Durum wheat improvement against fungal pathogens by using protein inhibitors of cell wall degrading enzymes.

Broad-spectrum and durable resistance is one of the most attracting perspective in breeding projects aimed at increasing crop resistance. Since most microbial pathogens need to surmount the plant cell wall to penetrate the host tissue, the reinforcement of this complex compartment should increase the capacity of the host plant to resist the attack of different pathogens. We pursued this goal by enhancing the host ability to abolish or limit the activity of Cell Wall Degrading Enzymes (CWDEs) secreted by the pathogens during the penetration and colonization of the host tissue. We concentrated our efforts on the containment of the activity of two different CWDEs: the PolyGalacturonases (PGs) and the xylanase inhibitors (XI). PolyGalacturonases (PGs) are among the first CWDEs secreted by fungal pathogens during infection and in some pathosystems they are virulence factors. PGs depolymerize the cell wall pectin, a minor components of the wheat cell wall, and are inhibited by Polygalacturonase Inhibiting proteins (PGIPs). PG activity is also negatively affected by an high degree of pectin methyl esterification. The level of pectin methyl esterification is controlled by the activity of pectin methylesterases (PMEs), which remove the methyl groups, and by its protein inhibitor called Pectin MethylEsterase Inhibitor (PMEI). Thus, indirectly PMEI may negatively affect the activity of PGs by maintaining the pectin with a high degree of methyl esterification. Xylanases are key enzymes in the degradation of arabinoxylans, a main component of the wheat cell wall. These enzymes have been shown to be virulence factors for the fungal pathogens Botrytis cinerea and Mycosphaerella graminicola. The activity of microbial xylanases is controlled in vitro by xylanase inhibitors (XIs) localized in the plant cell wall. However, no evidences in planta for a role of XIs in plant resistance against pathogens have not been reported yet. By using a transgenic approach we showed that PGIP or PMEI can endows durum wheat with new capacities to control the activity of fungal PGs, possibly through a direct interaction or indirectly by modifying the level and pattern of methyl esterification of cell wall pectin. Similarly, transgenic durum wheat plants over-expressing TAXI-III, a member of the TAXI-type XIs, showed new abilities to control fungal xylanases in all tissues, including those that normally do not accumulate this inhibitor. By phytopathogenic tests we demonstrated that these modifications are effective in limiting wheat diseases caused by the fungal pathogens Fusarium graminearum and Bipolaris sorokiniana. We showed also that the reduction of disease symptoms is associated with a reduced accumulation of mycotoxins. In conclusion, these results indicated that the host cell wall polysaccharides, irrespective of their amount and type, plays a key role as functional barrier against different pathogens and that the increased accumulation of glycosidase inhibitors can contribute to maintain their integrity and improve wheat resistance against fungal pathogens.