THE ROLE OF TARGETED SECRETION IN THE ESTABLISHMENT OF CELL POLARITY AND THE ORIENTATION OF THE DIVISION PLANE IN FUCUS ZYGOTES

In this study, we investigate the role of polar secretion and the resu lting asymmetry in the cell wall in establishing polarity in Fucus zyg otes, We have utilized brefeldin-A to selectively interrupt secretion of Golgi-derived material into the cell wall as assayed by toluidine b lue O staining of sulfated fucoidin, We show that the polar secretion of Golgi-derived material is targeted to a cortical site of the zygote identified by the localization of actin filaments and dihydropyridine receptors, The deposition of Golgi-derived material into the cell wal l at this target site is temporally coincident with and required for p olar axis fixation. We propose that local secretion of Golgi-derived m aterial into the cell wall transforms the target site into the fixed s ite of polar growth. We also found that polar secretion of Golgi-deriv ed material at the fixed site is essential for growth and differentiat ion of the rhizoid, as well as for the proper positioning of the first plane of cell division. We propose that the resulting asymmetry in th e cell wall serves as positional information for the underlying cortex to initiate these polar events, Our data supports the hypothesis that cell wall factors in embryos, previously shown to be responsible for induction of rhizoid cell differentiation, are deposited simultaneousl y with and are responsible for polar axis fixation, Furthermore, the p attern of polar growth is attributable to a positional signal at the f ixed site and appears to be independent of the orientation of the firs t cell division plane, Thus, the establishment of zygotic cell polarit y and not the position of the first division plane, is critical for th e formation of the initial embryonic pattern in Fucus.