Fig. 4.

(A) GUN4 retrograde signalling model (after Brzezowski et al., 2014). (i) GUN4 is proposed to be an activator of MgCh activity, interacting with the chlorophyll H subunit to promote the catalytic integration of Mg²⁺ with ProtoIX to form the chlorophyll biosynthesis pathway intermediate Mg-ProtoIX. (ii) The accumulation of excess tetrapyrrole intermediates, such as ProtoIX, in the chloroplast can lead to generation of ROS. GUN4 is proposed to bind ProtoIX, shielding its reaction with ROS. In shielding ProtoIX, GUN4 may be progressively modified or degraded, with degradation products hypothesized to act as the retrograde signals. (B) A contrasting model for GUN4 (after Tahari Tabrizi et al., 2016). Instead of having a ‘shielding’ effect when bound to ProtoIX, the GUN4–ProtoIX complex appeared to escalate ¹O₂ generation. The elevated ¹O₂ produced by GUN4–ProtoIX may be sensed by an ¹O₂-sensing system (like the Arabidopsis EXECUTER1/EXECUTER2 or EX1/EX2 system) yet to be discovered, that relays a signal to the nucleus.