Light is one of the most influential environmental cues regulating almost every facet of plant growth and development, from seed germination, chloroplast differentiation and photosynthesis, to flowering. Although it has been well known that phytochromes are red and far-red light receptors mediating most of the light responses, early signaling mechanisms linking light activation of phytochromes and the resulting morphological responses remain elusive. The PI developed a novel genetic screen specifically targeted at signaling components involved in one of the earliest light responses, the movement of phytochromes within the cell. This unique genetic screen identified a novel component for phytochrome signaling, HEMERA (HMR), in the plant model species Arabidopsis. Interestingly, HMR is required for multiple early phytochrome signaling events and is the first phytochrome-signaling component essential for the development of chloroplasts from their pro-plastid precursors (chloroplast differentiation). The research is aimed at uncovering HMR-mediated signaling mechanisms leading to light-dependent chloroplast differentiation. The specific aims are: (1) To determine how phytochromes regulate HMR; (2) To determine the location of HMR in cells in relationship to light signaling and chloroplast differentiation; (3) To identify and characterize components downstream of HMR by looking for mutations that restore normal traits in a plant where HMI is otherwise not functional. The outcomes of this project could contribute significantly to our basic knowledge of how plant development is regulated by environmental cues. A better understanding of this process will have profound implications in increasing yield and enhancing agronomically beneficial traits in crops. In particular, the elucidation of how chloroplast differentiation is initiated by light will ultimately provide the knowledge to facilitate scientists to improve photosynthetic capacity in biofuel crops. In addition, the project will provide excellent training in molecular genetic approaches for postdoctoral fellows, graduate students, and undergraduate students.
Major research and education activities The Specific Aims of the proposal are: To determine how phytochrome (phy) exerts its function through HEMERA (HMR). To determine which subcellular pool of HMR is required for phytochrome-mediated chloroplast differentiation. To identify and characterize components downstream of HMR by an hmr suppressor screen. The major accomplishments from the funding period 03/01/2014 to 02/28/2015 were: (1) We defined the nuclear and plastidial functions of HMR: HMR controls hypocotyl growth by regulating Phytochrome-Interacting Factors (PIFs) in phy signaling in the nucleus; in parallel, HMR plays a PIF-independent role in regulating chloroplast development; (2) We defined the biochemical function of HMR as a transcriptional coactivator that couples the degradation and transcriptional activity of PIFs. A manuscript reporting the function of HMR has been accepted by The Plant Cell. (3) We have found that SOH is a photobody constituent required for early phy signaling events, including photobody assembly, PIF degradation, and transactivation of PIF target genes. Together, all of the proposed Specific Aims have been accomplished. Define the functions of HMR in phy signaling. To dissect the nuclear and plastidial functions of HMR, we generated a hmr-5pif1pif3pif4pif5 (hmr-5pifq) quintuple mutant. The hmr-5pifq mutant rescued the long hypocotyl phenotype of hmr-5, indicating that HMR controls hypocotyl growth by regulating PIFs in the nucleus. In contrast, hmr-5pifq remains albino. Similar to hmr-5, hmr-5pifq fails to express the plastid-encoded photosynthetic genes, which are transcribed the Plastid-Encoded Plastid RNA polymerase (PEP). These results indicate that HMR plays an essential role in regulating the function of the PEP in chloroplasts. In addition, we found that HMR is a transcriptional coactivator binding directly with PIFs. HMR is required for the activation of a distinct set of PIF direct-target and growth-relevant genes. These results reveal a novel phy signaling mechanism, in which HMR couples the degradation and transcriptional activity of PIFs to quantitatively control hypocotyl growth. A manuscript reporting these findings has been accepted by The Plant Cell. Characterize the function of Son-Of-HMR (SOH) in phy signaling. SOH was identified by a forward genetic screen for hmr-like mutants with tall-and-albino phenotypes. In this funding period, we found that SOH is also dual-localized to the chloroplasts and the nucleus. In addition, within the nucleus, SOH is colocalized with phyB to photobodies, indicating that SOH is a constituent of photobodies. Moreover, SOH interacts directly with phyB. Surprisingly, knocking-out four PIFs, PIF1,3,4, and 5, the soh-10pifq quintuple mutant rescues the tall-and-albino phenotypes of soh-10, indicating that SOH controls both hypocotyl growth and chloroplast development through PIFs. We are currently preparing a manuscript reporting the identification of SOH and characterization of its function in phy signaling.
