A Biostimulant Based on <i>Ecklonia maxima</i> and Yeast Extract Increases the Resistance of Tomato Plants Toward <i>Pseudomonas syringae</i> pv. <i>tomato</i> DC3000

Plant biostimulants represent a promising option to improve agricultural production and stress resistance while reducing the use of fertilizers and pesticides. Despite various evidence demonstrating the beneficial role of biostimulants in preventing the negative effects of abiotic stress on plants, the ability of biostimulants to bolster defense mechanisms has been brought to light only recently. In this work, the impact of a biostimulant based on Ecklonia maxima and yeast extracts (S/Y) on the response of tomato infected with Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) was assessed. S/Y was selected after a screening to identify biostimulants capable of conferring resistance to Pst DC3000. S/Y boosts the early events of the plant's innate immunity. Indeed, biostimulation increased the Pst DC3000-induced oxidative burst by upregulating the NADPH oxidase/respiratory burst oxidase homolog and apoplastic class III peroxidases expression. Moreover, the deposition of callose was also promoted. Due to improved activation of early defense responses by S/Y, disease symptoms and bacterial spread 72 h after the infection were significantly reduced. Finally, levels of salicylic acid, a key hormone in plant innate immunity, were increased by S/Y, whilst those of jasmonic acid and auxin, which are negative regulators in defense responses to Pst DC3000, were hampered. Overall, these findings show that S/Y mitigates infection symptoms by acting on different defense mechanisms, thus providing evidence of the potential of the biostimulant to improve plants' response to biotic stresses.