Abstract
Plant steroid hormones, brassinosteroids (BRs), play essential roles in plant growth, development and stress responses. However, mechanisms by which BRs interfere with plant resistance to virus remain largely unclear. In this study, we used pharmacological and genetic approaches in combination with infection experiments to investigate the role of BRs in plant defense against Tobacco Mosaic Virus (TMV) in Nicotiana benthamiana. Exogenous applied BRs enhanced plant resistance to virus infection, while application of Bikinin (inhibitor of glycogen synthase kinase-3), which activated BR signaling, increased virus susceptibility. Silencing of NbBRI1 and NbBSK1 blocked BR-induced TMV resistance, and silencing of NbBES1/BZR1 blocked Bikinin-reduced TMV resistance. Silencing of NbMEK2, NbSIPK and NbRBOHB all compromised BR-induced virus resistance and defense-associated genes expression. Furthermore, we found MEK2-SIPK cascade activated while BES1/BZR1 inhibited RBOHB-dependent ROS production, defense gene expression and virus resistance induced by BRs. Thus, our results revealed BR signaling had two opposite effects on viral defense response. On the one hand, BRs enhanced virus resistance through MEK2-SIPK cascade and RBOHB-dependent ROS burst. On the other hand, BES1/BZR1 inhibited RBOHB-dependent ROS production and acted as an important mediator of the trade-off between growth and immunity in BR signaling.
Highlights
BRI1 EMS SUPPRESSOR 1 (BES1) and BRASSINAZOLE RESISTANT 1 (BZR1), acting as major regulators of BR-induced transcriptional changes, which become active[11,12]
Our results showed that transient expression of NbSIPK substantially increased H2O2 accumulation in Tobacco rattle virus (TRV):[00] and TRV:NbSIPK N. benthamiana plants infected with Tobacco mosaic virus (TMV)-green fluorescent protein (GFP), but not in TRV:NbRBOHB plants as compared with control plants (Fig. 7a)
Through the well-established TRV-based virus-induced gene silencing (VIGS) approach, we reveal that the BR
Summary
BRI1 EMS SUPPRESSOR 1 (BES1) and BRASSINAZOLE RESISTANT 1 (BZR1), acting as major regulators of BR-induced transcriptional changes, which become active[11,12]. Several recent studies reveal that BRs are involved in bacterial defense response[18,19,20] It is unclear at the moment how BR signaling fit into virus resistance in plants. Similar to WIPK and SIPK, virus-induced gene silencing (VIGS) of several other MAPK components NPK1 (MAPKKK), MEK1 (MAPKK), or NTF6 (MAPK) attenuate N gene- and Pto-mediated resistance against TMV26,27, indicating that the NPK1-MEK1-NTF6 pathway is another MAPK cascade involved in TMV resistance. These studies indicated that at least two MAPK cascades participated in disease resistance in tobacco plants. We showed that MEK2-SIPK cascade induced by BRs mediated RBOHB-dependent oxidative burst in N. benthamiana plants response to TMV
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