Relation betweenship strigolactones and branching in strawberry

Abstract:【Objective】Strigolactones (SLs) are novel plant hormones that regulate various aspects of plant architecture and development, such as shoot branching or tillering and root morphology forming.SLs are bio-synthetically derived from the carotenoids, and several genes have been reported to involve in the synthesis and signal transduction pathway. However, the mechanism of the regulation of branching by SLs is poorly understood in strawberry. Therefore, the aims of this study were to quantify the SLs in roots and stem of strawberry, explore the mechanism controlling strawberry branches at different developmental stages and detect the expression of the relative genesinvoled in the SLs synthesis and signal transduction pathways.【Methods】Two varieties of strawberry‘Benihoppe'and‘C5'characterized by different branching features were used. Standard conventional field practices (including fertilization, irrigation, and pest control) were carried out during the growing season. Branches, inflorescence, number of fruits and yield of twenty different single plants for each cultivar were investigated. A sensitive quantification assay, ultra-fast liquid chromatography-electrospray ionization-tandem mass spectrometry (UFLC-ESI-MS/MS) was used to quantify SLs in roots and stems at three branching developmental stages namely 15 DAP (days after planting) (S-1) , 30 DAP (S-2) , and 60 DAP (S-3) . The sample preparation mainly included hormone extraction, solid phase extraction and reconstitution. Then, the prepared samples were separated on a Waters ACQUITY UPLC T3 column (100 mm × 2.1 mm, 1.7 μm) with mobile phase of 0.1% formic acid aqueous solution and 0.1% formic acid acetonitrile solution. The flow rate was set to 0.4 mL · min-1 and the injection volume was 3 μL. Tandem MS analyses were carried out on in positive and negative ESI mode with multiple reaction monitoring (MRM) at unit resolution.Each UFLC-ESI-MS/MS assay was repeated thrice. Based on reported genes in other crops related to the synthesis and signal transduction of SLs, the sequence of seven homologous genes (FaD27, FaCDD7, FaCDD8, FaP450, FaD14, FaD53 and FaD3) were blasted in strawberry genome database.Total RNA was extracted using E.Z.N.A kit (Omega) according to the user manual, and the semi-quantitative PCR method was used to analyze the expression levels of those genes in roots and stems at three branching developmental stages of‘Benihoppe'and‘C5'.【Results】There was significant difference in yield-related traits between‘Benihoppe'and‘C5'. The number of branches, inflorescence, fruit numbers, and yield of‘C5'were 2.8, 2.6, 1.7, and 2.1 folds higher than those of‘Benihoppe', respectively.UFLC-ESI-MS/MS detected three derivatives of SLs, including 5-deoxystrigol, strigol and orobanchol.In the roots and the stems of the two cultivars, the content of 5-deoxystrigol and strigol was high, while the content of orobanchol was very low. They all had different trends in roots and new stems of‘Benihoppe'and‘C5'. The trend of 5-deoxystrigol in the roots of‘C5'was similar to the trend in the stems of‘Benihoppe'and‘C5', interestingly, both cultivars had the lowest content of 5-deoxystrigol at S-2 stage. The content of 5-deoxystrigol in the roots of‘Benihoppe'gradually increased with branches development. The content of strigol in‘Benihoppe'was different from that of‘C5'. Both the roots and the stems of‘Benihoppe'had the lowest strigol content at S-2 stage, while the trend of strigol in the roots and the stems of‘C5'was opposite. The strigol content was down-regulated in the roots and upregulated in the stems of‘C5'. The content of orobanchol was higher in the stems and the roots of‘Benihoppe'at S-2 stage, which was contrary to the content of strigol in both the roots and the stem of‘Benihoppe'. The content of orobanchol in the roots and stems of‘C5'was not consistent with branches development. Considering the relationship between the total amount of SLs and the development of branches, SLs had the similar trend in the stems of‘Benihoppe'and‘C5', but the trend was opposite in the roots between‘Benihoppe'and‘C5'. The opposite trend of SLs in the roots between‘Benihoppe'and‘C5'might influence the formation of branches in these two cultivars. The semi-quantitative PCR results of genes involved in the synthesis and the signal transduction of SLs demonstrated that the expression of the genes was higher at S-3 stage. FaD27 was up regulated with development and had a similar trend with the content of SLs in the roots and the stem of‘Benihoppe', which caused the lower number of branches in‘Benihoppe'.【Conclusion】‘Benihoppe'had lower yield and fewer branches, and‘C5'had higher yield and more branches. The number of new stem branches was positively correlated with the yield in both cultivars. Results indicated that branching features or number of branches were dependent on the content of SL. The lower number of branches in‘Benihoppe'or higher number of branches in‘C5'was directly associated with the increase or decrease of the content of SLs in the roots respectively. The high expression level of FaD27 gene might be involved in the higher content of SLs in‘Benihoppe'. This study would provide some references for the mechanism of SLs in the regulation of new stem branching in strawberry.