plants Article Seaweed Extract-Stimulated Priming in Arabidopsis thaliana and Solanum lycopersicum Md Tohidul Islam 1 , Tony Arioli 1,2 and David M. Cahill 1, *   Citation: Islam, M.T.; Arioli, T.; Cahill, D.M. Seaweed Extract- Stimulated Priming in Arabidopsis thaliana and Solanum lycopersicum. Plants 2021, 10, 2476. https:// doi.org/10.3390/plants10112476 Academic Editors: Attila L. Ádám and Lorant Király Received: 6 October 2021 Accepted: 10 November 2021 Published: 16 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Life and Environmental Sciences, Deakin University Geelong Waurn Ponds Campus, Waurn Ponds, VIC 3216, Australia; md.islam2@deakin.edu.au (M.T.I.); tonyarioli@seasol.com.au (T.A.) 2 Seasol International, Bayswater, VIC 3153, Australia * Correspondence: david.cahill@deakin.edu.au Abstract: Plant priming is an induced physiological state where plants are protected from biotic and abiotic stresses. Whether seaweed extracts promote priming is largely unknown as is the mechanism by which priming may occur. In this study, we examined the effect of a seaweed extract (SWE) on two distinct stages of plant priming (priming phase and post-challenge primed state) by characterising (i) plant gene expression responses using qRT-PCR and (ii) signal transduction responses by evaluating reactive oxygen species (ROS) production. The SWE is made from the brown algae Ascophyllum nodosum and Durvillaea potatorum. The priming phase was examined using both Arabidopsis thaliana and Solanum lycopersicum. At this stage, the SWE up-regulated key priming-related genes, such as those related to systemic acquired resistance (SAR) and activated the production of ROS. These responses were found to be temporal (lasting 3 days). The post-challenge primed state was examined using A. thaliana challenged with a root pathogen. Similarly, defence response-related genes, such as PR1 and NPR1, were up-regulated and ROS production was activated (lasting 5 days). This study found that SWE induces plant priming-like responses by (i) up-regulating genes associated with plant defence responses and (ii) increasing production of ROS associated with signalling responses. Keywords: seaweed; Ascophyllum nodosum; Durvillaea potatorum; Phytophthora cinnamomi; Arabidopsis thaliana; priming; priming phase; post-challenge primed state 1. Introduction Agricultural biostimulants made from seaweed extracts have received considerable attention in recent years due to their use in conventional, sustainable, and regenerative agriculture. Seaweed extracts have been demonstrated to increase crop productivity, increase nutrient use, and enhance plant tolerance to biotic and abiotic stress [1]. Extracts from a single macroalgae, such as Ascophyllum nodosum, can stimulate an increase in plant growth, and increase crop productivity [2–4]. Other extracts derived from two brown algal species, A. nodosum and Durvillaea potatorum, also stimulated tomato plant growth and productivity and improved soil health [5]. Seaweed extracts are compositionally diverse and complex in nature [3]. Recent re- views [6,7] confirm the great diversity in extracts derived from macroalgae and the wide variety of physiological responses to specific components of the extracts. For example, Ghaderiardakani et al. [8] found that extracts from Ulva species contained a range of hor- mones that had both inhibitory and stimulatory effects on plant growth and development. It is well recognised that extracts from various major groups of macroalgae, including the two species used in the current study, contain a wide range of biologically active com- pounds including plant hormones such as cytokinins [9], laminarins, alginates, phenolics, ulvans, and carrageenans [6,10,11]. However, the mechanisms that underly the effect of seaweed extracts on plants remains unclear. Also, it is unknown if extracts made from one or two macroalgae use the same stimuli. Plants 2021, 10, 2476. https://doi.org/10.3390/plants10112476 https://www.mdpi.com/journal/plants