Phytochrome B Controls Shoot Architecture by Regulating Phytochrome Interacting Factors and Phytohormones

Plant architectural responses to changes in the ratio of red light to far-red light (R: FR) are mediated by phytochromes (phy), especially phyB. phyB function is transduced through interactions with the PHYTOCHROME INTERACTING FACTORS (PIFs) family of transcription factors. This study assessed the roles of Arabidopsis thaliana PIF4, PIF5 and PIF7 in mediating shoot architectural responses to high and low R:FR. The genetic interactions between various PIFs and phyB were also examined. The results indicated that PIF4/PIF5 and PIF7 are required for suppression of branch outgrowth under low R:FR, or with the loss of functional phyB. Compared to wild-type, lower levels of axillary bud abscisic acid (ABA) were detected in the pif7 and pif4pif5 mutants under low R:FR. The loss of functional phyB elevated axillary bud sensitivity to exogenous ABA. It was also demonstrated that the abscisic acid biosynthetic enzyme NCED3 was essential for aspects of phyB mediated regulation of branching. The analysis of transcript abundances of a panel of auxin-responsive genes in pif and phyB mutants in the study suggested that PIF4/PIF5 may mediate branching responses by regulating the transcription of auxin-signaling genes. PIF7 mediated effects on bud outgrowth may involve regulation of both ABA abundances and sensitivity in buds. In summary, PIF4/PIF5 and PIF7 affect branching by regulating auxin-signaling in shoots, ABA biosynthesis and sensitivity in buds in response to the R:FR in coordination with phyB. Assessment of the kinetics of axillary bud outgrowth and ABA levels in buds revealed a significant change in bud ABA levels as early as 1 h after alteration of the R:FR. This indicates that buds are able to rapidly respond to variations in the R:FR. Ethylene is known to mediate plant responses to variation in the R:FR. The assessment of plant architectural changes in the ethylene insensitive mutants ein2-1 and etr1-2 revealed a minor contribution of ethylene in mediating branch outgrowth responses to the R:FR. EIN2 and ETR1 were shown to regulate normal gravitropic responses in rosette branches.