Two inositol hexakisphosphate kinases drive inositol pyrophosphate synthesis in plants

Abstract

SUMMARYInositol pyrophosphates are unique cellular signaling molecules with recently discovered roles in energy sensing and metabolism. Studies in eukaryotes have revealed that these compounds have a rapid turnover, and thus only small amounts accumulate. Inositol pyrophosphates have not been the subject of investigation in plants even though seeds produce large amounts of their precursor, myo-inositol hexakisphosphate (InsP 6 ). Here, we report that Arabidopsis and maize InsP 6 transporter mutants have elevated levels of inositol pyrophosphates in their seed, providing unequivocal identification of their presence in plant tissues. We also show that plant seeds store a little over 1% of their inositol phosphate pool as InsP 7 and InsP 8 . Many tissues, including, seed, seedlings, roots and leaves accumulate InsP 7 and InsP 8 , thus synthesis is not confined to tissues with high InsP 6 . We have identified two highly similar Arabidopsis genes, AtVip1 and AtVip2, which are orthologous to the yeast and mammalian VIP kinases. Both AtVip1 and AtVip2 encode proteins capable of restoring InsP 7 synthesis in yeast mutants, thus AtVip1 and AtVip2 can function as bonafide InsP 6 kinases. AtVip1 and AtVip2 are differentially expressed in plant tissues, suggesting nonredundant or non-overlapping functions in plants. These results contribute to our knowledge of inositol phosphate metabolism and will lay a foundation for understanding the role of InsP 7 and InsP 8 in plants.

VIH2 Regulates the Synthesis of Inositol Pyrophosphate InsP8 and Jasmonate-Dependent Defenses in Arabidopsis

“…To investigate VIH2 functions in planta, we analyzed inositol polyphosphates in 3-week-old seedlings of wild-type Figure 3C; Supplemental Figure 3A). However, in contrast to this previous report, we also detected two additional peaks more anionic than InsP 6 that eluted at elution times expected for InsP 7 and InsP 8 , respectively ( Figure 3C), supporting recent findings by Desai et al (2014).…”

“…We expressed VIH2 in a vip1D kcs1D ddp1D triple mutant yeast strain, which lacks inositol pyrophosphates and is devoid of Ddp1 (diadenosine-and-diphosphoinositol-polyphosphatephosphohydrolase)-dependent inositol pyrophosphatase activity, thus facilitating the detection of inositol pyrophosphates synthesized by ectopically expressed kinases (Safrany et al, 1998;Mulugu et al, 2007). VIH2 expression in this background resulted in a robust InsP 7 peak that eluted at a retention time identical to the 1-InsP 7 peak in transformants ectopically expressing Vip1 ( Figure 2E; Supplemental Figure 2D), supporting recent observations by Desai et al (2014). However, these results did not address whether VIH proteins have Vip1/PPIP5K or Kcs1/IP6K-like activities.…”

“…The existence of inositol species more polar than InsP 6 has been reported in Spirodela polyrrhiza (Flores and Smart, 2000), barley (Hordeum vulgare; Brearley and Hanke, 1996;Dorsch et al, 2003), guard cells of intact guard cells of potato (Solanum tuberosum; Lemtiri-Chlieh et al, 2000), and in extracts of Arabidopsis and maize (Zea mays; Desai et al, 2014). In the later study, the authors addressed a possible involvement of Arabidopsis Vip1 homologs in the synthesis of inositol pyrophosphates by performing yeast complementation assays (Desai et al, 2014).…”

“…In the later study, the authors addressed a possible involvement of Arabidopsis Vip1 homologs in the synthesis of inositol pyrophosphates by performing yeast complementation assays (Desai et al, 2014). Based on these assays, the authors proposed a function of Arabidopsis Vip1 homologs as IP6K enzymes with a possible role in InsP 7 biosynthesis.…”

“…Note that in a previous published work (Desai et al, 2014), which was published during the preparation of this article, Arabidopsis homologs of yeast Vip1 were named AtVIP1 and AtVIP2 (corresponding to VIH2 and VIH1, respectively). Because VIP1 is already in use for an unrelated protein (Arabidopsis VirE2 Interacting Protein 1, encoded by At1g43700), we propose to use the gene symbol VIH (VIP1 homolog) registered at the TAIR database (see http://www.arabidopsis.org).…”