Nature 426, 567-570 (2003)

Targeted degradation of TOC1 by ZTL modulates circadian function in Arabidopsis thalianaP. Más, W.Y. Kim, D.E. Somers and S.A. Kay

The underlying mechanism of circadian rhythmicity appears to be conserved among organisms, and is based on negative transcriptional feedback loops forming a cellular oscillator (or 'clock'). Circadian changes in protein stability, phosphorylation and subcellular localization also contribute to the generation and maintenance of this clock. In plants, several genes have been shown to be closely associated with the circadian system3, 4. However, the molecular mechanisms proposed to regulate the plant clock are mostly based on regulation at the transcriptional level. Here we provide genetic and molecular evidence for a role of ZEITLUPE (ZTL) in the targeted degradation of TIMING OF CAB EXPRESSION 1 (TOC1) in Arabidopsis thaliana (thale cress). The physical interaction of TOC1 with ZTL is abolished by the ztl-1 mutation, resulting in constitutive levels of TOC1 protein expression. The dark-dependent degradation of TOC1 protein requires functional ZTL, and is prevented by inhibiting the proteosome pathway. Our results show that the TOC1 ZTL interaction is important in the control of TOC1 protein stabilit−y, and is probably responsible for the regulation of circadian period by the clock.

ECOLOGICAL FUNCTION : PHYTOCHROME SPECIALZATION

Type I : phyA

Phytochrome

Type II : phyB, phyC, phyD, phyE

Phytochrome B Mediates Responses to Continuous Red or White Light

1. PhyB is involved in long hypocotyls under continuous white light.

2. PhyB mediates shade avoidance by regulating hypocotyl length in respones to red light given in low-fluence pulses or continuously.

3. PhyB involve perception of the Pfr.

4. PhyB interact other phytochrome.

5. PhyB mediates photoreversible seed germination.

Phytochrome A is Required for the Response to Continuous Far-Red

1. PhyA appers to have a limited role in photomorphogenesis.

2. PhyA restricted primarily to de-etiolation and far-red responses.

3. PhyA would be important when seeds germinate under a canopy.

4. PhyA appears to be involved in the germination VLER of Arabidopsis seed.

Developmental Roles for Phytochromes C,D and E Are Also

Emerging

1. PhyD and PhyE help mediate the shade avoidance response - a response mediated primarily by PhyB.

2. PhyB, PhyD plays a role in regulating leaf petiole elongation, as well as in flowering time.

3. PhyE acts redundantly with phyA and phyB inhibition of internode elongation.

4. In phyBDE mutant, there are differences in phytochrome-regulated gene expression function of PhyC.

In summary, phyC, phyD, and phyE appear to play roles that are for the most

part redundant with those of phyA and phyB.

Whereas phyB appears to be involved in regulating all stages of development,

the functions of the other phytochromes are restricted to specific

developmental step or responses

Phytochrome Interction Are Important Early in Germination

Phytochrome functioal domain