Newly Isolated Circadian Clock Components Conserved across Eukaryotes

Organisms maintain circadian rhythms that coordinate biological processes with daily environmental changes. The circadian clock molecular mechanism is self-sustaining, yet is also capable of entrainment to variable environmental conditions (reviewed in McClung, 2006). At its simplest, the circadian clock has a central oscillator that perpetuates a diurnal cycle and is synchronized to the environment by inputs such as light and temperature signals. The oscillator then produces outputs that control cellular processes. Outputs of the plant circadian clock include changes in gene expression, physical changes such as leaf movements, and the measurement of daylength to determine flowering time. Not surprisingly, the plant circadian clock is not governed by simplicity; instead, multiple interlocked oscillator loops are phased at different times. Recent work published in The Plant Cell highlights two novel and interesting components of the clock mechanism.

Martin-Tryon and Harmer (pages 1244–1259) investigate XAP5 CIRCADIAN TIMEKEEPER (XCT), which encodes a novel protein that may play a role in light input into the clock for the regulation of both circadian period and photomorphogenesis, but not flowering time. Loss-of-function xct mutants show a shorter circadian period under all light conditions but do not flower early. xct mutants also show photomorphogenesis defects, including delayed chloroplast development. Interestingly, different mutant phenotypes are sensitive to different wavelengths of light; for example, xct mutants have very long hypocotyls in red light but very short hypocotyls in blue light. By contrast, xct mutant sensitivity of circadian period to increasing intensities of light is increased in red light but not blue light. Thus, XCT appears to have both light-dependent and light-independent functions, and its light-dependent function inhibits red light inputs to the clock. XCT also may act on the oscillation mechanism in a light-independent manner, as its shortening of photoperiod phenotype appears in all light conditions, including complete darkness.

The XCT protein is nuclear localized (see figure) and shows good conservation across eukaryotes, with identity between plant and mammalian proteins at nearly 50%. Another interesting nuclear-localized clock component that shows strong conservation across kingdoms is the FIONA1 (FIO1) protein, published in the February issue of The Plant Cell (Kim et al., 2008). In contrast with xct mutants, fio1 mutants show a lengthened circadian rhythm that is independent of light and temperature conditions, indicating that FIO1 may not function in clock input, but rather acts directly in the oscillation mechanism. Also, fio1 mutants flower extremely early. Interestingly, most known core clock components are not conserved between plants and animals. Unfortunately, the protein sequences of XCT and FIO1 do not give us a clue to their protein functions, so determination of the relevance of their intriguing cross-kingdom sequence conservation awaits further study.

Figure 1.

XCT protein is localized to the nucleus. Fluorescence image of XCT-YFP fusion overlaid on a bright-field image (left) and alone (right): trichome (A); hypocotyl (B). (Figure reproduced from Martin-Tyron and Harmer [2008].)