Synthesis of vesicle cargo determines amplitude of Ca2+-sensitive exocytosis

Here we examine the potential coupling between the synthesis of secretory proteins and the sensitivity of exocytosis to the concentration of free Ca2+ in the cytosol ([Ca2+]i) in plant cell. We therefore monitor in tobacco protoplasts the excursion of the membrane capacitance in response to an elevation of [Ca2+]i as a measure for exocytotic activity. The data show that a ramp like elevation of [Ca2+]i generates in protoplasts from wild type plants and from transgenic plants, which overexpress the secreted α-amylase, an exocytotic burst with an initial steep and a subsequent slow phase. The largest capacitive burst is obtained in α-amylase producing plants and the amplitude of the [Ca2+]i evoked Cm excursion is a function of the amylase synthesis of the plants. The data support a model according to which plant cells have at least two serial [Ca2+]i sensitive processes in the final steps of their exocytotic pathway. The overproduction of a secreted cargo does not affect the kinetics of this process but the number of vesicles in pools upstream of the [Ca2+]i sensitive steps.