Phaeophyceaean (Brown Algal) Extracts Activate Plant Defense Systems inArabidopsis thalianaChallenged WithPhytophthora cinnamomi

Seaweed extracts are important sources of plant biostimulants that boost agricultural productivity to meet current world demand. The ability of seaweed extracts based on either of the Phaeophyceaean speciesAscophyllum nodosumorDurvillaea potatorumto enhance plant growth or suppress plant disease have recently been shown. However, very limited information is available on the mechanisms of suppression of plant disease by such extracts. In addition, there is no information on the ability of a combination of extracts fromA. nodosumandD. potatorumto suppress a plant pathogen or to induce plant defense. The present study has explored the transcriptome, using RNA-seq, ofArabidopsis thalianafollowing treatment with extracts from the two species, or a mixture of both, prior to inoculation with the root pathogenPhytophthora cinnamomi. Following inoculation, five time points (0-24 h post-inoculation) that represented early stages in the interaction of the pathogen with its host were assessed for each treatment and compared with their respective water controls. Wide scale transcriptome reprogramming occurred predominantly related to phytohormone biosynthesis and signaling, changes in metabolic processes and cell wall biosynthesis, there was a broad induction of proteolysis pathways, a respiratory burst and numerous defense-related responses were induced. The induction by each seaweed extract of defense-related genes coincident with the time of inoculation showed that the plants were primed for defense prior to infection. Each seaweed extract acted differently in inducing plant defense-related genes. However, major systemic acquired resistance (SAR)-related genes as well as salicylic acid-regulated marker genes (PR1,PR5, andNPR1) and auxin associated genes were found to be commonly up-regulated compared with the controls following treatment with each seaweed extract. Moreover, each seaweed extract suppressedP. cinnamomigrowth within the roots of inoculatedA. thalianaby the early induction of defense pathways and likely through ROS-based signaling pathways that were linked to production of ROS. Collectively, the RNA-seq transcriptome analysis revealed the induction by seaweed extracts of suites of genes that are associated with direct or indirect plant defense in addition to responses that require cellular energy to maintain plant growth during biotic stress.