Abstract
The maintenance of growth of hyphae of Saprolegnia ferax was dependent on the presence of external Ca 2+ and the growth rate increased with increased external Ca 2+ up to 5 × 10 −2 m Ca 2+. When Ca 2+ was greater than 5 × 10 −2 m, growth rates decreased. Internal membrane-associated Ca 2+ was localized with chlortetracycline. Internal Ca 2+ became depleted in hyphae grown in the absence of Ca 2+ and was increased in hyphae grown in high concentrations of Ca 2+, showing that internal Ca 2+ can be modulated by external Ca 2+. However, the range of the internal change was not as great as the range of external concentration used, indicating that the hyphae are capable of regulating Ca 2+ in the presence of a large concentration gradient. In the absence of external Ca 2+, growth can occur for a limited time through use of internal Ca 2+. The actin cytoskeleton was altered in hyphae grown in both high and low Ca 2+. Hyphae grown in 10 −3 m Ca 2+ had more actin in their apical network and peripheral plaques of actin were further from the apex than in more slowly growing hyphae in 10 −1 m and 0 Ca 2+. The tips of hyphae growing in low Ca 2+ also had a tendency to swell, giving these hyphae irregular shapes. Ca 2+ is known to affect cell wall rigidity and the consistency of actin gels, two factors that can be expected to affect hyphal growth. External Ca 2+ does play a role in hyphal growth possibly directly by acting on the cell wall and indirectly by altering internal Ca 2+, thus affecting the actin cytoskeleton and possibly other growth processes.
Published Version
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