Research Interests

Light is arguably the most important resource for plants, and an array of photosensory pigments enable plants to develop optimally in a broad range of ambient light conditions. The photosensory pigments include red and far-red light -absorbing phytochromes and UV-A/blue light-absorbing crytochromes and phototropins . Phytochromes and cryptochromes regulate seedling de-etiolation responses, photoperiodic flowering, and circadian rhythm , whereas phototropins enhance photosynthetic efficiency by controlling phototropic bending, stomatal opening, and chloroplast movement. The optimal performance of a plant therefore depends on coordination among the different light signaling pathways. It has been realized that a minimal level of active phytochrome seems to be necessary for full activity of cryptochromes or phototropins. One classic example is the enhancement by red light of phototropic bending toward unilateral blue light.

Along the lines of research, we have identified several mutants with defective seedling de-etiolation responses under more than a single wavelength. For example, hypersensitive to red and blue 1 or hrb1 has a short hypocotyl under red and blue light, a fairly unusual phenotypic spectrum among light signaling mutants. HRB1 encodes a nuclear ZZ-type zinc finger protein and is required for the proper expression of phytochrome interacting factor 4 or PIF4 under red and blue light. pif4 shows a very similar hypersensitive response as hrb1 to both red and blue light. Therefore, HRB1 and PIF4 together may define points where red light signaling and blue light signaling intersect. Mutation in HRB1 also causes late flowering and overexpression of HRB1 leads to early flowering .

Another mutant, red and far-red light insensitive 2-1 or rfi2-1 , has a long hypocotyl phenotype under red and far-red light. rfi2-1 is also impaired in other phytochrome-mediated de-etiolation responses. RFI2 was identified through the segregation of two T-DNA insertions into different recombinant lines, genetic rescue, and phenotypic characterization of rfi2-2 . RFI2 encodes a nuclear protein with a C 3 H 2 C 3 -type zinc finger or RING-domain known to mediate protein-protein interactions. Under both long-day and short-day photoperiods, rfi2-1 flowers early, similar to phyB-9 but in contrast to phyA-211 . The early flowering phenotype is accountable by an enhanced expression of CO , a gene that promotes floral transition, in rfi2-1 . RFI2 also shows a long-day photoperiod-enhanced expression and a free-running circadian rhythm similar to CO . RFI2 therefore reveals a previously unidentified step that integrates phytochrome and circadian signaling to control photoperidic flowering.

The third mutant, lir1-1 for light insensitive response 1-1 , has a long hypocotyl under red, far-red, and blue light. The long hypocotyl phenotype in lir1-1 is caused by an overaccumulation of LIR1 transcript, and is recapitulated by overexpression of LIR1 in transgenic Arabidopsis . However, lir1-2 , a knockout allele of LIR1 , exhibits a short hypocotyl phenotype only under blue light. Thus, LIR1 functions specifically in blue light signaling but overexpression of LIR1 expands its activity to modulate phytochrome signaling. Studies on both lir1-1 and lir1-2 suggest that LIR1 acts either positively or negatively. The positive action of LIR1 signaling may involve HFR1, a basic helix-loop-helix protein . LIR1 itself encodes a cytosolic protein similar to members of the SYG1 protein family from fungi, C. elegans , fly, mammals, and Arabidopsis . Our studies suggest a specific function of LIR1 in regulation of cry-mediated blue light responses in Arabidopsis , but also a possible involvement of LIR1-like proteins in either cry-mediated blue light signaling or other blue light responses in other organisms.