Molecular Genetic Analysis Of Plant Circadian Rhythms

Dartmouth College, Hanover NH

Investigators

Abstract

Increased understanding of plant responses to environmental input and to endogenous temporal cues has agricultural importance. The clock integrates temporal information and coordinates many aspects of biology, including basic metabolism and responses to biotic and abiotic stresses. Additionally, environmental cues and the circadian clock contribute to the photoperiodic flowering. Proper coordination of the endogenous timing mechanism with the external day confers adaptive advantage, and impaired circadian function is associated with reduced fitness. The model plant, Arabidopsis thaliana, offers a powerful and experimentally tractable system in which to investigate the molecular mechanisms of circadian rhythmicity. Because plants are closely related, it is quite likely that understanding derived from Arabidopsis studies will be readily transferred to agronomically important species. In the context of climate change and the need to exploit increasingly marginal habitats, fuller understanding of clock mechanism may offer strategies to improve crop productivity. This project focuses on the Arabidopsis thaliana family of five Pseudo-Response Regulators (PRRs). Mutational analysis indicates that each is important to the circadian system, although individuals play distinct roles. Indeed, the first bona fide clock mutant in Arabidopsis was prr1, known also as toc1. This mutant was isolated on the basis of a shortened circadian period as monitored by leaf movement and by gene expression. toc1 mutants also exhibit defects in photoperiodic induction of flowering. This project focuses on PRR9 and PRR7. Mutation of these loci confers phenotypes distinct from those of the other PRR loci-prr9 exhibits lagging phase and prr7 exhibits long period, whereas mutation of the other three family members, including TOC1, shortens the period. The double prr7 prr9 mutant has an exaggerated phenotype in white light (very long period, very lagging phase), which suggests partial functional redundancy between PRR7 and PRR9. The clock defects of PRR7 and PRR9 are seen following entrainment either to light-dark cycles or to temperature (warm-cool) cycles. This argues for a central role of PRR7 and PRR9 in the perception or transduction of environmental signals. It is also possible that these PRRs may act within the central oscillator, analogously to TOC1 (PRR1), which is accepted as a crucial component of the Arabidopsis clock. This project employs genetic and biochemical approaches to elucidate the roles of PRR7 and PRR9 as components of input pathways or of the oscillator itself. These include overexpression, use of double and triple mutants to assess genetic interaction, and screens for second mutations that enhance or suppress prr7 or prr9 mutant phenotypes. Physical interaction between PRR7 and PRR9 will be tested, and other interactors with PRR7 and PRR9 will be sought.