PS PhD Exit Seminar - Decrypting the cell-specific cellular signalling pathways and acclimation responses driving stress-resilient C4 photosynthesis

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Abstract - 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 nuclear 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 that drive C4 photosynthesis influence stress signalling networks. To answer this question, my PhD project interrogates cell-specific heat and high light responses in the model C4 species, Setaria viridis using a combination of live-cell imaging, genetic manipulation, and ‘omics technologies, including tissue-enriched metabolomics and transcriptomics, as well as single-cell transcriptomics.
Both time- and cell type-dependent differential patterns of ROS accumulation were uncovered in S. viridis mesophyll and bundle sheath cells under heat and high light stress. Correspondingly, the accumulation of ROS-responsive chloroplast stress signals, such as 3’-phosphoadenosine 5’-phosphate (PAP) and β-cyclocitral also occurred in a cell-specific manner. The cell-specific accumulation of these ROS and signalling metabolites correlated strongly with cell-specific gene expression responsive to these ROS and chloroplast signals. Indeed, the transcriptomes of enriched S. viridis tissue fractions suggested activation of distinct chloroplast-to-nucleus signalling pathways in mesophyll and bundle sheath cells during heat and high light stress, correlating with the specialised chloroplast architecture and photosynthetic machinery found in these cell types.
In parallel with the analysis above, I optimised and performed single-cell RNA sequencing (scRNA-seq) of isolated individual cells from S. viridis leaves under control and stress conditions. Analysis of the scRNA-seq data enabled further spatial separation of stress responses within a whole leaf and pinpointed the specific cell types responsible for driving acclimation responses to heat and high light stress. For instance, I identified: 1) an up-regulation of flavonoid metabolism and accumulation of UV scavengers specific to epidermal pavement cells; and 2) the cell-specific expression and suppression of ROS homeostasis genes in mesophyll, bundle sheath and vascular cells, which collectively provided a mechanistic explanation for the patterns of ROS accumulation observed in these cell types. Excitingly, transgenic plants with these ROS homeostasis genes manipulated in a cell-specific manner showed improved short-term stress resilience.
Together, these results provide preliminary evidence for chloroplast architecture and photosynthesis being a major contributor to cell-specific stress signalling communication. This is significant as current crop bioengineering approaches, such as engineering C4 photosynthesis into C3 plants have not incorporated stress signalling networks. I propose that a deeper understanding of how chloroplast- and cell type-specialisation intersects with stress signalling under C4 photosynthesis may provide novel strategies for enhancing crop productivity.

Biography - Prior to her PhD studies, Suyan completed a joint Honours year with the Pogson and Corry groups, combining protein modelling and chemical biology to characterise inhibitors capable of finetuning plant stress responses. She has also worked with the Price group within the ANU node of the Realising Increased Photosynthetic Efficiency (RIPE) Consortium, contributing to transferring functional transporters into plant carboxysomes using synthetic biology. Suyan commenced her PhD in 2022 in Dr. Kai Chan’s newly established research group within the RSB. Her long-term research goal is to safeguard plant productivity against environmental stresses exacerbated by climate change. Her current research works towards this goal by combining emerging technologies like single-cell transcriptomics with live-cell imaging to provide novel insights into plant responses under challenging environmental conditions.