Vesicles Accumulating Auxin in Vitro Are Not “Presynaptic”

Dear Editor,

I was astonished to read how Baluska and colleagues et al. (see Robinson et al., 2018) interpret the plant membrane vesicles we characterized 3 decades ago (Hertel et al., 1983; Hertel, 1987). These vesicles were extracted from zucchini hypocotyls and maize coleoptiles, and were purified by Suc density centrifugation. After transfer from pH 7 to pH 5, creating a pH gradient neutral inside and acidic outside, added radioactive IAA could be shown to accumulate inside the vesicles. This in vitro accumulation was sensitive to the H⁺-uncoupler FCCP, and net uptake was stimulated by auxin transport inhibitors such as NPA and 2,3,5-TIBA.

The analogy to presynaptic vesicles in animal neurons, as suggested by Baluska and colleagues et al. (discussed in Robinson et al., 2018), contradicts all evidence with enzyme markers in density gradients (Lützelschwab et al., 1989): The vesicles accumulating auxin and binding NPA were derived from the plasma membrane, not from endomembranes. The origin of these vesicles was confirmed by two-phase partitioning with polyethylene glycol (Thein and Michalke, 1988). To suggest otherwise is absurd, as Robinson et al. (2018) note.