Insight into the nodal cells transcriptome of the streptophyte green alga Chara braunii S276

The role of indole-3-acetic acid and characterization of PIN transporters in complex streptophyte alga Chara braunii

Auxin, indole-3-acetic acid (IAA), is a key phytohormone with diverse morphogenic roles in land plants, but its function and transport mechanisms in algae remain poorly understood. We therefore aimed to explore the role of IAA in a complex, streptophyte algae Chara braunii. Here, we described novel responses of C. braunii to IAA and characterized two homologs of PIN auxin efflux carriers: CbPINa and CbPINc. We determined their localization in C. braunii using epitope-specific antibodies and tested their function in heterologous land plant models. Further, using phosphoproteomic analysis, we identified IAA-induced phosphorylation events. The thallus regeneration assay showed that IAA promotes thallus elongation and side branch development. Immunolocalization of CbPINa and CbPINc confirmed their presence on the plasma membrane of vegetative and generative cells of C. braunii. However, functional assays in tobacco BY-2 cells demonstrated that CbPINa affects auxin transport, whereas CbPINc does not. The IAA is effective in the acceleration of cytoplasmic streaming and the phosphorylation of evolutionary conserved targets such as homolog of RAF-like kinase. These findings suggest that, although canonical PIN-mediated auxin transport mechanisms might not be fully conserved in Chara, IAA is involved in morphogenesis and fast signaling processes.

The impact of salt stress on the physiology and the transcriptome of the model streptophyte green alga Chara braunii

Chara braunii is a model for early land plant evolution and terrestrialization. Salt stress has a profound effect on water and ion transport activities, thereby interacting with many other processes, including inorganic carbon acquisition for photosynthesis. In this study, we analyzed the impact of salt stress (5 practical salt units, PSU) on the physiology and gene expression in C. braunii. Photosynthesis was only slightly affected 6 h after salt addition and returned to control levels after 48 h. Several organic compounds such as proline, glutamate, sucrose, and 2-aminobutyrate accumulated in salt-treated thalli and might contribute to osmotic potential acclimation, whereas the amount of K+ decreased. We quantified transcript levels for 17,387 genes, of which 95 were up-regulated and 44 down-regulated after salt addition. Genes encoding proteins of the functional groups ion/solute transport and cell wall synthesis/modulation were enriched among the up-regulated genes 24-48 h after salt stress, indicating their role in osmotic acclimation. However, a homolog to land plant ERD4 osmosensors was transiently upregulated after 6 h, and phylogenetic analyses suggested that these sensors evolved in Charophyceae. Down-regulated genes were mainly related to photosynthesis and carbon metabolism/fixation, consistent with the observed lowered growth after extended cultivation. The changed expression of genes encoding proteins for inorganic carbon acquisition might be related to the impact of salt on ionic relations and inorganic carbon uptake. The results indicate that C. braunii can tolerate enhanced salt concentrations in a defined acclimation process, including distinct gene expression changes to achieve new metabolic homeostasis.