Induction of phlorotannins in the brown macroalga Ecklonia radiata (Laminariales, Phaeophyta) in response to simulated herbivory--the first microscopic study.

We investigated, for the first time in a microscopic study, the accumulation of phlorotannins as a possible inducible chemical defence against herbivory. Ecklonia radiata (C. Agardh) was mechanically wounded with a cork borer, simulating grazer action, and at intervals of 0, 1, 2, 3, 5, 7 and 9 days after wounding the distribution of phlorotannins and other structural changes were examined by light, fluorescence and electron microscopy. In brown algal cells, phlorotannins (polyphenolic compounds) occur in vesicles known as physodes. In E. radiata, most of the physodes were found in the outer epidermal cell layer, but some were present in the cortical cells and in the innermost medullary cells (sieve elements and hyphal cells). The wound-healing process could be divided into three stages: (i) 'closing' of the medulla by the formation of new medullary cells, (ii) accumulation of phlorotannins (physodes) at the wound area (first in the medullary cells and then in the cortical cells) and in the medullary tissue further away from the wound, and (iii) formation of a new epidermis. The accumulation of phlorotannins started on day 1 and was evident from day 3 on. Our results show structural wound-healing and support wound-sealing functions for phlorotannins and the view that phlorotannins might be considered as inducible anti-herbivory agent in E. radiata. Our results strongly demonstrate the importance of detailed microscopic studies, in addition to chemical analysis, for revealing the localised nature of the brown algal response to wounding.