Arabidopsis AHL13 Protein Associates With Poly(ADP-ribosyl)ation To Regulate Innate Plant Immunity

Plants are affected by various pathogens during the growth process.An innate immune system that recognizes pathogens and builds a disease-resistant response has evolved during evolution.In this system,transcriptional activation of immune genes in plant nuclei in response to pathogen invasion is the key to the establishment of the entire defense mechanism.We found that PARP2 positively regulates the expression of immune genes in Arabidopsis,and its catalytic poly(ADP-ribosyl)ation or PARylation is an important regulatory mechanism of Arabidopsis innate immunity.In order to study its potential molecular mechanism,we screened for proteins that interact with PARP2 through yeast two-hybrid,one of which was an AT-hook motif nuclear localization(AHL)protein,AHL13.AHL proteins could recognize DNA minor groove and regulate chromatin structure and transcription.In model plant Arabidopsis,the AHL family members can regulate plant growth and development and response to environmental cues,such as floral organ formation,pollen wall construction,leaf development,hypocotyl growth,root-stem vascular bundle differentiation,hormone response,etc.However,about half of the 29 members of the AHL family in Arabidopsis have not been functionally characterized,and there are relatively few studies on the potential immune functions of AHL proteins.We conducted in-depth studies on the function and biological significance of AHL13 and its interaction with PARP2 in disease resistance of Arabidopsis.The sub-cellular localization of GFP fusion protein revealed that both AHL13 and PARP2 were localized to the nucleus,and their interaction was confirmed by co-immunoprecipitation,in vitro pull-down and bi-molecular fluorescence complementation(Bi FC)experiments.Interestingly,PARylation significantly enhances the interaction between AHL13 and PARP2.Furthermore,by analyzing the T-DNA insertion mutant of AHL13 and the over-expression transgenic lines,we found that AHL13 could suppress the resistance to the bacterial pathogen Pseudomonas syringae and the expression of immune genes.The PARylation of AHL13 by PARP2 was also detected in vitro.It is speculated that AHL13 and PARP2 catalyzed PARylation antagonize with each other and play an important role in fine-tuning the appropriate expression level of immune genes.In our in vitro assay,it was found that PARylation could suppress DNA binding activity of AHL13,providing a new molecular mechanism by which PARylation regulate transcription factor’s function.This study not only discovered a novel negative regulator in Arabidopsis innate immunity,but also revealed a new mechanism by which PARylation regulates the transcription of plant immune genes.