Regulation of plant biotic interactions and abiotic stress responses by inositol polyphosphates

Abstract

Inositol pyrophosphates (PP-InsPs), derivatives of inositol hexakisphosphate (phytic acid, InsP6) or lower inositol polyphosphates, are energy-rich signaling molecules that have critical regulatory functions in eukaryotes. In plants, the biosynthesis and the cellular targets of these messengers are not fully understood. This is because, in part, plants do not possess canonical InsP6 kinases and are able to synthesize PP-InsP isomers that appear to be absent in yeast or mammalian cells. This review will shed light on recent discoveries in the biosynthesis of these enigmatic messengers and on how they regulate important physiological processes in response to abiotic and biotic stresses in plants.

Crystal Structure and Enzymology of Solanum tuberosum Inositol Tris/Tetrakisphosphate Kinase 1 (StITPK1)

“…Recently, noncanonical PP-InsP 5 species have been identified in plants. , CE-MS peaks with chromatographic mobility and parent/daughter ion relationships identical with synthetic d - and/or l -4-PP-InsP 5 [4-PP-InsP 5 /6-PP-InsP 5 ] and the meso -compound 2-PP-InsP 5 have been detected. , Similar conclusions can be drawn from a previous study . It is, however, unknown whether plants possess a single [PP] 2 -InsP 4 , a mixture of the enantiomers 1,5-[PP] 2 -InsP 4 /3,5-[PP] 2 -InsP 4 or additional isomers and enantiomeric pairs . Recent work has identified a family of 5-β-phosphate-targeting phosphatases, showing that VIH1/2 is not the only agent of [PP] 2 -InsP 4 turnover.…”

“…We further suggest that enhanced accumulation of Ins-(3,4,5,6)P 4 in itpk1 mutants 9,15,27 and ipk1 mutants 4,15 amplifies reductions in PP-InsP species 9,15,27 that are widely reported to be the agents of much physiology. 3 Indeed, itpk1 and ipk1 mutants show a constitutive phosphate starvation response. 4,14,15 ■ DISCUSSION Our understanding of inositol pyrophosphate function rests heavily on the molecular genetic disruption of hydroxy-kinase and phosphokinase activities.…”

“…4,15 Ablation of AtITPK1 increases lower inositol phosphates 9,15,27 and reduces PP-InsP 5 and [PP] 2 -InsP 4 alike, in plants. While it has been widely assumed that [PP] 2 -InsP 4synthesizing activity belongs exclusively to VIH1/2, 3,6,15,23,24,42 until the recent detailed description of noncanonical PP-InsP 5 species 9,21,27 little consideration had been given to other possibilities. Similarly, rather little consideration has been given to the possibility that different inositol phosphates and inositol pyrophosphates are active site competitors of ATP-grasp kinases.…”

“…23,24 Even so, the activities of ITPK1/2 and VIH1/2 20,21,27 do not account for the full spectrum of inositol pyrophosphates detected in plants. 21,27,34 The possible isomers and enantiomers comprise 1-InsP 7 , 1-PP-Ins (2,3,4,5,6) The ITPK family, including members that synthesize inositol pyrophosphates 20,21,27 or do not, 20,22 is represented in early land plants 29 and in early aquatic vascular plants, the duckweeds in which a lipid-independent pathway of InsP 6 synthesis that uses the favored InsP 4 substrate of plant ITPK1 21 was described. 30−32 Herein, we have solved a crystal structure for a potato enzyme, StITPK1, in complex with ADP.…”

“…Since the discovery of inositol pyrophosphates (diphosphoinositol phosphates) in plants, , inositol pyrophosphates have emerged as participants in diverse aspects of plant physiology extending to phosphate starvation response, hormone signaling, symbiosis, and response to pathogens. − The same can also be claimed of inositol phosphates lacking anhydride bonds. ,− The claim of the contribution of inositol pyrophosphates to aspects of plant biology rests heavily on the characterization of enzymes and the analysis of mutants thereof. ITPKs and PPIP5Ks, variously referred to as VIH or Vip1 in plants and yeast, respectively, contribute to inositol pyrophosphate synthesis.…”