β-Cyclocitral is a conserved root growth regulator

Abstract

Natural compounds capable of increasing root depth and branching are desirable tools for enhancing stress tolerance in crops. We devised a sensitized screen to identify natural metabolites capable of regulating root traits in Arabidopsis β-Cyclocitral, an endogenous root compound, was found to promote cell divisions in root meristems and stimulate lateral root branching. β-Cyclocitral rescued meristematic cell divisions in ccd1ccd4 biosynthesis mutants, and β-cyclocitral-driven root growth was found to be independent of auxin, brassinosteroid, and reactive oxygen species signaling pathways. β-Cyclocitral had a conserved effect on root growth in tomato and rice and generated significantly more compact crown root systems in rice. Moreover, β-cyclocitral treatment enhanced plant vigor in rice plants exposed to salt-contaminated soil. These results indicate that β-cyclocitral is a broadly effective root growth promoter in both monocots and eudicots and could be a valuable tool to enhance crop vigor under environmental stress.

Keywords: abiotic stress; carotenoid; lateral root emergence; meristem; plant hormone.

Fig. 1.

Identification of β-cyclocitral, a root growth promoter in Arabidopsis. (A) The LR capacity of D15-treated plants, normalized to control plants. The IC₅₀ is highlighted in red. (B) Seedlings after treatment with 30 μM D15 and 25 μM volatile β-cyclocitral (β-cyc). (Scale bar, 5 mm.) (C) LR capacity of plants treated with 30 μM D15 and 25 μM volatile β-cyclocitral. (D) Arabidopsis seedlings treated directly with 750 nM β-cyclocitral. (Scale bar, 5 mm.) (E) Quantification of LR capacity of seedlings treated with increasing concentrations of β-cyclocitral. (F) Quantification of primary root length in β-cyclocitral–treated plants. *P = 0.05, **P = 0.01, ***P = 0.001, and ****P = 0.0001.

Fig. 2.

β-Cyclocitral induces meristematic cell divisions in Arabidopsis. (A) Confocal images of primary root meristems. (Scale bar, 50 μm.) Meristematic cortex cells are highlighted in orange. (B) Relative number of cortex cells in the primary root meristems of treated and control plants. (C) Relative number of GFP-positive cells in the root meristem of pCYCB1;1:CYCB1;1-GFP seedlings treated with β-cyclocitral. (D) Confocal images of root meristems in pCYCB1;1:CYCB1;1-GFP seedlings. (Scale bar, 25 μm.) GFP-positive cells are outlined in orange. (E) CCD1, CCD4, and CCD7 gene expression (log[3xFPKM]) in the three developmental zones at the root tip. (F) Relative number of cortex cells in the primary root meristems of WT and ccd1ccd4 double mutants with and without β-cyclocitral. *P = 0.05 and ****P = 0.0001.

Fig. 3.

β-Cyclocitral has conserved effects on root architecture in tomato and rice. (A) Tomato seedlings treated with β-cyclocitral. (Scale bar, 10 mm.) (Inset) The growth angle (θ) between the tip of the LR and the primary root measured to quantify the steepness of LRs is shown in blue. (B) Heat map depicting the increase (blue) or decrease (orange) in primary root length (PR ι), meristematic cell number (Cell #), LR length (LR), and angle of LR growth (LR θ) upon treatment with β-cyclocitral in Arabidopsis, tomato, and rice. (C) Root systems of 9311 rice seedlings treated with β-cyclocitral. (Scale bar, 10 mm.) The primary roots are highlighted in orange. (D) Quantification of the average depth of the crown roots in rice. (E) Quantification of the number of crown roots per seedling. ***P = 0.001 and ****P = 0.0001.

Fig. 4.

β-Cyclocitral promotes rice root growth under salt stress. (A) Rice roots treated with β-cyclocitral and grown in gel with 50 mM NaCl. (Scale bar, 10 mm.) (B) Root system depth in seedlings treated β-cyclocitral and grown in gel with 50 mM NaCl. (C) Primary root length in β-cyclocitral–treated rice plants grown in soil with salt stress. (D) Shoot height in β-cyclocitral–treated rice plants grown in soil with salt stress. (E) Representative images of rice seedlings grown in salt-contaminated soil with and without β-cyclocitral treatment. **P = 0.01, ***P = 0.001, and ****P = 0.0001.