Involvement of the putative Ca2+-permeable mechanosensitive channels, NtMCA1 and NtMCA2, in Ca2+ uptake, Ca2+-dependent cell proliferation and mechanical stress-induced gene expression in tobacco (Nicotiana tabacum) BY-2 cells

To gain insight into the cellular functions of the mid1-complementing activity (MCA) family proteins, encoding putative Ca2+-permeable mechanosensitive channels, we isolated two MCA homologs of tobacco (Nicotiana tabacum) BY-2 cells, named NtMCA1 and NtMCA2. NtMCA1 and NtMCA2 partially complemented the lethality and Ca2+ uptake defects of yeast mutants lacking mechanosensitive Ca2+ channel components. Furthermore, in yeast cells overexpressing NtMCA1 and NtMCA2, the hypo-osmotic shock-induced Ca2+ influx was enhanced. Overexpression of NtMCA1 or NtMCA2 in BY-2 cells enhanced Ca2+ uptake, and significantly alleviated growth inhibition under Ca2+ limitation. NtMCA1-overexpressing BY-2 cells showed higher sensitivity to hypo-osmotic shock than control cells, and induced the expression of the touch-inducible gene, NtERF4. We found that both NtMCA1-GFP and NtMCA2-GFP were localized at the plasma membrane and its interface with the cell wall, Hechtian strands, and at the cell plate and perinuclear vesicles of dividing cells. NtMCA2 transcript levels fluctuated during the cell cycle and were highest at the G1 phase. These results suggest that NtMCA1 and NtMCA2 play roles in Ca2+-dependent cell proliferation and mechanical stress-induced gene expression in BY-2 cells, by regulating the Ca2+ influx through the plasma membrane.