Fine Tuning of Reactive Oxygen Species Homeostasis Regulates Primed Immune Responses in Arabidopsis

Abstract

Selected stimuli can prime th e plant immune system for a faster and stronger defense reaction to pathogen attack. Pretreatment of Arabidopsis with the chemical agent β - aminobutyric acid (BABA) augmented H 2 O 2 and callose production after induction with the pathogen-associated molecular pattern (PAMP) chitosan, or inoculation with the necrotrophic fungus Plectosphaerella cucumerina. How- ever, BABA failed to prime H 2 O 2 and callose production after challenge with the bacterial PAMP Flg22. Analysis of Arabidopsis mutants in reactive oxygen species (ROS) pro- duction ( rbohD ) or ROS scavenging ( pad2 , vtc1 , and cat2 ) suggested a regulatory role for ROS homeostasis in prim- ing of chitosan- and P. c u c u m e r i n a -inducible callose and ROS. Moreover, rbohD and pad2 were both impaired in BABA-induced resistance against P. cucumerina . Gene ex- pression analysis revealed direct induction of NADPH/res- piratory burst oxidase protein D ( RBOHD ), γ -glutamylcys- teine synthetase 1 ( GSH1 ), and vitamin C defective 1 ( VTC1 ) genes after BABA treatment. Conversely, ascorbate peroxi- dase 1 ( APX1 ) transcription was repressed by BABA after challenge with chitosan or P. cucumerina , probably to pro- vide a more oxidized environment in the cell and facilitate augmented ROS accumulation. Measuring ratios between reduced and oxidized glutathione confirmed that augmented defense expression in primed plants is associated with a more oxidized cellular status. Together, our data indicate that an altered ROS equilibrium is required for augmented defense expression in primed plants.