Brassinosteroids (BRs) play essential roles in modulating plant growth, development and stress responses. Here, involvement of BRs in plant systemic resistance to virus was studied. Treatment of local leaves inNicotiana benthamianawith BRs induced virus resistance in upper untreated leaves, accompanied by accumulations of H2O2 and NO. Scavenging of H2O2 or NO in upper leaves blocked BR-induced systemic virus resistance. BR-induced systemic H2O2 accumulation was blocked by local pharmacological inhibition of NADPH oxidase or silencing of respiratory burst oxidase homolog geneNbRBOHB, but not by systemic NADPH oxidase inhibition orNbRBOHAsilencing. Silencing of the nitrite-dependent nitrate reductase geneNbNRor systemic pharmacological inhibition of NR compromised BR-triggered systemic NO accumulation, while local inhibition of NR, silencing ofNbNOA1and inhibition of NOS had little effect. Moreover, we provide evidence that BR-activated H2O2 is required for NO synthesis. Pharmacological scavenging or genetic inhibiting of H2O2 generation blocked BR-induced systemic NO production, but BR-induced H2O2 production was not sensitive to NO scavengers or silencing ofNbNR. Systemically applied sodium nitroprusside rescued BR-induced systemic virus defense inNbRBOHB-silenced plants, but H2O2 did not reverse the effect ofNbNRsilencing on BR-induced systemic virus resistance. Finally, we demonstrate that the receptor kinase BRI1(BR insensitive 1) is an upstream component in BR-mediated systemic defense signaling, as silencing ofNbBRI1compromised the BR-induced H2O2 and NO production associated with systemic virus resistance. Together, our pharmacological and genetic data suggest the existence of a signaling pathway leading to BR-mediated systemic virus resistance that involves local Respiratory Burst Oxidase Homolog B (RBOHB)-dependent H2O2 production and subsequent systemic NR-dependent NO generation.Significance Statement
Through pharmacological and genetic approaches in combination with infection experiments, we established a signaling pathway leading to BR-mediated systemic defense response that involved local RBOHB-dependent H2O2 production and subsequent systemic NR-dependent NO generation.