Brassinosteroids modulate the polyamines profile in enhancing resistance to manganese toxicity in Malus robusta Rehd.

Abstract Background and aims Manganese (Mn) toxicity is a widely spread phenomenon in soils, seriously restricting crop growth, quality and yield. However, mechanisms towards BRs in regulating PAs-enhanced Mn stress are unknown. This study aims to explore the effects of BRs on Mn resistance of apple seedlings by regulating PAs profile. Methods The effects of 2,4-epibrassinolide (EBL) on the expression of genes involved in BR signal pathway, Mn accumulation, polyamines-mediated responses (precursors, content, metabolic enzymes and genes) as well as growth parameters in Mn-stressed Malus robusta Rehd. were assessed. Results EBL application significantly modulated BR signaling genes (MdBRI, MdBSK, etc.) expression and reduced Mn accumulation, accompanied with strengthened growth rate of root length and plant height, RWC, chlorophyll content, Fv/Fm, ΦPSII, and decreased electrical conductivity. Meanwhile, it weakened Orn pathway, decreased ODC activity and increased biosynthesis from Put to Spm via elevating the PAO activity and expressions of MdSPDS, MdSPMS and MdPAO. ADC and DAO activities and MdDAO gene expression failed to restrain the loss of Put. The trends for free, PS-conjugated and PIS-bound PAs were similar to that of total PAs, except that no significant change was observed in free Spm, PS-conjugated Spd and Spm as well as PIS-bound Spd groups. As a result, EBL significantly reduced Put accumulation and enhanced Spm content and (Spd + Spm)/Put ratio. Conclusions This study contributes to clarify the utility of BR-induced PAs response as an important mitigative measure against Mn toxicity in fruit trees.