Identification of a new polyphosphoinositide in plants, phosphatidylinositol 5-monophosphate (PtdIns5P), and its accumulation upon osmotic stress

Polyphosphoinositides play an important role in membrane trafficking and cell signalling. In plants, two PtdIns P isomers have been described, PtdIns3 P and PtdIns4 P . Here we report the identification of a third, PtdIns5 P . Evidence is based on the conversion of the endogenous PtdIns P pool into PtdIns(4,5) P 2 by a specific PtdIns5 P 4-OH kinase, and on in vivo 32 P-labelling studies coupled to HPLC head-group analysis. In Chlamydomonas , 3–8% of the PtdIns P pool was PtdIns5 P , 10–15% was PtdIns3 P and the rest was PtdIns4 P . In seedlings of Vicia faba and suspension-cultured tomato cells, the level of PtdIns5 P was about 18%, indicating that PtdIns5 P is a general plant lipid that represents a significant proportion of the PtdIns P pool. Activating phospholipase C (PLC) signalling in Chlamydomonas cells with mastoparan increased the turnover of PtdIns(4,5) P 2 at the cost of PtdIns4 P , but did not affect the level of PtdIns5 P . This indicates that PtdIns(4,5) P 2 is synthesized from PtdIns4 P rather than from PtdIns5 P during PLC signalling. However, when cells were subjected to hyperosmotic stress, PtdIns5 P levels rapidly increased, suggesting a role in osmotic-stress signalling. The potential pathways of PtdIns5 P formation are discussed.