The MED7 subunit paralogs of Mediator function redundantly in development of etiolated seedlings in Arabidopsis
Plant J. 2018, 96(3):578-594

Kumar KRR, Blomberg J, Björklund S

Abstract
MED7 is a subunit of the Mediator middle module and is encoded by two paralogs in Arabidopsis. We generated MED7 silenced lines using RNAi to study its impact on Arabidopsis growth and development. Compared to wild type, etiolated seedlings of the MED7 silenced lines exhibit reduced hypocotyl length caused by reduced cell elongation when grown in dark. The hypocotyl length phenotype was rescued by exogenously supplied brassinosteroid. In addition, MED7 silenced seedlings exhibit defective hook opening in dark as well as defective cotyledon expansion in the presence of the brassinosteroid inhibitor brassinazole. Whole transcriptome analysis on etiolated seedlings using RNA sequencing revealed several genes known to be regulated by auxin and brassinosteroids, and a broad range of cell wall‐related genes being differentially regulated in the MED7 silenced lines. This was especially evident for genes involved in cell wall extension and remodeling, like EXPANSINs and XTHs. Conditional complementation with each MED7 paralog individually restored the hypocotyl phenotype as well as the gene expression defects. Additionally, conditional expression of MED7 had no effects independent of the Mediator complex on the observed phenotypes. We conclude that the MED7 paralogs function redundantly in regulating genes required for normal development of etiolated seedlings in Arabidopsis.

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