Endopolyploidy occurs when DNA replication takes place without subsequent mitotic nuclear division, resulting in cell-specific ploidy levels within tissues. In plants, endopolyploidy plays an important role in sustaining growth and development, but only a few studies have demonstrated a role in abiotic stress response. In this study, we investigated the function of ploidy level and nuclear and cell size in leaf expansion throughout development and tracked cell type-specific ploidy in the halophyte In addition to developmental endopolyploidy, we examined the effects of salinity stress on ploidy level. We focused specifically on epidermal bladder cells (EBC), which are modified balloon-like trichomes, due to their large size and role in salt accumulation. Our results demonstrate that ploidy increases as the leaves expand in a similar manner for each leaf type, and ploidy levels up to 512C were recorded for nuclei in EBC of leaves of adult plants. Salt treatment led to a significant increase in ploidy levels in the EBC, and these cells showed spatially related differences in their ploidy and nuclear and cell size depending on the positions on the leaf and stem surface. Transcriptome analysis highlighted salinity-induced changes in genes involved in DNA replication, cell cycle, endoreduplication, and trichome development in EBC. The increase in cell size and ploidy observed in under salinity stress may contribute to salt tolerance by increasing the storage capacity for sodium sequestration brought about by higher metabolic activity driving rapid cell enlargement in the leaf tissue and EBC.

Rubisco is a fundamental enzyme in photosynthesis and therefore for life. Efforts to improve plant Rubisco performance have been hindered by the enzymes’ complex chloroplast biogenesis requirements. New Synbio approaches however now allow the production of some plant Rubisco isoforms in E. coli. While this enhances opportunities for catalytic improvement, there remain limitations in the utility of the expression system. Here we generate, optimise and test a robust Golden Gate cloning E. coli expression system incorporating the protein folding machinery of tobacco chloroplasts. By comparing the expression of different plant Rubiscos in both E. coli and plastome transformed tobacco we show the E. coli expression system can accurately predict high level Rubisco production in chloroplasts but poorly forecasts the biogenesis potential of isoforms with impaired production in planta. We reveal heterologous Rubisco production in E. coli and tobacco plastids poorly correlates with Rubisco large subunit phylogeny. Our findings highlight the need to fully understand the factors governing Rubisco biogenesis if we are to deliver an efficient, low cost screening tool that can accurately emulate chloroplast expression.

Cyanobacterial CO2concentrating mechanisms (CCMs) sequester a globally significant proportion of carbon into the biosphere. Proteinaceous microcompartments, called carboxysomes, play a critical role in CCM function, housing two enzymes to enhance CO2fixation: carbonic anhydrase (CA) and Rubisco. Despite its importance, our current understanding of the carboxysomal CAs found in α-cyanobacteria, CsoSCA, remains limited, particularly regarding the regulation of its activity. Here, we present the first structural and biochemical study of CsoSCA from the cyanobacterium Cyanobium PCC7001. Our results show that the Cyanobium CsoSCA is allosterically activated by the Rubisco substrate ribulose-1,5-bisphosphate (RuBP), and forms a hexameric trimer of dimers. Comprehensive phylogenetic and mutational analyses are consistent with this regulation appearing exclusively in cyanobacterial α-carboxysome CAs. These findings clarify the biologically relevant oligomeric state of α-carboxysomal CAs and advance our understanding of the regulation of photosynthesis in this globally dominant lineage.