Abstract
The plant mediator is a highly conserved protein complex that interacts with transcription factors (TFs) and RNA polymerase II (RNAP II) to relay regulatory information during transcription. Plant immune response is one of the biological processes that is orchestrated by this regulatory mechanism. Brassica napus, an important oil crop, is severely attacked by a devastating disease Sclerotinia stem rot. Here, we explored broad-spectrum disease resistant roles of B. napus mediator subunit 16 (BnMED16) and its host defense mechanism against fugal pathogen Sclerotinia sclerotiorum. We found that BnMED16 expression was significantly increased by S. sclerotiorum infection, and its homologous overexpression resulted in rapid and comprehensive defense responses from the beginning to the end. This affected signal transduction with multiple channels including pathogen recognition, intracellular Ca2+ concentration, reactive oxygen species (ROS) accumulation and clearance, and activation of mitogen-activated protein kinase (MAPK) signaling cascades initially. Subsequently, pathogen-/defense-related genes and hormone-responsive pathways were highly activated, which resulted in enhanced cell wall and secretion of defense proteases. Furthermore, the biochemical analysis showed that BnMED16 interacts with BnMED25 and BnWRKY33. Additionally, BnMED25 also interacts with TFs BnMYC2, BnCOI1, and BnEIN3 of the JA/ET signal transduction pathway. Taken together, we proposed a hypothetical model that BnMED16 confers S. sclerotiorum resistance by enhancing BnMED25-mediated JA/ET defense pathways and BnWRKY33-activated defense signaling in B. napus. The BnMED16 overexpressing lines with enhanced broad-spectrum disease resistance could be useful for breeding Sclerotinia-resistant oilseed rape varieties, as well as serving as basis for further strategy development in resistance breeding.
Highlights
Oilseed rape (Brassica napus L.) is a major oil crop in the world
We further discovered that B. napus mediator subunit 16 (BnMED16) interacted with BnMED25 and BnWRKY33, while BnMED25 associates with jasmonic acid/ethylene (JA/ET) transcription factors (TFs) MYC2, CORONA TINE INSENSITIVE 1 (COI1), and ETHYLENE INSENSITIVE3 (EIN3)
Our results indicated that BnMED16 is a key regulator of both BnMED25mediated JA/ET defense pathways and the BnWRKY33-activated defense signaling in B. napus
Summary
It is prone to a devastating disease sclerotinia stem rot (SSR), which is caused by Sclerotinia sclerotiorum (Lib.) de Bary, a necrotrophic ascomycete (Bolton et al, 2006) This fungus has a broad host range on at least 408 known plant species of 278 genera in 75 families (Boland and Hall, 1994), causing serious crop losses worldwide (Derbyshire and Denton-Giles, 2016). When plants subjected to pathogens, transcriptional reprogramming signals were produced to shift plant normal growth to disease resistance before basal resistance is activated, and folds changes of the transcriptional signals would determine the resistance to disease (Maleck et al, 2000; Katagiri, 2004; Wu et al, 2016) In this process, the mediator complex (MED) links TFs and RNA polymerase II (RNAP II), and helps the TFs to target on gene promoters (Kidd et al, 2011; Moore et al, 2011). Since MEDs constitute the basis of transcriptional activation or inhibition in specific signal pathways, they are involved in various vital biology activities includes plant growth and development, response to biotic and abiotic stresses, and a variety of intracellular life activities (Kidd et al, 2011)
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