Plant Biology Select

Most organisms have daily rhythms set by environmental conditions such as light. Plants in particular are very sensitive to changes in their environment, responding to such changes with altered growth and development. At a molecular level, changes in the environment are detected and processed through an internal circadian clock. At the core of all circadian clocks is a transcriptional feedback loop; however, posttranslational modifications, such as phosphorylation and ubiquitination, are also important in the regulation of the circadian clock. Here we discuss recent studies on the plant circadian clock ranging from new posttranslational mechanisms that regulate the plant clock to how the plant clock mediates responses to the plant hormone auxin, thereby connecting the clock to growth and development. A central component of the plant circadian clock is TOC1 (TIMING OF CAB EXPRESSION 1). Degradation of TOC1 is carried out by the F-box protein ZTL (ZEITLUPE), which is part of an SCF E3 ubiquitin ligase complex. Although ZTL mRNA is relatively stable, the ZTL protein undergoes oscillations in its abundance through an unknown mechanism. Working in the model plant Arabidopsis thaliana, Kim et al. (2007) report that ZTL is a novel type of photoreceptor that is stabilized by the GIGANTEA (GI) protein specifically in blue light. While examining ZTL levels in circadian clock plant mutants, Kim et al. noticed that ZTL levels were drastically reduced in plants with mutations in the gigantea (gi) gene that had an altered cir-cadian periodicity. Conversely, overexpression of GI caused an increase in the amount of ZTL protein. Absence of ZTL also reduced the amounts of GI, indicating that these proteins must stabilize each other. The authors show that these two proteins interact with each other in vitro and in vivo, and in both systems, GI binds strongly to the LOV (light, oxygen, or voltage) domain of ZTL. This domain is important for signaling in response to blue light. Kim et al. next determined that the interaction between these two proteins is enhanced and stabilized in blue light. Moreover, mutations in one residue (C82A) of ZTL's LOV domain (but not mutations in other domains) abolished the interaction between ZTL and GI in blue light while still maintaining their interaction (albeit weakly) in other light conditions. Hence, blue light, sensed by ZTL, stabilizes ZTL through its interaction with GI. Thus, the stability of ZTL is not regulated at the tran-scriptional level but by a new kind …