HrCYP90B1 modulating brassinosteroid biosynthesis in sea buckthorn (Hippophae rhamnoides L.) against fruit fly (Rhagoletis batava obseuriosa Kol.) infection.

Sea buckthorn is an important ecological and economic tree species, and its berries have been severely damaged by sea buckthorn fruit fly, Rhagoletis batava obseuriosa Kol. (Diptera: Tephritidae) (RBO). Brassinosteroid (BR) is widely involved in stress tolerance of plant. However, limited knowledge exists regarding the molecular mechanisms underlying insect resistance. Here, we found that BR content was much higher in sea buckthorn fruits with RBO infection than non-infection, and the damage rates of fruit with BR treatment were significantly lower than that of non-treatment. It indicated that BR could enhance RBO resistance in sea buckthorn. Several BR biosynthesis-related HrCYPs genes (CYP85A1/85A2/90A1/90B1/90C1/90D1/92A6/724B/734A1) were obtained and identified based on transcriptome analysis, of which the most up-regulated gene in fruits was HrCYP90B1 under RBO and mechanical damage. Overexpression of HrCYP90B1 in Arabidopsis thaliana showed BR and salicylic acid (SA) content was significantly increased, and the substrate campesterol (CR) of HrCYP90B1 content decreased. Further studies revealed that silencing HrCYP90B1 by virus-induced gene silencing resulted in decrease of BR, SA and defense-related enzymes contents, and increase of CR content. Silencing HrCYP90B1 also caused suppression of SA and activation of jasmonic acid pathways, enabling enhanced RBO susceptibility and more damage of fruits. Taken together, we obtained evidence that HrCYP90B1 was a positive regulator in RBO resistance improvement in sea buckthorn, which will provide comprehensive insights into the tree defense system of sea buckthorn to pest infection.