Auxin Transport and Its Regulation by Flavonoids

Abstract

There is much debate about the contribution of changes in auxin concentration to the control of auxin responsive processes. Transport of auxin (IAA) could participate in plant signalling networks by influencing its availability to binding sites, whose occupancy is envisaged to modulate coupling and effector processes. However, the evidence for this is still fragmentary. Auxin movement across membranes has been extensively studied at the cellular level as well as in sealed vesicles [22, 38]. Such works lead [34] to the “chemiosmotic” interpretation of polar auxin transport (PAT) which is likely to be an important determinant of IAA’s role as a pattern-forming morphogen, particularly in vascular differentiation [15, 39]. PAT is preferentially basipetal in shoot tissue, energy-dependent, and has a velocity of 5–20 mm h^-1 [18]. The fundamental transport processes can be summarized as follows: IAA can cross membranes by both diffusive and carrier-mediated routes, responding to transmembrane pH (ΔpH) and electrical potential gradients (ΔΨ). The relatively alkaline cytoplasm (pH 7–7.4) can accumulate IAA from more acidic compartments such as the cell wall because of (i) the high diffusive membrane permeability of undissociated IAAH molecules (pK = 4.7) relative to IAA anions and (ii) a high-affinity, saturable uptake carrier which may operate by electroim-pelled IAA^-/2H⁺ co-transport [22, 38]. There is also a carrier, probably for IAA anions, catalyzing efflux down the electrochemical gradient set up by the accumulative uptake process [33, 38].