Main content area

Ion antagonism in phytochrome-mediated calcium-dependent germination of turions of Spirodela polyrhiza (L.) Schleiden

Appenroth, K.J., Gabrys, H., Scheuerlein, R.W.
Planta 1999 v.208 no.4 pp. 583-587
shoots, Spirodela polyrhiza, phytochrome, germination, cations, light, culture media, magnesium, barium, strontium, lanthanum, ion transport, inorganic ions, calcium
The light-dependent germination response of turions (resting fronds) is mediated by phytochrome and requires the presence of Ca(2+) in the medium (K.-J. Appenroth and H. Augsten, 1990, Photochem. Photobiol. 52: 61-65). The Ca(2+) requirement of germination is apparent only in the presence of exogenous Mg(2+). A competitive ion antagonism was demonstrated between Ca(2+) and Mg(2+) in this physiological response; Mg(2+) could also be replaced by Ba(2+) or Sr(2+). Without exogenous Mg(2+), a Ca(2+) concentration as low as 0.9 micromolar fulfilled the Ca(2+) requirement. This type of ion antagonism resembled the competitive Ca/Mg interaction reported previously for calcium-binding proteins. The physiological response was blocked by inhibitors of Ca(2+) uptake (verapamil, La(3+)). It was concluded that uptake of Ca(2+) from the external medium is an essential step in the phytochrome-mediated germination of turions. The results are in agreement with the assumption that the uptake of Ca(2+) is blocked at the side of entry by other alkaline earth ions. Treatment of turions with Mg(2+) (1 mM) for 24 h at varying times after the red light pulse in otherwise virtually Ca(2+)-free KNO(3) solution resulted in a response similar to a Ca(2+) stepdown treatment. This is in agreement with the assumption that the Ca(2+)- and the Mg(2+)-sensitive periods coincide. The ion interaction described here represents the first photophysiological example in plants of an antagonistic effect between Ca(2+) and Mg(2+) similar to that which occurs in vitro with calmodulin.