9.1.2 How mobile are plant hormones?

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In addition to biochemical control of synthesis and inactivation, hormone concentrations can be modified by import and export between different regions of the plant. Indeed, transport is an essential component of long-distance signalling systems. All plant hormones, being small molecules, can diffuse within and between cells. Some may pass readily across lipid membranes; others such as glucosides are more water soluble and may tend to accumulate in the vacuole, along with other cellular waste products. There is probably little a plant can do to prevent local diffusion of hormones, and plasmodesmal bridges (Section 10.1.2) allow intercellular cytosolic passage of most hormone-sized molecules. In addition, xylem and phloem sap analysis indicates that several hormones also move over much greater distances, for example perhaps 100 m from deep root tip to leaf of a large eucalypt.

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Figure 9.2 Long-distance transport of hormone signals through mass flow can be influenced by sap flow rates. Here, flux in tomato xylem is expressed as a delivery rate (molecules per second) measured as sap flow is altered by pressurising the root system. Flux of ABA in the xylem stream increases with sap flow, whereas flux of nitrate, a major inorganic nutrient, is constant.

(Based on Else et al. 1995)

Are mass-flow systems good channels for signal transport? Xylem flux varies massively on a diurnal basis as stomata generally open during the day and close at night, thus modifying transpiration rates. Superimposed on that are seasonal changes in temperature and water availability: with dry roots come slow flow rates; with hot, dry air, there is huge evaporative demand, and rapid sap flow subject to access to a water supply. Likewise, phloem flow is highly variable and sometimes bidirectional, making it difficult to specify source and target. A growing leaf will initially import sugars through the phloem, but with attainment of photosynthetic competence, it will instead export through the same channels. It sounds fraught with potential problems, but most physiologists believe that long-distance transport of hormones has functions in many regulatory processes. Consider that plants have evolved with variable mass flow: perhaps some hormone signalling is dependent on such oscillations rather than being defeated by them. There is evidence from tomato plants that xylem ABA flux (molecules delivered per hour) is influenced by the carrier solvent (sap) flow rate whereas flux of soil solutes such as NO3 is independent of flow rate (Figure 9.2; Else et al. 1995). In Section 9.2.2 we look further at fluctuations in xylem ABA and the consequences of ABA delivery from root to shoot. Here, we examine specific mechanisms for auxin transport.

Auxin polar transport

The best-studied aspect of hormone mobility is auxin polar transport, the only specific system presently known for move-ment of any plant hormone. It is termed polar because of its intrinsic directionality which is not altered even by drastic experimental procedures such as excision and tissue inversion. The phenomenon is commonly demonstrated in segments of young shoot tissues such as hypocotyls and cereal coleoptiles in which applied radioactive auxin moves from tip to base at about 1 cm per hour, but hardly at all in the opposite direction. This speed is faster than diffusion but slower than phloem sap flow. Not all cells within the tissue exhibit polar transport, and it is often restricted to ancillary cells within the vascular bundles such as phloem parenchyma. The mechanism of movement is chemiosmotic rather than active, and depends on three factors:

1. the dissociation kinetics of IAA between its neutral IAAH and anionic IAA forms;

2. a pH difference between cell wall and cytoplasm;

3. selective IAA channels in the plasma membrane.

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Figure 9.3 Auxin movement in plants operates partly through a polar (uni-directional) transport system. The acidic properties of auxin, together with a pH difference between cell wall and cytoplasm, and localised auxin anion efflux carrier channels in the plasma membrane, combine to generate a net basipetal (away from shoot tip) movement of auxin. (a) Diagram showing the components of this 'chemiosmotic' transport system. (b) Immunofluorescence components of this 'chemiosmotic' transport system. (b) Immunofluorescence micrograph showing presumed location of auxin channels in basal ends of vascular parenchyma cells (bright zones marked by arrows), cut in longitudinal section. The antibody used was raised against purified NPA-binding protein. NPA is an auxin transport inhibitor.

(Based on Jacobs 1983 and Jacobs and Gilbert 1986; (b) reproduced with permission of American Association for the Advancement of Science)

Figure 9.3(a) illustrates how IAA ions can pass through the normally ion-impermeant membrane via the IAA channels located predominantly in the basal membranes of the transporting cell files. In the cell wall compartment, IAA reassociates to IAAH due to the low pH and so the IAA does not readily re-enter the cytoplasm. Activity of the auxin channel protein is blocked by certain synthetic compounds such as naphthyl phthalamic acid (NPA) and tri-iodobenzoic acid (TIBA), as well as by natural plant flavonoids such as quercetin, and apigenin (Jacobs and Rubery 1988). This raises the intriguing possibility that plants may use these natural inhibitors to regulate their own auxin transport. Using anti-bodies against the protein to which NPA binds, immuno-fluorescence microscopy has shown that this protein, which is probably the auxin channel itself or a closely associated protein, is predominant in the plasma membrane at the basal end of cells (Figure 9.3b). Because IAA molecules will exit more through these channels, there is a net movement of auxin from top to base of the tissue. The relatively slow speed probably reflects the fact that each IAA molecule has to enter and exit every cell on its route down the tissue.

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