our model system

 
Plant growth and development are profoundly influenced by environmental light quality, quantity, direction, and periodicity. This is exemplified during seedling establishment, as a light-grown Arabidopsis seedling poses dramatic differences in morphology and physiology from one grown in the dark (Figure above). When a seed germinates underneath the ground, it first adopts the dark-grown program, and then switches to the light-grown program as soon as it encounters light. This critical transition in a plant’s life is called de-etiolation. It has been well known that phytochromes, an ancient family of red and far-red photoreceptors, are the prominent photoreceptors mediating de-etiolation. However, the signaling pathways between the perception of light by phytochromes and the phenotypical responses remain largely unknown.

 

project #1: subnuclear dynamics and signaling

One of the earliest effects of light at the cellular level is the subnuclear compartmentalization of a red/far-red photoreceptor, phytochrome. The above figure shows that Arabidopsis phytochrome B GFP fusion proteins (phyB::GFP) form phyB nuclear bodies in a light-intensity-dependent manner. The steady state pattern of phyB nuclear bodies is tightly correlated to phyB-mediated light responses. The function of phyB NBs in light signaling remains elusive. Using genetic approaches, we try to identify both intrinsic and extrinsic factors that are required for phyB NB formation. Using a microscopy-based genetic screen for phyB::GFP mislocalization mutants, we identified a novel mutant, hemera (hmr), which fails to form large phyB NBs and is defective in all phytochrome responses examined. Thus, HMR provides the missing link between phyB NBs and phytochrome signaling. Currently, we are characterizing the biochemical function of HMR and try to screen for additional hmr-like mutants to identify additional components in the HMR pathways. (HEMERA was named after the Greek goddess of "day", you can read more about her here).
 

project #2: light-dependent initiation of chloroplast differentiation

Chloroplast differentiation, as a vital part of the photomorphogenetic program in plants, is to establish photosynthetic plastids that enable plants for photoautotrophic growth in the light. This process is tightly controlled by environmental light cues, as dark-grown seedlings develop non-green etioplasts, light triggers a rapid greening process turning on chloroplast differentiation. Although it is well known that phytochromes are the major photoreceptors mediating photomorphogenesis through transcription regulation in the nucleus, the molecular mechanisms by which phytochromes initiate chloroplast differentiation is largely unknown. When the hmr mutant was isolated, surprisingly, it is an albino mutant, suggesting that HMR is also required for chloroplast differentiation. The discovery of the hmr mutant, which is the first photomorphogenetic mutant exhibiting an albino phenotype under all light conditions, provides an unprecedented opportunity to unravel the mechanistic links between phytochrome signaling and chloroplast differentiation.