Johan Memelink

The AP2/ERF domain transcription factor ORA59 integrates jasmonic acid and ethylene signals in plant defense

Plant defense against pathogens depends on the action of several endogenously produced hormones, including jasmonic acid (JA) and ethylene. In certain defense responses, JA and ethylene signaling pathways synergize to activate a specific set of defense genes. Here, we describe the role of the Arabidopsis (Arabidopsis thaliana) APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain transcription factor ORA59 in JA and ethylene signaling and in defense. JA- and ethylene-responsive expression of several defense genes, including PLANT DEFENSIN1.2 (PDF1.2), depended on ORA59. As a result, overexpression of ORA59 caused increased resistance against the fungus Botrytis cinerea, whereas ORA59-silenced plants were more susceptible. Several AP2/ERF domain transcription factors have been suggested to be positive regulators of PDF1.2 gene expression based on overexpression in stably transformed plants. Using two different transient overexpression approaches, we found that only ORA59 and ERF1 were able to activate PDF1.2 gene expression, in contrast to the related proteins AtERF1 and AtERF2. Our results demonstrate that ORA59 is an essential integrator of the JA and ethylene signal transduction pathways and thereby provide new insight into the nature of the molecular components involved in the cross talk between these two hormones.

A novel jasmonate- and elicitor-responsive element in the periwinkle secondary metabolite biosynthetic gene Str interacts with a jasmonate- and elicitor- inducible AP2-domain transcription factor, ORCA2

Jasmonate (JA) is an important plant stress hormone that induces various plant defense responses, including the biosynthesis of protective secondary metabolites. The induction of the secondary metabolite biosynthetic gene Strictosidine synthase (Str) in Catharanthus roseus (periwinkle) cells by elicitor requires JA as a second messenger. A 42 bp region in the Str promoter is both necessary and sufficient for JA‐ and elicitor‐responsive expression. This region is unlike other previously identified JA‐responsive regions, and contains a GCC‐box‐like element. Yeast one‐hybrid screening identified cDNAs encoding two AP2‐domain proteins. These octadecanoid‐derivative responsive Catharanthus AP2‐domain (ORCA) proteins bind in a sequence‐specific manner the JA‐ and elicitor‐responsive element. ORCA2 trans‐activates the Str promoter and its expression is rapidly inducible with JA and elicitor, whereas Orca1 is expressed constitutively. The results indicate that a GCC‐box‐like element and ORCA2 play key roles in JA‐ and elicitor‐responsive expression of the terpenoid indole alkaloid biosynthetic gene Str.

Biotechnology for the production of plant secondary metabolites

The production of plant secondary metabolites by means of large-scale culture of plant cells in bioreactors is technically feasible. The economy of such a production is the major bottleneck. For some costly products it is feasible, but unfortunately some of the most interesting products are only in very small amounts or not all produced in plant cell cultures. Screening, selection and medium optimization may lead to 20- to 30-fold increase in case one has producing cultures. In case of phytoalexins, elicitation will lead to high production. But for many of the compounds of interest the production is not inducible by elicitors. The culture of differentiated cells, such as (hairy) root or shoot cultures, is an alternative, but is hampered by problems in scaling up of such cultures. Metabolic engineering offers new perspectives for improving the production of compounds of interest. This approach can be used to improve production in the cell culture, in the plant itself or even production in other plant species or organisms. Studies on the production of terpenoid indole alkaloids have shown that the overexpression of single genes of the pathway may lead for some enzymes to an increased production of the direct product, but not necessarily to an increased alkaloid production. On the other hand feeding of such transgenic cultures with early precursors showed an enormous capacity for producing alkaloids, which is not utilized without feeding precursors. Overexpression of regulatory genes results in the upregulation of a series of enzymes in the alkaloid pathway, but not to an improved flux through the pathway, but feeding loganin does result in increased alkaloid production if compared with wild-type cells. Indole alkaloids could be produced in hairy root cultures of Weigelia by overexpression of tryptophan decarboxylase and strictosidine synthase. Alkaloids could be produced in transgenic yeast overexpressing strictosidine synthase and strictosidine glucosidase growing on medium made out the juice of Symphoricarpus albus berries to which tryptamine is added. Metabolic engineering thus seems a promising approach to improve the production of a cell factory.

ORCAnization of jasmonate-responsive gene expression in alkaloid metabolism

Jasmonic acid is an important plant stress signalling molecule. It induces the biosynthesis of defence proteins and protective secondary metabolites. In alkaloid metabolism, jasmonate acts by coordinate activation of the expression of multiple biosynthesis genes. In terpenoid indole alkaloid metabolism and primary precursor pathways, jasmonate induces gene expression and metabolism via ORCAs, which are members of the AP2/ERF-domain family of plant transcription factors. Other jasmonate-regulated (secondary) metabolic pathways might also be controlled by ORCA-like AP2/ERF-domain transcription factors. If so, such regulators could be used to improve plant fitness or metabolite productivity of plants or cell cultures.

Engineering secondary metabolite production in plants

Recent achievements have been made in the metabolic engineering of plant secondary metabolism. Various pathways have been altered using genes encoding biosynthetic enzymes or genes encoding regulatory proteins. In addition, antisense genes have been used to block competitive pathways, thereby increasing the flux towards the desired secondary metabolites.

Transcription factors controlling plant secondary metabolism: what regulates the regulators?

Plants produce secondary metabolites, among others, to protect themselves against microbial and herbivore attack or UV irradiation. Certain metabolite classes also function in beneficial interactions with other organisms. For example, anthocyanin pigments and terpenoid essential oils have key roles in attraction of flower pollinators. Secondary metabolites also have direct uses for man. Flavonoids and terpenoids for example have health-promoting activities as food ingredients, and several alkaloids have pharmacological activities. Controlled transcription of biosynthetic genes is one major mechanism regulating secondary metabolite production in plant cells. Several transcription factors involved in the regulation of metabolic pathway genes have been isolated and studied. There are indications that transcription factor activity itself is regulated by internal or external signals leading to controlled responses. The aim of this review is to discuss the regulation of transcription factors involved in secondary metabolism in plants at gene and protein levels, using phenylpropanoid and terpenoid indole alkaloid pathways as two well-studied examples.

Geraniol 10-hydroxylase1, a cytochrome P450 enzyme involved in terpenoid indole alkaloid biosynthesis

Geraniol 10‐hydroxylase (G10H) is a cytochrome P450 monooxygenase involved in the biosynthesis of iridoid monoterpenoids and several classes of monoterpenoid alkaloids found in a diverse range of plant species. Catharanthus roseus (Madagascar periwinkle) contains monoterpenoid indole alkaloids, several of which are pharmaceutically important. Vinblastine and vincristine, for example, find widespread use as anti‐cancer drugs. G10H is thought to play a key regulatory role in terpenoid indole alkaloid biosynthesis. We purified G10H from C. roseus cells. Using degenerate PCR primers based on amino acid sequence information we cloned the corresponding cDNA. The encoded CYP76B6 protein has G10H activity when expressed in C. roseus and yeast cells. The stress hormone methyljasmonate strongly induced G10h gene expression coordinately with other terpenoid indole alkaloid biosynthesis genes in a C. roseus cell culture.

Coordinated regulation of two indole alkaloid biosynthetic genes from Catharanthus roseus by auxin and elicitors

Catharanthus roseus (periwinkle) produces a wide range of terpenoid indole alkaloids, including several pharmaceutically important compounds, from the intermediate strictosidine. The complete mRNA sequence for the enzyme strictosidine synthase (SSS) was determined. Comparison of the primary structure of the encoded protein with the amino-terminal sequence of purified SSS indicated the presence of a signal peptide of 31 amino acids in the putative primary translation product. SSS is encoded by a single-copy gene indicating that isoenzymes reported by others are formed post-translationally from a single precursor. The sss gene and the tryptophan decarboxylase gene (tdc), encoding another enzyme essential for indole alkaloid biosynthesis, are coordinately regulated. In plants steady-state mRNA levels are highest in roots. In cell suspension cultures the genes are rapidly down-regulated by auxin. In contrast, both genes are strongly induced by fungal elicitors such as Pythium aphanidermatum culture filtrate or yeast extract. Induction is a rapid, transcriptional event occurring independent of de novo protein synthesis. These results show that a first important regulatory step in the complex process leading to indole alkaloid accumulation in C. roseus suspension cells is transcription of the biosynthetic genes.

Metabolic engineering of plant secondary metabolite pathways for the production of fine chemicals

The technology of large-scale plant cell culture is feasible for the industrial production of plant-derived fine chemicals. Due to low or no productivity of the desired compounds the economy is only in a few cases favorable. Various approaches are studied to increase yields, these encompass screening and selection of high producing cell lines, media optimization, elicitation, culturing of differentiated cells (organ cultures), immobilization. In recent years metabolic engineering has opened a new promising perspectives for improved production in a plant or plant cell culture.

Salicylic Acid Suppresses Jasmonic Acid Signaling Downstream of SCFCOI1-JAZ by Targeting GCC Promoter Motifs via Transcription Factor ORA59

Interactions between the plant hormones salicylic acid and jasmonic acid play an important role in the regulation of plant defense responses against pathogens and insects. This work provides mechanistic insight into this hormonal crosstalk by showing that salicylic acid antagonizes jasmonic acid–dependent defenses by targeting the transcriptional activator ORA59. Antagonism between the defense hormones salicylic acid (SA) and jasmonic acid (JA) plays a central role in the modulation of the plant immune signaling network, but the molecular mechanisms underlying this phenomenon are largely unknown. Here, we demonstrate that suppression of the JA pathway by SA functions downstream of the E3 ubiquitin-ligase Skip-Cullin-F-box complex SCFCOI1, which targets JASMONATE ZIM-domain transcriptional repressor proteins (JAZs) for proteasome-mediated degradation. In addition, neither the stability nor the JA-induced degradation of JAZs was affected by SA. In silico promoter analysis of the SA/JA crosstalk transcriptome revealed that the 1-kb promoter regions of JA-responsive genes that are suppressed by SA are significantly enriched in the JA-responsive GCC-box motifs. Using GCC:GUS lines carrying four copies of the GCC-box fused to the β-glucuronidase reporter gene, we showed that the GCC-box motif is sufficient for SA-mediated suppression of JA-responsive gene expression. Using plants overexpressing the GCC-box binding APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) transcription factors ERF1 or ORA59, we found that SA strongly reduces the accumulation of ORA59 but not that of ERF1. Collectively, these data indicate that the SA pathway inhibits JA signaling downstream of the SCFCOI1-JAZ complex by targeting GCC-box motifs in JA-responsive promoters via a negative effect on the transcriptional activator ORA59.