The effects of gibberellin on the expression of symbiosis-related genes in Paris-type arbuscular mycorrhizal symbiosis in Eustoma grandiflorum

Abstract

Arbuscular mycorrhiza (AM) is a symbiotic interaction in terrestrial plants that is colonized by fungi in the Glomeromycotina. The morphological types of AM, including the Arum-type and Paris-type, are distinct, depending on the host plant species. A part of the regulatory pathways in Arum-type AM symbiosis has been revealed because most model plants form the Arum-type AM with a model AM fungus, Rhizophagus irregularis. Moreover, gibberellin (GA) is known to severely inhibit AM fungal colonization in Arum-type AM symbiosis. Recently, we showed that exogenous GA treatment significantly promoted AM fungal colonization in Paris-type AM symbiosis in Eustoma grandiflorum. In this study, we focused on the transcriptional changes in AM symbiosis-related genes in GA-treated E. grandiflorum. The expression levels of all examined E. grandiflorum genes were maintained or increased by GA treatment compared with those of the control treatment. Our new results suggest that signaling pathway(s) required for establishing AM symbiosis in E. grandiflorum may be distinct from the well-characterized pathway for that in model plants.

Keywords: Eustoma grandiflorum; Paris-type; Arbuscular mycorrhizal symbiosis; Gibberellin.

Figure 1.

Expression levels of symbiosis-related genes in gibberellin-treated Eustoma grandiflorum roots. Quantitative RT-PCR was conducted to reveal the regulatory mechanism of Paris-type AM symbiosis in E. grandiflorum colonized by Rhizophagus irregularis (at 4 weeks post-inoculation). In this analysis, E. grandiflorum seedlings were treated with 0.01% ethanol as control treatment (NC: non-colonized roots, AM: colonized roots), 1 μM GA₃ (GA–AM), and 1 μM Uniconazole-P (Uni–AM). R. irregularis spores were also inoculated in the soil at 1000 spores per soil container. (a) The expression levels of E. grandiflorum orthologues of RAM1, RAD1, PT4, STR/STR2, Vpy. The inset graphs are added due to the relatively low expression levels of EgRAM1, EgRAD1, EgSTR, EgSTR2, and EgVpy. (b) The same analysis as (A) was performed to investigate the expression level of DLK2 (DWARF LIKE-2) in the conditions. The expression levels of these genes were normalized by that of EgACT. Mean values ± SDs were calculated for each samples (n = 3). This experiment was conducted at 3 times, and the representative data is shown here. Different alphabets indicate significant differences using Tukey–Kramer test (P < .05) (Red alphabets: P < .1).

Figure 2.

Proposed signaling pathway(s) underlying Paris-type AM symbiosis in Eustoma grandiflorum. In Arum-type AM symbiosis in model plants, the presence of bioactive gibberellins (GAs) triggers the degradation of DELLA protein, which suppresses the expression of AM symbiosis-related genes. In contrast, the exogenous GA treatment may somehow promote EgRAD1 expression (Fig. 1A), which supposes the existence of other protein X. If so, the protein would directly/indirectly regulate the expression of EgRAD1, which may result in the promotion of AM symbiosis-related genes’ expression (see the text for details). In addition, crosstalk between GA and other phytohormones might be included in the establishment of AM symbiosis in E. grandiflorum. The dotted lines indicate possible pathways regulating AM symbiosis in E. grandiflorum.