Hookless1 Regulates Responses to Pathogens and Abscisic Acid Through Interaction with Mediator18 and Acetylation of WRKY33 and ABI5 Chromatin
Arabidopsis histone acetyltransferase HLS1 has been implicated in the control of seedling development in response to ethylene. Here, we demonstrate that the loss of function hls1 mutant plants have broad effects on the plant development such as early senescence and flowering, seed insensitivity to ABA as well as plant defense responses including enhanced susceptibility to the necrotrophic pathogen Botrytis cinerea and the hemi-biotrophic pathogen Pseudomonas syringae DC3000 strain expressing the effector protein AvrRpm1 strain. We show that HLS1 modulates the expression of WRKY33 gene, a known defense regulator, as well as ABI5 in modulating ABA signaling pathway. The regulation of WRKY33 and ABI5 is through histone H3 acetylation (H3Ac) at the specific loci. Furthermore, pathogen infection enhances HLS1 association and H3Ac at WRKY33 chromatin. Interestingly, MEDIATOR18 (MED18), a component of Arabidopsis Mediator complex, is known to regulate gene expression underlying plant defense, response to ABA, ethylene, and flowering time which is also similar to the HLS1-regulated processes. Consistent with this functional overlap, MED18 and HLS1 physically interact in co-immunoprecipitation (co-IP) assay in Nicotiana benthamiana and transgenic Arabidopsis plants that co-express these proteins. In addition, the co-expression of HLS1 and MED18 proteins in Arabidopsis protoplasts enhances WRKY33 and HLS1 expression level, suggesting HLS and MED18 synergistically activate gene expression. The results of ChIP-qPCR assay showed that HLS1 is required for MED18 association with WRKY33 locus but not affected MED18-mediated plant resistance to B. cinerea. Plants overexpressing WRKY33 in the hls1 mutant were rescued to the wildtype level for the ABA insensitive and the susceptibility to pathogen. While plants pretreated with ABA prior to B. cinerea inoculation displayed enhanced plant resistance, the hls1 and wrky33 mutant plants failed to respond to ABA and remained susceptible to B. cinerea, suggesting that HLS1 and WRKY33 regulate plant resistance to pathogen in response to ABA. Further, we revealed that HLS1 protein accumulation is enhanced following B. cinerea inoculation and ABA treatment. WRKY33 expression is regulated through histone H3Ac in response to ABA treatment. Finally, the transcription repressor AUXIN RESPONSE FACTOR 2 (ARF2) has been known to interfere with the HLS1-mediated apical hook formation in seedling growth. The arf2 mutant shows extreme resistance to B. cinerea accompanied with ABA hypersensitivity and delayed senescence and flowering. The hls1 arf2 double mutant plants display the enhanced resistance to B. cinerea, delayed senescence indicating the suppression of the hls1 mutant phenotypes. The data suggest a genetic interaction and diverse functions of HLS1 and ARF2 in plant defense and development. However, HLS1 does not appear to associate with ARF2 locus. Our results indicated that the expression of transcription factor HOMEOBOX PROTEIN 33 (HB33) is increased in arf2, but reduced in hls1 mutant plants, implying HLS1 and ARF2 regulate the same gene(s) during responses to pathogens through independent pathways. HLS1 has a major regulatory function on plant defense and hormone responses through cooperation with Mediator complex and gene activation of ABI5 and WRKY33. ARF2 functions as a suppressor to neutralize HLS1 effects on plant defense as well as hormone responses.
Mengiste, Purdue University.
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