Circadian and Plastid Signaling Pathways Are Integrated to Ensure Correct Expression of the CBF and COR Genes during Photoperiodic Growth
Plant Physiol. 2016, 171(2):1392-1406

Noren L, Kindgren P, Stachula P, Ruhl M, Eriksson M, Hurry V, Strand A

Abstract
The circadian clock synchronizes a wide range of biological processes with the day-night cycle, and correct circadian regulation is essential for photosynthetic activity and plant growth. We describe here a mechanism where a plastid signal converges with the circadian clock to fine-tune the regulation of nuclear gene expression. Diurnal oscillations of tetrapyrrole levels in the chloroplasts contribute to the regulation of the nuclear encoded transcription factors C-REPEAT BINDING FACTORS (CBFs). The plastid signal triggered by tetrapyrrole accumulation inhibits the activity of cytosolic HEAT SHOCK PROTEIN90 (HSP90) and as a consequence the maturation and stability of the clock component ZEITLUPE (ZTL). ZTL negatively regulates the transcription factor LONG HYPOCOTYL5 (HY5) and PSEUDO-RESPONSE REGULATOR5 (PRR5). Thus, low levels of ZTL result in a HY5- and PRR5-mediated repression of CBF3 and PRR5-mediated repression of CBF1-2 expression. The plastid signal thereby contributes to the rhythm of CBF expression and the downstream COLD RESPONSIVE (COR) expression during day-night cycles. These findings provide novel insight into how plastid signals converge with, and impact upon, the activity of well-defined clock components involved in the circadian regulation.

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