The Mediator complex subunit MED25 interacts with HDA9 and PIF4 to regulate thermomorphogenesis.

Thermomorphogenesis is, among other traits, characterized by enhanced hypocotyl elongation due to the induction of auxin biosynthesis genes like YUCCA8 by transcription factors, most notably PHYTOCHROME INTERACTING FACTOR 4 (PIF4). Efficient binding of PIF4 to the YUCCA8 locus under warmth depends on HISTONE DEACETYLASE 9 (HDA9) activity, which mediates histone H2A.Z depletion at the YUCCA8 locus. However, HDA9 lacks intrinsic DNA-binding capacity, and how HDA9 is recruited to YUCCA8, and possibly other PIF4-target sites, is currently not well understood. The Mediator complex functions as a bridge between transcription factors bound to specific promoter sequences and the basal transcription machinery containing RNA polymerase II. Mutants of Mediator component Mediator25 (MED25) exhibit reduced hypocotyl elongation and reduced expression of YUCCA8 at 27°C. In line with a proposed role for MED25 in thermomorphogenesis in Arabidopsis (Arabidopsis thaliana), we demonstrated an enhanced association of MED25 to the YUCCA8 locus under warmth and interaction of MED25 with both PIF4 and HDA9. Genetic analysis confirmed that MED25 and HDA9 operate in the same pathway. Intriguingly, we also showed that MED25 destabilizes HDA9 protein. Based on our findings, we propose that MED25 recruits HDA9 to the YUCCA8 locus by binding to both PIF4 and HDA9.

A temperature regime that disrupts clock-controlled starch mobilization induces transient carbohydrate starvation, resulting in compact growth.

In nature plants are usually subjected to a light/temperature regime of warm day and cold night (referred to as +DIF). Compared to growth under +DIF, Arabidopsis plants show compact growth under the same photoperiod, but with an inverse temperature regime (cold day and warm night: -DIF). Here we show that -DIF differentially affects the phase and amplitude of core clock gene expression. Under -DIF the phase of the morning clock gene CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) is delayed, similar to that of plants grown on low sucrose. Indeed, under -DIF carbohydrate (CHO) starvation marker genes are specifically upregulated at the End of the Night (EN) in Arabidopsis rosettes. However, only in inner-rosette tissue (small sink leaves and petioles of older leaves) sucrose levels are lower under -DIF compared to under +DIF, suggesting that sucrose in source leaf blades is not sensed for CHO status and that sucrose transport from source to sink may be impaired at EN. CHO-starvation under -DIF correlated with increased starch breakdown during the night and decreased starch accumulation during the day. Moreover, we demonstrate that different ways of inducing CHO-starvation all link to reduced growth of sink leaves. Practical implications for control of plant growth in horticulture are discussed.

HISTONE DEACETYLASE 9 stimulates auxin-dependent thermomorphogenesis in Arabidopsis thaliana by mediating H2A.Z depletion.

Many plant species respond to unfavorable high ambient temperatures by adjusting their vegetative body plan to facilitate cooling. This process is known as thermomorphogenesis and is induced by the phytohormone auxin. Here, we demonstrate that the chromatin-modifying enzyme HISTONE DEACETYLASE 9 (HDA9) mediates thermomorphogenesis but does not interfere with hypocotyl elongation during shade avoidance. HDA9 is stabilized in response to high temperature and mediates histone deacetylation at the YUCCA8 locus, a rate-limiting enzyme in auxin biosynthesis, at warm temperatures. We show that HDA9 permits net eviction of the H2A.Z histone variant from nucleosomes associated with YUCCA8, allowing binding and transcriptional activation by PHYTOCHROME INTERACTING FACTOR 4, followed by auxin accumulation and thermomorphogenesis.

Functional intron-derived miRNAs and host-gene expression in plants.

BACKGROUND: Recently, putative pre-miRNAs locations have been identified in the introns of plant genes, raising the question whether such genes can show a dual functionality by having both correct maturation of the host gene pre-mRNA and maturation of the miRNAs from the intron. Here, we demonstrated that such dual functionality is indeed possible, using as host gene the firefly luciferase gene with intron (ffgLUC), and different artificial intronic miRNAs (aimiRNA) placed within the intron of ffgLUC. RESULTS: The miRNAs were based on the structure of the natural miR319a. Luciferase (LUC) activity in planta was used to evaluate a correct splicing of the ffgLUC mRNA. Different target sequences were inserted into the aimiRNA to monitor efficiency of silencing of different target mRNAs. After adjusting the insertion cloning strategy, the ffgLUCaimiR-319a gene showed dual functionality with correct splicing of ffgLUC and efficient silencing of TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR1 transcription factor genes targeted in-trans by aimiR-319a or targeting the transgene ffLUC in-cis by an aimiR-LUC. Silencing of endogenous target genes by aimiRNA or amiRNA is efficient both in transient assays and stable transformants. A behave as strong phenotype the PHYTOCHROME B (PHYB) gene was also targeted by ffgLUCaimiR-PHYB. The lack of silencing of the PHYB target was most likely due to an insensitive target site within the PHYB mRNA which can potentially form a double stranded stem structure. CONCLUSION: The combination of an overexpression construct with an artificial intronic microRNA allows for a simultaneous dual function in plants. The concept therefore adds new options to engineering of plant traits that require multiple gene manipulations.