Helical growth of stage-IVb sporangiophores of Phycomyces blakesleeanus: the relationship between rotation and elongation growth rates

An understanding of the relationship between the two components of helical growth (rotation rate and elongation rate) is fundamental to understanding the biophysical and molecular mechanism(s) of cell wall extension in algal cells, fungal cells, and plant stems and roots. Helical growth occurs throughout development of the sporangiophores of Phycomyces blakesleeanus. Previous studies within the growth zone of stage-IVb sporangiophores have reported conflicting conclusions. An implicit assumption in the previous studies [E.S. Castle (1937) J Cell Comp Physiol 9:477—489; R. Cohen and M. Delbruck (1958) J Cell Comp Physiol 52:361—388; J.K.E. Ortega et al. (1974) Plant Physiol 53:485—490] was that the relationship between rotation rate and elongation rate was independent of the magnitude of the elongation rate. In the present study, for stage-IVb sporangiophores growing at a steady rate, it is shown that the ratio of rotation rate and elongation rate decreases as the elongation rate increases. Previously proposed biophysical and molecular mechanisms cannot account for the observed behavior. The previously postulated fibril- reorientation mechanism [J.K.E. Ortega and R.I. Gamow (1974) J Theor Biol 47:317—332; J.K.E. Ortega et al. (1974) Plant Physiol 53:485—490] is modified to accommodate this new finding. Other experiments were conducted to determine how the ratio of rotation rate and elongation rate behaves during a pressure response (a transient decrease in elongation rate produced by a large step-up in turgor pressure using the pressure probe). Results of these experiments indicate that this ratio increases during the pressure response.