Stacey Harmer

Stacey Harmer is a chronobiologist whose work centers on the study of circadian rhythms in plants. Her research focuses on the molecular workings of the plant circadian clock and its influences on plant behaviors and physiology. She is a professor in the Department of Plant Biology at the University of California, Davis.

Education Harmer achieved her bachelor's degree in Biochemistry from the University of California, Berkeley in 1991, then earned a PhD at the University of California, San Francisco in 1998. At UC San Francisco, she was a Howard Hughes Predoctoral Fellow in Tony DeFranco's lab, while researching the systems involving signal transduction by the B-cell antigen receptor.

Harmer Lab The Harmer Lab is a research group dedicated to studying the plant circadian clock, in particular the molecular processes and physiology underlying plant development and responses to environmental stimuli. The Harmer Lab was established when Harmer was recruited to the Department of Plant Biology at the University of California, Davis.

Over time, the Harmer Lab has made several important contributions to the field of plant chronobiology:

Identifying several crucial genes involved in plant circadian growth including LHY and CCA1. Exploring timekeeping genes and rhythmic feedback loops in the Arabidopsis thaliana. Mapping detailed chronobiological models and factors of circadian rhythms in plants.

The Harmer Lab also collaborates with a number of outside research groups including:

The Blackman Lab at the University of California, Berkeley. The Jones Lab at the University of Glasgow. Circadian rhythms enable plants to anticipate seasonal changes and adjust accordingly in order to promote survival and overall fitness through the facilitation of leaf movement, growth, pollination, and more. A prime example of a model plant is Arabidopsis thaliana, also used by Harmer in many of her papers. Due to its relatively small and non-repetitive genome, Arabidopsis thaliana was also used to elucidate the existence of a TTFL (transcription-translation feedback loop) that facilitates the workings of an endogenous clock.

While research is still being conducted on the intricacies of the plant TFFL, many proteins and genes have been identified such as CCA1, TOC1, LHY. CCA1 and LHY are two, relatively well-researched transcription factors that work as repressors in the plant TFFL. These repressors target genes like ELF4, LUX, TOC1, GIGANTEA (GI), and more. The combination of activators and repressors and their oscillations within the plant circadian clock ultimately control phenotypic and physiological outputs.

The circadian mechanism within these plants involves numerous transcription factors that contribute to multiple transcriptional feedback loops that form a highly detailed, modeled network revolving around morning and evening outputs. Harmer suggests a simpler model that incorporates morning genes including CCA1 and LHY, and afternoon genes like RVE 4,6,8 within a regulatory system to investigate the robustness of plant rhythms in the face of changing environmental conditions. This central transcriptional feedback serves as a core part of the plant circadian clock and provides clues as to how solar tracking, water efficiency, and daily growth operate in plant systems.

The lab is currently exploring the role the eukaryote protein XCT to find out more about its role in stunting plant growth and regulating the circadian oscillator. With Harmer's work on the Arabidopsis thaliana, XCT has been found to rescue growth in yeast mutants. This method allows researchers to directly record and monitor the rhythmicity of circadian genes in plants.

Harmer investigated the effects of auxin dosage on the rhythmic expression in Arabidopsis. She found that exogenous application of the auxin IAA to plants causes a lengthening of the plants' free-running period.

Since the sunflowers did not have pulvini, organs that controlled solar tracking for other plants, Harmer hypothesized that stem growth may cause heliotropism. She monitored growth of stems and solar tracking in dwarf2 (dw2) sunflowers, which lack gibberellin growth hormones. Due to this deficiency, dw2 sunflowers have short stems and no heliotropism. After treating these flowers with gibberellin hormones, heliotropism was restored. As a result, this day and night movement was caused by the stem's elongation.

Harmer further hypothesized that heliotropism occurs from the irregular growth rates on the opposite sides of the stem. On the east side, the stem had more growth during the day and less growth during the night, but on the west side, the stem experienced the opposite. This contrast indicated that the east side of the stem lengthened during day and the west side lengthened during night, which enabled it to move east to west during day and west to east at night. This uneven growth was controlled by genes influenced by light and circadian rhythm. Harmer's findings showed how circadian rhythms regulated the sunflowers movement during light and dark.

Circadian processes maintain temperature compensation. During constant darkness at 18 °C, 25 °C, and 30 °C, the ovary and stamen development maintained free-running rhythms. The general periods of these growths were similar across all temperatures. The internal, rhythmic activity that modulated floret's anthesis was temperature-compensated, further pushing the hypothesis on the plant clock. Based on her studies, Harmer concluded that the circadian clock, light, and temperature signals modulate the developmental timing of florets. Harmer continues to investigate the pathways that control when late-stage florets grow. Her findings opened up the possibility that the floret's anthesis may seduce pollinators, thus encouraging reproductive performance.

Honors Harmer is the recipient of several honors, both from professional societies and her institution: 2011 - University of California, Davis Chancellor's Fellow: awarded for expertise in her field 2020 - Fellow of the American Association for the Advancement of Science (AAAS): awarded for her scientific efforts in advancing the field of chronobiology 2021 - Fellow of American Society of Plant Biologists: awarded for sustained contributions to the field of plant biology NIH National Research Service Award: awarded to scientists-in-training to gain research opportunities American Society of Photobiology's New Investigator Award: awarded for impactful research at the beginning of an investigator's career

Publications