Interaction of brassinosteroid functions and sucrose transporter SlSUT2 regulate the formation of arbuscular mycorrhiza

Abstract

Transgenic tomato plants with reduced expression of the sucrose transporter SlSUT2 showed higher efficiency of mycorrhization suggesting a sucrose retrieval function of SlSUT2 from the peri-arbuscular space back into the cell cytoplasm plant cytoplasm thereby limiting mycorrhiza fungal development. Sucrose uptake in colonized root cells requires efficient plasma membrane-targeting of SlSUT2 which is often retained intracellularly in vacuolar vesicles. Protein-protein interaction studies suggested a link between SISUT2 function and components of brassinosteroid biosynthesis and signaling. Indeed, the tomato DWARF mutant d(x) defective in BR synthesis (1) showed significantly reduced mycorrhization parameters. (2) The question has been raised whether the impact of brassinosteroids on mycorrhization is a general phenomenon. Here, we include a rice mutant defective in DIM1/DWARF1 involved in BR biosynthesis to investigate the effects on mycorrhization. A model is presented where brassinolides are able to impact mycorrhization by activating SUT2 internalization and inhibiting its role in sucrose retrieval.

Keywords: AM, arbuscular mycorrhiza; BR, brassinosteroids; DIM, diminuto; DRM, detergent resistant membrane; GO, gene ontology; LRR, leucine-rich repeat; MSBP, membrane steroid binding protein; Oryza sativa; PCR, polymerase chain reaction; RNA, ribonucleic acid; Rhizophagus irregularis; SNARE, soluble N-ethylmaleimide-sensitive-factor attachment receptor; SUC, sucrose carrier; SUT, sucrose transporter; arbuscular mycorrhiza; brassinosteroid; membrane trafficking; protein-protein interactions; sucrose transport.

Figure 1.

Mycorrhizal parameters of Oryza sativa Nipponbare wild type rice plants compared to brd2-1 mutant plants defective in BR biosynthesis 6 weeks after inoculation with Rhizophagus irregularis. F%: infection frequency, M%: absolute mycorrhizal colonization, m% relative mycorrhizal colonization, A% absolute arbuscule abundance, a% relative arbuscular abundance. Shown are mean values and standard deviations. Significant differences between WT and brd2-1 mutant plants are indicated by asterisks (Students-t-test, P ≤ 0.05; n = 4-6).

Figure 2.

Quantitative real time PCR analysis of transcript levels of the DIM/DWARF1- (Os10g0397400) and OsSUT4-encoding (Q6YK44) genes relative to ubiquitin gene (XR_423446) transcript levels in Nipponbare wild type rice plants compared to the BR-synthesis defective brd2-1 mutant. Wild type plants were non-mycorrhizal (co) or inoculated with the AM fungi R. irregularis (myc). Shown are mean values and standard deviations. Significant differences between WT and brd2-1 mycorrhizal plants are indicated by asterisks (Students-t-test, P ≤ 0.05; n = 3-5).

Figure 3.

Hypothetical model describing the involvement of SlSUT2-interacting proteins in the subcellular localization of the sucrose transporter. SlSUT2 interacts with MSBP1, the LRR-receptor kinase BAK1-like and DIM1 that is associated to membrane microdomains. Endocytosis inhibits SlSUT2 transport activity, whereas recycling to the plasma membrane potentially via SNARE proteins is required for activation and sucrose retrieval from the periarbuscular space away from the AM fungus via SlSUT2. Brassinolide is assumed to affect mycorrhization via enabling MSBP1-SUT2 interaction for internalisation of SUT2 and inactivation its sucrose retrieval function.