Nicotinamide Adenine Dinucleotide (NAD ⁺ ) Induces Defense Responses Against Powdery Mildew and Regulates <i>LecRLK</i> Gene Expression in Squash

Nicotinamide adenine dinucleotide (NAD + ) is a ubiquitous electron carrier in many metabolic pathways. Exogenous application of NAD + induces systemic acquired resistance-like defense responses in Arabidopsis thaliana, and potential NAD + -binding receptors in Arabidopsis, the lectin receptor-like kinase (LecRLK) family, have been reported to influence plant development and stress response. To evaluate the effects of exogenous NAD + treatment of squash against powdery mildew disease, caused by the fungal pathogen Podosphaera xanthii, greenhouse trials were conducted. NAD + at concentrations of 0.25, 0.5, 0.75, 1, and 5 mM, applied by leaf infiltration, significantly lowered powdery mildew disease severity compared with the untreated control. The hyphal development of P. xanthii was inhibited after infiltrating NAD + at 1 mM into squash leaves. Additionally, an RNA-sequencing study revealed the upregulation of different function categories of genes in squash, including those in the stress and defense response pathways, after NAD + treatment and during pathogen invasion and disease development. The LecRLK family in squash was also screened and analyzed as potential receptors. A total of 51 LecRLK genes were found in squash, and phylogenetic analysis revealed a close relationship between these genes and the Arabidopsis LecRLK genes. The RNA-sequencing results showed that this gene family is positively regulated by NAD + treatment and pathogen inoculation in squash. Taken together, the results from this study show that NAD + has significant effects on reducing powdery mildew disease severity and imply the potential role of the LecRLK gene family in disease resistance to powdery mildew in squash. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .