XAP5 CIRCADIAN TIMEKEEPER Affects Both DNA Damage Responses and Immune Signaling in Arabidopsis.

Roderick W. Kumimoto,Hongtao Zhang,Stacey L. Harmer,Clare L. Casteel,Tania Y. Toruño,Cory T. Ellison,Gitta Coaker,Aurélie Bak

doi:10.3389/fpls.2021.707923

Abstract

Numerous links have been reported between immune response and DNA damage repair pathways in both plants and animals but the precise nature of the relationship between these fundamental processes is not entirely clear. Here, we report that XAP5 CIRCADIAN TIMEKEEPER (XCT), a protein highly conserved across eukaryotes, acts as a negative regulator of immunity in Arabidopsis thaliana and plays a positive role in responses to DNA damaging radiation. We find xct mutants have enhanced resistance to infection by a virulent bacterial pathogen, Pseudomonas syringae pv. tomato DC3000, and are hyper-responsive to the defense-activating hormone salicylic acid (SA) when compared to wild-type. Unlike most mutants with constitutive effector-triggered immunity (ETI), xct plants do not have increased levels of SA and retain enhanced immunity at elevated temperatures. Genetic analysis indicates XCT acts independently of NONEXPRESSOR OF PATHOGENESIS RELATED GENES1 (NPR1), which encodes a known SA receptor. Since DNA damage has been reported to potentiate immune responses, we next investigated the DNA damage response in our mutants. We found xct seedlings to be hypersensitive to UV-C and γ radiation and deficient in phosphorylation of the histone variant H2A.X, one of the earliest known responses to DNA damage. These data demonstrate that loss of XCT causes a defect in an early step of the DNA damage response pathway. Together, our data suggest that alterations in DNA damage response pathways may underlie the enhanced immunity seen in xct mutants.

Highlights

The genome sequencing of a wide range of organisms has enabled the identification of genes that are widely conserved and yet generally maintained in a single copy per genome
XAP5 CIRCADIAN TIMEKEEPER was originally identified in a screen for Arabidopsis mutants with altered free-running circadian rhythms (Martin-Tryon et al, 2007). xct-1 is a reduction-of-function allele predicted to produce a protein with an internal deletion of three amino acids, while xct-2 is a presumptive null allele with a T-DNA inserted within an exon (Martin-Tryon and Harmer, 2008)
The dark to light transition is a powerful clock-resetting cue in Arabidopsis (Covington et al, 2001), so we anticipated the modest differences in circadian phase between genotypes harvested at this time would have negligible effects on levels of gene expression

Summary

Introduction

The genome sequencing of a wide range of organisms has enabled the identification of genes that are widely conserved and yet generally maintained in a single copy per genome. Conserved XCT homologs are present in most eukaryotes and have been shown to be nuclear localized in Arabidopsis, fission yeast, and Chlamydomonas (Matsuyama et al, 2006; Martin-Tryon and Harmer, 2008; Li et al, 2018). In most organisms, these homologs are termed XAP5 genes, named for the highly conserved domain of unknown function found in the C-termini of the encoded proteins. Subsequent genetic and biochemical analyses in this yeast revealed that Xap is a chromatin-associated protein affecting chromatin regulation in a manner similar to the variant histone H2A.Z

Methods

Results

Conclusion