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Abstract
The PIN-FORMED (PIN) proteins are plasma-membrane-associated facilitators of auxin transport. They are often targeted to one side of the cell only through subcellular mechanisms that remain largely unknown. Here, we have studied the potential roles of the cytoskeleton and endomembrane system in the localisation of PIN proteins. Immunocytochemistry and image analysis on root cells from Arabidopsis thaliana and maize showed that 10-30% of the intracellular PIN proteins mapped to the Golgi network, but never to prevacuolar compartments. The remaining 70-90% were associated with yet to be identified structures. The maintenance of PIN proteins at the plasma membrane depends on a BFA-sensitive machinery, but not on microtubules and actin filaments. The polar localisation of PIN proteins at the plasmamembrane was not reflected by any asymmetric distribution of cytoplasmic organelles. In addition, PIN proteins were inserted in a symmetrical manner at both sides of the cell plate during cytokinesis. Together, the data indicate that the localisation of PIN proteins is a postmitotic event, which depends on local characteristics of the plasma membrane and its direct environment. In this context, we present evidence that microtubule arrays might define essential positional information for PIN localisation. This information seems to require the presence of an intact cell wall.
Highlights
A widely accepted model proposes that gradients of the hormone auxin are the basis of differential cell behaviour during pattern formation in higher plants (Benková et al, 2003; Friml et al, 2003; Reinhardt et al, 2003)
A BLAST analysis revealed that several PIN proteins have sequences that are close to the p74 peptide used for raising the antibody
Double immunostaining with AtPIN-antibody and antibodies against calreticulin, an endoplasmic reticulum (ER) marker (Kluge et al, 2004), showed that there was no colabelling of the two antibodies in differentiating cells. These results show that the Golgi network is actively involved in trafficking of the PIN proteins
Summary
A widely accepted model proposes that gradients of the hormone auxin are the basis of differential cell behaviour during pattern formation in higher plants (Benková et al, 2003; Friml et al, 2003; Reinhardt et al, 2003). Auxin distribution throughout the whole plant is controlled by at least two families of plasma-membrane associated proteins, called AUX/LAX and PIN-FORMED (PIN) These so-called transport facilitators, regulate auxin fluxes in and out of the cells (Gälweiler et al, 1998; Swarup et al, 2001). Since these proteins are often localised on one side of the cell only, it has been proposed that they can generate auxin fluxes through tissues, creating differences in hormone concentrations. For example, these proteins occur on the apical or basal pole of cells in the same cell file In this way, they supposedly create upwards or downwards directed auxin fluxes. The cellular basis of PIN localisation remains poorly understood
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