Isolation and characterization of a possible polygalacturonase: inhibiting protein from wheat

English: The presence and possible role of polygalacturonase-inhibiting protein (PGIP) in wheat (Triticum aestivum) as part of the plant's defense reaction following leaf rust (Puccinia triticina) infection were investigated. Through its ability to inhibit fungal endopolygalacturonase (EPG) that breaks down the plant cell wall during colonization, this protein is known to play an important role in the natural defense arsenal of dicotyledonous plants. The presence of PGIP in monocotyledonous cereals has never before been conclusively proved. A preliminary investigation using a polyclonal antibody raised against a purified bean PGIP (PGIP-I) revealed the induction of a possible PGIP of ±37.0 kDa following fungal infection, while an inhibition assay of EPG from AsperglÏ/us niger showed a decrease in PGIP activity. Through ion-exchange and size exclusion chromatography the presence of wheat PGIP was subsequently confirmed by the purification of a ±36.0 kDa inhibitor, which proved specific for the EPG of Coch/iobo/us sativus and not A. niger. Using a more specific anti-PGIP antibody (PGIP-II) the presence of this protein in wheat was also confirmed through immunoblotting. The expression of PGIP in wheat following salicylic acid (SA) treatment and fungal infection in terms of C sativus EPG inhibition was recorded. While SA treatment showed an induction of PGIP at protein and activity levels, fungal infection repeated the reduction in PGIP activity as previously observed. Using PGIP-II in immunogold localization the expression of PGIP in wheat leaves was confined to the plant cell wall and the periphery of the haustorium in the cytosol. Attempts to clone the wheat pgip gene through the polymerase chain reaction (peR) using degenerate primers were inconclusive, as fragments amplified did not exhibit significant similarity to PGIP from dicotyledonous plants. These results therefore indicate that wheat expresses a ±36.0 kDa PGIP in reaction to fungal and SA treatment, but fungus-related factors originating from either the plant or the fungus apparently induce the EPG activity to higher levels, or suppress the PGIP activity to lower levels, both recordable as a decrease in PGIP activity and having the potential to enhance plant disease.