Effect of restrictive conditions on the growth and morphology of a temperature-sensitive mannose-requiring mutant of Aspergillus nidulans

Abstract

When incubated at 45 degrees C in the absence of added mannose, pregrown hyphae of a temperature-sensitive, mannose-relief mutant (mnrA455) of Aspergillus nidulans grew normally for a short time (4-5 h) before exhibiting an abnormal morphology consisting of the production by hyphae of discrete spherical swellings called balloons. These swellings could be up to 10 microns in diameter and were produced either at or behind the hyphal apex. Often only one swelling was produced in association with each hyphal tip, but in a significant minority of cases (approximately 19.6%) a second balloon was produced in close association with the first. Hyphal tip extension slowed before and during balloon formation, but growth at individual tips did not usually stop when a balloon began to be formed in the same hypha. All tip extension ceased after approximately 8 h in cultures maintained at 45 degrees C. However, normal growth resumed 45-60 min after transfer of such a culture to the permissive temperature of 37 degrees C even after 48 h at 45 degrees C. Electron microscopic examination indicated that balloons consistently had thicker walls than the surrounding hyphae but that no accumulation of cytoplasmic vesicles was apparent within them. This indicates that a modification of wall structure, probably including deposition of new wall material, was caused by a mannose deficiency, but that this altered wall synthesis and attendant hyphal swelling was not due to diversion of the normal vesicle-mediated tip-extension system to the side walls of hyphae.