Development of haustoria of Melampsora lini

Abstract

An initial step in haustorium formation is the contact of a terminal intercellular mycelial cell with the wall of a host cell. The former differentiates to form a haustorial mother cell having a thickened wall in the region of contact. A penetration peg develops in the thickened region of the cell wall; it elongates, penetrates the host cell wall, and extends into the lumen of the host cell. A dark-staining region, the neck ring, develops in the wall of the penetration peg and persists throughout subsequent stages of development. The distal end of the penetration peg expands to form a somewhat spherical haustorial body. Subsequently, one to several lobes form on, and extend from, the body of the haustorium. The more ephemeral stages of development are the initiation of the spherical haustorial body and the elongation of the penetration peg. The less ephemeral stages are prepenetration and early postpenetration of the host cell wall and the intermediate stages of haustorial expansion.