ITPK1-Dependent Inositol Polyphosphates Regulate Auxin Responses in Arabidopsis thaliana

The combinatorial phosphorylation of results in the generation of different inositol phosphates (InsP), of which phytic acid (InsP) is the most abundant species in eukaryotes. InsP is also the precursor of higher phosphorylated forms called inositol pyrophosphates (PP-InsPs), such as InsP and InsP, 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 kinases. Recently, it was shown that Arabidopsis ITPK1 catalyzes the phosphorylation of InsP to the natural 5-InsP isomer . Here, we demonstrate that Arabidopsis ITPK1 contributes to the synthesis of InsP . 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 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 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, 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.