Auxin appears to regulate a wide range of plant developmental processes owing to its asymmetrical distribution across adjacent cells and tissues during important stages of growth and development. Auxin is synthesised in meristematic tissues and is distributed throughout the plant either through the phloem or by a more controlled cell-to-cell, polar transport system. A now well-accepted model for polar auxin transport is known as the chemiosmotic hypothesis.
As already shown in Figure 7.8 , there is a pH gradient across the cell, from about pH 5.5 in the matrix of the cell wall to pH 7.0 in the cytoplasm. In the cell wall the IAA molecule exists primarily in the protonated form (IAAH) and enters the cell passively through the plasma membrane. It dissociates once inside the cell and the IAA- ion becomes trapped inside the cytoplasm. Specific auxin efflux carriers are required to transport the dissociated form out of the cell. These are predicted to be asymmetrically located in the cell to account for unidirectional auxin transport. We now know that an auxin carrier molecule also exists to further enable IAA- uptake (Figure 7.10).
cytoplasm cell wall direction of auxin transport
I—| , auxin-efflux carrier, PIN protein Q , auxin-influx carrier, AUX protein cytoplasm cell wall direction of auxin transport
I—| , auxin-efflux carrier, PIN protein Q , auxin-influx carrier, AUX protein
Figure 7.10 The chemiosmotic model for polar auxin (IAA) transport in xylem parenchyma cells (from Vieten et al., 2007). See text for further details.
The nature of the auxin-efflux carrier was identified from studies with the pin-1 mutant of Arabidopsis thaliana. These mutants have needle- l ike stems that lack flowers and molecular analysis of the PIN1 gene revealed in 1998 that it encoded a transmembrane carrier protein. A further seven PIN genes have been found and all phenotypes related to aberrant auxin accumulation. All PIN proteins are localised in a polar fashion, according to the predictions of the chemiosmotic hypothesis.
Arabidopsis mutants were also used to discover the auxin influx carrier in 1998. The AUX-1 mutant was identified as having resistance to movement of 2,4-D. The AUX1 gene was cloned and found to encode a protein with similarity to amino acid permeases. The AUX1 protein is also localised asymmetrically in some cells.
An intriguing question is 'how do these auxin carriers end up at distinct sides of the cell?' t Current thinking, as reviewed by Vieten et al. (2007) t suggests that their movement and targeting results from the vesicle trafficking pathways in the cytoplasm. An accessory protein AXR4 is also thought to ensure that the protein is localised in the plasma membrane, although how this is achieved is not known.
In Arabidopsist PIN proteins are thought to rapidly cycle between the plasma membrane and the endoplasmic reticulum via endosomes. This process is termed constitutive cycling and, according to Paciorek and colleagues t2005) t is inhibited by auxin. This novel finding, shared by biologically active auxins, leads to increased auxin-efflux carriers at the cell surface where auxin concentration is least. Thus, auxins may promote their own efflux.
Inhibition of auxin transport has been often thought to be a potentially effective mechanism for herbicidal exploitation. Interestingly, one compound has emerged in recent years with these properties, diflufenzopyr (BAS 662H) (Figure 7.11).
When applied alone, diflufenzopyr stunts weed growth, but when combined with an auxin-type herbicide it appears to result in enhanced translocation of the auxin-herbicide to the weed apices to give a more effective broadleaf weed control. The combination of dicamba with diflufenzopyr appears to be particularly effective for a broad spectrum of weed control and tolerance in maize (Bowe et al. t 1999), with relatively low dose rates of 100-300 gai ha-1.
Diflufenzopyr acts by binding to a specific protein involved in transporting auxin away from the meristematic apices. It has a high affinity for this site, with an It 0 of 19 nM diflufenzopyr . Could this be a PIN protein? Thus, both natural and synthetic auxins accumulate at these apices to induce an 'auxin-overdose' response. Interestingly, root geotropism is also inhibited by this treatment, with an Ito of 0.6 nM.
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