Stiff Mutant Genes of Phycomyces Affect Turgor Pressure and Wall Mechanical Properties to Regulate Elongation Growth Rate

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.

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses

“…Also relevant in this context is the finding that some of the so‐called ‘stiff” mutants of Phycomyces with reduced gravitropic bending possess an increased turgor pressure but reduced rates of cell wall deformation (Ortega et al . ). An increased focus on the nanomechanics would very likely lead to integrative gravitropism concepts that have already been better explored for plants (reviewed in Baluska & Volkmann ).…”

“…Cultures of Phycomyces cultivated for 19.5 days in microgravity in the Russian biosatellite Cosmos-782 maintained normal vegetative and sexual development, but had completely disoriented sporangiophores, forming twists and loops. The normal growth pattern of sporangiophores could be recovered on a centrifuge operating at an acceleration of 1 g (Parfyonov et al 1979). The effectiveness of gravitropism depends to some extent on the developmental stage, as stage 1 sporangiophores, which lack a sporangium ( Fig.…”

“…The experiment of Dennison (see above) could serve as a guidepost for future fungal gravitropism studies that scrutinise more closely the relationship between statolith-and wall stress-mediated bending. Also relevant in this context is the finding that some of the so-called 'stiff" mutants of Phycomyces with reduced gravitropic bending possess an increased turgor pressure but reduced rates of cell wall deformation (Ortega et al 2012). An increased focus on the nanomechanics would very likely lead to integrative gravitropism concepts that have already been better explored for plants (reviewed in Baluska & Volkmann 2011).…”

Dimensionless numbers to study cell wall deformation of stiff mutants of Phycomyces blakesleeanus

“…This increased chitin synthase activity could explain the light growth response if newly synthesized chitin produces wall loosening by competing for load‐bearing bonds. Previously, it was shown that stiff mutant sporangiophores have shorter growth zones compared to wild type (Ortega et al, ). It may be deduced that the stiff mutant sporangiophores have less chitin synthase activity due simply to having shorter growing zone, or the shorter growth zones are the result of less chitin synthase activity.…”

“…As a result, the mutants were termed “stiff” mutants, which indicate that the elastic deformation rate of the cell wall is reduced. Subsequent experimental research employing the pressure probe shows that the irreversible cell wall extensibility, ϕ , for stiff mutants C216 and C149 is significantly smaller than that of the wild type, but the magnitude of the critical turgor pressure, P c , is smaller as well (Ortega et al, ). The results indicate that the plastic deformation rate of the cell wall is reduced but the magnitude of the change could not be definitively determined.…”