Cell Wall Synthesis in Apical Hyphal Growth

This chapter discusses the cell wall synthesis in apical hyphal growth. It appears that the hyphal apex is best viewed as a highly polarized system of exocytosis. Wall materials, extracellular enzymes, and probably other substances are excreted at the growing end of a tubular cell. The most obvious cellular features that accompany this polarized system are (1) the unidirectional flow of vesicles in the cytoplasm fusing with the plasma membrane at the apex, (2) the gradients in wall synthesis at the apex, and (3) the cytoplasmic gradients in ion distribution that are maintained at the apex. New microscopic techniques reveal a cytoskeletal organization of the cytoplasm at the apex, which may be crucial to its polarized activity. Growth of the wall at the hyphal apex requires that the wall in this region has plastic properties, which contrast with the requirement of rigidity elsewhere in the hypha. A widely held view involves the participation of wall-lytic enzymes in plasticizing the wall at the apex and in allowing new wall material to be inserted. A critical evaluation of the evidence presented to support this view makes this hypothesis less attractive. As an alternative a steady-state model is discussed based on recent observations in the author's laboratory. This model holds that the assemblage of polymers synthesized at the apex is inherently plastic. However, this assemblage develops rigidity by interactions, in the wall, between and among the various individual polymers present while the wall segment moves in subapical directions during elongation. This model seems to fit many of the original observations made on living hyphae.