Arabidopsis. However, the molecular basis underlying the multifunctionality of FLK remains to be determined. Here we provide data to support a role of FLK in regulating reactive oxygen species (ROS) homeostasis, which is dependent on the primary catalase gene CAT2. The flk mutations confer reduced ROS burst upon elicitation and higher resistance to ROS-inducing stress conditions, including treatments with the herbicide paraquat, UV, and salinity. CAT2 is required for flk-conferred abiotic stress response and pathogen defense. We further demonstrate a negative regulation of the catalase enzyme activity by FLK. In addition, our data show that the ROS-regulatory role of FLK is independent of its function in flowering time control. Together our data illustrate a mechanism underlying FLK’s defense role and decouple the multifunctionality of FLK in defense and development in Arabidopsis. 

Abstract Plant disease resistance is a complex process that is maintained in an intricate balance with development. Increasing evidence indicates the importance of posttranscriptional regulation of plant defense by RNA binding proteins. In a genetic screen for suppressors of Arabidopsis (Arabidopsis thaliana) accelerated cell death 6-1 (acd6-1), a small constitutive defense mutant whose defense level is grossly in a reverse proportion to plant size, we identified an allele of the canonical flowering regulatory gene FLOWERING LOCUS K HOMOLOGY DOMAIN (FLK) encoding a putative protein with triple K homology (KH) repeats. The KH repeat is an ancient RNA binding motif found in proteins from diverse organisms. The relevance of KH-domain proteins in pathogen resistance is largely unexplored. In addition to late flowering, the flk mutants exhibited decreased resistance to the bacterial pathogen Pseudomonas syringae and increased resistance to the necrotrophic fungal pathogen Botrytis cinerea. We further found that the flk mutations compromised basal defense and defense signaling mediated by salicylic acid (SA). Mutant analysis revealed complex genetic interactions between FLK and several major SA pathway genes. RNA-seq data showed that FLK regulates expression abundance of some major defense- and development-related genes as well as alternative splicing of a number of genes. Among the genes affected by FLK is ACD6, whose transcripts had increased intron retentions influenced by the flk mutations. Thus, this study provides mechanistic support for flk suppression of acd6-1 and establishes that FLK is a multifunctional gene involved in regulating pathogen defense and development of plants.