LEAFY COTYLEDON1-CASEIN KINASE I-TCP15-PHYTOCHROME INTERACTING FACTOR4 network regulates somatic embryogenesis by regulating auxin homeostasis

Abstract

Somatic embryogenesis (SE) is an efficient tool for the propagation of plant species and is also a useful model  for  studying  the  regulatory  networks  in  embryo development. However, the regulatory networks underlying  the  transition  from  non-embryogenic  callus  to  somatic  embryos  during  SE  remain  poorly understood. Here, we describe a Gossypium hirsutum CASEIN KINASE I gene, GhCKI, which is  a  novel key regulatory factor that strongly affects SE.  Overexpressing  GhCKI  halted  the  formation  of  embryoids and plant regeneration due to a block in the transition  from  non-embryogenic  callus  to  somatic  embryos.  In contrast, defective GhCKI in plants facilitated SE.  To  better  understand  the  mechanism  by  which GhCKI regulates  SE,  the  regulatory  network  was  analyzed.  A  direct  upstream  negative  regulator  protein, Gossypium  hirsutum  LEAFY  COTYLEDON1  (GhLEC1) ,  was  identified  to  be  targeted  to a cis-element, CTTTTC, in the  promoter of GhCKI.  Moreover,  GhCKI  interacted  with  and  phosphorylated  GhTCP15 by coordinately regulating the expression of  Gossypium  hirsutum  PHYTOCHROME  INTERACTING  FACTOR4 (PIF4), finally disrupting auxin homeostasis, which led to increased cell  proliferation  and  aborted  somatic embryo formation in GhCKI-overexpressing somatic cells. Our results demonstrate a complex process of SE that is negatively regulated by GhCKI through a complex regulatory network.