A phenylalanine ammonia-lyase ortholog (PkPAL1) from Picrorhiza kurrooa Royle ex. Benth: molecular cloning, promoter analysis and response to biotic and abiotic elicitors

PkGPPS.SSU interacts with two PkGGPPS to form heteromeric GPPS in Picrorhiza kurrooa: Molecular insights into the picroside biosynthetic pathway

Geranyl geranyl pyrophosphate synthase (GGPPS) is known to form an integral subunit of the heteromeric GPPS (geranyl pyrophosphate synthase) complex and catalyzes the biosynthesis of monoterpene in plants. Picrorhiza kurrooa Royle ex Benth., a medicinally important high altitude plant is known for picroside biomolecules, the monoterpenoids. However, the significance of heteromeric GPPS in P. kurrooa still remains obscure. Here, transient silencing of PkGGPPS was observed to reduce picroside-I (P-I) content by more than 60% as well as picroside-II (P-II) by more than 75%. Thus, PkGGPPS was found to be involved in the biosynthesis of P-I and P-II besides other terpenoids. To unravel the mechanism, small subunit of GPPS (PkGPPS.SSU) was identified from P. kurrooa. Protein-protein interaction studies in yeast as well as bimolecular fluorescence complementation (BiFC) in planta have indicated that large subunit of GPPS PkGPPS.LSUs (PkGGPPS1 and PkGGPPS2) and PkGPPS.SSU form a heteromeric GPPS. Presence of similar conserved domains such as light responsive motifs, low temperature responsive elements (LTRE), dehydration responsive elements (DREs), W Box and MeJA responsive elements in the promoters of PkGPPS.LSU and PkGPPS.SSU documented their involvement in picroside biosynthesis. Further, the tissue specific transcript expression analysis vis-à-vis epigenetic regulation (DNA methylation) of promoters as well as coding regions of PkGPPS.LSU and PkGPPS.SSU has strongly suggested their role in picroside biosynthesis. Taken together, the newly identified PkGPPS.SSU formed the heteromeric GPPS by interacting with PkGPPS.LSUs to synthesize P-I and P-II in P. kurrooa.

Structural and functional characterisation of a class I endochitinase of the carnivorous sundew (Drosera rotundifolia L.)

Chitinase gene from the carnivorous plant, Drosera rotundifolia , was cloned and functionally characterised. Plant chitinases are believed to play an important role in the developmental and physiological processes and in responses to biotic and abiotic stress. In addition, there is growing evidence that carnivorous plants can use them to digest insect prey. In this study, a full-length genomic clone consisting of the 1665-bp chitinase gene (gDrChit) and adjacent promoter region of the 698 bp in length were isolated from Drosera rotundifolia L. using degenerate PCR and a genome-walking approach. The corresponding coding sequence of chitinase gene (DrChit) was obtained following RNA isolation from the leaves of aseptically grown in vitro plants, cDNA synthesis with a gene-specific primer and PCR amplification. The open reading frame of cDNA clone consisted of 978 nucleotides and encoded 325 amino acid residues. Sequence analysis indicated that DrChit belongs to the class I group of plant chitinases. Phylogenetic analysis within the Caryophyllales class I chitinases demonstrated a significant evolutionary relatedness of DrChit with clade Ib, which contains the extracellular orthologues that play a role in carnivory. Comparative expression analysis revealed that the DrChit is expressed predominantly in tentacles and is up-regulated by treatment with inducers that mimick insect prey. Enzymatic activity of rDrChit protein expressed in Escherichia coli was confirmed and purified protein exhibited a long oligomer-specific endochitinase activity on glycol-chitin and FITC-chitin. The isolation and expression profile of a chitinase gene from D. rotundifolia has not been reported so far. The obtained results support the role of specific chitinases in digestive processes in carnivorous plant species.

The phenylalanine ammonia-lyase gene family in Isatis indigotica Fort.: molecular cloning, characterization, and expression analysis

Phenolic compounds, metabolites of the phenylpropanoid pathway, play an important role in the growth and environmental adaptation of many plants. Phenylalanine ammonia-lyase (PAL) is the first key enzyme of the phenylpropanoid pathway. The present study was designed to investigate whether there is a multi-gene family in I. Indigotic and, if so, to characterize their properties. We conducted a comprehensive survey on the transcription profiling database by using tBLASTn analysis. Several bioinformatics methods were employed to perform the prediction of composition and physicochemical characters. The expression levels of IiPAL genes in various tissues of I. indigotica with stress treatment were examined by quantitative real-time PCR. Protoplast transient transformation was used to observe the locations of IiPALs. IiPALs were functionally characterized by expression with pET-32a vector in Escherichia colis strain BL21 (DE3). Integration of transcripts and metabolite accumulations was used to reveal the relation between IiPALs and target compounds. An new gene (IiPAL2) was identified and both IiPALs had the conserved enzymatic active site Ala-Ser-Gly and were classified as members of dicotyledon. IiPAL1 and IiPAL2 were expressed in roots, stems, leaves, and flowers, with the highest expression levels of IiPAL1 and IiPAL2 being observed in stems and roots, respectively. The two genes responded to the exogenous elicitor in different manners. Subcellular localization experiment showed that both IiPALs were localized in the cytosol. The recombinant proteins were shown to catalyze the conversion of L-Phe to trans-cinnamic acid. Correlation analysis indicated that IiPAL1 was more close to the biosynthesis of secondary metabolites than IiPAL2. In conclusion, the present study provides a basis for the elucidation of the role of IiPALs genes in the biosynthesis of phenolic compounds, which will help further metabolic engineering to improve the accumulation of bioactive components in I. indigotica.

Discerning picroside-I biosynthesis via molecular dissection of in vitro shoot regeneration in Picrorhiza kurroa

Expression analysis of primary and secondary metabolic pathways genes vis-à-vis shoot regeneration revealed developmental regulation of picroside-I biosynthesis in Picrorhiza kurroa. Picroside-I (P-I) is an important iridoid glycoside used in several herbal formulations for treatment of various disorders. P-I is synthesized in shoots of Picrorhiza kurroa and Picrorhiza scrophulariiflora. Current study reports on understanding P-I biosynthesis in different morphogenetic stages, viz. plant segment (PS), callus initiation (CI), callus mass (CM), shoot primordia (SP), multiple shoots (MS) and fully developed (FD) stages of P. kurroa. Expression analysis of genes involved in primary and secondary metabolism revealed that genes encoding HMGR, PMK, DXPS, ISPE, GS, G10H, DAHPS and PAL enzymes of MVA, MEP, iridoid and shikimate/phenylpropanoid pathways showed significant modulation of expression in SP, MS and FD stages in congruence with P-I content compared to CM stage. While HK, PK, ICDH, MDH and G6PDH showed high expression in MS and FD stages of P. kurroa, RBA, HisK and CytO showed high expression with progress in regeneration of shoots. Quantitative expression analysis of secondary metabolism genes at two temperatures revealed that 7 genes HMGR, PMK, DXPS, GS, G10H, DAHPS and PAL showed high transcript abundance (32-87-folds) in FD stage derived from leaf and root segments at 15 °C compared to 25 °C in P. kurroa. Further screening of these genes at species level showed high expression pattern in P. kurroa (6-19-folds) vis-à-vis P. scrophulariiflora that was in corroboration with P-I content. Therefore, current study revealed developmental regulation of P-I biosynthesis in P. kurroa which would be useful in designing a suitable genetic intervention study by targeting these genes for enhancing P-I production.

Rice phenylalanine ammonia-lyase gene OsPAL4 is associated with broad spectrum disease resistance

Most agronomically important traits, including resistance against pathogens, are governed by quantitative trait loci (QTL). QTL-mediated resistance shows promise of being effective and long-lasting against diverse pathogens. Identification of genes controlling QTL-based disease resistance contributes to breeding for cultivars that exhibit high and stable resistance. Several defense response genes have been successfully used as good predictors and contributors to QTL-based resistance against several devastating rice diseases. In this study, we identified and characterized a rice (Oryza sativa) mutant line containing a 750 bp deletion in the second exon of OsPAL4, a member of the phenylalanine ammonia-lyase gene family. OsPAL4 clusters with three additional OsPAL genes that co-localize with QTL for bacterial blight and sheath blight disease resistance on rice chromosome 2. Self-pollination of heterozygous ospal4 mutant lines produced no homozygous progeny, suggesting that homozygosity for the mutation is lethal. The heterozygous ospal4 mutant line exhibited increased susceptibility to three distinct rice diseases, bacterial blight, sheath blight, and rice blast. Mutation of OsPAL4 increased expression of the OsPAL2 gene and decreased the expression of the unlinked OsPAL6 gene. OsPAL2 function is not redundant because the changes in expression did not compensate for loss of disease resistance. OsPAL6 co-localizes with a QTL for rice blast resistance, and is down-regulated in the ospal4 mutant line; this may explain enhanced susceptibility to Magnoporthe oryzae. Overall, these results suggest that OsPAL4 and possibly OsPAL6 are key contributors to resistance governed by QTL and are potential breeding targets for improved broad-spectrum disease resistance in rice.

Arbuscular mycorrhizal fungal inoculation increases phenolic synthesis in clover roots via hydrogen peroxide, salicylic acid and nitric oxide signaling pathways

Arbuscular mycorrhizal fungi can increase the host resistance to pathogens via promoted phenolic synthesis, however, the signaling pathway responsible for it still remains unclear. In this study, in order to reveal the signaling molecules involved in this process, we inoculated Trifolium repense L. with an arbuscular mycorrhizal fungus (AMF), Glomus mosseae, and monitored the contents of phenolics and signaling molecules (hydrogen peroxide (H(2)O(2)), salicylic acid (SA), and nitric oxide (NO)) in roots, measured the activities of l-phenylalanine ammonia-lyase (PAL) and nitric oxide synthase (NOS), and the expression of pal and chs genes. Results demonstrated that AMF colonization promoted the phenolic synthesis, in parallel with the increase in related enzyme activity and gene expression. Meanwhile, the accumulation of all three signaling molecules was also up-regulated by AMF. This study suggested that AMF increased the phenolic synthesis in roots probably via signaling pathways of H(2)O(2), SA and NO in a signaling cascade.