Interactions between HLH and bHLH Factors Modulate Light-Regulated Plant Development

Phytochromes （Phy） and phytochrome-interacting factor （PIF） transcription factors constitute a major signal- ing module that controls plant development in response to red and far-red light. A low red：far-red ratio is interpreted as shading by neighbor plants and induces cell elongation--a phenomenon called shade-avoidance syndrome （SAS）. PAR1 and its closest homolog PAR2 are negative regulators of SAS; they belong to the HLH transcription factor family that lacks a typical basic domain required for DNA binding, and they are believed to regulate gene expressions through DNA binding transcription factors that are yet to be identified. Here, we show that light signal stabilizes PAR1 protein and PAR1 interacts with PIF4 and inhibits PIF4-mediated gene activation. DNA pull-down and chromatin immunoprecipitation （CHIP） assays showed that PAR1 inhibits PIF4 DNA binding in vitro and in vivo. Transgenic plants overexpressing PAR1 （PARIOX） are insensitive to gibberellin （GA） or high temperature in hypocotyl elongation, similarly to the pifq mutant. In addition to PIF4, PAR1 also interacts with PRE1, a HLH transcription factor activated by brassinosteroid （BR） and GA. Overexpression of PRE1 largely suppressed the dwarf phenotype of PARIOX. These results indicate that PAR1-PRE1 and PAR1-PIF4 het- erodimers form a complex HLH/bHLH network regulating cell elongation and plant development in response to light and hormones.

Chemical control of receptor kinase signaling by rapamycin-induced dimerization

Membrane-localized leucine-rich repeat receptor kinases(LRR-RKs)sense diverse extracellular signals,and coordinate and specify cellular functions in plants.However,functional understanding and identification of the cellular signaling of most LRR-RKs remain a major challenge owing to their genetic redundancy,the lack of ligand information,and subtle phenotypes of LRR-RK overexpression.Here,we report an engineered rapamycin-inducible dimerization(RiD)receptor system that triggers a receptor-specific LRR-RK signaling independent of their cognate ligands or endogenous receptors.Using the RiD-receptors,we demonstrated that the rapamycin-mediated association of chimeric cytosolic kinase domains from the BRI1/BAK1 receptor/co-receptor,but not the BRI1/BRI1 or BAK1/BAK1 homodimer,is sufficient to activate downstream brassinosteroid signaling and physiological responses.Furthermore,we showed that the engineered RiD-FLS2/BAK1 could activate flagellin-22-mediated immune signaling and responses.Using the RiD system,we also identified the potential function of an unkmown orphan receptor in immune signaling and revealed the differential activities of SERK co-receptors of LRR-RKs.Our results indicate that the RiD method can serve as a synthetic biology tool for precise temporal manipulation of LRR-RK signaling and for understanding LRR-RK biology.