Nitric oxide is involved in brassinosteroid-induced alternative respiratory pathway in Nicotiana benthamiana seedlings' response to salt stress.

Recent studies reported that brassinosteroids (BRs) can induce plant tolerance to different environmental stresses via the nitric oxide (NO) signaling pathway. Previous reports have indicated that alternative oxidase (AOX) plays an important role in plants under various stresses. The mechanisms governing how NO is involved as a signal molecule which connects BR with AOX in regulating stress tolerance are still unknown. Recently, we found that Nicotiana benthamiana seedlings which were pretreated with BR have more tolerance to salt stress, accompanied with an increase of CN-resistant respiration. Our results suggested that pretreatment with 0.1 μM brassinolide (BL, the most active brassinosteroid) alleviated salt-induced oxidative damage and increased the NbAOX1 transcript level. Application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-1-oxyl-3-oxide (cPTIO, an NO scavenger) or virus-induced gene silencing of nitrate reductase (NR) and nitric oxide synthase (NOS)-like enzyme compromised the BRs-induced alternative respiratory pathway. Furthermore, pretreatment with specific chemical inhibitors of NR and NOS or gene silencing experiments decreased plant resistance to salt stress which also compromised BRs-induced salt stress tolerance. In conclusion, NO is involved in BRs-induced AOX capability which plays essential roles in salt tolerance in N. benthamiana seedlings.