The expression of pNFkB in PC and NPC corals supports the involvement of autophagy/symbiophagy in P. acuta bleaching
pBcl-2 and other involved proteins are the effectors in the PCD signaling, not the triggers. That’s why we analyzed the expression rate of NFkB, a pleiotropic transcription factor (TF) involved in many crucial cellular processes. In response to various stressors, NFkB is translocated to nucleus where it usually triggers transcription of pro-survival genes such as Bcl-2 (Luo, Kamata, & Karin, 2005). Increased expression levels of NFkB as a result of hyperthermal shock have been observed in coral Acropora palmata (DeSalvo, Sunagawa, Voolstra, & Medina, 2010). Moreover, Bellantuono et al. observed increased levels of NFkB inhibitor (NFKBI ) in thermally PC corals (Bellantuono et al., 2012). Our results also show increasedNFKBI expression levels in PC vs NPC corals (p(0h) = 0.003, p(1h) = 0.013) (Fig. 5B). It suggests the role of NFkB in corals is to promote, not to prevent cell death. It is known that NFkB is involved in innate immune system across species (Gilmore & Wolenski, 2012). It promotes organismal immune reaction to get rid of the pathogen while it enhances the capability of the host cells to survive. One of the algal-coral symbiosis theories suggests that coral symbiont forms an intracellular organelle called the symbiosome, which is a late endosome in the state of arrested phagocytosis (Davy, Allemand, & Weis, 2012). Under stress conditions, the host innate immune response is reactivated leading to a specific type of autophagy called symbiophagy. Autophagy and NFkB pathway share common upstream signals and can control each other (Trocoli & Djavaheri-Mergny, 2011). During signaling, NFkB represses autophagy through activation of mTOR, but under specific conditions such as presence of reactive oxygen species (ROS) or after heat shock, NFkB directly activates autophagy (Trocoli & Djavaheri-Mergny, 2011). It has been well documented that symbionts release increased ROS during heat stress due to overexcitation of their photosynthetic systems, and symbiophagy is one of the main observed mechanisms of algal loss in hyperthermia (Downs et al., 2009; Dunn et al., 2007). NFkB has the potential to link these two processes.