Abstract
distribution of auxin within plant tissues is of great importance for developmental plasticity, including root gravitropic growth. Auxin flow is directed by the subcellular polar distribution and dynamic relocalisation of auxin transporters such as the PIN-FORMED (PIN) efflux carriers, which can be influenced by the main natural plant auxin indole-3-acetic acid (IAA). Anthranilic acid (AA) is an important early precursor of IAA and previously published studies with AA analogues have suggested that AA may also regulate PIN localisation. Using Arabidopsis thaliana as a model species, we studied an AA-deficient mutant displaying agravitropic root growth, treated seedlings with AA and AA analogues and transformed lines to over-produce AA while inhibiting its conversion to downstream IAA precursors. We showed that AA rescues root gravitropic growth in the AA-deficient mutant at concentrations that do not rescue IAA levels. Overproduction of AA affects root gravitropism without affecting IAA levels. Treatments with, or deficiency in, AA result in defects in PIN polarity and gravistimulus-induced PIN relocalisation in root cells. Our results revealed a previously unknown role for AA in the regulation of PIN subcellular localisation and dynamics involved in root gravitropism, which is independent of its better known role in IAA biosynthesis.
Highlights
Auxin distribution in controlled concentration gradients within certain tissues plays an important role in regulating the dynamically plastic growth and development of plants (Vanneste & Friml, 2009)
As Anthranilic acid (AA) is a precursor of indole-3-acetic acid (IAA), we investigated the possibility that the rescue of root gravitropic growth by Endosidin 8 (ES8) and AA might indirectly result from increased IAA biosynthesis
As AA is a precursor of auxin, which is known to affect transcription of PIN genes (Vieten et al, 2005; Paponov et al, 2008), we investigated gene expression levels for all the plasma membrane-localised PIN proteins (PIN1, PIN2, PIN3, PIN4 and PIN7) in WT and mutant seedlings at 9 d old, the age at which we performed our root gravitropic growth and length studies
Summary
Auxin distribution in controlled concentration gradients within certain tissues plays an important role in regulating the dynamically plastic growth and development of plants (Vanneste & Friml, 2009). An intense research effort has revealed many of the complex mechanisms by which plasma membrane-localised auxin carrier proteins are polarly distributed to direct the flow of auxin in plant tissues and maintain these gradients (reviewed by Luschnig & Vert, 2014; and Naramoto, 2017). These proteins, including the well studied PIN-FORMED (PIN) auxin efflux carriers, are remarkably dynamic in that they rapidly relocalise within the cell in response to signals, resulting in changes in their polarity.
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