Endo-PGs from species of the Gibberella fujikuroi complex provide new insight into the PG-PGIP interaction

Several Fusarium species of the Gibberella fujikuroi complex are toxigenic and able to induce diseases on many crops, especially cereals. At early stages of the infection process, these fungi produce endo-polygalacturonase (PG) activity which degrades the host cell wall pectic polymers. Plants, on the other hand, contain polygalacturonase-inhibiting proteins (PGIPs) in their tissues which may contrast the fungal infection by inhibiting the PG activity. We observed that the PGs from several species of the Gibberella fujikuroi complex (F. verticillioides, F. sacchari, F. fujikuroi, F. proliferatum, F. subglutinans, F. thapsinum, F. anthophilum, F. nygamai, F. circinatum) are not inhibited by PGIPs extracted from asparagus and leek monocot plants. Instead, these PGs were inhibited to variable extents by the PGIP extracted from bean. The PGs ranked into four groups of inhibition when assayed against the bean PGIP. Specifically, the PGs from F. verticillioides and F. nygamai were the least inhibited and those from F. thapsinum were low inhibited; the PGs from F. sacchari, F. fujikuroi and F. proliferatum were inhibited at an intermediate level, while the PGs from F. anthophilum, F. circinatum and F. subglutinans were the most inhibited. The genes encoding these fungal PGs were cloned, and the multiple alignments of the deduced amino acid sequences allowed identifying the few amino acid substitutions likely involved in the different binding with the bean PGIP. Site-directed mutagenesis experiments showed that the amino acid in position 122 is crucial for recognition of the Fusarium PGs by the bean PGIP.