Coordinated cpSRP43 and cpSRP54 Abundance Is Essential for Tetrapyrrole Biosynthesis While cpSRP43 Is Independent of Retrograde Signaling

The chloroplast signal recognition particle (cpSRP) components cpSRP43 and cpSRP54 not only form a complex with light-harvesting chlorophyll (Chl)-binding proteins to direct them to the thylakoid membrane, but also serve other functions. cpSRP43 independently acts as a chaperone for some tetrapyrrole biosynthesis enzymes, while cpSRP54 participates in co-translational targeting of plastid-encoded proteins. However, it remains unclear to what extent the two cpSRP components are coregulated—despite their distinct functions—and whether both participate in genomes uncoupled (GUN)-mediated retrograde signaling. Here, we demonstrate that cpSRP43 and cpSRP54 accumulation is strongly interdependently controlled: overexpression of one protein increases the level of the other, while a deficiency in one of the two proteins leads to a simultaneous decrease of the other component. Disruption of this balance, e.g., by combining overexpression of one component with a knockout of the other, results in a severe chlorosis, stunted growth, and reduced levels of Chl and tetrapyrrole intermediates. Moreover, cpSRP43 deficiency exacerbates the pale-green phenotype of gun4 and gun5 mutants, highlighting a synergistic impact on TBS; however, cpSRP43 overexpression fails to rescue these defects. Remarkably, loss of cpSRP43 does not affect the expression of nuclear-encoded photosynthetic genes under intrinsic plastid stress, clearly demonstrating that cpSRP43 is not involved in plastid-to-nucleus retrograde signaling. Overall, our findings underscore that fine-tuned expression of cpSRP43 and cpSRP54 is crucial for proper chloroplast function and pigment biosynthesis, while cpSRP43 alone does not participate in the retrograde signaling pathway.