Abstract
The PDR-type ABCG transporter, ABCG36/PDR8/PEN3, is thought to be implicated in the export of a few structurally unrelated substrates, including the auxin precursor, indole-3-butyric acid (IBA), although a clear-cut proof of transport is lacking. An outward facing, lateral root (LR) location for ABCG36 fuelled speculations that it might secrete IBA into the rhizosphere. Here, we provide strong evidence that ABCG36 catalyzes the export of IBA – but not of indole-3-acetic acid – through the plasma membrane. ABCG36 seems to function redundantly with the closely related isoform ABCG37/PDR9/PIS1 in a negative control of rootward IBA transport in roots, which might be dampened by concerted, lateral IBA export. Analyses of single and double mutant phenotypes suggest that both ABCG36 and ABCG37 function cooperatively in auxin-controlled plant development. Both seem to possess a dual function in the control of auxin homeostasis in the root tip and long-range transport in the mature root correlating with non-polar and polar expression profiles in the LR cap and epidermis, respectively.
Highlights
indole-3-butyric acid (IBA) was described as a synthetic auxin that elicited auxin-like effects such as root initiation and was used for plant propagation in a process called rooting
By heterologous expression, ABCG37/PDR9/PIS1 was shown to function as a transporter of IBA and 2.4-D but not of indole-3-acetic acid (IAA) (Ruzicka et al, 2010)
By heterologous expression in yeast and HeLa cells, ABCG37/PDR9/PIS1 was shown to function as a transporter of IBA, 2.4-D and naphthylphtalamic acid (NPA) but not of IAA (Ruzicka et al, 2010)
Summary
IBA was described as a synthetic auxin that elicited auxin-like effects such as root initiation and was used for plant propagation in a process called rooting (reviewed in Frick and Strader, 2018). It has been established that IBA is an endogenous compound in a variety of plant species examined (Frick and Strader, 2018) Its existence, in the model plant Arabidopsis thaliana has been questioned (Novak et al, 2012). It appears that this might be a question of extraction and detection methods (Frick and Strader, 2018; Matern et al, 2019). IBA Transport by ABCG36/PEN3/PDR8 reaction (Zolman et al, 2008) and peroxisomal import is thought to occur by PEROXISOMAL TRANSPORTER1/ COMATOSE/ABCD1 (PXA1/CTS/ABCD1) belonging to the ABCD family of ABC transporters (Zolman et al, 2001; Footitt et al, 2002; Hooks et al, 2007)
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