Brassinosteroids and sucrose transport in mycorrhizal tomato plants.

Silencing of SlSUT2 expression in tomato plants leads to a dwarfed phenotype, reduced pollen vitality and reduces pollen germination rate. Male sterility of flowers, together with a dwarfed growth behavior is reminiscent to brassinosteroid defective mutant plants. Therefore we aimed to rescue the SlSUT2 silencing phenotype by local brassinosteroid application. The phenotypical effects of SlSUT2 down-regulation could partially be rescued by epi-brassinolide treatment suggesting that SlSUT2 interconnects sucrose partitioning with brassinosteroid signaling. We previously showed that SlSUT2 silenced plants show increased mycorrhization and, this effect was explained by a putative sucrose retrieval function of SlSUT2 at the periarbuscular membrane. More recently, we reported that the symbiotic interaction between Solanaceous hosts and AM fungi is directly affected by watering the roots with epi-brassinolide. Here we show that the SlSUT2 effects on mycorrhiza are not only based on the putative sucrose retrieval function of SlSUT2 at the periarbuscular membrane. Our analyses argue that brassinosteroids as well as SlSUT2 per se can impact the arbuscular morphology/architecture and thereby affect the efficiency of nutrient exchange between both symbionts and the mycorrhizal growth benefit for the plant.

Effects of Jasmonate on Ethylene Function during the Development of Tomato Stamens.

The phenotype of the tomato mutant jasmonate-insensitive1-1 (jai1-1) mutated in the JA-Ile co-receptor COI1 demonstrates JA function in flower development, since it is female-sterile. In addition, jai1-1 exhibits a premature anther dehydration and pollen release, being in contrast to a delayed anther dehiscence in the JA-insensitive Arabidopsis mutant coi1-1. The double mutant jai1-1 Never ripe (jai1-1 Nr), which is in addition insensitive to ethylene (ET), showed a rescue of the jai1-1 phenotype regarding pollen release. This suggests that JA inhibits a premature rise in ET to prevent premature stamen desiccation. To elucidate the interplay of JA and ET in more detail, stamen development in jai1-1 Nr was compared to wild type, jai1-1 and Nr regarding water content, pollen vitality, hormone levels, and accumulation of phenylpropanoids and transcripts encoding known JA- and ET-regulated genes. For the latter, RT-qPCR based on nanofluidic arrays was employed. The data showed that additional prominent phenotypic features of jai1-1, such as diminished water content and pollen vitality, and accumulation of phenylpropanoids were at least partially rescued by the ET-insensitivity. Hormone levels and accumulation of transcripts were not affected. The data revealed that strictly JA-regulated processes cannot be rescued by ET-insensitivity, thereby emphasizing a rather minor role of ET in JA-regulated stamen development.

Brassinosteroids Affect the Symbiosis Between the AM Fungus Rhizoglomus irregularis and Solanaceous Host Plants.

Together with several proteins involved in brassinosteroid (BR) signaling and synthesis, the membrane steroid binding protein 1 (MSBP1) was identified within the interactome of the sucrose transporter of tomato (SlSUT2). We asked whether MSBP1 is also involved in BR signaling as assumed for the AtMSBP1 protein from Arabidopsis and whether it impacts root colonization with arbuscular mycorrhizal (AM) fungi in a similar way as shown previously for SlSUT2. In addition, we asked whether brassinosteroids per se affect efficiency of root colonization by AM fungi. We carried out a set of experiments with transgenic tobacco plants with increased and decreased MSBP1 expression levels. We investigated the plant and the mycorrhizal phenotype of these transgenic plants and tested the involvement of MSBP1 in BR metabolism by application of epi-brassinolide and brassinazole, an inhibitor of BR biosynthesis. We show that the phenotype of the transgenic tobacco plants with increased or reduced MSBP1 expression is consistent with an inhibitory role of MSBP1 in BR signaling. MSBP1 overexpression could be mimicked by brassinazole treatment. Interestingly, manipulation of MSBP1 expression in transgenic tobacco plants not only affected plant growth and development, but also the host plant responses toward colonization with AM fungi, as well as arbuscular architecture. Moreover, we observed that brassinosteroids indeed have a direct impact on the nutrient exchange in AM symbiosis and on the biomass production of colonized host plants. Furthermore, arbuscular morphology is affected by changes in MSBP1 expression and brassinolide or brassinazole treatments. We conclude that host plant growth responses and nutrient exchange within the symbiosis with AM fungi is controlled by brassinosteroids and might be impeded by the MSBP1 protein.