Resistance of plants to infection by phytopathogenic microorganisms is the result of multiple defense reactions comprising both constitutive and inducible barriers. In grapevine, the most frequently observed and best characterized defense mechanisms are the accumulation of phytoalexins and the synthesis of PR-proteins. Particular attention has been given here to stilbene phytoalexins produced by Vitaceae, specifically, their pathway of biosynthesis (including stilbene phytoalexin gene transfer experiments to other plants) and their biological activity together with fungal metabolism.

Phytoalexins from the Vitaceae: biosynthesis, phytoalexin gene expression in transgenic plants, antifungal activity, and metabolism.

Developmental regulation of fungal cell-wall formation

Candida lipolytica synthetisiert das Antibioticum Tryptanthrin aus 1 Mol Tryptophan und 1 Mol Anthranilsaure. Bei Verfutterung von Tryptophan und substituierter Anthranilsaure, bzw. substituiertem Tryptophan und Anthranilsaure, konnten die zu erwartenden Tryptanthrinderivate isoliert und identifiziert werden. Die Enzyme der Tryptanthrinbiosynthese wiesen in bezug auf diese Substrate, mit Ausnahme von Bromtryptophan, keine Spezifitat auf. Parallel zu diesen Versuchen wurden durch chemische Synthese substituierte Tryptanthrine hergestellt. Die Dierivate wurden auf ihre antibiotische Wirksamkeit gepruft; als besonders wirksam erwiesen sich die halogenierten Verbindungen.

Stoffwechselprodukte von Mikroorganismen

Interactions between dual cultures of Septoria nodorum and several other cereal phylloplane fungi were studied in vitro to determine their antagonistic ability and their tolerance of antagonism. No fungi tested were outstandingly successful antagonists and interference reactions between opposed hyphae rarely occurred. Septoria caused the hyphae of some test fungi to fail to plasmolyse and others to burst. In experiments designed to examine volatile and non-volatile metabolites growth of Septoria was rarely affected by colonies of the test fungi.

Colony interactions and hyphal interference between Septoria nodorum and phylloplane fungi

A novel mathematical model is proposed to explain how a tubular shape (e.g., a fungal hypha) is generated by a tip-growing cell. The model derived from a computer simulation of morphogenesis assumes that: i) the cell surface expands from materials discharged by wall-destined vesicles, ii) vesicles are released from a postulated vesicle supply center (VSC), iii) vesicles move from the VSC to the surface in any random direction. The position and movement of the VSC become the critical determinant of morphogenesis: a stationary VSC releases vesicles that reach the cell surface in about equal numbers in all directions, and the cell grows as a sphere. Any displacement of the VSC from its original central position distorts the spherical shape. A sustained linear displacement of the VSC generates the typical cylindroid shape of fungal hyphae. The model yields the equation

$$y = x\cot \frac{{V \cdot x}}{N}$$

which defines both the shape and size (diameter) of a hypha by two parameters, to which physiological significance can be ascribed:N, the amount of wall-destined vesicles released from the VSC per unit time;V, the rate of linear displacement of the VSC. There is a remarkable coincidence between the position of the VSC in the model and the position of the Spitzenkorper in real hyphae. The model affords a simple mechanism to generate a tubular shape from a tip-growing cell; it obviates the need to postulate specific targets for vesicles on the apical cell surface or to invoke gradients in the properties of the apical wall. Other common morphogenetic transitions of fungi and other organisms can be simulated with the same basic model.

Computer simulation of fungal morphogenesis and the mathematical basis for hyphal (tip) growth

An autoradiographic study revealed two different patterns of cell wall constructioni associated with two types of morphogenesis in Mucor rouxii. In hyphae, the cell wall was preferentially synthesized in the apical region; these cylindrical walls seemed to be generated by a sharply descending gradient of wall synthesis radiating from the apex. In spherical cells (germinating spores, yeast cells), wall formation occurred largely, if not entirely, in uniformly dispersed fashion over the entire cell periphery.

https://science.sciencemag.org/content/sci/165/3890/302.full.pdf

Fungal Morphogenesis: Cell Wall Construction in Mucor rouxii

SUMMARY: A wide diversity of treatments can cause rapid and extensive bursting of hyphal tips of Mucor rouxii. Most hyphal tips from colonies grown on full-strength agar medium burst readily when flooded with distilled water. In contrast, hyphal tips from colonies grown on diluted medium survived flooding with distilled water but succumbed to dilute aqueous solutions (particularly acids, but also neutral salts, EDTA, alcohols, acetone, detergents). Apex bursting was generally inhibited by alkaline solutions but took place at certain concentrations of ethanolamine or NH40OH. Mg2+, Mn2+ or Ca2+ caused pronounced swelling of the hyphal apex followed by an occasional burst. Sharp temperature increases also brought about apical disintegration regardless of nutrient concentration.
Colonies of Aspergillus fumigatus, Fusarium sp., Neurospora crassa (wild-type and two morphological mutants), Penicillium claviforme, Rhizoctonia solani and Rhizopus arrhizus also underwent apical bursting when flooded with water or rapidly heated. Schizophyllum commune, several Phytophthora spp., and some morphological mutants of N. crassa were refractory.
The bursting of hyphal tips by flooding colonies with water is not simply an osmotic phenomenon; its temperature coefficient (Q10 1.3 to 2.0) suggests the additional participation of biochemical reaction(s) as a rate-limiting step in the bursting process. Probably, enzymatic degradation and hence weakening of the wall is a prerequisite for bursting. These observations are offered as circumstantial evidence supporting the following conclusions: (i) the growing tips of fungi have a large wall lytic potential; (ii) the release of this activity during growth must be gradual and delicately co-ordinated with wall synthesis; (iii) this balance can be easily disturbed by a wide variety of external stimuli and the ensuing surge of lytic activity results in the violent disintegration of the hyphal apex.

/pdf/the-bursting-tendency-of-hyphal-tips-of-fungi-presumptive-2cwfmiiawi.pdf

The Bursting Tendency of Hyphal Tips of Fungi: Presumptive Evidence for a Delicate Balance between Wall Synthesis and Wall Lysis in Apical Growth

Summary: Polyoxin D is a strong inhibitor of growth and spore germination of Mucor rouxii. This antibiotic inhibited the growth of both the yeast and mycelial forms of M. rouxii. It caused some minor morphological alterations but did not decisively affect the pattern of dimorphic development. Polyoxin D is a powerful competitive inhibitor of chitin synthetase (K
i, = 0·6 μm). Organisms growing at inhibitory concentrations of the antibiotic exhibited weakened walls that were susceptible to bursting.

Inhibition of Mucor rouxii by Polyoxin D: Effects on Chitin Synthetase and Morphological Development

Polarization of cell wall synthesis during spore germination of Mucor rouxii

Cell-free extracts from the wall-less slime mutant of Neurospora crassa and the mycelium of wild type exhibit similar chitin synthetase properties in specific activity, zymogenicity and a preferential intracellular localization of chitosomes. The yield of chitosomal chitin synthetase from sline cells was essentially the same irrespective of cell breakage procedure (osmotic lysis or ballistic disruption) —an indication that chitosomes are not fragments of larger membranes produced by harsh (ballistic) disruption procedures. The plasma membrane fraction, isolated from slime cells treated with concanavalin A, contained only a minute portion of the total chitin synthetase of the fungus. Most of the activity was in the cytoplasmic fraction; isopycnic sedimentation of this fraction on a sucrose gradient yielded a sharp band of chitosomes with a buoyant density=1.125 g/ cm3. Approximately 76% of the total chitin synthetase activity of the slime mutant was recovered in the chitosome band. Because of their low density, chitosomes could be cleanly separated from the rest of the membranous organelles of the fungus. Apparently, the lack of a cell wall in the slime mutant is not due to the absence of either chitosomes or zymogenic chitin synthetase.

Eleanor Lippman

Papers

Fungal Morphogenesis: Cell Wall Construction in Mucor rouxii

The Bursting Tendency of Hyphal Tips of Fungi: Presumptive Evidence for a Delicate Balance between Wall Synthesis and Wall Lysis in Apical Growth

Inhibition of Mucor rouxii by Polyoxin D: Effects on Chitin Synthetase and Morphological Development

Polarization of cell wall synthesis during spore germination of Mucor rouxii

Chitosomes from the wall-less "slime" mutant of Neurospora crassa