Cell autonomous and cell-type specific circadian rhythms in Arabidopsis.

The circadian system of plants regulates a wide range of rhythmic physiological and cellular output processes with a period of about 24 h. The rhythms are generated by an oscillator mechanism that, in Arabidopsis, consists of interlocking feedback loops of several components including CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), LATE ELONGATED HYPOCOTYL (LHY), TIMING OF CAB EXPRESSION 1 (TOC1) and CCA1 HIKING EXPEDITION (CHE). Over recent years, researchers have gained a detailed picture of the clock mechanism at the resolution of the whole plant and several tissue types, but little information is known about the specificities of the clock mechanism at the level of individual cells. In this paper we have addressed the question of cell-type-specific differences in circadian systems. Using transgenic Arabidopsis plants with fluorescence-tagged CCA1 to measure rhythmicity in individual leaf cells in intact living plants, we showed that stomatal guard cells have a different period from surrounding epidermal and mesophyll leaf cells. By comparing transcript levels in guard cells with whole plants, we identified differences in the expression of some oscillator genes that may underlie cell-specific differences in clock properties. In addition, we demonstrated that the oscillators of individual cells in the leaf are robust, but become partially desynchronized in constant conditions. Taken together our results suggest that, at the level of individual cells, there are differences in the canonical oscillator mechanism that has been described for plants.