Brassinosteroids Benefit Plants Performance by Augmenting Arbuscular Mycorrhizal Symbiosis

Abstract

Arbuscular mycorrhizal (AM) play an important role in improving plant growth and development. The interaction between phytohormones and AM symbiosis is gradually revealed. Here we examined the effect of Brassinosteroids (BR) on AM symbiosis and discussed the synergistic promotion of plant growth by BR and AM symbiosis. The xylophyta Eucalyptus grandis Hill (E. grandis) was inoculated with AM fungi Rhizoglomus irregularis R197198 (R. irregularis) and treated with different concentrations (0, 1, 10, and 100 nM) of 24-epibrassinolide (24-epiBL) for 6 weeks. With the increase of 24-epiBL concentration, E. grandis growth was firstly promoted and then inhibited, but inoculation with AM fungi alleviated this inhibition. 24-epiBL and R. irregularis colonization significantly improved E. grandis growth and antioxidant system response, and the synergistic effect was the best. Compared with the control group, 24-epiBL treatment significantly increased the mycorrhizal colonization and arbuscular abundance of AM fungi R. irregular in E. grandis roots. The expression of AM symbiosis maker genes was significantly increased by 24-epiBL treatment. Both 24-epiBL treatment and AM colonization upregulated gibberellins (GA) synthesis genes, but no inhibition caused by GA levels was found. 24-epiBL is a kind of synthetic highly active BR. Based on the results of 24-epiBL treatment, we hypothesized that BR actively regulates AM symbiosis regulates AM symbiosis without affecting GA-INSENSITIVE DWARF1 (GID1)-DELLA expression. The synergistic treatment of BR and AM symbiosis can significantly promote the growth and development of plants. IMPORTANCE Brassinosteroids (BR) and Arbuscular mycorrhizas (AM) symbiosis play an important role in improving plant growth and development. Previous studies have shown that there is a complex regulatory network between phytohormones and AM symbiosis. However, the interactions of BR-signaling and AM symbiosis are still poorly understood. Our results suggest that BR actively regulates the colonization and development of AM fungi, and AM fungal colonization can alleviate the inhibition of plant growth caused by excessive BR. In addition, BR actively regulates AM symbiosis, but does not primarily mediate gibberellins-DELLA interaction. The synergistic treatment of BR and AM symbiosis can significantly promote the growth and development of plants. The conclusions of this study provide a reference for phytohormones-AM symbiosis interaction.

Keywords: Arbuscular mycorrhizal; Brassinosteroids; symbiosis; xylophyta.

FIG 1

Effects of AM fungi R. irregularis colonization and 24-epiBL application on the growth of E. grandis. 30-day-old E. grandis seedlings were inoculated (AM) and not inoculated (NM) with AM fungi R. irregularis, and then treated with different concentrations of 24-epiBL (1, 10 and 100 nM). After 6 weeks of treatment, the whole E. grandis plant sample was harvested, and the measurement results of its roots fresh weight (A), shoots fresh weight (B), tap roots length (C) and plant height (D). (E) Shoot growth performance of AM and NM E. grandis plants after 6 weeks of treatment. The data are shown as the means ± SE of six biological replicates (n = 6). Different letters indicate significant differences at P < 0.05, according to Duncan's new multiple range test.

FIG 2

Effects of application of different concentrations of 24-epiBL (1, 10 and 100 nM) on the colonization of AM fungi R. irregularis in E. grandis roots. Total mycorrhizal frequency (A), mycorrhizal intensity (B), and arbuscule abundance (C) in the R. irregularis colonized roots estimated after WGA488 staining. The data are shown as the means ± SE of three biological replicates (n = 3). Different letters indicate significant differences at P < 0.05, according to Duncan's new multiple range test.

FIG 3

Responses of E. grandis roots antioxidant system to AM fungi R. irregularis colonization and 24-epiBL application. Analysis of CAT (A), POD (B), SOD (C), MDA (D), H2O2 (E), and OFR (F) content in R. irregularis roots after 6 weeks of treatment (R. irregularis colonization and 24-epiBL application). The data are shown as the means ± SE of three biological replicates (n = 3). Different letters indicate significant differences at P < 0.05, according to Duncan's new multiple range test.

FIG 4

Effects of 24-epiBL treatment on gene expression related to BR signaling, AM symbiosis and GA signaling in E. grandis roots. Total RNA was isolated from AM and NM E. grandis roots. The EgUBI3 (Ubiquitin 3) gene from E. grandis was used as the housekeeping gene for normalization. Expression of the BR-related gene (EgBRI1, EgBAK1 and EgCYP85A1) (A–C), the GA-related gene (EgDELLA, EgGID1 and EgGA20ox1) (D–F), and the AM fungal-induced gene (EgPT4, EgPT5 and EgRAM1) (G–I) in plant roots after 6 weeks of BR treatment. The data are shown as the means ± SE of three biological replicates (n = 3). Different letters indicate significant differences at P < 0.05, according to Duncan's new multiple range test.