Isolation and identification of ginseng PgCYP76B93 genes
All cytochrome P450 (CYP) genes homologous with known CYP family genes were identified from the expressed sequence tag (EST) libraries constructed from the embryogenic calli, 4- and 14-year-old roots, leaves, bud, and methyl jasmonic acid (MeJA)-treated adventitious roots [25]. One EST clone showing similarity with CYP76B family was further chosen, and the recovery of its full-length cDNA was confirmed by rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) [26]. The full-length cDNA of CYP76B was annotated again as PgCYP76B93 (CYP76B93 from P. ginseng) after consulting with the CYP nomenclature committee [27]. The original EST clone coding for PgCYP76B93 was obtained from MeJA-treated adventitious roots, indicating that it can be induced by an elicitor and is expressed weakly in normal conditions. PgCYP76B93 is a 1551 bp long gene encoding 517 amino acids. Other amino acid sequences of the characterized CYPs in plants were collected from various gene banks and were used to generate a phylogenic tree (Fig. 1a). PgCYP76B93 was clustered in between the CYP76B family enzymes involved in terpenoid biosynthesis [28, 29] and several CYP76B and CYP76C family enzymes [15, 16, 18], which were previously reported to be involved in phenylurea herbicide resistance (Fig. 1a). The enzymes belonging to the A-type subfamily of the CYP76 family enzymes were clustered relatively closer with PgCYP76B93, and three ginseng CYP enzymes involved in ginsenoside biosynthesis [6,7,8] were clustered far more distantly (Fig. 1a). This indicates that PgCYP76B93 likely plays a role in herbicide resistance and/or terpenoid biosynthesis.
The PgCYP76B93 enzyme was predicted to have a molecular weight of 58,390 kDa and a pI of 7.74 using the ProtParam program [30]. The CYP family enzymes were reported to share low similarity in their amino acid sequences, except for several conserved domains, which are necessary for their tertiary structure and enzymatic function [31]. The most well-conserved motif is the heme-binding region FxxGxRxCxG (also known as the CxG motif), in which C (cysteine) binds to the heme group. The ExxR and PER motifs form the E–R–R triad and play important roles in locking the structure of the heme pocket. The most conserved amino acids in the CYP family are glutamic acid (E) and arginine (R) in the ExxR motif and cysteine in the CxG motif, and these amino acids are also found in PgCYP76B93 (Fig. 1b). The least conserved motif, (A/G)Gx(D/E)T(T/S) (also known as AGxDTT), contributes to oxygen binding and activation. The AGxDTT motif is also well conserved in PgCYP76B93, but the PER motif is replaced by the PKR motif (Fig. 1b). Understanding the significance of the substitution of K (lysine) for E requires further investigation.
Transcript levels of PgCYP76B93 are gradually increased by chlorotoluron
In one step to understand the function of PgCYP76B93, modulation of transcripts against two phenylurea herbicides were evaluated according to the recent report [20]. Differential mRNA levels of PgCYP76B93 was quantified in time-dependent manner by treatment of 10 μM of chlorotoluron and isoproturon (Fig. 2). The transcripts of PgCYP76B93 were increased by chlorotoluron gradually from 1 to 24 h after treatments, and were initially upregulated by isoproturon upto 4 h (Fig. 2). It indicates that PgCYP76B93 might play roles in herbicide detoxification, especially chlorotoluron.
Molecular docking study shows putative phenylurea herbicide interaction with PgCYP76B93
The major secondary structure of PgCYP76B93 is pretty identical to Zebra Fish CYP-450 17A1 mutant Abiraterone complex [21]. The heme site of simulated PgCYP76B93 was not appeared but it could be predicted by superimposing the simulated PgCYP76B93 on the Zebra Fish CYP-450 17A1 Abiraterone complex (Additional file 2: Fig. S1). When isoproturon and chlorotoluron were added to the AutoDock program to test the binding possibility with PgCYP76B93, possible docking simulation for the interaction was observed. Total 5 of 20 docking poses for isoproturon and 6 of 20 docking poses for chlorotoluron are resided around heme in active site (Fig. 3a, b). These positions are interestingly located on the similar site of abiraterone which is a substrate of the template protein, Zebra Fish CYP-450 17A1. Residues interacting with isoproturon and chlorotoluron around heme of CYP76B93 was identified by Ligplot+ program [22] (Fig. 3). Isoproturon dominantly forms hydrophobic interaction with Arg102, Val119, Trp120, Met214, Asp315, Val318, Ala319, Thr323, Pro383, Ala384, Leu387, Ile388, Arg456, Pro459, Gly460 and Ile500 in 5 poses, and minor hydrogen bond with Ile388 and Arg390 in 1 poses near heme (Fig. 3c). Chlorotoluron majorly contacts with Val119, Trp120, Asp315, Val318, Ala319, Asp322, Thr323, Pro383, Ala384, Leu387, Ile388, Arg390, Arg456, Ile500 and Thr501 in 6 poses by hydrophobic interaction and minorly interacts with Ile388, Arg390 and Thr501 by hydrogen bond in 3 poses (Fig. 3d). Val119, Trp120, Val318, Ala319, Thr323, Thr384, Leu387, Ile388, Ile500 involved in interaction with isoproturon or chlorotoluron are frequently observed in each poses, so it can be further considered as a target for mutagenesis study. The closest distance from heme to isoproturon is 9.9 Å and to chlorotoluron is 4.3 Å (Fig. 3e). This result is believed to support the data that transgenic Arabidopsis possessing overexpression of CYP76B93 gene exhibits resistance against chlorotoluron other than isoproturon. Closely bound chlorotoluron to heme of PgCYP76B93 might be used as a substrate for undergoing detoxification mechanism.
Overexpression of PgCYP76B93 reduced plant height and leaf size
In order to understand the functional characteristics of PgCYP76B93 in plant growth and development, its full-length coding sequence was overexpressed in heterologous Arabidopsis plants. Of the several T1 transgenic lines selected on the media containing an antibiotic, two were characterized further. These two transgenic lines showed on average 3.6 times more upregulation of PgCYP76B93 (Fig. 4a) and displayed reduced plant height (Fig. 4b). In seedling stage, one strongly expressing line No.9 displayed reduced hypocotyl length (Additional file 2: Fig. S2). It could be resulted in slightly reduced plant height in overexpression lines. Rosette leaves were smaller than those in the control lines (Fig. 4c). This implies that PgCYP76B93 might be involved in plant growth and development via phytohormone regulation which is involved in stem and leaf development. However, the overexpression of PgCYP76B93 did not alter the root growth phenotype (Fig. 4d, e) suggesting that the phytohormone-regulated PgCYP76B93 gene is more functional in aerial parts than in underground parts.
Transcripts of gibberellin oxidases are altered by the overexpression of PgCYP76B93
Gibberellins (GAs) are well-known plant hormones that promote stem elongation as well as seed germination [32]. The PgCYP76B93-mediated reduced plant height (Fig. 4b) led us to analyze whether the transcript levels of gibberellin biosynthesis-related genes were changed. Gibberellins are biosynthesized from geranylgeranyl diphosphate (GGDP), which is a common C20 precursor for diterpenoids, using three different classes of enzymes [33]: (1) terpene synthases (2) cytochrome P450 monooxygenases, and (3) 2-oxoglutarate-dependent dioxygenases. To uncover a possible link with bioactive GA biosynthesis, four GA monooxidase genes were analyzed. Two GA oxidase enzymes, GA20ox1 and GA3ox1, are involved in catalyzing the reaction of bioactive GA biosynthesis, whereas GA2ox1 and GA2ox2 are involved in the conversion of bioactive gibberellins into an inactive form. In two different developmental stages, all transcripts of GA20ox1 and GA3ox1 were significantly downregulated, whereas GA2ox1 and GA2ox2 were more upregulated (Fig. 5a, b). Though the mRNA levels of GA2ox2 were higher in the 28-day-old seedlings than in the 14-day-old seedlings, the general tendency was a decrease in the expression of the more bioactive form of GA oxidase gene, suggesting that the reduced height (Fig. 4b) of the PgCYP76B93 overexpression lines is caused by reduction in the levels of the more bioactive forms of gibberellins.
Constitutive overexpression of PgCYP76B93 confers herbicide resistance
PgCYP76B93 is closely clustered with the CYP76B family of enzymes, which have been reported to be involved in herbicide metabolism [15,16,17]. Also upregulation of PgCYP76B93 transcripts against chlorotoluron supported this notion. Thus, the PgCYP76B93 overexpression lines were evaluated for their ability to display resistance to phenylurea herbicide chlorotoluron. Several concentrations (0.5, 1, 3, 7, 10 μM) of chlorotoluron were analyzed from the germination, and the seedlings exposed to 7 and 10 μM of chlorotoluron showed distinct tolerant phenotypes compared to the control plants (Fig. 6a). Statistical analysis also suggests that the survival rate (%) on 1/2 MS media containing herbicide is significant (Fig. 6b). To evaluate the resistance of already germinated seedlings, 4-day-old seedlings from Col-0 and PgCYP76B93 overexpression lines (OE lines) were analyzed in the presence and absence of two different concentrations of chlorotoluron, and found OE lines show apparent tolerant growth phenotype (Additional file 2: Fig. S3).