Mechanosensitive (MS) ion channels respond to mechanical stress and convert it into intracellular electric and ionic signals. Five MS channel families have been identified in plants, including the Mid1-Complementing Activity (MCA) channel; however, its activation mechanisms have not been elucidated in detail. We herein demonstrate that the MCA2 channel is a Ca²⁺-permeable MS channel that is directly activated by membrane tension. The N-terminal 173 residues of MCA1 and MCA2 were synthesized in vitro, purified, and reconstituted into artificial liposomal membranes. Liposomes reconstituted with MCA1(1-173) or MCA2(1-173) mediate Ca²⁺ influx and the application of pressure to the membrane reconstituted with MCA2(1-173) elicits channel currents. This channel is also activated by voltage. Blockers for MS channels inhibit activation by stretch, but not by voltage. Since MCA proteins are found exclusively in plants, these results suggest that MCA represent plant-specific MS channels that open directly with membrane tension.

MscS is a mechanosensitive channel that is ubiquitous among bacteria. Recent progress in the genome projects has revealed that homologs of MscS are also present in eukaryotes, but whether they operate as ion channels is unknown. In this study we cloned MSC1 , a homolog of MscS in Chlamydomonas , and examined its function when expressed in Escherichia coli . Full-length MSC1 was not functional when expressed in E. coli cells. However, removal of the N-terminal signal sequence (ΔN-MSC1) reversed this effect. ΔN-MSC1 was found to open in response to membrane stretch and displayed a preference for anions over cations as permeable ions. ΔN-MSC1 exhibited marked hysteretic behavior in response to ascending and descending stimuli. That is, channel gating occurred in response to significant stimuli but remained open until the stimulus was almost completely removed. Indirect immunofluorescence revealed that MSC1 is present as punctate spots in the cytoplasm and chloroplasts. Moreover, knockdown of MSC1 expression resulted in the abnormal localization of chlorophyll. These findings show that MSC1 is an intracellular mechanosensitive channel and is responsible for the organization of chloroplast in Chlamydomonas .

Temperature is physiologically critical for all living organisms, which cope with temperature stress using metabolic and behavioral responses. In unicellular and some multicellular organisms, thermotaxis is a behavioral response to avoid stressful thermal environments and promote accumulation in an optimal thermal environment. In this study, we examined whether Chlamydomonas reinhardtii, a unicellular green alga, demonstrated thermotaxis. We found that between 10 °C and 30 °C, Chlamydomonas cells migrated toward lower temperatures independent of cultivation temperature. Interestingly, when we applied reagents to change intracellular reduction-oxidation (redox) conditions, we saw that thermotaxis was enhanced, suppressed, or reversed, depending on the redox conditions and cultivation temperature. Thermotaxis was almost absent in ppr2 and ppr3 mutants, which cannot swim backward because of a defect in generating calcium current in flagella. The frequency of spontaneous backward swimming was lower at more favorable temperature, suggesting a pivotal role of spontaneous backward swimming generated by flagellar membrane excitation.

The physical environment exerts a profound influence on microbial life. The directional movement of cells in response to their physical environment is understood as taxis, which has been studied in biology as chemotaxis, phototaxis, gravitaxis and so forth. These taxis are induced by physiological, physical or both factors. Nevertheless, the physical picture of cellular responses to the environment at the individual cell scale remains unclear. In this review paper, we therefore aim to provide an overview of the recent progress in the physical depiction of microbial taxis and their behaviour in various environments. Four types of responses are discussed: responses to (i) gravity, (ii) light, (iii) surface and (iv) flow. For each response, a simple physical picture is presented that describes the mechanism of behaviours. Knowledge of physical mechanisms will help us to understand the behaviour of cells in various environments at both the individual and population levels. Finally, the future challenges and prospects in this research field are discussed. This article is part of the theme issue ‘Biological fluid dynamics: emerging directions’.

Cilia are organelles involved in motility and sensory transduction, but how these two functions coexist has not been elucidated in depth. Here, the involvement of the ciliary transient receptor potential (TRP) channel TRP11 in mechanoresponses is studied in Chlamydomonas reinhardtii using a TRP11-knockout mutant. The mutant has defects in the conversion of the bending mode of the cilium from forward to reverse when tapped with a glass rod, the detachment of cilia when shear is applied, the increase in ciliary beat frequency upon application of mechanical agitation by vortex mixing, and the initiation of gliding while both cilia are attached in opposite directions to a glass surface. These observations indicate that TRP11 can perceive mechanical stimuli with distinct intensities and durations and induce various types of ciliary responses.