Secondary Metabolism and Differentiation in Fungi

Bennett

• Éditeur: CRC Press
• Année: 2020
• ISBN:
  9781000146165
• Édition: 1
• Sujet: Scienze biologiche
• Sous-sujet: Biologia

The first source to unite secondary fungal metabolism and morphogenesis in one volume, Secondary Metabolism and Differentiation in Fungi treats biological systems as parts of a whole rather than as a series of individual elements, highlighting research in genetics, molecular biology, and ecology. Featuring the expertise of 19 international authorities, each chapter is a rich source of experimentation ideas. The book facilitates the application of novel techniques to existing problems in molecular mycology and explores potentials for major new research. This indispensable guide to a key scientific field benefits biologists, chemists, and other scientists.

1 Differentiation and Secondary Metabolism in Mycelial Fungi

J. W. Bennett / Tulane University, New Orleans, Louisiana

INTRODUCTION

"Differentiation" and "secondary metabolism" are terms which encompass an enormous body of accumulated scientific knowledge and evoke an even larger body of unanswered questions. Neither term is easily defined and many overlapping and sometimes ambiguous definitions have been used by different writers. Table 1 presents a list of pertinent terminology with the definitions that will be employed in this chapter.

Historically, development has been viewed as two separate, although frequently simultaneous, processes: growth and differentiation. A more modern conception might be that there is no such thing as an undifferentiated cell-there are simply changes in states of differentiation. Nevertheless, this arbitrary distinction between undifferentiated and differentiated states is useful for purposes of discussion and will be retained with a recognition of the inherent limitations.

In multicellular plants and animals, after an initial stage of undifferentiated growth, the processes of growth and differentiation occur concomitantly. Indeed, "cellular differentiation is the necessary condition of multicellular life" (Gross, 1968) or, put another way, "in order for organisms to become large, they must divide the labor; the two phenomena are inseparable" (Bonner, 1974, p. 25).

Many aspects of development are common to both plants and animals, but the different nature of the biological systems has profound effects upon the pattern of development. Plant cells generally have rigid cell walls and are not motile. In plants, development is indeterminate, with the growing points remaining permanently embryonic and with specific organ systems exhibiting limited growth; most animals show a distinct embryonic phase which ends when adult structure is achieved. These distinctions are important because most biologists who call themselves "developmentalists" and theorize on the molecular basis of differentiation are trained in the animal sciences. They frequently view

Table 1 Definitions

Major term	Related terms	Definition
Development		Process of growth and differentiation by which the potentialities of a zygote or spore are established; sequence of progressive changes resulting in increased biological complexity.
	Epigenesis	Development involving gradual diversification and differentiation of an initially undifferentiated entity such as a zygote or sphore. Development is an epigenetic phenomenon.
	Morphogenesis	The formation of tissues and organs; the developmental process leading to the characteristic mature form of an organism; those aspects of development related to morphological changes.
Differentiation		Progressive diversification of the structure and function of cells in an organism; acquisition of differences during development.
Growth		An increase in size, especially an increase in the quantity of cellular material; frequently dry weight is used as an equivalent.
Mycelium		The vegetative part (thallus) of a fungus consisting of one or more hyphae; a mass of hyphae.
	Hyphae	Branched filamentous cells; may be septate or aseptate.
Secondary metabolites		Diverse natural products unnecessary for growth, of restricted taxonomic distribution, generally produced during a limited stage of the cell cycle from a few simple precursors derived from primary metabolism; among microorganisms the term “idiolite” may be used as a synonym.

Source: Ainsworth (1971); King (1972); Turner (1971); Webster’s Third (1972).

differentiation as an ultimately self-limiting process in development whereby differentiation proceeds through a number of significant restrictions in developmental potential (Rutter et al., 1973).

The concept of "pluripotency" is an important one in embryology. Pluripotency is the condition of having a large indeterminate number of possible fates; it is viewed as a property lost during the developmental process in animals. The historical context of "pluripotency" led botanists to adopt the term "totipotency" to express the idea that any somatic, nucleated plant cell could, under the influence of appropriate stimuli, dedifferentiate and regain the ability to act like a zygote and produce an entire new plant (Needham, 1950; Street, 1976; Steward and Mohan Ram, 1961).

Among prokaryotes and unicellular eukaryotes, growth and differentiation are phenomena observed in populations, not simultaneously in single cells. A given cell may divide vegetatively for a number of generations, or it may form a spore. When viewing an entire population, some cells are differentiated, some are not; but when viewing any given cell (organism), it is either vegetative, reproductive, or in transition. Moreover, this transition from a vegetative to a reproductive form is dependent upon the external environment. For this reason, microorganisms have been useful model systems for studying certain forms of differentiation; for example, experimental manipulation of the environment can be used as a "trigger" to initiate the transition between "undifferentiated" and "differentiated" states.

The fungi hold a unique and useful position for experimentalists. Like plants and animals, they are multicellular and eukaryotic; like plants, individual cells are totipotent; and like prokaryotes and unicellular forms, differentiation is generally triggered by changes in the environment. Some of the most elegant experiments on fungal morphogenesis involve the water mold Blastocladiella emersonii (Cantino, 1966), the acellular slime molds or Mycomycetes (Alexopoulos, 1962; 1966), and the cellular slime molds or Acrasiales (Ashworth, 1971; Gregg, 1966; Sussman and Brackenbury, 1976). But these are not "mycelial fungi."

What are "mycelial fungi"? Formal fungal taxonomy is by no means settled and a variety of classification schemes have been adopted by different workers. Alexopoulos (1962), Hawker (1966), and others have accepted a general scheme in which the true fungi (Eumycota) are subdivided as follows:

• Lower fungi (Phycomycetes). Unicellular or with a mycelium that is generally aseptate.
• Higher fungi. The mycelium is regularly septate, with the exception of the yeasts. The higher fungi are usually divided into three classes:
• Ascomycetes: Sexual spores (ascospores) borne endogenously.
• Basidiomycetes. Sexual spores (basidiospores) born exogenously.
• Deuteromycetes. Sexual reproduction lacking. This form class is also called the Fungi Imperfecti.

In the broadest sense, the mycelial fungi encompass all filamentous species of both the lower and the higher fungi. However, perhaps because Vuillemin restricted the term "hyphae" to septate forms (Ainsworth, 1971, p. 281), usage of "mycelial fungi" is often accepted as meaning only the higher fungi. I will follow this limited definition and exclude the Phycomycetes from my discussion. The subject of yeast-mold dimorphism will also be excluded, but it is reviewed by Cutler and Hazen in Chapter 10 of this volume. Breitenback and Lachkovics discuss the role of the yeast genome in differentiation from a contemporary molecular perspective in Chapter 11 of this volume.

MORPHOGENESIS

Spores can be considered as "both a beginning and an end of fungal development" (Ainsworth, 1976, p. 81). Spores possess many attributes in common with the totipotent cells of plants and animals and more attention has been given to the spore than any other fungal structure. Spore morphology and development are basic to fungal systematics and most studies on fungal differentiation involve aspects of spore formation or germination.

"Spore" is a general term for a reproductive structure. In fungi, spores may be produced asexually or sexually, and in many species both forms of reproduction occur. Generally, the expression "vegetative" is associated with the mycelial phase of the fungal life cycle by mycologists. This is in contrast to another common usage of the term which equates "vegetative" to "asexual."

Spores vary in shape, size, ornamentation, origin, modes of liberation, function, and ontogeny. Asexual spores (conidia) have received the most detailed attention by classical mycologists because they provide the basic taxonomic criteria within many groups. The earliest and the majority of studies on spore differentiation are descriptive. Vuillemin was among the first to emphasize spore development as a taxonomic criterion rather than the characteristics of the spores themselves. He distinguished spores which are not separated from the hypae producing them (thallospores) from spores borne upon special hyphae which separate upon maturity (conidiospores) (see Fig. 1).

The extraordinary diversity of spore types in nature has challenged mycologists to find a satisfactory nomenclature. Ainsworth's Dictionary of the Fungi (1971) lists 100 spore names. In face of this proliferation, many workers simply call all asexual fungal spores "conidiospores." This in turn creates a terminological backlash. Vullemin's conidiospores are now designated "phiallospores" or "conidia vera." The major spore types delineated by Tubaki (1966) for the Fungi Imperfecti are illustrated in Fig. 2. An extended treatment of spore types and terminology within this group has been given by Kendrick (1971). Other recent monographs on conidial fungi include Cole and Samson (1979) and Cole and Kendrick (1981).

Figure 1 Examples of thallospores and conidiospores; Vuillemin's spore groups (after Tubaki, 1966).

Figure 2 Examples of different spore morphologies. (A) Blastospore of Cladosporium avellaneium (from Hughes after de Vries, 1953, p. 588), (B) radula spores of Brachysporium obovatum (after Hughes, 1953, p. 598), (C) chla...