Arabidopsis HIT4 encodes a novel chromocentre-localized protein involved in the heat reactivation of transcriptionally silent loci and is essential for heat tolerance in plants

Abstract

The Arabidopsis mutant heat-intolerant 4-1 (hit4-1) was isolated from an ethyl methanesulphonate-mutagenized M2 population on the basis of its inability to withstand prolonged heat stress (4 days at 37°C). Further characterization indicated that hit4-1 was impaired specifically in terms of basal but not acquired thermotolerance. Map-based cloning revealed that the HIT4 gene encoded a plant-specific protein for which the molecular function has yet to be studied. To investigate the cellular role of HIT4 and hence elucidate better its protective function in heat tolerance in plants, a GFP-HIT4 reporter construct was created for a protoplast transient expression assay. Results showed that fluorescently tagged HIT4 was localized to the chromocentre, a condensed heterochromatin domain that harbours repetitive elements for which transcription is normally suppressed by transcriptional gene silencing (TGS). DAPI-staining analysis and FISH with a probe that targeted centromeric repeats showed that heat-induced chromocentre decondensation was inhibited in nuclei of hit4-1 subjected to direct heat treatment, but not in those that were allowed to acquire thermotolerance. Moreover, heat reactivation of various TGS loci, regardless of whether they were endogenous or transgenic, or existed as a single copy or as repeats, was found to be attenuated in hit4-1. Meanwhile, the levels of transcripts of heat shock protein genes in response to heat stress were similar in both hit4-1 and wild-type plants. Collectively, these results demonstrated that HIT4 defines a new TGS regulator that acts at the level of heterochromatin organization and is essential for basal thermotolerance in plants.

The Arabidopsis heat‐intolerant 5 (hit5)/enhanced response to aba 1 (era1) mutant reveals the crucial role of protein farnesylation in plant responses to heat stress

“…To better characterize the effect of the hit5‐1 mutation on other forms of heat stress, we then compared the thermotolerance of hit5‐1 with that of hit2 and hit4‐1 mutants. These two mutants were selected because they have well‐characterized phenotypes that are defective in terms of tolerance to both prolonged heat stress (37°C for 4 d) and sudden heat shock (44°C for 30 min) treatments (Wu et al ., ; Wang et al ., ). Unexpectedly, the results showed that the hit5‐1 mutant, unlike hit2 and hit4‐1 , did not lose the capacity to endure heat shock treatment at 44°C for 30 min.…”

“…For example, Arabidopsis HIT1 , which encodes a membrane trafficking regulator involved in thermal adaptation of the plasma membrane, is essential for plants to tolerate prolonged heat stress (37°C for 4 d), but not heat shock (44°C for 30 min) (Wang et al ., ). In contrast, HSP101 , which encodes a molecular chaperone, is required for plants to tolerate heat shock but not prolonged heat stress (Hu et al ., ; Wang et al ., ). Therefore, a plausible scenario would be that there are at least two HSR regulators that are substrates for PFT.…”

“…Accumulated evidence has indicated that, of the complex family of Hsfs, members within the HsfA1 group play roles in the immediate heat‐induced transcription of a subset of genes that encode not only heat shock proteins (HSPs) but also other Hsfs such as HsfA2, the dominant Hsf in thermotolerant cells (Scharf et al ., ). Meanwhile, recent studies have shown that the transcription start site‐enriched histone variant H2A.Z might act as a thermosensor, and large‐scale heterochromatin decondensation is correlated with the release of transcriptional gene silencing during extended periods of heat stress, indicating that HSRs at the transcriptional level also involve remodeling of chromatin structure and configuration (Kumar & Wigge, ; Wang et al ., , ).…”

Stress-induced structural changes in plant chromatin

“…but does not reduce the induction of heat shock proteins. HIT4 associates with chromocenters and is involved in their decondensation during extended periods of heat stress [87 ]. Preceding the decondensation, HIT4 is delocalized to the nucleoli but this is not sufficient to achieve decondensation [88].…”

Overexpression of ZmMAPK1 enhances drought and heat stress in transgenic Arabidopsis thaliana

“…Since there is much evidence for cross-talk between signaling pathways regulating the response to abiotic stress (Jin et al 2011;Frey et al 2012;Wang et al 2013), transgenic Arabidopsis plants overexpression the ZmMAPK1 gene were generated. Three-week-old A. thaliana homozygous T 3 seedlings of the transgenic lines and WT plants were used for drought, heat and salt stress experiments.…”