Once the fungus penetrates the wheat cell wall, the fungal cell wall materials (i.e. chitin) act as elicitors which interact with plant receptors and induce the plant defence mechanism. One product of this is reinforcement of the cell wall in an attempt to stop the invasion. The plant starts forming the papillae - a callose matrix (cellulose, suberin, protein, gums, calcium and silicon) which provides extra resistance to penetration, however this barrier does remain effective for long if the level of micro-organisms present is high.