Regulation of suberin biosynthesis and Casparian strip development in the root endodermis by two plant auxins

The biological function of the auxin phenylacetic acid (PAA) is not well characterized in plants. Although some aspects of its biology; transport, signaling and metabolism have recently been described. Previous work on this phytohormone has suggested that PAA behaves in an identical manner to IAA (indole-3-acetic acid) in promoting plant growth, yet plants require greater concentrations of PAA to elicit the same physiological responses. Here we show that normalized PAA treatment results in the differential expression of a unique list of genes, suggesting that plants can respond differently to the two auxins. This is further explored in endodermal barrier regulation where the two auxins invoke striking differences in the deposition patterns of suberin. We further show that auxin acts antagonistically on Casparian strip (CS) formation as it can circumvent the CS transcriptional machinery to repress CS related genes. Additionally, altered suberin biosynthesis reduces endogenous levels of PAA and CS deficiency represses the biosynthesis of IAA and the levels of both auxins. These findings implicate auxin as a regulator of endodermal barrier formation and highlight a novel role for PAA in root development and differentiation.