Mutations in the exoribonuclease 5ʹ-3ʹ XRN4 cause a delay in the clock period. (A) Expression of the night core-clock reporter pTOC1:LUC was measured for 7 d in LL after entrainment under LD conditions. WT (Col-0), WT Columbia ecotype (squares); xrn4-3 mutant (circles). (B) Circadian rhythm of leaf movement. Leaf angles were measured for the first pair of leaves in seedlings entrained under LD conditions, and plants were then transferred to LL. WT (Col-0) (squares). xrn4-3 mutant (circles, n = 6); xrn4-5 mutant (triangle). (C, D) The period length of bioluminescence and leaf movement rhythms was estimated by the fast Fourier transform nonlinear least squares (FFT-NLLS) test. (E, F) Relative amplitude error (R.A.E.) were plotted for each genotype. Error bars represent the mean with range. **P < 0.001 (Student’s t-test).