Abstract
Auxin is a key regulator of plant growth and development. Within the root tip, auxin distribution plays a crucial role specifying developmental zones and coordinating tropic responses. Determining how the organ-scale auxin pattern is regulated at the cellular scale is essential to understanding how these processes are controlled. In this study, we developed an auxin transport model based on actual root cell geometries and carrier subcellular localizations. We tested model predictions using the DII-VENUS auxin sensor in conjunction with state-of-the-art segmentation tools. Our study revealed that auxin efflux carriers alone cannot create the pattern of auxin distribution at the root tip and that AUX1/LAX influx carriers are also required. We observed that AUX1 in lateral root cap (LRC) and elongating epidermal cells greatly enhance auxin's shootward flux, with this flux being predominantly through the LRC, entering the epidermal cells only as they enter the elongation zone. We conclude that the nonpolar AUX1/LAX influx carriers control which tissues have high auxin levels, whereas the polar PIN carriers control the direction of auxin transport within these tissues.
Highlights
The plant hormone auxin is an important regulator of plant growth and development (Benjamins and Scheres, 2008) and plays a key role in organ initiation and in tropic responses (Marchant et al, 1999; Benková et al, 2003)
Recent models that use regular cell geometries have suggested that the pattern of PIN efflux carriers can create the auxin distribution at the root tip (Grieneisen et al, 2007, 2012; Stoma et al, 2008; Mironova et al, 2012) with an auxin maxima in the quiescent center (QC) region (Petersson et al, 2009)
We first considered the role of the PINs and predicted what auxin distribution they create when modeling transport within a root tip comprised of actual root cell geometries
Summary
The plant hormone auxin is an important regulator of plant growth and development (Benjamins and Scheres, 2008) and plays a key role in organ initiation and in tropic responses (Marchant et al, 1999; Benková et al, 2003). Genetic studies suggest that the auxin distribution in the root is governed by the PINs and by the AUX1/LAX (A) Schematic showing cell types labeled on the wild-type root geometry. (I) Mean predicted auxin concentration in the different cell types. (J) Mean predicted DII-VENUS concentration in the different cell types. (K) Mean measured DII VENUS levels in the different cell types. In (I) to (K), error bars show 1 SE.
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