Fusarium oxysporum NAD+ hydrolase FonNADase1 is essential for pathogenicity and inhibits plant immune responses

Abstract

Plants use nicotinamide adenine dinucleotide (NAD⁺) as a key signaling molecule to activate immune responses. However, whether pathogens secrete specific NAD⁺ hydrolases (NADases) to affect plant NAD⁺ levels for infection remains unclear. Here, we report the function and possible mechanism of fungal NADases in watermelon Fusarium wilt fungus Fusarium oxysporum f. sp. niveum (Fon) pathogenicity. Fon secretes two NADases, FonNADase1 and FonNADase2, both of which harbor a secretory signal peptide and an NADase-active tuberculosis necrotizing toxin (TNT) domain. FonNADase1 and FonNADase2 are not involved in the growth, development, or stress responses of Fon. Moreover, only FonNADase1 is essential for Fon pathogenicity, and FonNADase1 deletion results in decreased invasive growth and spread within watermelon plants. FonNADase1 and FonNADase2 are functional NADases capable of decreasing plant NAD⁺ levels and FonNADase1 inhibits INF1- and BAX-induced cell death and chitin-triggered immune responses in Nicotiana benthamiana leaves in an NADase activity-dependent manner. Furthermore, FonNADase1 inhibited INF1- and BAX-induced expression of defense genes, such as NbPR1a, NbPR2, NbLOX, NbERF1, NbHIN1, and NbHSR203J, in N. benthamiana leaves and affected the expression of a set of immunity-associated genes in watermelon plants. These findings suggest that FonNADase1 plays a key role in Fon pathogenicity by affecting fungal invasive growth and spread within plants as well as modulating host immune responses, thus highlighting the critical role of fungal NADases in pathogenicity.