A role for phosphoinositides in regulating plant nuclear functions

Abstract

Nuclear localized inositol phospholipids and inositol phosphates are important for regulating many essential processes in animal and yeast cells such as DNA replication, recombination, RNA processing, mRNA export and cell cycle progression. An overview of the current literature indicates the presence of a plant nuclear phosphoinositide (PI) pathway. Inositol phospholipids, inositol phosphates, and enzymes of the PI pathway have been identified in plant nuclei and are implicated in DNA replication, chromatin remodeling, stress responses and hormone signaling. In this review, the potential functions of the nuclear PI pathway in plants are discussed within the context of the animal and yeast literature. It is anticipated that future research will help shed light on the functional significance of the nuclear PI pathway in plants.

Keywords: chromatin remodeling; inositol phosphate; nucleus; phosphatidylinositol; phosphoinositide; plant; signaling.

Figure 1

Schematic representation of inositol phospholipids and inositol phosphate metabolism associated with the phosphoinositide (PI) pathway. A generalized PI pathway is shown. Lipid metabolites are represented by ovals, inositol and inositol phosphate derivatives are depicted by boxes and enzymes are underlined. Well characterized pathways are indicated with bold arrows. Enzymes and metabolites that have been found in plants to date are shown in red. Enzyme abbreviations are as follows: IPS, myo-inositol phosphate synthase; IMP, inositol monophosphatase; PIS, phosphatidylinositol synthase; PI3K, PtdIns 3-kinase; PI4K, PtdIns 4-kinase; PIP5K, PtdIns4P 5-kinase; FAB/PIPKfyve, PtdIns3P 5-kinase; PI-PLC, PtdIns/PtdInsP-Phospholipase C; IPK, inositol phosphate kinase; DGK, diacylglycerol kinase; PAK, PtdOH kinase; KCS1, kinase C suppressor 1 (inositol hexakisphosphate (InsP₆) kinase); VIP1, InsP₆, and inositol heptakisphosphate (InsP₇) kinase. Animal Type I PtdIns3K enzyme activity that produces PtdIns(3,4,5)P₃ has not been identified in plants and is marked with one (*) asterisk. KCS1 and VIP1 activities have been identified in yeast and animals and are marked with two (**) asterisks.

Figure 2

Phosphoinositide pathway functions reported in animal and yeast nuclei. In animals and yeast, phospholipids and inositol phosphates are associated with many critical nuclear processes such as DNA repair, chromatin remodeling and RNA editing and export as described in the text. Nuclear enzymes affected by the PI pathway include: polyadenylate polymerase (PAP), topoisomerase, DNA polymerase and RNA polymerase. Nuclear proteins include: transcription factors (TF), inositol phosphate kinase 2 (IPK2, with transcription factor activity), inositol trisphosphate receptor (InsP₃R), histone H1 (H1), histones 2A, 2B, 3 and 4 (Histone Octomer). Nuclear complexes include: DNA repair complex with Non-homologous end joining (NHEJ), RNA editing, mRNA export, Spliceosome, Remodeling complexes, Telomerase complex and the NPC – nuclear pore complex. Data are from both in vivo and in vitro assays. Small circles represent different phospholipids and small squares represent different inositol phosphates as indicated.