The observations by Darwin on tropisms (Darwin and Darwin 1881) and subsequent experiments by plant biologists such as Went (Went and Thimann 1937) not only led to the identification of auxin, but also revealed directional transport of this hormone from source (biosynthesis) tissues to sites of action. Transport measurements using radio-labeled auxin showed the existence of two types of auxin transport systems: fast and non-directional transport through the phloem, and slow and directional cell-to-cell transport that is referred to as polar auxin transport (PAT). The transport through the phloem was first detected by Morris and Thomas (Morris and Thomas 1978) and occurs in both basi- and acropetal directions at approximately 5-20 cm/h (Nowacki and Bandurski 1980). Experiments performed by Baker
(Baker 2000) indeed revealed significant presence of IAA in the phloem. A connection between the fast transport of auxin and PAT was demonstrated in experiments performed in pea, in which radio-labeled IAA initially present in the phloem was detected in the polar transport system at a later time point (Cambridge and Morris 1996).
In contrast to the phloem-mediated auxin transport, PAT is restricted to free IAA, is unidirectional and occurs in a cell-to-cell manner. The velocity is much slower, and has been estimated at approximately 5-20 mm/h. In dicotyledon plants, polar auxin transport is known to travel long-distance from the shoot apex to the root via the vascular cambium and xylem parenchyma and differentiating xylem vessels. On its way down, lateral transport of auxin from the vascular cells to the outer cell layers is needed to stimulate shoot elongation, inhibit lateral bud outgrowth and initiate lateral root primordia. At the root tip, PAT is redirected upwards, proceeding basipetally through the root epidermis towards the root elongation zone. Polar auxin transport inhibitors, such as 1-N-naphphthlylphtalamic acid (NPA) have been used to show that this cell-to-cell transport of auxin directs plant development (Morris et al. 2004). Auxin induces cell division and cell expansion, and the dynamic auxin gradients and maxima established by its polar transport not only direct tropic growth responses, as described by the Cholodny and Went hypothesis (Went 1926; Cholodny 1927; Went and Thimann 1937), but are also instructive for embryo patterning (Friml et al. 2003; Weijers et al. 2005b), root meristem maintenance (Sabatini et al. 1999) and the initiation and positioning of lateral root and aerial organ primordia (phyllotaxis) (Benkova et al. 2003; Reinhardt et al. 2003).
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