Meta-regulation of Arabidopsis auxin responses depends on tRNA maturation

Abstract

Polar transport of the phytohormone auxin throughout plants shapes morphogenesis and is subject to stringent and specific control. Here, we identify basic cellular activities connected to translational control of gene expression as sufficient to specify auxin-mediated development. Mutants in subunits of Arabidopsis Elongator, a protein complex modulating translational efficiency via maturation of tRNAs, exhibit defects in auxin-controlled developmental processes, associated with reduced abundance of PIN-formed (PIN) auxin transport proteins. Similar anomalies are observed upon interference with tRNA splicing by downregulation of RNA ligase (AtRNL), pointing to a general role of tRNA maturation in auxin signaling. Elongator Protein 6 (ELP6) and AtRNL expression patterns underline an involvement in adjusting PIN protein levels, whereas rescue of mutant defects by auxin indicates rate-limiting activities in auxin-controlled organogenesis. This emphasizes mechanisms in which auxin serves as a bottleneck for plant morphogenesis, translating common cellular activities into defined developmental readouts.

Graphical abstract

Figure 1

Loss of Elongator Complex in Arabidopsis Interferes with Auxin-Controlled Morphogenesis and Post-transcriptional Regulation of PINs (A) Comparison of wild-type Col-0 and elp mutant seedlings at 7 DAG. (B) Orientation of primary root growth of Col-0 and elp seedlings at 9 DAG. A total of 50 seedlings were analyzed for each genotype and plotted as percentage of seedlings displaying <15°, <30°, <45°, and >45° deviation from the vertical growth axes. (C) Comparison of primary root length of Col-0 and elp seedlings at 5 DAG. 20 seedlings were analyzed for each genotype. SDs are indicated. (D) Quantification of PIN1 and PIN2 protein (left panels) and mRNA (right panels) in Col-0 and elp seedlings at 7 DAG. Relative signal intensities are indicated below (Col-0 = 1); “coom.” displays Coomassie staining of samples used. qPCR performed with Col-0 (= 1) and elp mutants displaying transcript levels of PIN1 and PIN2. SDs from three biological repeats with three technical replicates are indicated (^∗Student’s two-tailed t test; p < 0.05). (E) Comparison of PIN1::PIN1:GFP signals (green) in Col-0 and elp mutant root meristems at 5 DAG. Propidium iodide staining (PI, red) was used for visualizing cell boundaries. Size bars in represent 10 mm (A) and 50 μm (E). See also Figures S1 and S2.

Figure 2

Elp/mop Alleles Are Partially Rescued by PIN1 Overexpression and Characterized by Defects in Ribonucleotide Modifications (A) Primary root length of Col-0, elp6^mop2, RP40p::PIN1 and isogenic elp6^mop2RP40p::PIN1 seedlings at 7 DAG, grown on PNS (left) or PNS supplemented with NPA (right). 30–40 seedlings were analyzed for each genotype/condition. SDs are indicated (^∗Student’s two-tailed t test; p < 0.001). (B and C) Comparison of elp6^mop2 and elp6^mop2RP40p::PIN1 primary root meristems at 14 DAG grown on PNS (B) or on PNS supplemented with 1 μM NPA (C). White arrowheads indicate increased cell proliferation of elp6^mop2 in response to the auxin transport inhibitor. (D) Total tRNA fractions isolated from Col-0, elp3, and elp6^mop2 were analyzed by HPLC. Left: parts of chromatograms between retention times 40.0 and 53.0 min are displayed. Arrow indicates the expected retention time of mcm⁵s²U. Right: parts of chromatograms between retention times 10.0 and 18.0 min: Arrow indicates the expected retention time of ncm⁵U. Chromatograms were monitored at 314 and 254 nm, respectively. Size bars represent 50 μm (B and C). See also Figure S3.

Figure 3

Expression of Kluyveromyces lactis γ-Toxin in Arabidopsis Downregulates Abundance of tRNAs and Causes Defects in Morphogenesis Resembling elp Mutants (A) qPCR performed with cDNA from RP40p::gam lines 14-2 and 18-1. Transcript levels were normalized to expression of At5g60390 (= 1). (B) qPCR displaying abundance of ${\text{tRNA}}_{\text{UUG}}^{\text{Gln}}$ and ${\text{tRNA}}_{\text{UUC}}^{\text{Glu}}$ in Arabidopsis seedlings at 5 DAG, after normalization to levels of either ${\text{tRNA}}_{\text{GUG}}^{\text{His}}$ or ${\text{tRNA}}_{\text{AGA}}^{\text{Ser}}$ as internal standards (relative tRNA abundance in Col-0 = 1). Results presented are obtained from two biological repeats each with three technical repeats. SDs are indicated (^∗t test < 0.05; ^∗∗t test < 0.01; ^∗∗∗t test < 0.001). (C) Comparison of Col-0 and RP40p::gam seedlings at 6 DAG. Alterations in cotyledon formation and directional root growth are indicated by white arrowheads. (D) Orientation of primary root growth of Col-0 and RP40p::gam seedlings at 6 DAG. A total of 30–40 seedlings was analyzed for each data set and plotted as percentage of seedlings displaying <15°, <30°, <45°, and >45° deviation from the vertical growth axes. (E and F) Comparison of Col-0 and RP40p::gam inflorescences at 35 DAG. White arrowhead: aberrations in inflorescence architecture. (G and H) Expression of PIN1::PIN1:GFP (green) in Col-0 (G) and RP40p::gam (H) seedlings at 6 DAG. White arrowheads indicate aberrations in root patterning. Roots were stained with PI (red). (I) qPCR displaying PIN1 transcript levels in RP40p::gam lines 14-2 and 18-1 normalized to Col-0 (= 1). Results from three biological repeats with three technical replicates are shown. SDs are indicated. Size bars represent 10 mm (C, E, and F) and 50 μm (G and H). See also Figure S4.

Figure 4

Expression Analysis of ELP Reporter Genes (A and B) ELP6p::VENUS:ELP6 expression (yellow) in developing true leaf (A) and primary root meristem (B) at 6 DAG. (C) Cytoplasmic distribution of ELP6p::VENUS:ELP6 in root meristem stele cells. Areas with reduced signal intensities represent nuclei. Seedlings were stained with PI (red) to visualize cell and organ boundaries. (D) Expression of 35S::YFP:ELP3 (yellow signals) in 6-day-old Col-0 root meristem cells results in cytoplasmic and additional nuclear signals. (E) 35S::YFP:ELP3 root meristem epidermis cells (6 DAG) displaying pronounced cytoplasmic signals. To visualize nuclei, seedlings were fixed and counterstained with DAPI (blue). (F) Expression of 35S::YFP:GCN5, encoding a nuclear-localized protein, in Col-0 root meristem cells under conditions identical to those in (D). Size bars represent 100 μm (A), 50 μm (B, D, and F), and 10 μm (C and E).

Figure 5

Analysis of AtRNL RNA Ligase Knockdown Lines (A) qPCR displaying AtRNL transcript levels in RP40p:amiRNL-b lines 2–8 and 6–8 at 6 DAG (Col-0 = 1). Results from three biological repeats with three technical replicates are shown. SDs are indicated (^∗Student’s two-tailed t test, p < 0.01). (B) Quantification of ${\text{tRNA}}_{\text{GUA}}^{\text{Tyr}}$ levels in Col-0 and RP40p::amiRNL-b silencer lines by PCR. Upper band corresponds to non-spliced ${\text{tRNA}}_{\text{GUA}}^{\text{Tyr}}$ precursor (black arrowhead); lower band corresponds to processed ${\text{tRNA}}_{\text{GUA}}^{\text{Tyr}}$ (red arrowhead). Ratios of spliced and precursor band intensities are displayed below. (C and D) Col-0 (C) and RP40p::amiRNL-b (D) seedlings at 6 DAG. White arrowhead in (D) points to fused cotyledons. (E) Orientation of primary root growth of Col-0 and RP40p::amiRNL-b lines 2–8 and 6–8 seedlings at 8 DAG. A total of 40–50 seedlings was analyzed for each genotype and plotted as percentage of seedlings displaying <15°, <30°, <45°, and >45° deviation from the vertical growth axes. (F and G) Cotyledon vasculature in Col-0 (F) and RP40p::amiRNL-b (G). White arrowhead indicates vasculature discontinuity not observed in wild-type. (H and I) Detail of inflorescence axes of flowering Col-0 (H) and RP40p::amiRNL-b (I) at 28 DAG. White arrowheads indicate fused, fasciated stem. (J and K) Comparison of wild-type (J) and RP40p::amiRNL-b (K) primary root meristems expressing auxin-responsive DR5_rev::GFP (yellow). Roots were stained with PI (red) to visualize cell boundaries. White arrowheads point to alterations in root meristem patterning. Size bars represent 10 mm (C, D, H, and I), 1 mm (F and G), and 50 μm (J and K). See also Figures S5 and S6.

Figure 6

Elongator and AtRNL during Lateral Root Formation (A and B) AtRNLp::GUS activity at early (A) and later (B) stages of lateral root development. Arrowheads point to early-stage lateral root primordium. (C and D) ELP6p::VENUS:ELP6 expression (yellow) at early (C, white arrowhead) and later (D) stages of lateral root development. Roots were counterstained with PI to visualize cell boundaries. (E–H) PIN1p::PIN1:GFP signals in Col-0 (E and G) and elp6^mop2 (F and H) at early (E and F) and later (G and H) stages of lateral root development. (I and J) Expression of PIN1p::PIN1:GFP in lateral roots of Col-0 (I) and RP40p::amiRNL-b (J). (K) Number of lateral roots formed by Col-0, elp6^mop2, and RP40p::amiRNL-b at 10 DAG (^∗Student’s two-tailed t test; p < 0.001; n = 25–30 for each genotype and condition). Size bars represent 15 μm (A), 50 μm (B, D, and E–J), and 25 μm (C). See also Figure S6.