TOC1

TOC1, short for TIMING OF CAB EXPRESSION 1, is a gene in Arabidopsis thaliana also known as PRR1 (pseudo-response regulator 1). It encodes a nuclear protein that is a member of the pseudo-response regulator family and plays a central role in the plant circadian clock. The TOC1 gene is one of several PRR family members that coordinate daily rhythms in gene expression and physiological processes.

The TOC1 protein contains a receiver-like domain at the N-terminus but lacks the conserved residues required for typical phosphorelay activity, which classifies it as a pseudo-response regulator. Its expression is tightly circadian, with mRNA levels peaking in the evening, and the protein localizes to the nucleus where it can influence transcriptional activity. TOC1 is part of a regulatory network that helps generate and maintain a roughly 24-hour cycle in biological processes.

In the circadian oscillator, TOC1 acts in a negative feedback loop by repressing the expression of morning-phased genes such as CCA1 and LHY during the evening. This repression helps shape the timing of morning gene expression and contributes to the overall period and robustness of the clock. TOC1 is part of the larger PRR family, which includes PRR3, PRR5, PRR7, and PRR9, and these proteins collectively support the function and stability of the plant circadian system.

Research on TOC1 has advanced understanding of how plants synchronize internal rhythms with environmental cues, including light and temperature, and how circadian timing influences photoperiodic responses and flowering. Its study remains central to broader investigations of plant biology and chronobiology.