Interpreting Cytokinin Action as Anterograde Signaling and Beyond

Abstract

Among the major phytohormones, the cytokinin exhibits unique features for its ability to positively affect the developmental status of plastids. Even early on in its research, cytokinins were known to promote plastid differentiation and to reduce the loss of chlorophyll in detached leaves. Since the discovery of the components of cytokinin perception and primary signaling, the genes involved in photosynthesis and plastid differentiation have been identified as those directly targeted by type-B response regulators. Furthermore, cytokinins are known to modulate versatile cellular processes such as promoting the division and differentiation of cells and, in concert with auxin, initiating the de novo formation of shoot apical meristem (SAM) in tissue cultures. Yet how cytokinins precisely participate in such diverse cellular phenomena, and how the associated cellular processes are coordinated as a whole, remains unclear. A plausible presumption that would account for the coordinated gene expression is the tight and reciprocal communication between the nucleus and plastid. The fact that cytokinins affect plastid developmental status via gene expression in both the nucleus and plastid is interpreted here to suggest that cytokinin functions as an initiator of anterograde (nucleus-to-plastid) signaling. Based on this viewpoint, we first summarize the physiological relevance of cytokinins to the coordination of plastid differentiation with de novo shoot organogenesis in tissue culture systems. Next, the role of endogenous cytokinins in influencing plastid differentiation within the SAM of intact plants is discussed. Finally, a presumed plastid-derived signal in response to cytokinins for coupled nuclear gene expression is proposed.

The LONELY GUY gene family: from mosses to wheat, the key to the formation of active cytokinins in plants

“…As cytokinin is intimately involved in the response of the plant to the light environment (reviewed in Cortleven et al ., 2019a ; Ikeda et al ., 2021 ; Pavlü et al ., 2018 ), it is not surprising that light‐responsive elements are over‐represented. CMA3 (conserved DNA module array) is a binding site for 3AF3, which, along with other proteins, activates light transcription of the small subunit of RUBISCO (Sarokin and Chua, 1992 ).…”

The Passage of H2O2 from Chloroplasts to Their Associated Nucleus during Retrograde Signalling: Reflections on the Role of the Nuclear Envelope

“…More recently, this relationship has received growing attention since the juxtaposition of a subset of chloroplasts with the nucleus is suggested to be a crucial feature in the communication and coordination of highly complex processes between these organelles in response to developmental and environmental cues. [4,11,[16][17][18]. Since some signalling molecules could originate from Plants 2022, 11, 552 2 of 11 multiple cellular sources, the close association between the nucleus and a subset of chloroplasts may provide the necessary specificity for retrograde signal transduction.…”