ITPK1-Dependent Inositol Polyphosphates Regulate Auxin Responses in <i>Arabidopsis thaliana</i>

Abstract

ABSTRACT The combinatorial phosphorylation of myo-inositol results in the generation of different inositol phosphates (InsP), of which phytic acid (InsP 6 ) is the most abundant species in eukaryotes. InsP 6 is also the precursor of higher phosphorylated forms called inositol pyrophosphates (PP-InsPs), such as InsP 7 and InsP 8 , which are characterized by a diphosphate moiety and are also ubiquitously found in eukaryotic cells. While PP-InsPs regulate various cellular processes in animals and yeast, their biosynthesis and functions in plants has remained largely elusive because plant genomes do not encode canonical InsP 6 kinases. Recently, it was shown that Arabidopsis ITPK1 catalyzes the phosphorylation of InsP 6 to the natural 5-InsP 7 isomer in vitro . Here, we demonstrate that Arabidopsis ITPK1 contributes to the synthesis of InsP 7 in planta . We further find a critical role of ITPK1 in auxin-related processes including primary root elongation, leaf venation, thermomorphogenic and gravitropic responses, and sensitivity towards exogenously applied auxin. Notably, 5-InsP 7 binds to recombinant auxin receptor complex, consisting of the F-Box protein TIR1, ASK1 and the transcriptional repressor IAA7, with high affinity. Furthermore, a specific increase in 5-InsP 7 in a heterologous yeast expression system results in elevated interaction of the TIR1 homologs AFB1 and AFB2 with various AUX/IAA-type transcriptional repressors. We also identified a physical interaction between ITPK1 and TIR1, suggesting a dedicated channeling of an activating factor, such as 5-InsP 7 , to the auxin receptor complex. Our findings expand the mechanistic understanding of auxin perception and lay the biochemical and genetic basis to uncover physiological processes regulated by 5-InsP 7 .