The MEDIATOR genes MED12 and MED13 control Arabidopsis root system configuration influencing sugar and auxin responses

Abstract

Arabidopsis med12 and med13 mutants exhibit shoot and root phenotypes related to an altered auxin homeostasis. Sucrose supplementation reactivates both cell division and elongation in primary roots as well as auxin-responsive and stem cell niche gene expression in these mutants. An analysis of primary root growth of WT, med12, aux1-7 and med12 aux1 single and double mutants in response to sucrose and/or N-1-naphthylphthalamic acid (NPA) placed MED12 upstream of auxin transport for the sugar modulation of root growth. The MEDIATOR (MED) complex plays diverse functions in plant development, hormone signaling and biotic and abiotic stress tolerance through coordination of transcription. Here, we performed genetic, developmental, molecular and pharmacological analyses to characterize the role of MED12 and MED13 on the configuration of root architecture and its relationship with auxin and sugar responses. Arabidopsis med12 and med13 single mutants exhibit shoot and root phenotypes consistent with altered auxin homeostasis including altered primary root growth, lateral root development, and root hair elongation. MED12 and MED13 were required for activation of cell division and elongation in primary roots, as well as auxin-responsive and stem cell niche gene expression. Remarkably, most of these mutant phenotypes were rescued by supplying sucrose to the growth medium. The growth response of primary roots of WT, med12, aux1-7 and med12 aux1 single and double mutants to sucrose and application of auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) revealed the correlation of med12 phenotype with the activity of the auxin intake permease and suggests that MED12 acts upstream of AUX1 in the root growth response to sugar. These data provide compelling evidence that MEDIATOR links sugar sensing to auxin transport and distribution during root morphogenesis.

MEDIATOR18 influences Arabidopsis root architecture, represses auxin signaling and is a critical factor for cell viability in root meristems

“…With this aim, we analyzed the effects of leaf applied sucrose on root development, according to Raya‐González et al . (). In WT and med18‐1 seedlings, sucrose supplied to the shoot increased lateral root formation (Figure S4), suggesting that vascular transport was not compromised by the lack of a functional MED18.…”

“…Because cell death in med18 mutants is specific of initial cells of a tissue that later differentiates into vasculature, we assessed whether the phloem transport could be affected in med18-1 roots. With this aim, we analyzed the effects of leaf applied sucrose on root development, according to Raya-Gonz alez et al (2017). In WT and med18-1 seedlings, sucrose supplied to the shoot increased lateral root formation (Figure S4), suggesting that vascular transport was not compromised by the lack of a functional MED18.…”

“…The Mediator (MED) complex is a multi‐subunit complex that acts as a bridge between promoter‐bound transcription factors and the RNA polymerase II transcription initiation complex to control gene expression. In plants, MED subunits participate in several developmental processes, hormonal signaling pathways, and biotic and abiotic stress tolerance (Manners and Kazan, ; Gillmor et al ., , ; Ito et al ., ; Xu and Li, ; Çevik et al ., ; Rai et al ., ; Yang et al ., ; Zhang et al ., ; Raya‐González et al ., , ). However, how MED subunits could be interacting to determine the MED functioning per se on plant development remains largely unknown.…”

MEDIATOR18 modulates viability of root initial cells

“…MED25, part of the tail module) (Muñoz‐Parra et al ., ) although the functions of some others (e.g. MED12 and MED13, components of the kinase module) appear to overlap (Raya‐González et al ., ). It might be expected that knockout mutations in any one component would yield similar phenotypes, but that is not the case.…”

“…They examined the effect of light because they previously showed that the phenotypes of med12 and med13 seedlings, which have very short roots, were corrected when the medium was supplemented with sucrose (Raya‐González et al ., ). Sucrose did not correct the med18 phenotype.…”