Figure 1.

Effect of calcium-signalling inhibitors on germination and CAT-mediated cell fusion. (A) Chelation of extracellular Ca²⁺ with BAPTA inhibits the formation of CATs and thus blocks cell fusion very effectively. Germ-tube formation and elongation are unaffected. (B) Calmidazolium, which also blocks calmodulin/calcineurin signalling, already affects germination, germ-tube elongation and CAT-mediated cell fusion at low concentrations. Nevertheless, CAT formation is much more sensitive compared to germ-tube formation. (C) Cyclosporin A, on the other hand, which also acts on calcineurin signalling, has a stronger inhibitory effect on CAT-mediated cell fusion. (D) Inactivation of calcineurin signalling with FK506 has a strong inhibitory effect on CAT formation but not on germ-tube development. (E) Inhibition of mitochondrial calcium uniporters with RU360 results in a very similar phenotype compared to verapamil, though at much lower effective concentrations. (F) Inhibition of Ca²⁺ influx into the ER with thapsigargin has only a weak but specific effect on CAT formation. (G) Inactivation of Ca²⁺ plasma-membrane channels with verapamil supresses germination altogether. (H) In the positive control (1% PDB + NaNO₃ fusion medium), germination, germ-tube elongation and germling fusion (*) occur with high frequencies, whereas (I) in the absence of NaNO₃ (1% PDB only), CAT formation (and hence germling fusion) cannot take place, and only elongated germ tubes are produced. Scale bars, 10 µm.