Minisymposium 7: Photomorphogenesis

Abs # 17001: Early Phytochrome Signaling and Regulation of Transcriptional Networks

Presenter:	Khanna, Rajnish , rajnish@uclink.berkeley.edu
Authors	Khanna, Rajnish  (A) (B)  Lanzatella, Christina L. (A) (B)  Toledo-Ortiz, Gabriela  (A) (B)  Huq, Enamul  (A) (B)  Tepperman, James M. (A) (B)  Quail, Peter H. (A) (B)
Affiliations:	(A): University of California, Berkeley (B): USDA / Plant Gene Expression Center

Elegant photosensory systems have evolved in plants. The phytochromes (phy) are the primary sensors for red and far-red light. Recent studies have suggested that the environmental light signal triggers translocation of the phy molecule from cytoplasm to nucleus where it binds specifically in its biologically active form to promoter-bound basic helix-loop-helix (bHLH) proteins. Previously, we have reported that the bHLH protein, PIF3, is a phy-interacting factor, which binds selectively to the biologically active form of phyB. A comprehensive computational analysis of the Arabidopsis genome-sequence databases has revealed a large family of bHLH encoding genes. We have focused our efforts to study the family members most closely related in sequence to PIF3. Biochemical analysis and reverse genetics have identified a novel phy-interacting bHLH factor, PIF5. PIF5 interacts selectively with the biologically active form of phyB. While pif5 mutants display a hypersensitive morphological phenotype, the PIF5 (OX) transgenics exhibit a hyposensitive phenotype. PIF5 appears to function as a negative regulator of phyB signaling in light grown seedlings. Another bHLH factor, PIF4, identified in parallel using forward genetic screens and yeast-two-hybrid screens followed by reverse genetic analysis, can form heterodimers with PIF3. Furthermore, PIF3 and PIF4 homo- and heterodimers can all specifically bind to the G-box DNA sequence motif CACGTG. This suggests a "bHLH combinatorial code" that has the potential to generate a high degree of diversity and complexity in phy mediated transcriptional regulation. Oligonucleotide microarray experiments have revealed that multiple transcription-factor genes are early targets of phyA signaling. Our recent microarray-based expression profiling of phyB signaling in response to continuous red (Rc) light during the induction of seedling deetiolation has suggested very early convergence of phyA and phyB photosensory pathways to control a largely common transcriptional network. In addition, the data indicate that one or more other members of the phy family have a major role in mediating Rc-regulated expression. Together with the visible morphogenic phenotypes, these data provide evidence that different members of the phy family regulate seedling deetiolation in an organ-specific manner.