Environmental memory from a circadian oscillator: the Arabidopsis thaliana clock differentially integrates perception of photic vs. thermal entrainment

Abstract

The constraint of a rotating earth has led to the evolution of a circadian clock that drives anticipation of future environmental changes. During this daily rotation, the circadian clock of Arabidopsis thaliana (Arabidopsis) intersects with the diurnal environment to orchestrate virtually all transcriptional processes of the plant cell, presumably by detecting, interpreting, and anticipating the environmental alternations of light and temperature. To comparatively assess differential inputs toward phenotypic and physiological responses on a circadian parameter, we surveyed clock periodicity in a recombinant inbred population modified to allow for robust periodicity measurements after entrainment to respective photic vs. thermal cues, termed zeitgebers. Lines previously thermally entrained generally displayed reduced period length compared to those previously photically entrained. This differential zeitgeber response was also detected in a set of diverse Arabidopsis accessions. Thus, the zeitgebers of the preceding environment direct future behavior of the circadian oscillator. Allelic variation at quantitative trait loci generated significant differences in zeitgeber responses in the segregating population. These were important for periodicity variation dependent on the nature of the subsequent entrainment source. Collectively, our results provide a genetic paradigm for the basis of environmental memory of a preceding environment, which leads to the integrated coordination of circadian periodicity.

Figure 1

Quantitative features of CCR2 periodicity after photic vs. after thermal entrainment. Variation of free-running period of CCR2 after entrainment to the two different protocols exemplified by two representative RILs: CvL6 and CvL47. LD denotes the free running rhythmicity of luminescence driven from the CCR2 promoter after entrainment by 12 hr light:12 hr dark at a constant 22°, and TMP denotes the free-running rhythmicity after entrainment to constant light with thermal cycles of 12 hr at 22°:12 hr 16°. Note that all assay conditions were under constant light at 22°. Relative luminescence is depicted. Assay started at time 0 and is the onset of lights for photic entrainment, or the onset of warm temperature for thermal entrainment. Note that CvL6 has smaller differences in free-running period than CvL47, after the two-entrainment protocols. Period variation of CCR2 period in CvL6 and CvL47 RILs. Line names are indicated. Dark blue, averaged period of CCR2 after photic entrainment in CvL47; pink, averaged period of CCR2 after thermal entrainment in CvL47; orange, averaged period of CvL6 after photic entrainment; light blue, averaged period of CvL6 after thermal entrainment.

Figure 2

Frequency distribution of CCR2 periodicity after photic vs. after thermal entrainment. Normal frequency distribution of CCR2 periodicity in individuals of the CvL population. Blue-colored bars represent periodicity after photic entrainment and pink-colored bars represent periodicity after thermal entrainment. Cvi and Ler denote the periodicity of CCR2 in the parental genotypes. Note the skew of temperature-entrained plants to shorter periodicity, when compared to photic-entrained plants.

Figure 3

Free-running-period differences in CvL lines. (A) The free-running period estimates of each RIL was plotted for oscillator speed after photic entrainment (red squares) or thermal entrainment (green squares). Note that the vast majority of RILs have a faster running oscillator after thermal, compared to after photic, entrainment. (B) The period difference for RILs depicted in A, with the parental lines included for comparison. Pairwise differences (in hours) were calculated by extracting periodicity after photic from after thermal entrainment. x-axis displays the RILs, named on the basis of its defined number (Alonso-Blanco et al. 1998), and the y-axis displays the difference (in hours) in periodicity after subtracting TMP periodicity from LD periodicity.

Figure 4

Free-running pairwise-period differences of CCR2:LUC in natural accessions. Accessions oscillator speed was plotted as the difference after thermal vs. after photic entrainment, as in Figure 3B. Accessions were ordered on the basis of their TMP periodicity from LD periodicity difference. Note extensive positive and several negative differences. Most lines ran a faster oscillator after thermal entrainment (P > 0.01). Green represents accessions with a statistically significant thermal enhancement difference, pink represents accessions with no statistically significant difference, and red represents accessions with a statistically significant photic enhancement.