Plants heat shock factors (HSFs) are encoded by large gene families with variable structure, expression, and function. HSFs are components of complex signaling systems that control responses not only to high temperatures but also to a number of abiotic stresses such as cold, drought, hypoxic conditions, soil salinity, toxic minerals, strong irradiation, and to pathogen threats. Here we provide an overview of the diverse world of plant HSFs through compilation and analysis of their functional versatility, diverse regulation, and interactions. Bioinformatic data on gene expression profiles of Arabidopsis HSF genes were re-analyzed to reveal their characteristic transcript patterns. While HSFs are regulated primarily at the transcript level, alternative splicing and post-translational modifications such as phosphorylation and sumoylation provides further variability. Plant HSFs are involved in an intricate web of protein–protein interactions which adds considerable complexity to their biological function. A list of such interactions was compiled from public databases and published data, and discussed to pinpoint their relevance in transcription control. Although most fundamental studies of plant HSFs have been conducted in the model plant, Arabidopsis, information on HSFs is accumulating in other plants such as tomato, rice, wheat, and sunflower. Understanding the function, interactions, and regulation of HSFs will facilitate the design of novel strategies to use engineered proteins to improve tolerance and adaptation of crops to adverse environmental conditions.

Heat shock factors regulate responses to high temperature, salinity, water deprivation, or heavy metals. Their function in combinations of stresses is, however, not known. Arabidopsis HEAT SHOCK FACTOR A4A (HSFA4A) was previously reported to regulate responses to salt and oxidative stresses. Here we show, that the HSFA4A gene is induced by salt, elevated temperature, and a combination of these conditions. Fast translocation of HSFA4A tagged with yellow fluorescent protein from cytosol to nuclei takes place in salt-treated cells. HSFA4A can be phosphorylated not only by mitogen-activated protein (MAP) kinases MPK3 and MPK6 but also by MPK4, and Ser309 is the dominant MAP kinase phosphorylation site. In vivo data suggest that HSFA4A can be the substrate of other kinases as well. Changing Ser309 to Asp or Ala alters intramolecular multimerization. Chromatin immunoprecipitation assays confirmed binding of HSFA4A to promoters of target genes encoding the small heat shock protein HSP17.6A and transcription factors WRKY30 and ZAT12. HSFA4A overexpression enhanced tolerance to individually and simultaneously applied heat and salt stresses through reduction of oxidative damage. Our results suggest that this heat shock factor is a component of a complex stress regulatory pathway, connecting upstream signals mediated by MAP kinases MPK3/6 and MPK4 with transcription regulation of a set of stress-induced target genes.

The fine tuning of hormone (e.g., auxin and gibberellin) levels and hormone signaling is required for maintaining normal embryogenesis. Embryo polarity, for example, is ensured by the directional movement of auxin that is controlled by various types of auxin transporters. Here, we present pieces of evidence for the auxin-gibberellic acid (GA) hormonal crosstalk during embryo development and the regulatory role of the Arabidopsis thaliana Calcium-Dependent Protein Kinase-Related Kinase 5 (AtCRK5) in this regard. It is pointed out that the embryogenesis of the Atcrk5-1 mutant is delayed in comparison to the wild type. This delay is accompanied with a decrease in the levels of GA and auxin, as well as the abundance of the polar auxin transport (PAT) proteins PIN1, PIN4, and PIN7 in the mutant embryos. We have previously showed that AtCRK5 can regulate the PIN2 and PIN3 proteins either directly by phosphorylation or indirectly affecting the GA level during the root gravitropic and hypocotyl hook bending responses. In this manuscript, we provide evidence that the AtCRK5 protein kinase can in vitro phosphorylate the hydrophilic loops of additional PIN proteins that are important for embryogenesis. We propose that AtCRK5 can govern embryo development in Arabidopsis through the fine tuning of auxin-GA level and the accumulation of certain polar auxin transport proteins.

The Arabidopsis AtCRK5 protein kinase is involved in the establishment of the proper auxin gradient in many developmental processes. Among others, the Atcrk5-1 mutant was reported to exhibit a delayed gravitropic response via compromised PIN2-mediated auxin transport at the root tip. Here, we report that this phenotype correlates with lower superoxide anion (O₂^•−) and hydrogen peroxide (H₂O₂) levels but a higher nitric oxide (NO) content in the mutant root tips in comparison to the wild type (AtCol-0). The oxidative stress inducer paraquat (PQ) triggering formation of O₂^•− (and consequently, H₂O₂) was able to rescue the gravitropic response of Atcrk5-1 roots. The direct application of H₂O₂ had the same effect. Under gravistimulation, correct auxin distribution was restored (at least partially) by PQ or H₂O₂ treatment in the mutant root tips. In agreement, the redistribution of the PIN2 auxin efflux carrier was similar in the gravistimulated PQ-treated mutant and untreated wild type roots. It was also found that PQ-treatment decreased the endogenous NO level at the root tip to normal levels. Furthermore, the mutant phenotype could be reverted by direct manipulation of the endogenous NO level using an NO scavenger (cPTIO). The potential involvement of AtCRK5 protein kinase in the control of auxin-ROS-NO-PIN2-auxin regulatory loop is discussed.

The spatiotemporal control of transcription and the maintenance of germline genome integrity depend on dynamic chromatin architecture. In Drosophila , the actin-related protein Arp6—a core subunit of the SWR1-like Domino chromatin remodeling complex—mediates the deposition of the histone variant H2Av. Previous studies have established H2Av as a key transcriptional regulator that modulates the +1 nucleosome barrier to promote RNA Polymerase II (Pol II) pause release and productive elongation. Conversely, H2Av is also integral to heterochromatin assembly and gene silencing. Here we demonstrate that Arp6 and H2Av are essential for female fertility and the global repression of transposable elements (TEs) in the Drosophila ovary. Rather than repressing TEs directly, we show that Arp6 and H2Av maintain genomic stability indirectly by driving the transcription of core PIWI-interacting RNA (piRNA) pathway genes. Depletion of either chromatin factor leads to a significant loss of piRNAs and reduced non-canonical transcription of dual-strand piRNA clusters. This defect stems from a failure to express the Rhino-Deadlock-Cutoff (RDC) complex, alongside the downregulation of multiple other piRNA biogenesis factors. Genomic profiling confirms that H2Av acts predominantly as an activating signal at host gene promoters. Upon H2Av or Arp6 depletion, genes that rely on H2Av for their expression exhibit a distinct upstream shift and more precise spatial localization of the Pol II peak at the TSS, indicating an impaired transition from transcription initiation into productive elongation. Together, our findings build upon the known transcriptional activation functions of the Arp6-H2Av axis, revealing that this established chromatin mechanism is critical for licensing piRNA-mediated genome defense and ensuring germline maintenance.

The PIWI-interacting RNA (piRNA) pathway protects the germline from the deleterious effects of uncontrolled transposon expression in animals. Although the piRNA pathway and its components, such as Piwi, have been extensively studied, their regulation is not fully understood. We revealed a reduced mCherry-Piwi signal in Drosophila nurse cell nuclei and decreased mCherry-Piwi protein levels in His2Av depleted ovaries. In contrast, we did not detect any significant changes in piwi mRNA levels. Here, we report that His2Av is implicated in maintaining Piwi protein abundance in the Drosophila female germline, however its precise role remains to be elucidated.