The involvement of a G‐protein in phytochrome‐regulated, Ca2+‐dependent swelling of etiolated wheat protoplasts

The red light (R)‐induced swelling of mesophyll protoplasts, isolated from dark‐grown wheat (Triticum aestivum L. cv. Arminda) leaves, was inhibited by guanosine‐5′‐0‐(2‐thiodiphosphate) (GDP‐β‐S). In darkness or after control irradiation with far‐red light (FR), guanosine‐5′‐O‐(3‐thiotriphosphate) (GTP‐γ‐S) induced swelling to the same extent as after R. Both GDP‐β‐S and GTP‐γ‐S were introduced into the cytoplasm by means of electroporation. The possibility of R‐induced activation of the phosphatidylinositol pathway of transmembrane signalling was investigated. Neomycin, Li+ and l‐(5‐isoquinolinesulfonyl)‐2‐methylpiperazine (H7) inhibited the R‐induced swelling. Phorbol 12‐myristate 13‐acetate (PMA) induced swelling after control irradiation with FR. Neomycin and Li+ also inhibited GTP‐γ‐S‐induced swelling. These results suggest that a GTP‐binding protein is involved in the phytochrome‐regulated swelling response. Addition of N6, 2′‐0‐dibutyryladenosine 3′:5′‐cyclic monophosphate (DB‐cAMP) induced swelling to the same extent as R‐irradiation. The calmodulin antagonist N‐(6‐aminohexyl)5‐chloro‐l‐naphthalenesulfonamide (W7) induced swelling after FR, while R‐induced swelling was not affected. The less active analogue N‐(6‐aminohexyl)‐l‐naphthalenesulfonamide (W5) induced no swelling after FR. It is speculated that the protoplast volume is correlated with the cytoplasmic concentration of free Ca2+.