A shift in sensitivity to auxin within development of maize seedlings

The auxin-induced changes in cytosolic concentrations of Ca2+ and H+ ions were investigated in protoplasts from maize coleoptile cells at 3rd, 4th and 5th day of development of etiolated seedlings. The shifts in [Ca2+]cyt and [H+]cyt were detected by use of fluorescence microscopy in single protoplasts loaded with the tetra[acetoxymethyl]esters of the fluorescent calcium binding Fura 2, or pH-sensitive carboxyfluorescein, BCECF, respectively. Both the auxin-induced shifts in the ion concentrations were specific to the physiologically active synthetic auxin, naphthalene-1-acetic acid (1-NAA), and not to the non-active naphthalene-2-acetic acid (2-NAA). Regardless of the age of the seedlings, the rise in [Ca2+]cyt was prior to the acidification in all investigated cases. The maximal acidification coincided with the highest amplitude of [Ca2+]cyt change, but not directly depended on the concentration of 1-NAA. Within aging of the seedlings the amplitude of auxin-induced [Ca2+]cyt elevation decreased. The shift in auxin-induced acidification was almost equal at 3rd and 4th day, but largely dropped at 5th day of development. The acidification was related to changes in the plasma membrane H+-ATPase activity, detected as phosphate release. The decrement in amplitude of both the tested auxin-triggered reactions well coincided with the end of the physiological function of the coleoptile. Hence the primary auxin-induced increase in [Ca2+]cyt, which is supposed to be an important element of hormone signal perception and transduction, can be used as a test for elucidation of plant cell sensitivity to auxin.