PIF4/HEMERA-mediated daytime thermosensory growth requires the Mediator subunit MED14

Authors: BAJRACHARYA, ABHISHESH - University Of Mississippi; XI, JING - Oak Ridge Institute For Science And Education (ORISE); KARLIE, GRACE - University Of Mississippi; BAYER, EDEN - University Of Mississippi; GRANT, CHLOE - University Of Mississippi; CLUTTON, CAROLINE - University Of Mississippi; Baerson, Scott; AGARWAL, AMEETA - University Of Mississippi; QIU, YONGJIAN - University Of Mississippi

Submitted to: bioRxiv
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2022
Publication Date: 3/4/2022
Citation: Bajracharya, A., Xi, J., Karlie, G., Bayer, E., Grant, C., Clutton, C., Baerson, S.R., Agarwal, A.K., Qiu, Y. 2022. PIF4/HEMERA-mediated daytime thermosensory growth requires the Mediator subunit MED14. bioRxiv. https://doi.org/10.1101/2022.03.02.482654.
DOI: https://doi.org/10.1101/2022.03.02.482654

Interpretive Summary: Plants are extremely sensitive to environmental temperature changes. For example, non-stressful elevated temperatures, near the upper range of normal ambient temperatures to which plants are typically exposed, trigger a suite of specific growth responses. These responses include rapid stem and root elongation, enhanced leaf growth, early flowering and reductions in leaf water loss, and are collectively referred to as thermomorphogenesis. Thermomorphogenic growth is accompanied by major changes in gene expression, some of which are regulated by the protein PHYTOCHROME-INTERACTING FACTOR 4 (PIF4). This work investigated the specific mechanism whereby PIF4 activates gene expression during thermomorphogenesis. Using both genetic and biochemical approaches, it was discovered that a protein referred to as MED14 is essential for mediating specific gene expression changes associated with thermomorphogenesis. It was determined that MED14 interacts directly with the proteins PIF4 and HEMERA, and in this manner causes the expression of specific genes which play a role in the thermomorphogenic growth response in plants.

Technical Abstract: Warm ambient temperatures do not trigger stress responses in plants but stimulate the growth of specific organs. The basic helix-loop-helix transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) plays a central role in regulating thermomorphogenetic hypocotyl elongation in various plant species, including the model dicotyledonous plant Arabidopsis thaliana. Although it is well known that PIF4 promotes plant thermosensory growth by activating key genes involved in the biosynthesis and signaling of the phytohormone auxin, the detailed molecular mechanism of such transcriptional activation is still not clear. Our previous studies demonstrated that HEMERA (HMR), a transcription co-activator of PIF4, promotes the thermo-induced expression of PIF4 target genes through the nine-amino-acid transactivation domain (9aaTAD) in its carboxy-terminus. In this report, we investigate the role of the Mediator complex in the PIF4/HMR-mediated thermoresponsive gene expression. Through the characterization of various mutants of the Mediator complex, a tail subunit named MED14 is identified as an essential factor for thermomorphogenetic hypocotyl growth. MED14 is required for the thermal induction of key PIF4 target genes but has a marginal effect on the levels of PIF4 and HMR. Further transcriptomic analyses confirm that the expression of numerous PIF4/HMR-dependent, auxin-related genes requires MED14 at a warm temperature. Moreover, PIF4 and HMR physically interact with MED14 and both are indispensable for the association of MED14 with the promoters of these thermoresponsive genes. Together, these results unveil an important thermomorphogenetic mechanism, in which PIF4 and its coactivator HMR recruit the Mediator complex to activate auxin-related growth-promoting genes when plants sense moderate increases in ambient temperature.