**Can Inositol Pyrophosphates Inform Strategies for Developing Low Phytate Crops?**

### **Catherine Freed, Olusegun Adepoju and Glenda Gillaspy \***

Department of Biochemistry, Virginia Tech, Blacksburg, VA 24061, USA; freedc@vt.edu (C.F.); ade1704@vt.edu (O.A.)

**\*** Correspondence: gillaspy@vt.edu; Tel.: +1-540-231-3062

Received: 24 December 2019; Accepted: 15 January 2020; Published: 17 January 2020

**Abstract:** Inositol pyrophosphates (PP-InsPs) are an emerging class of "high-energy" intracellular signaling molecules, containing one or two diphosphate groups attached to an inositol ring, that are connected with phosphate sensing, jasmonate signaling, and inositol hexakisphosphate (InsP6) storage in plants. While information regarding this new class of signaling molecules in plants is scarce, the enzymes responsible for their synthesis have recently been elucidated. This review focuses on InsP6 synthesis and its conversion into PP-InsPs, containing seven and eight phosphate groups (InsP7 and InsP8). These steps involve two types of enzymes: the ITPKs that phosphorylate InsP6 to InsP7, and the PPIP5Ks that phosphorylate InsP7 to InsP8. This review also considers the potential roles of PP-InsPs in plant hormone and inorganic phosphate (P*i*) signaling, along with an emerging role in bioenergetic homeostasis. PP-InsP synthesis and signaling are important for plant breeders to consider when developing strategies that reduce InsP6 in plants, as this will likely also reduce PP-InsPs. Thus, this review is primarily intended to bridge the gap between the basic science aspects of PP-InsP synthesis/signaling and breeding/engineering strategies to fortify foods by reducing InsP6.

**Keywords:** inositol; inositol phosphate; inositol pyrophosphate; inositol phosphate signaling; inositol phosphate kinases; PPIP5K; ITPK; phytate
