Light response inPhycomyces blakesleeanus: Evidence for roles of chitin biosynthesis and breakdown

Abstract

Chitin synthetase activity, both basal and zymogenic, fromPhycomyces sporangiophores was stimulated by lightin vitro andin vivo. AmadB mutant did not display these activations, whereas in amadE mutant only chitin synthetase zymogen was increased by illuminationin vivo. Light also produced a transient alteration in cell wall structure at the apical region of the sporangiophore revealed by accessibility of chitin to binding by wheat germ agglutinin and by an increased limited breakage of chitin microfibrils. This last response was absent in bothmadB andmadE mutants. Accordingly, it is suggested that the light growth response in the sporangiophore fromPhycomyces is due to a transient softening of the cell wall at the growing region followed by an elongation due to the turgor pressure of the cell and an enhanced chitin biosynthesis by the apically localized chitin synthetase which restores normal strength to the cell wall. A hypothetical scheme to account for these results is presented.