Production of reactive oxygen species (ROS) is an important immune response in plant multilayer defense mechanisms; however, direct modification of ROS homeostasis to breed plants with broad-spectrum resistance to disease has not yet been successful. In Arabidopsis, the receptor-like cytosolic kinase AtRIPK regulates broad-spectrum ROS signaling in multiple layers of the plant immune system. Upon treatment with immune elicitors, AtRIPK is activated and phosphorylates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which leads to ROS production. In this study, we identified an AtRIPK ortholog in tomatoes and generated knockdown mutants using CRISPR/Cas9 technology. slripk mutants displayed reduced ROS production in response to representative immune elicitors and were susceptible to pathogenic bacteria and fungi from different genera, including Ralstonia solanacearum, Pectobacterium carotovorum, Botrytis cinerea, and Fusarium oxysporum, which are leaf and root pathogens with hemibiotrophic and necrotrophic infection strategies. In contrast, transgenic tomato plants overexpressing SlRIPK are more resistant to these pathogens. Remarkably, the slripk mutants and SlRIPK-overexpressing transgenic plants did not exhibit significant growth retardation or yield loss. These results suggest that overexpression of SlRIPK confers broad-spectrum disease resistance without a yield penalty in tomato plants. Our findings suggest that modifying ROS homeostasis by altering the regulatory components of ROS production in plant immunity could contribute to engineering or breeding broad-spectrum disease-resistant crops without yield penalty.

Fungal infection is a major cause of crop and fruit losses. Recognition of chitin, the component of fungal cell walls, endows plants with enhanced fungal resistance. Here, we found that mutation of tomato LysM receptor kinase 4 (SlLYK4) and chitin elicitor receptor kinase 1 (SlCERK1) impaired chitin-induced immune responses in tomato leaves. Compared with the wild type, sllyk4 and slcerk1 mutant leaves were more susceptible to Botrytis cinerea (gray mold). SlLYK4 extracellular domain showed strong binding affinity to chitin, and the binding of SlLYK4 induced SlLYK4-SlCERK1 association. Remarkably, the qRT-PCR analysis indicated that SlLYK4 was highly expressed in tomato fruit, and β-GLUCURONIDASE (GUS) expression driven by the SlLYK4 promoter was observed in tomato fruit. Furthermore, SlLYK4 overexpression enhanced disease resistance not only in leaves but also in fruit. Our study suggests that chitin-mediated immunity plays a role in fruit, providing a possible way to reduce fungal infection-related fruit losses by enhancing the chitin-induced immune responses.