Auxin induces an increase of Ca 2+ concentration in the cytosol of wheat leaf protoplasts

Abstract

Auxin addition to protoplasts isolated from leaves of 6-day-old wheat seedlings ( Triticum aestivum L. cv. Kadett) induced a rapid increase in the cytosolic calcium concentration [Ca 2+] cyt. The shifts in [Ca 2+] cyt were detected by use of fluorescence microscopy in single protoplasts loaded with the calcium binding tetra[acetoxymethyl]ester of the fluorescent dye, Fura 2. Addition of the synthetic auxin naphthyl acetic acid, 1-NAA, induced an increase in [Ca 2+] cyt within 5–10 s, while the physiologically non-active analogue, 2-NAA, did not. The amplitude of calcium increase depended on the concentration of 1-NAA. Since the process was affected by different concentrations of Ca 2+ in the external medium, and since the calcium channel blockers (nifedipine and verapamil) postponed and inhibited the reaction, it is suggested that auxin primarily activates Ca 2+-permeable channels in the plasma membrane. In the presence of low external calcium concentration (0.1 mM), 5 mM LiCl almost totally blocked the increase in [Ca 2+] cyt, indicating a possible involvement of tonoplast Ca 2+-channels in the auxin-induced [Ca 2+] cyt shift. Thus, calcium signalling induced by auxin involves both external and internal Ca 2+ pools.