Assignment of UVB-responsive cis-element and protoplastization-(dilution-) and elicitor-responsive ones in the promoter region of a carrot phenylalanine ammonia-lyase gene (gDcPAL1)

Dual RNA-Seq analysis of Medicago truncatula and the pea powdery mildew Erysiphe pisi uncovers distinct host transcriptional signatures during incompatible and compatible interactions and pathogen effector candidates

Powdery mildew (PM) is a serious fungal disease of legumes. To gain novel insights into PM pathogenesis and host resistance/susceptibility, we used dual RNA-Seq to simultaneously capture host and pathogen transcriptomes at 1 d post-inoculation of resistant and susceptible Medicago truncatula genotypes with the PM Erysiphe pisi (Ep). Differential expression analysis indicates that R-gene mediated resistance against Ep involves extensive transcriptional reprogramming. Functional enrichment of differentially expressed host genes and in silico analysis of co-regulated promoters suggests that amplification of PTI, activation of the JA/ET signaling network, and regulation of growth-defense balance correlate with resistance. In contrast, processes that favor biotrophy, including suppression of defense signaling and programmed cell death, and weaker cell wall defenses are important susceptibility factors. Lastly, Ep effector candidates and genes with known/putative virulence functions were identified, representing a valuable resource that can be leveraged to improve our understanding of legume-PM interactions.

UV regulates the expression of phenylpropanoid biosynthesis genes in cucumber (Cucumis sativus L.) in an organ and spectrum dependent manner

A combination of a substantial number of cis-acting regulatory elements (MREs, ACEs, and G-boxes) is present in promoters of copies of phenylpropanoid biosynthesis genes that show strongly enhanced expression under UV-B-containing light in Cucumis sativus.

Possible involvement of a tetrahydrobiopterin in photoreception for UV-B-induced anthocyanin synthesis in carrot

Our previous studies of action spectra for UV-B-induced anthocyanin accumulation in cultured carrot cells indicated that a reduced form of pterin, possibly tetrahydrobiopterin, contributes to UV-B photoreception. In this report, we provide additional evidence for the involvement of pterin in UV-B light sensing. UV-B-induced phenylalanine ammonia-lyase (PAL) activity was considerably suppressed by N-acetylserotonin (an inhibitor of tetrahydrobiopterin biosynthesis), and this suppression was partially recovered by adding biopterin or tetrahydrobiobiopterin. In addition, protein(s) specifically bound to biopterin were detected by radiolabeling experiments in N-acetylserotonin-treated cells. Furthermore, diphenyleneiodonium, a potent inhibitor of electron transfer, completely suppressed UV-B-induced PAL activity. These results suggest the occurrence of an unidentified UV-B photoreceptor (other than UVR8, the tryptophan-based UV-B sensor originally identified in Arabidopsis) with reduced pterin in carrot cells. After reexamining published action spectra, we suggest that anthocyanin synthesis is coordinately regulated by these two UV-B sensors.

Transcriptional regulation of tocopherol biosynthesis in tomato

Tocopherols, compounds with vitamin E (VTE) activity, are potent lipid-soluble antioxidants synthesized only by photosynthetic organisms. Their biosynthesis requires the condensation of phytyl-diphosphate and homogentisate, derived from the methylerythritol phosphate (MEP) and shikimate pathways (SK), respectively. These metabolic pathways are central in plant chloroplast metabolism and are involved in the biosynthesis of important molecules such as chlorophyll, carotenoids, aromatic amino-acids and prenylquinones. In the last decade, few studies have provided insights into the regulation of VTE biosynthesis and its accumulation. However, the pathway regulatory mechanism/s at mRNA level remains unclear. We have recently identified a collection of tomato genes involved in tocopherol biosynthesis. In this work, by a dedicated qPCR array platform, the transcript levels of 47 genes, including paralogs, were determined in leaves and across fruit development. Expression data were analyzed for correlation with tocopherol profiles by coregulation network and neural clustering approaches. The results showed that tocopherol biosynthesis is controlled both temporally and spatially however total tocopherol content remains constant. These analyses exposed 18 key genes from MEP, SK, phytol recycling and VTE-core pathways highly associated with VTE content in leaves and fruits. Moreover, genomic analyses of promoter regions suggested that the expression of the tocopherol-core pathway genes is trancriptionally coregulated with specific genes of the upstream pathways. Whilst the transcriptional profiles of the precursor pathway genes would suggest an increase in VTE content across fruit development, the data indicate that in the M82 cultivar phytyl diphosphate supply limits tocopherol biosynthesis in later fruit stages. This is in part due to the decreasing transcript levels of geranylgeranyl reductase (GGDR) which restricts the isoprenoid precursor availability. As a proof of concept, by analyzing a collection of Andean landrace tomato genotypes, the role of the pinpointed genes in determining fruit tocopherol content was confirmed. The results uncovered a finely tuned regulation able to shift the precursor pathways controlling substrate influx for VTE biosynthesis and overcoming endogenous competition for intermediates. The whole set of data allowed to propose that 1-deoxy-D-xylulose-5-phosphate synthase and GGDR encoding genes, which determine phytyl-diphosphate availability, together with enzyme encoding genes involved in chlorophyll-derived phytol metabolism appear as the most plausible targets to be engineered aiming to improve tomato fruit nutritional value.

Constitutive expression of bergaptol O-methyltransferase in Glehnia littoralis cell cultures

We investigated whether exogenously supplied precursors of bergapten, namely umbelliferone, psoralen and bergaptol, could be utilized to produce bergapten without elicitation in Glehnia littoralis cell suspension cultures. The levels of added psoralen and bergaptol in the medium soon decreased, and this was followed by the detection of bergapten in both culture fluid and cells. Umbelliferone was also incorporated but in this case no bergapten was produced; instead, skimmin, umbelliferone monoglucoside, was detected. To determine whether conversion of psoralen to bergapten was due to enzyme induction by precursor feeding, the transcript accumulations and enzyme activities of bergaptol O-methyltransferase (BMT, EC 2.1.1.69), which catalyzes the last step of bergapten synthesis, and of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5), which catalyzes the initial step of the phenylpropanoid biosynthetic pathway and is known as a marker enzyme of elicitation, were examined. The results showed that both the expression and the activity of BMT were always detected in all cells, including control cells. Since PAL was slightly induced in the cells supplied with/without precursors, phenylethyl alcohol (PEA, a competitive inhibitor of PAL) was applied to suspension cells prior to the addition of psoralen. PAL activity was effectively inhibited by PEA at 1-5 mM concentrations. Under these conditions, PEA did not affect bergapten production by cell cultures fed with psoralen at all. These results demonstrate that BMT is constitutively expressed in G. littoralis cell cultures.

Upregulation of the promoter activity of the carrot (Daucus carota) phenylalanine ammonia-lyase gene (DcPAL3) is caused by new members of the transcriptional regulatory proteins, DcERF1 and DcERF2, which bind to the GCC-box homolog and act as an activator to the DcPAL3 promoter

The phenylalanine ammonia-lyase (PAL) gene, DcPAL3, was expressed during the synthesis of anthocyanin in suspension-cultured cells of carrot (Daucus carota). There were two putative cis-elements in the DcPAL3 promoter region: the box-L and GCC-box homologs. Both of these are committed to the upregulation of promoter activity. Although box-L is known as the conserved cis-element present in the promoter region of most PAL genes of many plant species targeted by the R2R3-MYB protein, among PAL genes, the GCC-box homolog is unique to the promoter region of the DcPAL3 gene. We have isolated two proteins belonging to the ethylene-responsive element-binding factor (ERF) family, DcERF1 and DcERF2, from two different cDNA libraries prepared from anthocyanin-synthesizing cells of different cultured cell lines of carrot. The methodology employed was yeast one-hybrid screening with the GCC-box homolog as a bait. Both DcERF1 and DcERF2 bound to the GCC-box homolog sequence in vitro. Transient expression analysis showed that, in carrot protoplasts, DcERF1 was able bind to the GCC-box homolog and act as an activator of the DcPAL3 promoter. In contrast, DcERF2 itself had no ability to activate DcPAL3 promoter activity, possibly because transiently expressed DcERF2 may not be exported into the nucleus. These results suggest that DcERF1 and DcERF2 may function in different ways in committing to the upregulation of the DcPAL3 promoter activity in anthocyanin-synthesizing cells of carrot.

DcMYB1 acts as a transcriptional activator of the carrot phenylalanine ammonia-lyase gene (DcPAL1) in response to elicitor treatment, UV-B irradiation and the dilution effect

Expression of a carrot phenylalanine ammonia-lyase (PAL) gene (DcPAL1) in suspension-cultured carrot cells is induced by treatment with a fungal elicitor, ultraviolet B (UV-B) irradiation, and by transferring and diluting cells with fresh medium (the dilution effect). Box-L-like sequences are known as important cis-elements of genes for enzymes involved in the phenylpropanoid biosynthetic pathway. Six sequences, box-L0 to box-L5, exist in the DcPAL1 gene promoter region. In this study, we isolated cDNA encoding the R2R3 type of MYB transcription factor, DcMYB1, using yeast one-hybrid screening with box-L1 or box-L5 as target elements. DcMYB1 bound to boxes-L0, L1, L3/4, and L5 sequences (ACC(A/T)(A/T)CC) in vitro, and in yeast cells and carrot protoplasts. Transient expression of DcMYB1 could up-regulate DcPAL1 promoter activity in carrot protoplasts. Results of the transient expression experiment for the deletion-mutated promoters of boxes-L0, L1, L3, and L5 suggest that these box-L-like sequences were required for the complete activation of the DcPAL1 promoter by DcMYB1. Expression of DcMYB1 transcripts was induced 0.5 h after elicitor treatment or UV-B irradiation, and 2 h after the dilution effect. Induction of DcPAL1 expression occurred 1 h after DcMYB1 expression in all stress treatments, and repression of DcMYB1 expression by RNA interference caused cessation of the up-regulation of DcPAL1 expression in the elicitor treatment or with UV-B irradiation. These results suggest that DcMYB1 is the main regulatory factor acting on box-L sequences in the DcPAL1 gene that respond to environmental cues.

Putative cis-elements in the promoter region of the carrot phenylalanine ammonia-lyase gene induced during anthocyanin synthesis

Deletion mutants of the carrot phenylalanine ammonia-lyase gene promoter were used to survey cis-elements for their effect on expression of promoter activity by transient expression. Two putative cis-elements were required to give full activity, but a third might be the most important in regulation of the promoter by 2,4-dichlorophenoxyacetic acid.