Rapid accumulation and metabolism of polyphosphoinositol and its possible role in phytoalexin biosynthesis in yeast elicitor-treated<Emphasis Type="Italic"> Cupressus lusitanica</Emphasis> cell cultures

Inositol 1,4,5-trisphosphate Ins(1,4,5)P₃] rapidly accumulates in elicited Cupressus lusitanica Mill. cultured cells by 4- to 5-fold over the control, and then it is metabolized. Correspondingly, phospholipase C (PLC) activity toward phosphatidylinositol 4,5-bisphosphate PtdIns(4,5)P₂] is stimulated to high levels by the elicitor and then decreases whereas Ins(1,4,5)P₃ phosphatase activity declines at the beginning of elicitation and increases later. These observations indicate that elicitor-induced biosynthesis and dephosphorylation of Ins(1,4,5)P₃ occur simultaneously and that the Ins(1,4,5)P₃ level may be regulated by both PtdIns(4,5)P₂–PLC and Ins(1,4,5)P₃ phosphatases. Studies on the properties of PLC and Ins(1,4,5)P₃ phosphatases indicate that PLC activity toward PtdIns(4,5)P₂ was optimal at a lower Ca²⁺ concentration than activity toward phosphatidylinositol whereas Ins(1,4,5)P₃ phosphatase activity is inhibited by high Ca²⁺ concentration. This suggests that Ins(1,4,5)P₃ biosynthesis and degradation may be regulated by free cytosolic Ca²⁺. In addition, a relationship between Ins(1,4,5)P₃ signaling and accumulation of a phytoalexin (-thujaplicin) is suggested because inhibition or promotion of Ins(1,4,5)P₃ accumulation by neomycin or LiCl affects elicitor-induced production of -thujaplicin. Moreover, ruthenium red inhibits elicitor-induced accumulation of -thujaplicin while thapsigargin alone induces -thujaplicin accumulation. These results suggest that Ca²⁺ released from intracellular calcium stores may mediate elicitor-induced accumulation of -thujaplicin via an Ins(1,4,5)P₃ signaling pathway, since it is widely accepted that Ins(1,4,5)P₃ can mobilize Ca²⁺ from intracellular stores. This work demonstrates an elicitor-triggered Ins(1,4,5)P₃ turnover, defines its enzymatic basis and regulation, and suggests a role for Ins(1,4,5)P₃ in elicitor-induced phytoalexin accumulation via a Ca²⁺ signaling pathway.Abbreviations Ins(1,4,5)P₃ Inositol-1,4,5-trisphosphate - Ins(1,4)P₂ Inositol-1,4-bisphosphate - Ins(4,5)P₂ Inositol-4,5-bisphosphate - Ins(1)P Inositol 1-phosphate - Ins(4)P Inositol 4-phosphate - PLC Phospholipase C - PtdIns Phosphatidylinositol - PtdIns(4,5)P₂ Phosphatidylinositol 4,5-bisphosphate - YE Yeast elicitor