Functional insights into two Ocimum kilimandscharicum 4-coumarate-CoA ligases involved in phenylpropanoid biosynthesis.

Plant 4-coumarate-CoA ligase (4CL) catalyzes the ligation of CoA to cinnamic acid and its derivatives. Activated CoA esters are utilized for the biosynthesis of phenolic metabolites and lignin that play essential function in plants. Here, we characterize the diversity of Ocimum kilimandscharicum 4CLs (Ok4CLs). Phylogenetic analysis suggest that Ok4CLs could be grouped into three classes, class I - enzymes mostly involved in lignin biosynthesis, class II - non-structural phenylpropanoid biosynthesis and class III - yet to be characterized for specific role(s). We selected two Ok4CLs namely Ok4CL7 and Ok4CL15 for further characterization. Gene expression analysis suggested that Ok4CL7 is highly expressed in leaf trichomes, whereas Ok4CL15 is abundant in the roots. The recombinant Ok4CL7 and Ok4CL15 had optimal enzyme activities at 40 °C in pH 8 and 7, respectively. Ok4CL7 showed substrate preference towards p-coumaric acid, ferulic acid and caffeic acid. While, Ok4CL15 preferred p-coumaric acid, ferulic acid and sinapic acid. Feruloyl adenylate showed higher number of contacts and lowers binding energy with Ok4CL7 and 15 compared to cinnamoyl adenylate. Based on root-specific expression and preference for sinapic acid, Ok4CL15 might be involved in lignin biosynthesis. Further exploration is needed to unravel the role of diverse Ok4CLs in O. kilimandscharicum.

Comparative transcriptome analysis to identify putative genes related to trichome development in Ocimum species.

Genus Ocimum is known to have species possessing important therapeutic essential oil. The major phytoconstituents of essential oil in Ocimum species are phenylpropanoids and terpenoids. The essential oil is accumulated in the trichomes; the specialized structures predominantly found on leaves and other tissues. The development of trichome is integrated with development of plant and leaf and also tightly coordinated with the primary and secondary metabolic pathways producing essential oil constituents. In continuation to our studies on elucidating/understanding the mechanism of biosynthesis of  essential oil pathways in Ocimum species, we have performed comparative transcriptome analysis to investigate the role of trichome-related gene expression in the regulation of biosynthetic pathways of essential oil. The essential oil biogenesis is tightly integrated with primary metabolic activities, the analysis for the expression pattern of genes related to primary metabolism and its relationship with secondary metabolism was evaluated in comparative manner. Physiological parameters in relation to primary metabolism such as photosynthetic pigment content, soluble sugar content, and invertase enzymes along with morphological parameters were analysed in O. basilicum and O. sanctum. Differential expression profiling uncovered about 8116 and 2810 differentially expressed transcripts in O. basilicum and O. sanctum, respectively. Enrichment of differentially expressed genes were analysed in relation to metabolic pathways, primary metabolism and secondary metabolism. Trichome related genes identified from the Ocimum species vis-à-vis their expression profiles suggested higher expression in O. basilicum. The findings in this study provide interesting insights into the role of trichome-related transcripts in relation to essential oil content in Ocimum species. The study is valuable as this is the first study on revealing the transcripts and their role in trichome development and essential oil biogenesis in two major species of Ocimum.

Comparative functional characterization of eugenol synthase from four different Ocimum species: Implications on eugenol accumulation.

Isoprenoids and phenylpropanoids are the major secondary metabolite constituents in Ocimum genus. Though enzymes from phenylpropanoid pathway have been characterized from few plants, limited information exists on how they modulate levels of secondary metabolites. Here, we performed phenylpropanoid profiling in different tissues from five Ocimum species, which revealed significant variations in secondary metabolites including eugenol, eugenol methyl ether, estragole and methyl cinnamate levels. Expression analysis of eugenol synthase (EGS) gene showed higher transcript levels especially in young leaves and inflorescence; and were positively correlated with eugenol contents. Additionally, transcript levels of coniferyl alcohol acyl transferase, a key enzyme diverting pool of substrate to phenylpropanoids, were in accordance with their abundance in respective species. In particular, eugenol methyl transferase expression positively correlated with higher levels of eugenol methyl ether in Ocimum tenuiflorum. Further, EGSs were functionally characterized from four Ocimum species varying in their eugenol contents. Kinetic and expression analyses indicated, higher enzyme turnover and transcripts levels, in species accumulating more eugenol. Moreover, biochemical and bioinformatics studies demonstrated that coniferyl acetate was the preferred substrate over coumaryl acetate when used, individually or together, in the enzyme assay. Overall, this study revealed the preliminary evidence for varied accumulation of eugenol and its abundance over chavicol in these Ocimum species. Current findings could potentially provide novel insights for metabolic modulations in medicinal and aromatic plants.

Functional characterization and transient expression manipulation of a new sesquiterpene synthase involved in ß-caryophyllene accumulation in Ocimum.

The genus Ocimum has a unique blend of diverse secondary metabolites, with major proportion of terpenoids including mono- and sesquiterpenes. Although, ß-Caryophyllene, bicyclic sesquiterpene, is one of the major terpene found in Ocimum species and known to possess several biological activities, not much is known about its biosynthesis in Ocimum. Here, we describe isolation and characterization of ß-caryophyllene synthase gene from Ocimum kilimandscharicum Gürke (OkBCS- GenBank accession no. KP226502). The open reading frame of 1629 bp encoded a protein of 542 amino acids with molecular mass of 63.6 kDa and pI value of 5.66. The deduced amino acid sequence revealed 50-70% similarity with known sesquiterpene synthases from angiosperms. Recombinant OkBCS converted farnesyl diphosphate to ß-caryophyllene as a major product (94%) and 6% α-humulene. Expression variation of OkBCS well corroborated with ß-caryophyllene levels in different tissues from five Ocimum species. OkBCS transcript revealed higher expression in leaves and flowers. Further, agro-infiltration based transient expression manipulation with OkBCS over-expression and silencing confirmed its role in ß-caryophyllene biosynthesis. These findings may potentially be further utilized to improve plant defense against insect pests.

Characterization and functional analysis of eugenol O-methyltransferase gene reveal metabolite shifts, chemotype specific differential expression and developmental regulation in Ocimum tenuiflorum L.

Eugenol-O-methyltransferase (EOMT) catalyzes the conversion of eugenol to methyleugenol in one of the final steps of phenylpropanoid pathway. There are no comprehensive reports on comparative EOMT gene expression and developmental stage specific accumulation of phenylpropenes in Ocimum tenuiflorum. Seven chemotypes, rich in eugenol and methyleugenol, were selected by assessment of volatile metabolites through multivariate data analysis. Isoeugenol accumulated in higher levels during juvenile stage (36.86 ng g(-1)), but reduced sharply during preflowering (8.04 ng g(-1)), flowering (2.29 ng g(-1)) and postflowering stages (0.17 ng g(-1)), whereas methyleugenol content gradually increased from juvenile (12.25 ng g(-1)) up to preflowering (16.35 ng g(-1)) and then decreased at flowering (7.13 ng g(-1)) and post flowering (5.95 ng g(-1)) from fresh tissue. Extreme variations of free intracellular and alkali hydrolysable cell wall released phenylpropanoid compounds were observed at different developmental stages. Analyses of EOMT genomic and cDNA sequences revealed a 843 bp open reading frame and the presence of a 90 bp intron. The translated proteins had eight catalytic domains, the major two being dimerisation superfamily and methyltransferase_2 superfamily. A validated 3D structure of EOMT protein was also determined. The chemotype Ot7 had a reduced reading frame that lacked both dimerisation domains and one of the two protein-kinase-phosphorylation sites; this was also reflected in reduced accumulation of methyleugenol compared to other chemotypes. EOMT transcripts showed enhanced expression in juvenile stage that increased further during preflowering but decreased at flowering and further at postflowering. The expression patterns may possibly be compared and correlated to the amounts of eugenol/isoeugenol and methyleugenol in different developmental stages of all chemotypes.

4-coumarate: CoA ligase partitions metabolites for eugenol biosynthesis.

Biosynthesis of eugenol shares its initial steps with that of lignin, involving conversion of hydroxycinnamic acids to their corresponding coenzyme A (CoA) esters by 4-coumarate:CoA ligases (4CLs). In this investigation, a 4CL (OS4CL) was identified from glandular trichome-rich tissue of Ocimum sanctum with high sequence similarity to an isoform (OB4CL_ctg4) from Ocimum basilicum. The levels of OS4CL and OB4CL_ctg4-like transcripts were highest in O. sanctum trichome, followed by leaf, stem and root. The eugenol content in leaf essential oil was positively correlated with the expression of OS4CL in the leaf at different developmental stages. Recombinant OS4CL showed the highest activity with p-coumaric acid, followed by ferulic, caffeic and trans-cinnamic acids. Transient RNA interference (RNAi) suppression of OS4CL in O. sanctum leaves caused a reduction in leaf eugenol content and trichome transcript level, with a considerable increase in endogenous p-coumaric, ferulic, trans-cinnamic and caffeic acids. A significant reduction in the expression levels was observed for OB4CL_ctg4-related transcripts in suppressed trichome compared with transcripts similar to the other four isoforms (OB4CL_ctg1, 2, 3 and 5). Sinapic acid and lignin content were also unaffected in RNAi suppressed leaf samples. Transient expression of OS4CL-green fluorescent protein fusion protein in Arabidopsis protoplasts was associated with the cytosol. These results indicate metabolite channeling of intermediates towards eugenol by a specific 4CL and is the first report demonstrating the involvement of 4CL in creation of virtual compartments through substrate utilization and committing metabolites for eugenol biosynthesis at an early stage of the pathway.