The Photoresponse of Phycomyces: Analysis Using Manual Techniques and an Automated Machine Which Precisely Tracks and Measures Growth During Programmed Stimuli

Citation

Foster, Kenneth William (1972) The Photoresponse of Phycomyces: Analysis Using Manual Techniques and an Automated Machine Which Precisely Tracks and Measures Growth During Programmed Stimuli. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SR6R-P445. https://resolver.caltech.edu/CaltechETD:etd-08072006-150918

Abstract

The sporangiophores of the fungus Phycomyces are sensitive to light, to gravity, to mechanical stretch, and possibly to a gas. In order to understand the processes by which these stimuli evoke response from the organism, the relationships between these input stimuli, and the output responses of the fungus have beenn studied. The light stimulus was chosen because of its ease of quantification. To study these responses better than in the past, automated equipment has been developed to improve the precision of observation, the control of the environment, and the collection of data. The heart of this equipment is a tracking system which stabilizes in space the sporangium, which serves as a convenient spherical marker just above the sensing and responding growing zone. Stabilization of this zone makes uniform stimulation relatively easy. Further the automatic system makes possible 6 or more hours of measurement on a single sporangiophore without requiring much operator intervention. After suppression of noise by means of a low pass filter of bandwidth 0.07 Hz, the growth velocity of typically 50 microns/min was measured to a precision of better than 5%. This compares favorably with manual techniques which optimally have a precision of about 20% for the average growth velocity over a 30 sec period. Use of this machine as well as of conventional experimental techniques on the newly available mutants of Phycomyces has provided a clearer understanding of the stimulus-response system. The effect of light scattering and of absorption of light inside the sporangiore on the response to light has been investigated. These studies have resulted in appreciation of the quantitative effect of the lens focusing of the cell and in an improved action spectrum. Evidence is presented that a major factor causing phototropism is the cell's natural twist (spiral growth) of about 6°/min in the middle of the growing zone. Each region of the cell locally adapts to the local light intensity. However with the twist of the cell, at one edge of the focused band which is produced by the lens focusing of the cell there will be a continuous entry of dark adapted area into the higher intensity band, thus giving rise to transient positive growth responses in a succession of surface elements. The response to this stimulus is expressed slowly in time and as it is expressed, it is carried around at the twist rate of the cell. This model has successfully explained both our experiments and a number from the literature. Using the machine, latency and shape of response have been characterized as a function of temperature, stimulus size, level of adaptation and incident light wavelength. Latency is dependent on the logarithm of the absolute adaptation level and there is no wavelength dependence of the stimulus response curve. For larger stimuli the shape of the response changes markedly, with the rising of an early peak in the growth velocity and inhibition of a later second peak. Study of mutants defective in the chemical pathway from light stimulation to response show a great variety of mutant types implying a relatively large number of biochemical steps from stimulus to response.

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