Chloroplast-to-nucleus communication: current knowledge, experimental strategies and relationship to drought stress signaling

Abstract

In order for plant cells to function efficiently under different environmental conditions, chloroplastic processes have to be tightly regulated by the nucleus. It is widely believed that there is inter-organelle communication from the chloroplast to the nucleus, called retrograde signaling. Although some pathways of communication have been identified, the actual signals that move between the two cellular compartments are largely unknown. This review provides an overview of retrograde signaling including its importance to the cell, candidate signals, recent advances, and current experimental systems. In addition, we highlight the potential of using drought stress as a model for studying retrograde signaling.

Figure 1

Model for retrograde signaling during drought and EL stress. Drought shares a common feature with EL stress in that both stresses induce ABA-mediated signaling, which activates transcription of stress response genes such as ZAT10, APX2 and ELIP2. The production of ROS species during drought can also induce changes in gene expression leading to a drought response. At PSII, ¹O₂ accumulation can induce programmed cell death; this is also regulated by EXECUTER (EX), but the pathway beyond the chloroplast is unknown. ROS produced at PSI also influence gene transcription. O₂⁻ accumulation can influence gene transcription but the mechanism is unclear. While the role of H₂O₂ in stress response, particularly in ABA-signaling, is well-studied, it is unknown if H₂O₂ can directly move to the nucleus to influence transcription. Another interesting possibility is that retrograde signals may actually be metabolites normally regarded as by-products and broken down by catabolic enzymes; these molecules may accumulate during stress and influence nuclear transcription (question mark). All dashed lines indicate areas that require more research.