No abstract

Summary• Until now, no information has been available on the transcriptional response to the transduction of defense signals in brown seaweeds that leads to active resistance against pathogens or grazers.• Using a polymerase chain reaction (PCR)-based, subtractive cDNA approach combined with filter arrays we demonstrated that Laminaria digitata exhibits a rapid response to oligoguluronate elicitors. The transcription levels of several genes were validated by quantitative real-time reverse-transcription PCR and further analysed using pharmacological approaches.• Fifty upregulated genes were identified by differential screening in elicited algae over a 24-h time-course. These genes were related to oxidative stress responses, production of antimicrobial secondary compounds or cell wall strengthening. Moreover, pharmacological tests showed that intracellular signal transduction is likely to involve reactive oxygen species. A new oligoguluronate-inducible vanadiumdependent haloperoxidase (vHPO), specific to iodide was also characterized. The transcription of several vHPO genes was shown to be tightly regulated.• Taken together, our data show that early transcriptional defense responses in L. digitata are similar to those in land plants but also include novel defense responses that involve tightly regulated iodine metabolism.

Arachidonic acid, linolenic acid and methyl jasmonate (MeJA) were found to be strong triggers of an oxidative burst in the kelp Laminaria digitata. These findings constitute the first report of an oxidative burst in an algal system induced by free fatty acids. The source of reactive oxygen species can be at least partially inhibited by diphenylene iodonium (DPI). Treatment with arachidonic acid increases the levels of a number of free fatty acids [including myristic (C14:0), linoleic (C18:2), linolenic (C18:3) and eicosapentaeneoic (C20:5) acids] and hydroxylated derivatives [such as 15-hydroxyeicosatetraenoic acid (15-HETE), 13-hydroxyoctadecatrienoic acid (13-HOTE) and 15-hydroxyeicosapentaenoic acid (15-HEPE)]. Similar to a previous report of the function of an alginate oligosaccharide-triggered oxidative burst in the establishment of resistance in L. digitata against infection by its brown algal endophyte Laminariocolax tomentosoides, C20:4- and MeJA-induced oxidative bursts seem to be involved in establishing the same protection in L. digitata. Altogether, this study supports the notion that lipid oxidation signaling plays a key role in defense induction in marine brown algae.