The role of ion-transporting proteins in the evolution of salt tolerance in charophyte algae

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.

Molecular membrane separation: plants inspire new technologies

Plants draw up their surrounding soil solution to gain water and nutrients required for growth, development and reproduction. Obtaining adequate water and nutrients involves taking up both desired and undesired elements from the soil solution and separating resources from waste. Desirable and undesirable elements in the soil solution can share similar chemical properties, such as size and charge. Plants use membrane separation mechanisms to distinguish between different molecules that have similar chemical properties. Membrane separation enables distribution or retention of resources and efflux or compartmentation of waste. Plants use specialised membrane separation mechanisms to adapt to challenging soil solution compositions and distinguish between resources and waste. Coordination and regulation of these mechanisms between different tissues, cell types and subcellular membranes supports plant nutrition, environmental stress tolerance and energy management. This review considers membrane separation mechanisms in plants that contribute to specialised separation processes and highlights mechanisms of interest for engineering plants with enhanced performance in challenging conditions and for inspiring the development of novel industrial membrane separation technologies. Knowledge gained from studying plant membrane separation mechanisms can be applied to developing precision separation technologies. Separation technologies are needed for harvesting resources from industrial wastes and transitioning to a circular green economy.

The cell biology of charophytes: Exploring the past and models for the future

Charophytes (Streptophyta) represent a diverse assemblage of extant green algae that are the sister lineage to land plants. About 500-600+ million years ago, a charophyte progenitor successfully colonized land and subsequently gave rise to land plants. Charophytes have diverse but relatively simple body plans that make them highly attractive organisms for many areas of biological research. At the cellular level, many charophytes have been used for deciphering cytoskeletal networks and their dynamics, membrane trafficking, extracellular matrix secretion, and cell division mechanisms. Some charophytes live in challenging habitats and have become excellent models for elucidating the cellular and molecular effects of various abiotic stressors on plant cells. Recent sequencing of several charophyte genomes has also opened doors for the dissection of biosynthetic and signaling pathways. While we are only in an infancy stage of elucidating the cell biology of charophytes, the future application of novel analytical methodologies in charophyte studies that include a broader survey of inclusive taxa will enhance our understanding of plant evolution and cell dynamics.

The molecular identity of the characean OH- transporter: a candidate related to the SLC4 family of animal pH regulators

Characeae are closely related to the ancient algal ancestors of all land plants. The long characean cells display a pH banding pattern to facilitate inorganic carbon import in the acid zones for photosynthetic efficiency. The excess OH^-, generated in the cytoplasm after CO₂ is taken into the chloroplasts, is disposed of in the alkaline band. To identify the transporter responsible, we searched the Chara australis transcriptome for homologues of mouse Slc4a11, which functions as OH^-/H⁺ transporter. We found a single Slc4-like sequence CL5060.2 (named CaSLOT). When CaSLOT was expressed in Xenopus oocytes, an increase in membrane conductance and hyperpolarization of resting potential difference (PD) was observed with external pH increase to 9.5. These features recall the behavior of Slc4a11 in oocytes and are consistent with the action of a pH-dependent OH^-/H⁺ conductance. The large scatter in the data might reflect intrinsic variability of CaSLOT transporter activation, inefficient expression in the oocyte due to evolutionary distance between ancient algae and frogs, or absence of putative activating factor present in Chara cytoplasm. CaSLOT homologues were found in chlorophyte and charophyte algae, but surprisingly not in related charophytes Zygnematophyceae or Coleochaetophyceae.

Salinity-induced Changes in Gene Expression in the Streptophyte Alga Chara: The Critical Role of a Rare Na+ -ATPase

The primarily freshwater genus Chara is comprised of many species that exhibit a wide range of salinity tolerance. The range of salt tolerance provides a good platform for investigating the role of transport mechanisms in response to salt stress, and the close evolutionary relationship between Charophytes and land plants can provide broader insights. We investigated the response to salt stress of previously identified transport mechanisms in two species of Chara, Chara longifolia (salt-tolerant), and Chara australis (salt-sensitive): a cation transporter (HKT), a Na⁺ /H⁺ antiport (NHX), H⁺ -ATPase (AHA), and a Na⁺ -ATPase (ENA). The presence of these candidate genes has been confirmed in both species of Chara, with the exception of the Na⁺ -ATPase, which is present only in salt-tolerant Chara longifolia. Time-course Illumina transcriptomes were created using RNA from multiple time points (0, 6, 12, 24 and 48 h) after freshwater cultures for each species were exposed to salt stress. These transcriptomes verified our hypotheses of these mechanisms conferring salt tolerance in the two species examined and also aided in identification of specific transcripts representing our genes of interest in both species. The expression of these transcripts was validated through use of qPCR, in a similar experimental set-up used for the RNAseq data described above. The RNAseq and qPCR data showed significant changes of expression mechanisms in C. longifolia (respectively), a down-regulation of HKT and a substantial up-regulation of ENA. Significant responses to salt stress in salt-sensitive C. australis show up-regulation of NHX and AHA.