Plants use an internal circadian clock to temporally coordinate biochemical and physiologic activities with the daily cycle imposed by the rotation of the earth on its axis. The matching of this internal clock with the daily environmental cycle contributes significantly to optimal field performance. This research investigates two key aspects of the plant clock mechanism. The first is a genetic and biochemical analysis of temperature compensation, the property by which clock function remains consistent across a range of temperatures. The second is the global regulation of gene expression by the circadian clock. Mutational analysis has identified a novel clock component, AtPRMT5, which encodes a chromatin modification enzyme that serves as a key regulator of gene expression. Specific experimental goals include the identification of targets of PRMT5 activity within the clock mechanism itself and the elucidation of the mechanism by which PRMT5 is regulated. The circadian clock integrates temporal information and coordinates many aspects of biology, including basic metabolism, auxin signaling and responses as well as responses to biotic and abiotic stress. Insights into the molecular details of the clock mechanism will inform an understanding of both the clock mechanism itself as well as of the regulatory network by which temporal information from the clock is used to regulate the expression of suites of genes and coordinate the activities of biochemical pathways. This, in turn, will inform and direct efforts to breed crops whose clock function is optimized for specific environmental (e.g., latitudinal) conditions. Educational efforts will emphasize the training of undergraduate, graduate and post-doctoral scientists able to work comfortably across scales of complexity, from the regulation of expression of individual genes through analyses at the global genomic level.
@font-face { font-family: "Times"; }@font-face { font-family: "Cambria"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: "Times New Roman"; }div.Section1 { page: Section1; } C.R. McClung, PI, IOS-0960803, 02/15/10-02/14/11, $100,000: "Molecular genetic analysis of the Arabidopsis circadian clock" Circadian rhythms are the product of an endogenous biological clock and serve to coordinate the biological processes of an organism with the day-night cycle of the earth. An underlying premise to the study of circadian rhythms has been that this coordination with the temporal environment enhances fitness. This premise has now been experimentally verified in several organisms. In the higher plant, Arabidopsis, the performance advantage that accrues from proper circadian function has been attributed to enhanced net photosynthesis and to improved starch metabolism. However, the circadian clock also coordinates many other aspects of plant biology, including basic metabolism, hormone signaling and response, and responses to biotic and abiotic stress. Increased understanding of plant responses to environmental input and to endogenous temporal cues has agricultural importance, in particular because both environmental cues and the circadian clock contribute to the photoperiodic decision to flower. Thus, understanding and manipulation of the circadian clock may contribute to improvement in crop productivity, particularly in crops grown over broad latitudinal ranges. In this work we addressed the roles of a number of clock components in the buffering of clock function against fluctuations in temperature and defined critical roles for two plant genes in this process. This may contribute to the development of crops better able to function across a wide range of temperatures and at temperature extremes. We also identified a new component of the plant circadian clock, PROTEIN ARGININE METHYLTRANSFERASE 5 (PRMT5). PRMT5 catalyzes the modification of proteins and this modification, the symmetric demethylation of specific arginine residues, alters protein function. These modifications are important in gene expression. One target of PRMT5 is histone proteins, and the methylation of histones is an important mechanism in the coordinate expression of suites of genes. A second set of PRMT targets includes components of the splicing machinery. Splicing plays an essential role in the processing of the primary RNA transcript into a mature mRNA that can be translated into protein. Thus PRMT5 affects the expression of large sets of genes by at least two distinct mechanisms. The circadian clock modulates the expression of at least 1/3 of all plant genes and PRMT5 offers a hitherto unexpected regulatory mechanism that contributes to this large-scale regulation of gene expression. Our work, therefore, offers new insight into circadian clock regulation of gene expression and offers new avenues for the enhancement of crop productivity via modulation of circadian clock function.
