Isolation of soluble metabolites of the phosphatidylinositol cycle from Samanea saman

Compared selectivities of the phosphatidylinositol-synthase from maize coleoptiles either in microsomal membranes or after solubilization

PI-synthase selectivity from etiolated maize coleptiles was studied either associated with the microsomal membranes or after solubilization by CHAPS and prepurification on a DEAE-trisacryl M column. When maize microsomes were incubated with [3H]inositol without any exogenous CPM-PA, the most heavily labelled molecular species were 16:0/18:2-PI (77%), 16:0/18:3-plus 18:2/18:2-PI (15%), 16:0/18:1-PI (4%) and 18:0/18:2-PI (4%). Addition to the incubation medium of up to 300 microM 16:0/16:0-CMP-PA unexpectedly resulted in the formation of very little labelled 16:0/16:0-PI. When the solubilized fraction from microsomes was incubated with [3H]inositol in absence of 16:0/16:0-CPM-PA, the same PI molecular species as above were synthesized. However, with increasing concentrations of 16:0/16:0-CMP-PA in the medium, increasing amounts of labelled 16:0/16:0-PI appeared as well. With prepurified PI-synthase eluted from a DEAE column, endogenous CMP-PA was poorly utilized for PI biosynthesis whereas the exogenous 16:0/16:0-CPM-PA was used actively. With time, the endogenous CMP-PA was utilized first and the exogenous substrate was utilized, albeit, much more slowly. The results demonstrate that the selectivity displayed by PI-synthase towards various molecular species of CMP-PA depends on the integration of the enzyme in the membrane structure. Solubilization of the enzyme, i.e., inclusion of the protein in micelles with detergents and lipids, results in an apparent loss of the selectivity for CMP-PA.

Evidence that generation of inositol 1,4,5-trisphosphate and hydrolysis of phosphatidylinositol 4,5-bisphosphate are rapid responses following addition of fungal elicitor which induces phytoalexin synthesis in lucerne (Medicago sativa) suspension culture cells

Treatment of lucerne suspension culture cells with glycoprotein elicitor from the phytopathogenic fungus Verticillium albo-atrum R & B triggers Ca(2+)-mediated induction of antimicrobial secondary metabolites termed phytoalexins. The present study investigated the possible role of polyphosphoinositide signal transduction in phytoalexin elicitation. Within 1 min of addition of elicitor to lucerne suspension culture cells we found a 100-160% (15-25 pmol/g fresh wt) increase in the level of compound with chromatographic and electrophoretic properties expected for an inositol trisphosphate (InsP3) and which was strongly bound by an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-specific binding protein; after 3 min the level of this compound had fallen below that observed prior to elicitor challenge. In 32P-prelabelled cells, the relative proportion of radioactivity which cochromatographed with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) was found to have decreased by 48% 1 min after elicitor addition and that rapid depletion of membrane lipid radioactivity was specific to this lipid fraction. The rapid, transient increase in level of Ins(1,4,5)P3 and concomitant fall in PtdIns(4,5)P2 suggests that Ins(1,4,5)P3 generated by hydrolysis of PtdIns(4,5)P2 may provide a Ca(2+)-mobilizing signal in phytoalexin elicitation in lucerne.

IP3 levels and their modulation FY fusicoccin measured by a novel [3H] IP3 binding assay

A recently developed sensitive assay based on the binding reaction of IP3 to bovine adrenal preparations has been utilized for determining the level of endogenous inositol-1,4,5 trisphosphate (IP3) in maize roots and coleoptiles. The amount of IP3 found in these tissues ranges from 0.1 to 1.0 nmol g-1 fresh weight. Reproducible results were obtained with extracts of tissues from a same harvest, while they showed a 2-3 fold variation when different batches of plantlets were compared. The fungal phytotoxin fusicoccin (FC) known to affect several physiological processes in higher plants, increases the level of IP3 in coleoptiles. This observation suggests that IP3 might be involved in the transduction of the FC encoded signal from its receptors at the plasmalemma level to the cell machinery.