The Evolution of Polar Transport Models, and some Possibilities for the Regulation of Auxin Carriers

Abstract

In 1928, Went used the Avena curvature bioassay to measure transport of the newly discovered “Wuchsstoff” (later identified with IAA), from agar donor to receiver blocks, through short (2–4.2 mm) sections of oat coleoptile segments [48]. With these methods, the transport appeared exclusively basipetal and fulfilled Paal’s 1919 prediction [33] of an internally-secreted, basally-moving, correlation carrier in photo- tropically-stimulated coleoptiles. Although quantitative velocity measurements (10 to 15 mm h⁻¹, see [14, 25] for discussion of “velocity”) had to await Van der Weij’s work [47], Went considered that simple diffusion could not explain the speed of movement and that protoplasmic streaming was responsible. Although Van der Weij had argued in 1932 [47] that the temperature insensitivity of transport velocity (see also [21]) excluded such a role, it was over 40 years before experiment [6] and theory [29] showed that streaming made only a small contribution to the velocity of polar auxin transport. Indeed, Van der Weij anticipated other contemporary ideas [47]: he coined the term “polar diffusion” (cf. [13]) and suggested (in translation) that “pure diffusion occurred within each cell, with polarity being conferred by a polar permeability [4] of the boundary between two cells so that auxin can only pass in a basal direction”. A layer of unstirred cytoplasm adjacent to the cell wall was suggested as the intracellular transport route. Seeking to minimise transport barriers, Went had [48] suggested that Wuchsstoff did not penetrate the vacuole during transport - a concept whose consequences have recently been considered [30].

The following contributions (pp. 197–246) are part of the workshop Auxin Transport