Ca(2+)-Transport through Plasma Membrane as a Test of Auxin Sensitivity.

Auxin is one of the crucial regulators of plant growth and development. The discovered auxin cytosolic receptor (TIR1) is not involved in the perception of the hormone signal at the plasma membrane. Instead, another receptor, related to the ABP1, auxin binding protein1, is supposed to be responsible for the perception at the plasma membrane. One of the fast and sensitive auxin-induced reactions is an increase of Ca(2+) cytosolic concentration, which is suggested to be dependent on the activation of Ca(2+) influx through the plasma membrane. This investigation was carried out with a plasmalemma enriched vesicle fraction, obtained from etiolated maize coleoptiles. The magnitude of Ca(2+) efflux through the membrane vesicles was estimated according to the shift of potential dependent fluorescent dye diS-C₃-(5). The obtained results showed that during coleoptiles ageing (3rd, 4th and 5th days of seedling etiolated growth) the magnitude of Ca(2+) efflux from inside-out vesicles was decreased. Addition of ABP1 led to a recovery of Ca(2+) efflux to the level of the youngest and most sensitive cells. Moreover, the efflux was more sensitive, responding from 10(-8) to 10(-6) M 1-NAA, in vesicles containing ABP1, whereas native vesicles showed the highest efflux at 10(-6) M 1-NAA. We suggest that auxin increases plasma membrane permeability to Ca(2+) and that ABP1 is involved in modulation of this reaction.