Involvement of fungal pectin methylesterase activity in the interaction between Fusarium graminearum and wheat

Fusarium graminearum, the causal agent of Fusarium Head Blight (FHB) of wheat, barley and other small grain cereals, starts infection by colonizing the soft anther and ovary tissue. The involvement of a fungal polygalacturonase (PG) in wheat spikelet infection has been previously established by pg knock out mutants. Since the PG hydrolyzes the de-esterified pectin (homogalacturonan) of the plant cell wall, a pectin methylesterase (PME) able to demethylate the wheat pectin is likely involved at the early stage of fungal infection. To investigate the importance of pectin de-methylesterification in FHB of wheat we are following two approaches: the knock-out of the F. graminearum PME activity, and the identification of wheat PME inhibitors (PMEIs) able to inhibit the fungal PME activity. By an in silico analysis of the F. graminearum sequenced genome, two PME encoding genes have been identified. Based on their nucleotide sequence, these genes putatively encode two isoforms. However, in liquid culture containing pectin as the sole carbon source, F. graminearum secretes only a PME isoform with a basic pI. This enzyme is under purification to identify the corresponding pme gene in order to perform the knock out by targeted homologous recombination. In a complementary approach, three PMEI encoding genes have been identified in the wheat genome and have been expressed heterologously; these inhibitors will be tested for their ability to inhibit the purified PME isoform of F. graminearum.