Modification of sexual development and carotene production by acetate and other small carboxylic acids in Blakeslea trispora and Phycomyces blakesleeanus

Abstract

In Phycomyces blakesleeanus and Blakeslea trispora (order Mucorales, class Zygomycetes), sexual interaction on solid substrates leads to zygospore development and to increased carotene production (sexual carotenogenesis). Addition of small quantities of acetate, propionate, lactate, or leucine to mated cultures on minimal medium stimulated zygospore production and inhibited sexual carotenogenesis in both Phycomyces and Blakeslea. In Blakeslea, the threshold acetate concentration was <1 mmol/liter for both effects, and the concentrations that had one-half of the maximal effect were <2 mmol/liter for carotenogenesis and >7 mmol/liter for zygosporogenesis. The effects on Phycomyces were similar, but the concentrations of acetate had to be multiplied by ca. 3 to obtain the same results. Inhibition of sexual carotenogenesis by acetate occurred normally in Phycomyces mutants that cannot use acetate as a carbon source and in mutants whose dormant spores cannot be activated by acetate. Small carboxylic acids may be signals that, independent of their ability to trigger spore germination in Phycomyces, modify metabolism and development during the sexual cycle of Phycomyces and Blakeslea, uncoupling two processes that were thought to be linked and mediated by a common mechanism.

FIG. 1.

(A) Modification of sexual responses by acetate: mated cultures of Phycomyces and Blakeslea wild-type strains grown on minimal medium with or without 10 mmol/liter sodium acetate and on potato dextrose medium. (B) Single-strain and mated cultures of Blakeslea strains F921 and F986 grown for the numbers of days indicated next to the images on minimal medium (upper rows of plates) or on minimal medium with 10 mmol/liter sodium acetate (lower rows of plates).

FIG. 2.

Dependence of zygospore production on acetate concentration. Mated cultures of Phycomyces wild-type strains (○) and Blakeslea wild-type strains (▴) were grown on minimal medium with various concentrations of sodium acetate. The values are means for 3 to 17 determinations in one to four independent experiments relative to the values obtained for the controls in the absence of acetate. The average relative standard errors of the means were 12% for Phycomyces and 10% for Blakeslea. The acetate concentration is plotted on a logarithmic scale.

FIG. 3.

Dependence of β-carotene content on acetate concentration. Single-strain and mated cultures of Phycomyces wild-type strains (○, NRRL1555; □, NRRL1554; ▴, mated) and Blakeslea wild-type strains (○, F921; □, F986; ▴, mated) were grown on minimal medium with various concentrations of sodium acetate. The values are means and standard errors for 2 to 15 determinations in one to four independent experiments. The acetate concentration is plotted on a logarithmic scale.

FIG. 4.

Time courses for phytoene and β-carotene contents of single-strain and mated cultures of Phycomyces wild-type strains (• and ○, NRRL1555; ▪ and □, NRRL1554; ▴ and ▵, mated) and Blakesla wild-type strains (• and ○, F921; ▪ and □, F986; ▴ and ▵, mated) grown on minimal medium (○, □, and ▵) or minimal medium with 10 mmol/liter sodium acetate (•, ▪, and ▴). The values are means and standard errors for 4 to 15 determinations in two to four independent experiments.