Conidiophore Morphogenesis

The development of an Aspergillus conidiophore can be divided into steps that occur in sequence. It begins with the differentiation of a hyphal cell into a thick-walled foot cell that extends into an aerial stalk of approximately 100 ^m (Figure 7.1) and allows the asexual reproductive cells to be pushed out of substratum for dissemination. Second, the stalk swells at the tip to form a globose, multinucleate vesicle which has a diameter of about 10 ^m. Third, the vesicle buds out about 60 cigar-shaped uninucleate cells called metulae. Fourth, each metula in turn buds out two phialide cells. Fifth, a single nucleus enters into the phialide and by successive mitotic divisions each phialide buds out a vertical chain of conidia with the oldest conidium at the top of the chain. Each conidium is approximately 3 ^m in diameter. By this form of development, each conidiophore can produce over 1000 conidia with economy of space and in a short time. We are beginning to understand how genes specify the development of a conidiophore stalk of a definite height that swells into a vesicle, from which are formed a number of metulae- and phialides-precise structures budding a number of spores. The principles elucidated from this relatively simple system apply to embryonic development—how does the embryo mark and measure space and time so that organs and tissues develop on schedule and in the correct locations?

The time line of conidiophore development in A. nidulans is as follows (Boylan et al., 1987): Undifferentiated hyphae (0 h) ^ aerial stalk (5 h) ^ vesicle (10 h) ^ metula and phialide (15 h) ^ immature conidia (20 h) ^ mature dark green conidia (25 h).





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Figure 7.1 Diagrammatic structure of a conidiophore of Aspergillus nidulans. Redrawn from Moore (1998).

Figure 7.1 Diagrammatic structure of a conidiophore of Aspergillus nidulans. Redrawn from Moore (1998).

Aspergillus nidulans: A Model for Study of Form and Asexual Reproduction 109 7.1.1 Developmental Competence

A. nidulans does not conidiate in submerged liquid cultures. However, if the mycelia from the submerged cultures are exposed to air and light for at least 15 to 30 min, conidiophore differentiation takes place. The action spectrum of conidiation showed that light of 680 nm is maximally effective, i.e., the specific wavelength that elicits conidiation is red light. The chemical nature of the photoreceptor in A. nidulans is not yet known. Interestingly, the red-light effect is inhibited by far-red light—a property well-known for the phyto-chrome-mediated responses in the green plants. The veA1 mutants conidiate both in light and dark conditions (Mooney and Yager, 1990), suggesting that the light requirement is dependent upon the veA gene.

By exposing liquid-grown mycelia to air and light at different time intervals, it was determined vegetative cells become competent to form conidiophores after 18 h of submerged growth. It was therefore hypothesized that a defined amount of vegetative mycelium must be formed and developmental competence acquired before mycelium can produce a conidiophore. The nature of competence remains unknown. Mutant strains have been isolated in which the length of growth period before conidiation is altered, suggesting that the switch from vegetative growth to conidiophore development is genetically determined (Axelrod et al., 1973).

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