Elongator Is Required for Root Stem Cell Maintenance by Regulating SHORTROOT Transcription

Abstract

SHORTROOT (SHR) is essential for stem cell maintenance and radial patterning in Arabidopsis (Arabidopsis thaliana) roots, but how its expression is regulated is unknown. Here, we report that the Elongator complex, which consists of six subunits (ELP1 to ELP6), regulates the transcription of SHR. Depletion of Elongator drastically reduced SHR expression and led to defective root stem cell maintenance and radial patterning. The importance of the nuclear localization of Elongator for its functioning, together with the insensitivity of the elp1 mutant to the transcription elongation inhibitor 6-azauracil, and the direct interaction of the ELP4 subunit with the carboxyl-terminal domain of RNA polymerase II, support the notion that Elongator plays important roles in transcription elongation. Indeed, we found that ELP3 associates with the premessenger RNA of SHR and that mutation of Elongator reduces the enrichment of RNA polymerase II on the SHR gene body. Moreover, Elongator interacted in vivo with SUPPRESSOR OF Ty4, a well-established transcription elongation factor that is recruited to the SHR locus. Together, these results demonstrate that Elongator acts in concert with SUPPRESSOR OF Ty4 to regulate the transcription of SHR.

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“…Downregulated expression of transcription factors was confirmed by means of qPCR, and additionally, by the analysis of transgenic lines transformed with constructs containing reporter fluorescent protein fused to promoter, or promoter and coding sequence of a given transcription factor. Low expression of SHR was shown by similar methods and by in situ hybridization in another elo/elp mutant-elo1, and supported by results of qPCR assay revealing decreased levels of SHR transcriptional targets including SCR [62].The increased resistance of elo/elp mutants to oxidative [42] and drought [41] stress is very likely associated with higher accumulation of anthocyanins in the mutants, resulting from much lower expression of the MYBL2 gene encoding negative regulator of anthocyanins biosynthesis [42].…”

“…ELP6 subunit expressed in fusion with yellow fluorescent protein VENUS and under its own autologous promoter was detected in the cytoplasm of Arabidopsis thaliana root meristem stele cells, while in the 35S::YFP:ELP3 overexpression line, both cytoplasmic and nuclear localization was observed [48]. In the most recent report, Qi and co-workers identified ELP3-GFP expressed from the autologous promoter in stele meristem cells, predominantly in the cytoplasm and to lesser extent in the nucleus [62]. In the case of the p35S:GFP-ELP3 line, nuclear localization was more evident in the columella and epidermis cells in the root elongation zone.…”

“…Finally, cell fractionation approach combined with immunodetection of endogenous ELP1 and ELP3 proved that Arabidopsis Elongator is located in both the cytoplasm and the nucleus. To prove that Elongator is indeed active in the nucleus, ELP3-GFP was fused with SV40 nuclear localization sequence, the resulting protein was detected in the nucleus and shown to fully rescue root growth defects of elp3 mutants [62].…”

“…This phenotype was later explained by a decreased palisade cell number [24], indicating that Elongator stimulates cell proliferation. Another phenotypic characteristic observed in elo mutants is the shorter primary root resulting from a reduced cell number and length pointing to the role of Elongator in both cell proliferation and cell elongation [24,38,62]. The meristem zone was smaller with a decreased cell number and also the elongation zone was shorter due to a reduced cell length [38].…”

“…The meristem zone was smaller with a decreased cell number and also the elongation zone was shorter due to a reduced cell length [38]. Elongator is also required to maintain cell identity in the root stem cell niche [38,62] and to regulate radial patterning in roots [62]. Root phenotype together with the reduced apical dominance, abnormal inflorescence phyllotaxis, and open leaf venation observed in elo/elp mutants are all signs of defective auxin signaling and distribution [25].…”