Specialisation of photosynthetic biochemistry in C4 chloroplasts potentiates distinct cell-specific functionality of different chloroplast-to-nucleus retrograde signalling pathways.

Abstract

Chloroplasts are key environmental sensors for plant cells. Upon exposure to abiotic stresses such as excess sunlight and heat, reactive oxygen species (ROS) in chloroplasts are sensed to trigger chloroplast-to-nucleus signalling pathways for acclimatory nuclear gene expression. There is extensive biochemical- and cell type-specialisation of chloroplasts, such as those in heat-resilient plants performing C4 photosynthesis, but how these specialisations intersect with chloroplast-to-nucleus signalling remains unclear. Here we show that specific chloroplast-to-nucleus signalling networks are functional in specialised leaf cell types of the C4 model species, Setaria viridis. RNA-sequencing, electron paramagnetic resonance (EPR) spectroscopy and metabolomics on cell type-enriched fractions, complemented with mutant analyses provided evidence for cell-specific enrichment of chloroplast-to-nucleus retrograde signalling pathways correlating with the photosynthetic specialisation in these cell types. Singlet oxygen (1O2)-responsive beta-cyclocitral ({beta}-CC) /dihydroactinidiolide (DHA) signalling and hydrogen peroxide (H2O2)-responsive 3'-phosphoadenosine 5'-phosphate (PAP) signalling pathways were specific to mesophyll cells, in line with enrichment of photosystem II and linear electron flow to photosystem I, respectively in this cell type. In contrast, bundle sheath cells were enriched for the rapid, ROS-independent retrograde signalling pathway mediated by the Calvin cycle product, dihydroxyacetone phosphate (DHAP), consistent with the exclusive localisation of the Calvin cycle to C4 bundle sheath cells. Taken together, our results provide insights into how chloroplast- and cell type-specialisation intersect with stress signalling in plant cells and identify photosynthetic biochemistry as a key driver of cell-specific cellular signalling.