Abiotic stresses, like heat and excess sunlight perturb photosynthesis in chloroplasts and induce the accumulation of reactive oxygen species (ROS). Subsequently, this activates signalling pathways that alter gene expression towards acclimation. Thus, chloroplasts can act as environmental stress sensors. However, these insights have been gained exclusively from plants performing C3 photosynthesis. Virtually nothing is known about how the biochemically- and cell type-specialised chloroplasts performing C4 photosynthesis impact stress signalling networks.
To answer this question, we integrated live-cell imaging and metabolomics with single-cell transcriptomics on the model C4 species, Setaria viridis. We observed time- and cell type-dependent differential patterns of ROS accumulation under heat and high light stress. Correspondingly, chloroplast stress signals, like 3’-phosphoadenosine 5’-phosphate (PAP) accumulated in a cell-specific manner.
Additionally, the cell-specific accumulation of these signalling metabolites correlated with gene expression from single-cell RNA sequencing (scRNA-seq) of isolated individual cells from S. viridis. The scRNA-seq data suggests cell-specific modulation of chloroplast-to-nucleus signalling pathways during heat and high light stress, correlating with cell type-specialisation of chloroplast architecture and photosynthetic machinery, as well as the aforementioned differential levels of ROS and PAP.
Our results provide preliminary evidence for chloroplast architecture and photosynthesis being a major contributor to cell-specific stress signalling networks. This is significant as current crop bioengineering approaches, such as engineering C4 photosynthesis into C3 plants tend not to consider stress signalling networks. We propose that a deeper understanding of how chloroplast- and cell type-specialisation intersect with stress signalling under C4 photosynthesis may provide novel strategies for enhancing crop productivity.