Download Mathematical modeling reveals additional links between

Mathematical modeling reveals additional links between the circadian clock
and the redox rhythm in Arabidopsis through a master immune regulator
SARG I S K AR APE T YAN1 , W EI WA NG1, MI A N ZHOU1, X INNI A N D ONG1, NICOL AS BUCHL ER1
1
Duke University, Durham, NC, United States
Recent studies have shown that many organisms, including Arabidopsis, have a circadian redox
rhythm driven by the organism’s metabolic activities . It has been hypothesized that the redox rhythm
in plant is linked to the circadian clock, but the mechanism of this link remains largely unknown . Our
experimental work shows that the master immune regulator NPR1 (non-expressor of PR1) of Arabidopsis
is a sensor of the plant’s redox state and regulates transcription of core circadian clock gene TOC1
(TIMING OF CAB2 EXPRESSION 1) through TGA transcription factors . Strikingly, acute perturbation
in the redox status triggered by the immune hormone salicylic acid (SA) leads to enhancement rather
than perturbation of the TOC1 expression in an NPR1-dependent manner . The stability of the period
and the phase to SA perturbation suggest that NPR1 activates other clock genes, in addition to the
evening-phased TOC1 .
To systematically search for other possible clock components that are regulated by NPR1, we
performed mathematical modeling using the P2012 circadian model (Pokhilko, et . al ., 2013) . This
model includes most of the known components of the Arabidopsis circadian clock . Based on our
data, we made the assumption that NPR1 is a transcriptional activator of other clock genes, as it is
for TOC1 . We systematically coupled NPR1 to TOC1 and to two other circadian clock genes X and Y
from the P2012 model . For each X, Y pair, we used nonlinear least squares fitting to find those NPR1
parameters that best fit our TO C1p:LUC data sets . We repeated this procedure for all X, Y pairwise
combinations of the circadian clock genes included in P2012 model .
Our modeling results unambiguously show that NPR1 must activate the expression of the morningphased
PRR7 (PSEUDO-RESPONSE REGULATO R 7) and LHY (LATE ELONGATE D HYPOCOTY L) genes .
We experimentally confirmed these regulatory links using qPCR of LH Y and PRR 7 transcripts . By
regulating multiple (different phased) components simultaneously, this network balance architecture
by NPR1 enables the organism to respond to redox perturbation caused by pathogen challenge and
to help restore normal rhythmic metabolic activities through a reinforced circadian clock .