Fungal strategies to elude polygalacturonase inhibition by plant PGIPs

Polygalacturonase inhibiting proteins (PGIPs) are plant defence molecules able to block the polygalacturonase activity of fungal pathogens. However, fungi have evolved strategies to overcome PGIP inhibition. For example, Sclerotinia sclerotiorum in the early stage of soybean infection produces considerable amount of PG activity encoded by a basic pg gene. By quantitative RT-PCR it was shown that initially the transcript of this pg gene overcomes by one hundred fold the transcript of the soybean pgip gene. Thereafter, pgip gene was noticeably induced and a second pg gene was expressed. This gene encodes a PG that, in the acidic pH conditions determined in diseased tissue by fungal secretion of oxalic acid, is less sensitive to PGIP. In other cases, PGIP may fail to recognise specific PGs produced by some fungi. For example, a PG refractory to all PGIPs was isolated from a Fusarium moniliforme strain. The comparison of the amino acidic sequence of this PG with that obtained from a different F. moniliforme isolate susceptible to PGIP inhibition allowed the identification of a few amino acidic substitutions responsible for escaping inhibition. Further studies using overlap extension of the two PG sequences and site-directed mutagenesis will permit to identify specific amino acids responsible for the lack of recognition by plant PGIP.