The occurrence and cellular significance of physodes in brown algae

This review discusses the biogenesis and the role of physodes in brown algal cells and thalli. Phlorotannins, packaged within membrane-bound physodes, appear to play multiple roles in brown algae, both at the cellular and at the organismal level. Physodes are a major cytoplasmic constituent of adult plants, as well as of gametes, zygotes, embryos and spores. They are formed in the perinuclear region of the cell by the Golgi apparatus and the endoplasmic reticulum, and are moved around the cell by actin microfilaments and microtubules. During fucoid development, phenolics are the first component of the primary cell wall to be secreted after fertilization. Following germination, they are conspicuous in the tips of developing rhizoids. The first indication of cross-wail formation is the accumulation of phenolic vesicles in the plane of cell division, where they play a role in the formation of the cell plate and the ensuing cross-walls. Phenolics play a critical role in cell wall formation and early development. Ever since physodes were first described, there has been much discussion on then possible function. Phenolic compounds are integral structural components of brown algal cell walls. They may act as a polyspermy block on fertilization and form part of the adhesive in some species. They have been found to play a role as a herbivore deterrent and have been suggested to play a role in a heavy metal resistance mechanism. Phlorotannins may also play a role in the protection of the plant against ultraviolet radiation.