Abstract
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.
Highlights
Brassinosteroids are phytohormones involved in cell elongation and expansion, in pollen and stamen development (Clouse and Sasse, 1998; Zhu et al, 2015), as well as in fruit ripening (Symons et al, 2006)
HIGHLIGHTS - Behavior of transgenic tobacco plants with up- or down-regulated membrane steroid binding protein 1 (MSBP1) expression is consistent with an inhibitory role of MSBP1 in BR signaling. - We show for the first time that brassinosteroids per se positively affect the symbiosis of tobacco and tomato plants with an arbuscular mycorrhizal fungus. - MSBP1 expression, and brassinolide treatment affects arbuscular morphology as well as host plant growth responses
The first hypothesis of the current study stated that MSBP1 is an inhibitor of brassinosteroid (BR) signaling in tobacco as previously postulated for AtMSBP1 in Arabidopsis thaliana (Song et al, 2009)
Summary
Brassinosteroids are phytohormones involved in cell elongation and expansion, in pollen and stamen development (Clouse and Sasse, 1998; Zhu et al, 2015), as well as in fruit ripening (Symons et al, 2006). The physiological role of brassinosteroids (BRs) in plant development, formation of the vascular system and pollen maturation has been investigated intensively during the past few years. A role in plant defense against pathogens could be shown (Zhang et al, 2015). It is still unclear whether the inhibitory role of BRs in defense responses against plant pathogens is effective in other biotic interactions such as mycorrhizal symbioses. Epibrassinolide treatment in wheat and rice has slight synergistic effects on mycorrhization under salt stress conditions (Tofighi et al, 2017). Tomato dx mutants defective in BR biosynthesis showed decreased mycorrhization (Bitterlich et al, 2014a,b). BR-deficient lkb pea mutants, did not show changes in root colonization (Foo et al, 2013) suggesting speciesdependent differences
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