Cellular efflux of auxin catalyzed by the Arabidopsis MDR/PGP transporter AtPGP1

Abstract

SummaryDirectional transport of the phytohormone auxin is required for the establishment and maintenance of plant polarity, but the underlying molecular mechanisms have not been fully elucidated. Plant homologs of human multiple drug resistance/P-glycoproteins (MDR/PGPs) have been implicated in auxin transport, as defects in MDR1 (AtPGP19) and AtPGP1 result in reductions of growth and auxin transport in Arabidopsis (atpgp1, atpgp19), maize (brachytic2) and sorghum (dwarf3). Here we examine the localization, activity, substrate specificity and inhibitor sensitivity of AtPGP1. AtPGP1 exhibits non-polar plasma membrane localization at the shoot and root apices, as well as polar localization above the root apex. Protoplasts from Arabidopsis pgp1 leaf mesophyll cells exhibit reduced efflux of natural and synthetic auxins with reduced sensitivity to auxin efflux inhibitors. Expression of AtPGP1 in yeast and in the standard mammalian expression system used to analyze human MDR-type proteins results in enhanced efflux of indole-3-acetic acid (IAA) and the synthetic auxin 1-naphthalene acetic acid (1-NAA), but not the inactive auxin 2-NAA. AtPGP1-mediated efflux is sensitive to auxin efflux and ABC transporter inhibitors. As is seen in planta, AtPGP1 also appears to mediate some efflux of IAA oxidative breakdown products associated with apical sites of high auxin accumulation. However, unlike what is seen in planta, some additional transport of the benzoic acid is observed in yeast and mammalian cells expressing AtPGP1, suggesting that other factors present in plant tissues confer enhanced auxin specificity to PGP-mediated transport.

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

“…Channels, in contrast, when open let ions diffuse rapidly down electrical and concentration gradients resulting in detectable currents. ABCBs were clearly shown to utilize ATP as a direct energy source and to transport auxin steadily across cellular membranes, suggesting ABCBs act in most cases as primary auxin pumps [4,6,127]. PINs and AUX1/LAXs are thought to act as secondary active auxin exporters dependent on an electrochemical gradient.…”

“…Members of plasma membrane located AUX1/LAX family are grouped as auxin importers [24]. Auxin exporters are primarily represented by plasma membrane localized PINs (so-called long PINs, see below) and members of the ABCB family, which have so far all been found to be plasma membrane-embedded [6,[25][26][27]. However, it should be mentioned that recently ABCB4 and ABCB21 were characterized as facultative importer/exporters whose transport directionality seems to be triggered by intracellular auxin levels [28].…”

“…Long PINs (PIN1-4,7) comprising a large cytoplasmic loop [5] are often polarly localized to a specific face of the plasma membrane in Arabidopsis root cells, in good correlation with the direction of the auxin flow [31]. By contrast, short PINs (PIN5, 6,8) are localized in the endomembrane system (most likely the endoplasmic reticulum (ER)) and appear to function in intracellular auxin transport and homeostasis [32,33]. Recently, a second family of ER-localized auxin transporters, called PILS (for PIN-LIKES), was also shown to regulate auxin homeostasis [34], although their role in auxin transport is less clear.…”

“…Therefore, auxin transport has been a matter of extensive interest and investigation ever since the emergence of the auxin concept more than a century ago [3]. Although seemingly a simple problem, it turned out to be very difficult to rigorously address the physiology and function of auxin transport proteins at the molecular level [4]: auxin transport studies have been found to be complicated by the diffusion component of auxins ( [5,6]; see Section 2 and Box 1). Moreover, membrane proteins are generally difficult to analyze functionally because of their low solubility at conditions that preserve their native structure and function.…”

A novel putative auxin carrier family regulates intracellular auxin homeostasis in plants

“…The distribution of auxin depends on both auxin metabolism (biosynthesis, conjugation and degradation) [8][9][10] and cellular auxin transport [11][12][13][14][15] . We identified in silico a novel putative auxin transport facilitator family, called PIN-LIKES (PILS).…”