Doctorate Student, University of Colorado Denver
Cell wall loosening in the fungus Phycomyces blakesleeanus
Elucidating the mechanical properties of fungal cell wall expansion during growth can help identify novel means of fungal control. Cell wall-loosening, a critical component of expansive growth in fungi, has been a debated topic. Work with plant and algal cells shows up to a 40% extension when cell walls are loosened during cell growth under low (acidic) pH. During this loosening stage, cell walls may show increased chemical and environmental susceptibility, providing a possible avenue for prevention of growth. The control of fungal growth for medical and non-medical purposes is challenging due to the structural nature of the cell wall. The objective of the work here was to obtain insight into the wall chemistry and wall loosening mechanism in the cell wall of the fungus Phycomyces blakesleeanus. Constant-tension extension experiments were conducted on frozen-thawed sporangiophore walls while being subjected to a 4.6 pH environment. In this study, the cell walls exhibited 13% increase in extension under low pH conditions. A decrease in pH from neutral to 4.6 produced a creep extension for up to 5 minutes. When metabolic activity was removed from the cell walls via boiling, cell wall extension was reduced and creep was no longer observed. This finding suggested that low pH may be involved in the in vivo wall extension process and that metabolic support of the process was needed. This study is the first of its kind to take a structural approach to modeling cell wall growth in fungi, the results of which will complement existing biological studies into fungal control.
Abstract: A considerable amount of research has been conducted to determine how cell walls are loosened to produce irreversible wall deformation and expansive growth in plant and algal cells. The same cannot be said about fungal cells. Almost nothing is known about how fungal cells loosen their walls to produce irreversible wall deformation and expansive growth. In this study, anoxia is used to chemically isolate the wall from the protoplasm of the sporangiophores of Phycomyces blakesleeanus. The experimental results provide direct evidence of the existence of chemistry within the fungal wall that is responsible for wall loosening, irreversible wall deformation and elongation growth. In addition, constant-tension extension experiments are conducted on frozen-thawed sporangiophore walls to obtain insight into the wall chemistry and wall loosening mechanism. It is found that a decrease in pH to 4.6 produces creep extension in the frozen-thawed sporangiophore wall that is similar, but not identical, to that found in frozen-thawed higher plant cell walls. Experimental results from frozen-thawed and boiled sporangiophore walls suggest that protein activity may be involved in the creep extension.
Pub.: 01 Jan '15, Pinned: 14 Jun '17
Abstract: Regulation of cell growth is paramount to all living organisms. In plants, algae and fungi, regulation of expansive growth of cells is required for development and morphogenesis. Also, many sensory responses of stage IVb sporangiophores of Phycomyces blakesleeanus are produced by regulating elongation growth rate (growth responses) and differential elongation growth rate (tropic responses). "Stiff" mutant sporangiophores exhibit diminished tropic responses and are found to be defective in at least five genes; madD, E, F, G, and J. Prior experimental research suggests that the defective genes affect growth regulation, but this was not verified. All the growth of the single-celled stalk of the stage IVb sporangiophore occurs in a short region termed the "growth zone." Prior experimental and theoretical research indicates that elongation growth rate of the stage IVb sporangiophore can be regulated by controlling the cell wall mechanical properties within the growth zone and the magnitude of the turgor pressure. A quantitative biophysical model for elongation growth rate is required to elucidate the relationship between wall mechanical properties and turgor pressure during growth regulation. In this study, it is hypothesized that the mechanical properties of the wall within the growth zone of stiff mutant sporangiophores are different compared to wild type (WT). A biophysical equation for elongation growth rate is derived for fungal and plant cells with a growth zone. Two strains of stiff mutants are studied, C149 madD120 (-) and C216 geo- (-). Experimental results demonstrate that turgor pressure is larger but irreversible wall deformation rates within the growth zone and growth zone length are smaller for stiff mutant sporangiophores compared to WT. These findings can explain the diminished tropic responses of the stiff mutant sporangiophores. It is speculated that the defective genes affect the amount of wall-building material delivered to the inner cell wall.
Pub.: 05 Jun '12, Pinned: 14 Jun '17