The fungal cell

8.1.1. The fungal cell wall

Both the structure and composition of the fungal cell wall have unique characteristics. The polysaccharide β(1,4) N-acetil-glucose-amin, generally known as chitin, is a unique cell wall component of the fungi, but the amount of chitin of cell walls of different groups may differ strikingly, e.g., baker’s yeast has only a small amount (1% of cell wall dry weight) of chitin in its cell wall. In some groups (e.g. in some zygomycetes) the chitin is partially deacetylized by an enzyme (chitin deacetylase) and so the cell wall will contain chitosan (deacetylized chitin). The chitin is synthesized by the membrane attached chitin synthase enzyme. Its zymogene is transported into the cell membrane by the vesicles called chitosomes. The fungal cell walls contain polysaccharides mainly β(1,3) and β(1,6) glucans synthesized by the transmembrane glucan synthase enzymes. Most proteins of the cell wall are present in glycolized form (even 90% of the proteins could be glycolized). A majority of the glycoproteins are mannans or mannoproteins, the proteins being connected by mannose. Some proteins are anchored in the cell membrane and play crucial roles in cell wall integrity.

Schematic structure of the cell membrane and cell wall fungi.1: cell membrane; 2: ergosterol; 3: chitin; 4:

anchore-proteins; 5: β(1,3) glucan; 6: β(1,6) glucan; 7: mannoproteins.

The Oomycetes have also cell walls and some species have a small amount of chitin in their walls, but the vast majority has no chitin at all. On the other hand, their cell walls contain cellulose, a β(1,4) glucan, which is never present in true fungi.

8.1.2. The fungal cell membrane

The organization of the fungal cell membrane is the same as the general eukaryotic cell membrane. Nevertheless, special fungal sterols affect membrane fluidity of the fungi. The most widely known are the ergosterols, but

metilen-Fungi

and etilene cholesterols, brassicosterols and cholesterols occur in some fungal lineages. The special membrane components have great importance in antifungal therapy; several fungicides target those molecules or their synthesis pathway. For example, the azols target the ergosterols and disintegrate the membrane, whereas poliens target the synthesis pathway of ergosterols.

8.1.3. The flagellum

Among the true fungi only chytrids have flagella. The motile zoospores have one posterior flagellum (opisthokont) with no tinsel (whiplash). The flagellum has 9+2 microtubules arranged in the typical eukaryotic pattern. The chytrid rumen fungi have posteriorly multiflagellate (up to 16 whiplash flagella) zoospores. The organization of the flagellar apparatus at the base of the flagellum is a characteristic feature of different groups of the chytrids.

Zoospores of fungi belonging to the Straminopila lineage have one or two flagella in anterior (akrokont) or lateral (pleurokont) position. At least one of the flagella is tinsel, the special straminipilous flagellum type; on their surfaces, at the position of two definite microtubules, these flagella have two rows of very fine hairs with special branching structures. The tips of the flagella are narrower than the main body of the flagellum as the two central microtubules are longer than the surrounding nine.

8.1.4. The nucleus

The fungal genomes belong to the small eukaryotic genomes, though some have relatively large or extreme small genomes, too. Considerable differences occur between the genome sizes of oomycetes (e.g.Albugo laibachii37 Mb,Phytophtora infestans240 Mb). The fungal chromosomes and nuclei are relatively small, and the chromosome number generally is also low (4-8), although much higher chromosome numbers may occur as well (e.g. 21 of Ustilago maydis(corn smut)).

The number of nuclei in single cells may vary albeit is characteristic of different fungal groups and/or life stages.

Coenocytic hyphae and thalli have many nuclei in each cell or thallus. For example, the species of the coenocytic Glomeromycota could have even thousands of nuclei in one asexual chlamydospore. Special stages of the lifecycle of fungi with monokaryotic and dikaryotic hyphae: Ascomycota of the family Morchellaceae belonging to the order Pezizales (cup fungi) have multinuclear ascospores, sometimes even more than eight nuclei in one spore. Dikaryotic hyphae, i.e. with two nuclei in one cell, are unique to the Ascomycota and Basidiomycota. That is why the clade of those two groups is called Dikarya. The dikaryotic hyphal stage is relatively short in the life cycle of Ascomy-cota, where such hyphae are formed only at the sexual reproductive, ascomata and ascus stages. In contrast, the life cycle of the Basidiomycota is dominated by the dikaryotic phase.

8.1.5. Cell division

Baker’s yeast has long served as the model organism of the eukaryotic cell cycle and cell division. Accordingly, our knowledge of fungal cell division is relatively strong though far from complete, especially about the regulatory steps of meiotic cell division. Many fungi perform special, so-called closed cell division (both meiotic and mitotic), during which the nuclear membranes, thus the nuclei, remain intact. These cell divisions are endonuclear, as chromosomes are segregated within the nuclei. The special organelles, the spindle pole bodies (SPB) play a crucial role in the orientation of microtubules (MTs); in some fungi, other MT-organizing-centers (MTOCs) also appear during cell division. The kinetochore MTs connected to the SPBs segregate the chromosomes within the nuclei, while the interpolar MTs, also connected to SPBs, help separate the two poles of the nucleus. The astral MTs are outside the nucleus, and connected to SPBs. The size, form and organization of the SPBs may vary between different fungal groups.

Fungi

Schematic structures of the mitosis of budding yeast.Cell in theA: G2-M phaseB: anaphaseC: early thelophase.

1: nuclear membrane; 2: astral microtubules (MT); 3: interpolar MT; 4: kinetochor; 5: kinetochor MT; 6: spindle pole body (SPB).

Closed, endonuclear mitosis has great importance in hyphal growth, which is based not on cell-division but on hyphal tip-growth, hence the divisions of nuclei follow the extending growing hyphae. Unicellular fungi have special cell division types. During the mitosis of budding yeasts no equal offspring cells appear, but during the budding the new cell grows out from the mother cell. After endonuclear mitosis ends the new cell separates from the mother cell. In fission yeasts (Schizosaccharomyces) the mitosis shows different mechanisms that are called fission. After endonuclear mitosis of the nucleus and migration of the new nuclei into the poles, an actin ring forms and constricts as the new septum develops and segregats the two cells. Both yeast-types serve as model organisms not only for cell cycle regulation but also for the mechanisms of different hyphal growth and developmental pro-cesses. The budding cells of baker’s yeasts emerge at a certain point of the cell, and the budding itself is a polarized grow similar to the directional growth of hyphal tips.

8.1.6. Special cell organelles

Several cell organelles as mentioned above are unique to fungi. For example, the chitosomes which transfer the zymogenes of the chitin synthase to the cell membrane, or the spindle pole body (SBP) which plays a crucial role in endonuclear cell divisions as well as many other processes such as synchronized nucleus migration of Dikarya and the migration of the nuclei into the basidiospores.

Nevertheless, several other organelles are characteristic of fungi. The hyphal ascomycetes have a special cell or-ganelle in their hyphae, the Woronin-body. These oror-ganelles generally are located around the septal pores. They are peroxisome-related organelles with single-layered membrane covers, and their body is mainly formed by various proteins;the exact chemical composition of Woronin-bodies is not well understood. The number and the shape of Woronin-bodies varies, albeit probably characteristic to a taxon, e.g. different families in the order of cup fungi (Pezizales) have Woronin-bodies with different forms. Plugging of the septal pores when hyphae are damaged could be one of their main functions. However, inMagnaporthe grisea, the plant pathogen causing brown rot of rice, Woronin-bodies help the fungus ito resist host plant defenses and survive in tissues poor in nutrients.

Fungi