Bennett Lab Research - Auxin Transport

Auxins represent important hormonal regulators of plant development (reviewed by Swarup et al, 2001). Uniquely amongst plant signalling molelcules, auxins are transported in a polar fashion in plant tissues. Plants employ specialised influx and efflux carriers to mobilise the major form of auxin, indole-3-acetic acid (IAA) from cell to cell (reviewed by Parry et al, 2001). Molecular genetic studies in Arabidopsis thaliana have identified putative auxin influx and efflux carrier components encoded by the AUX1 ( bennett et al, 1996) and AtPIN/AGR/EIR gene sequences respectively.

The coordinated movement of IAA within or between plant organs is essential to execute many developmental programmes. Studies using auxin efflux carrier inhibitors have demonstrated that the polarity of auxin movement provides an important developmental signal during embryogenesis, lateral root initiation (Casimiro et al, 2001), gravitropism (Marchant et al, 1999), vascular patterning and leaf phyllotaxis. In collaboration with Alain delbarre (CNRS, France), ourselves and others have recently characterised a new class of auxin influx carrier inhibitor (Parry et al, 2001). Plants treated with this new class of auxin transport inihibitor exhibit defects in root gravitropism (Parry et al, 2001) and embryo patterning (James & Bennett, unpub. results).

AUX1 is most closely related to three Arabidopsis sequences termed LAX1,LAX2, and LAX3 (Like AUX1) (Parry et al, 2001). AUX1 shares between 73% and 82% identity at the amino acid level with other LAX sequences, indicative of a conservation of transport function (fig 1).

We have adopted a reverse genetic strategy to investigate the developmental functions of every member of the AUX1 gene family in Arabidopsis. In collaboration with Jonathan Jones (JIC) our laboratory has identified dSpm insertions on the coding frames of the LAX1, LAX2 and LAX3 genes.

Protein localisation studies have recently revealed AUX1 appears asymmetrically localised in root protophloem pole cells (see fig 2). Double-labelling experiments have revealed that AUX1 and the auxin influx carrier AtPIN1 are preferentially targeted to the upper and lower protophloem PM domains respectivley. Localisation of AUX1 to the upper PM of protophloem cell files is proposed to facilitate the acropetal, post-phloem movement of IAA to the root apical meristem (RAM).

Direct auxin measurements using GC-SRM-MS analysis has demonstrated that IAA accumulation was impaired in aux1 mutant RAM, consistent with an AUX1 phloem unloading function (Swarup et al, 2001). We have also recently demonstrated that mutant leaves are defective for phloem loading of higher order vascular (Marchant et al, 2001). GC-SRM-MS and reporter genes represent a powerful combination of approaches to monitor IAA tissue levels/distribution and will be critical for understanding the hormonal basis of AUX/LAX mutant phenotypes.

The molecular resources and expertise that have been assembled in our laboratory and through collaborations provide an unprecedented oppportunity to investigate the cellular pathway(s) mediating auxin transport in higher plants.

The Laboratory currently has BBSRC funding to study the role of auxin transport during : -