Bifunctional inositol kinase that acts in concert with the IP6K kinases to synthesize the diphosphate group-containing inositol pyrophosphates diphosphoinositol pentakisphosphate, PP-InsP5, and bis-diphosphoinositol tetrakisphosphate, (PP)2-InsP4. PP-InsP5 and (PP)2-InsP4, also respectively called InsP7 and InsP8, may regulate a variety of cellular processes, including apoptosis, vesicle trafficking, cytoskeletal dynamics, and exocytosis. Phosphorylates inositol hexakisphosphate (InsP6) at position 1 to produce PP-InsP5 which is in turn phosphorylated by IP6Ks to produce (PP)2-InsP4. Alternatively, phosphorylates PP-InsP5 at position 1, produced by IP6Ks from InsP6, to produce (PP)2-InsP4 (PubMed:25231822). Probably involved in vitamin E homeostasis via the regulation of gamma-tocopherol biosynthesis (PubMed:17077148). Catalyzes the conversion of InsP7 to InsP8. Regulates jasmonic acid (JA) perception and plant defenses against herbivorous insects (e.g. P.rapae) and necrotrophic fungi (e.g. M.brassicae, B.cinerea and A.brassicicola) by triggering the production of jasmonate-induced pools of InsP8 and subsequent activation of SCF(COI1) E3 ubiquitin ligase complexes with JAZ proteins (e.g. TIFY10A/JAZ1) (PubMed:25901085). 1D-myo-inositol hexakisphosphate + ATP = 1-diphospho-myo-inositol 2,3,4,5,6-pentakisphosphate + ADP 5-diphospho-1D-myo-inositol 1,2,3,4,6-pentakisphosphate + ATP + H(+) = 1,5-bis(diphospho)-1D-myo-inositol 2,3,4,6-tetrakisphosphate + ADP Additional isoforms seem to exist. Mostly expressed in vegetative tissues (e.g. leaves and stems), and, to a lower extent, in roots, shoots and reproductive tissues (e.g. flowers and siliques) (PubMed:25231822, PubMed:25901085). Also present in mature pollen (PubMed:25901085). Abnormal accumulation of InsP7 and reduced levels of InsP8. Increased weight increase of P.rapae and M.brassicae larvae feeded on mutant plants thus leading to a reduced resistance. Increased susceptibility to the necrotrophic fungi B.cinerea and A.brassicicola. Increased levels of jasmonic acid (JA) and bioactive conjugates such as JA-Leu/Ile and JA-Val upon mechanical wounding, but reduced induction of JA-responsive genes in challenged mutant plants. Belongs to the histidine acid phosphatase family. VIP1 subfamily.