Transcriptional Regulation of the Ambient Temperature Response by H2A.Z Nucleosomes and HSF1 Transcription Factors in Arabidopsis

Temperature influences the distribution, range, and phenology of plants. The key transcriptional activators of heat shock response in eukaryotes, the heat shock factors （HSFs）, have undergone large-scale gene amplification in plants. While HSFs are central in heat stress responses, their role in the response to ambient temperature changes is less well understood. We show here that the warm ambient temperature transcriptome is dependent upon the HSFA1 clade ofArabidopsis HSFs, which cause a rapid and dynamic eviction of H2A.Z nucleosomes at target genes. A transcriptional cascade results in the activation of multiple downstream stress-responsive transcription factors, triggering large-scale changes to the transcriptome in response to elevated temperature. H2A.Z nucleosomes are enriched at temperature-responsive genes at non-inducible temperature, and thus likely confer inducibility of gene expression and higher responsive dynamics. We propose that the antagonistic effects of H2A.Z and HSF1 provide a mechanism to activate gene expression rapidly and precisely in response to temperature, while preventing leaky transcription in the absence of an activation signal.

Raman Spectroscopy Detection of Phytic Acid in Plant Seeds Reveals the Absence of Inorganic Polyphosphate

Dear Editor The fully phosphorylated myo-inositol ring, inositol hexakisphos- phate （IP6） is commonly known as phytic acid. It is the most abundant form of organic phosphate in soil （Raboy, 2003）. Phytic acid is able to regulate a broad range of intracellular processes, controlling many aspects of cell physiology both directly or as its derivative inositol pyrophosphates （Wilson et al., 2013）. One such process is phosphate homeostasis, as IP6 regulates the level of inorganic polyphosphate （polyP）, a linear polymer of phosphate groups that buffers cellular phosphate by regulation of its synthesis/degradation （Saiardi, 2012）. The quantification of seed phosphate levels, and the identification and localization of phosphate-containing molecular species, is of fundamental importance for the biotechnology industry. Several studies have reported the biochemical extrac- tion and analysis of the levels of phytic acid in plant seeds （Thavarajah and Thavarajah, 2014）. Direct experimental tools to visualize the phytic acid cellular distribution have not yet been developed, although DAPI fluorescence shift has been used to visualize the cellular presence of polyP.

ITPK1 is an InsP_(6)/ADP phosphotransferase that controls phosphate signaling in Arabidopsis.

In plants, phosphate (Pi) homeostasis is regulated by the interaction of PHR transcription factors with stand-alone SPX proteins, which act as sensors for inositol pyrophosphates. Here, we combined different methods to obtain a comprehensive picture of how inositol (pyro)phosphate metabolism is regulated by Pi and dependent on the inositol phosphate kinase ITPK1. We found that inositol pyrophosphates are more responsive to Pi than lower inositol phosphates, a response conserved across kingdoms. With CE-ESI-MS we could separate different InsP7 isomers in Arabidopsis and rice, and identify 4/6-InsP7 and a PP-InsP4 isomer hitherto not reported in plants. We found that the inositol pyrophosphates 1/3-InsP7, 5-InsP7 and InsP8 increase severalfold in shoots after Pi resupply and that tissue-specific accumulation of inositol pyrophosphates relies on ITPK1 activities and MRP5-dependent InsP6 compartmentalization. Notably, ITPK1 is critical for Pi-dependent 5-InsP7 and InsP8 synthesis in planta and its activity regulates Pi starvation responses in a PHR-dependent manner. Furthermore, we demonstrate that ITPK1-mediated conversion of InsP6 to 5-InsP7 requires high ATP concentrations and that Arabidopsis ITPK1 has an ADP phosphotransferase activity to dephosphorylate specifically 5-InsP7 under low ATP. Collectively, our study provides deeper insights into Pi-dependent changes in nutritional and energetic states with the synthesis of regulatory inositol pyrophosphates.