In this article, we will discuss about the transport of auxin in plants.
The auxins are synthesized in growing menistematic regions of plant from where they are transported to other plant parts. The transport of auxin in plant is predominantly polar. In stems, polar transport of auxin is basipetal i.e., it takes place from apex towards base. In roots also, the auxin transport is polar but is primarily acropetal.
Jacobs (1961) found polar transport of auxin in Coleus stem sections to be both basipetal and acropetal in the ratio of 3: 1. According to Audus (1959) some of the auxin synthesized by leaves may be transported to other plant parts through phloem in a rather non-polar manner. Phototropic and geotrophic movements indicate towards lateral transport of auxins in stem tip and root tip respectively.
Polar basipetal transport of auxin in plants is strongly developed in coleoptile of monocot seedlings such as Avena coleoptile which is illustrated in Fig. 17.12. Some coleoptile segments are removed from Avena seedling growing in vertical position. An agar block containing auxin (‘donor’ agar block) is appressed to the tip end (upper end) of the segment and a plain agar block (without auxin i.e., acceptor agar block) is appressed to the basal end (lower end) of the coleoptile segment.
After several hours, the two agar blocks are bio-assayed for auxin (IAA). It is observed that the auxin has moved through the coleoptile segment from morphological apical end to the base in the acceptor agar block (Fig. 17.12 A). In case, the coleoptile segment is turned upside down and the same experiment is repeated, no movement of auxin takes place into the acceptor agar block (Fig. 17.12 B).
Polar transport of auxin in plant is not a simple diffusion process in response to a concentration gradient, but it involves the activity of living cells and can take place against the concentration gradient too. Polar auxin transport is temperature sensitive and requires metabolic energy. It is inhibited under anaerobic conditions and by metabolic inhibitors.
Chemiosmotic Model of Polar Auxin Transport:
There are two main features of chemiosmotic model of polar auxin transport in plants which is now generally accepted:
(i) The main driving force for auxin influx (auxin uptake) is a pH gradient or proton motive force across the plasma-membrane and
(ii) The auxin efflux is driven by the presence of specific auxin efflux carriers (or proteins) located at the base of the auxin conducting cells.
(i) Auxin Influx (Uptake):
According to chemiosmotic model (Fig. 17.13), the auxin uptake by plant cells may take place from any direction. IAA may exist in two forms; one is protonated or un-dissociated form (IAAH) which is highly lipophilic and can cross the plasma-membrane easily. The other form is dissociated or anionic form (IAA–) that does not cross plasma-membrane unaided.
i. In more acidic pH (low pH), the IAAH form predominates such as that exists in cell- wall space (apoplast). The low apoplastic pH (about 5) is maintained due to activities of ATPases present all around in the plasma-membrane. Any IAA that may be present in cell wall, rapidly associates with H+ to form IAAH. The latter diffuses passively across the plasma-membrane easily.
ii. The anionic form (IAA ) may also cross the plasma-membrane from cell wall by secondary active co-transport mechanism through 2H+/IAA– symporter (i.e., influx carrier protein) which is uniformly distributed around the cell, These permease type of influx carrier proteins or 2H+/IAA– symporters have been called as AUX1 and were first identified in Arabidopsis roots by Bennett et al (1996).
(ii) Auxin Efflux:
In cytosol, the pH is relatively higher or neutral (about 7) in comparison to cell wall so that IAAH dissociates into H+ and IAA. It is in this dissociated form that auxin predominates in cytosol. In the dissociated or anionic form, the IAA” exits the cell only through basally located auxin efflux carriers called as PIN proteins (Fig. 17.13). The exit of IAA from the cell is driven by inside negative membrane potential.
The basal location of auxin efflux carriers in each cell in longitudinal pathway establishes polarity in auxin transport and as mentioned earlier, is one of the two main features of chemiosmotic model of polar auxin transport.
(Geldner et al (2001) have recently shown experimentally that although PIN proteins are stable but they do not remain permanently on plasma membrane. Instead, there is actin filaments-dependent cycling of PIN proteins from plasma membrane to some endosomal compartment through endocytotic vesicles and their recycling back to plasma membrane).
i. 2, 3, 5-triiodobenzoic acid (TIBA) and naphthylpthalamic acid (NPA) also strongly inhibit polar auxin transport, although they do not interfere with energy metabolism. These substances are called as antiauxins or phytotropins or auxin transport inhibitors (ATIs). Structures of TIBA and NPA are given in Fig. 17.14.
ii. It is believed that TIBA & NPA inhibit polar auxin transport by interfering with actin dependent cycling & recycling of PIN proteins and thus auxin efflux.
iii. TIBA & NPA are not found in plants. Naturally occurring auxin transport inhibitors are possibly some flavonoids such as genistein and quercetin).