Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules downstream of receptors/sensors that transduce extracellular stimuli into intracellular responses in eukaryotes. Plant MAPK cascades play pivotal roles in signaling plant defense against pathogen attack. In this review, we summarize recent advances in the identification of upstream receptors/sensors and downstream MAPK substrates. These findings revealed the molecular mechanisms underlying MAPK functions in plant disease resistance. MAPK cascades have also emerged as battlegrounds of plant-pathogen interactions. Activation of MAPKs is one of the earliest signaling events after plant sensing of pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) and pathogen effectors. MAPK cascades are involved in signaling multiple defense responses, including the biosynthesis/signaling of plant stress/defense hormones, reactive oxygen species (ROS) generation, stomatal closure, defense gene activation, phytoalexin biosynthes...

MAPK cascades in plant disease resistance signaling.

Molecular communication between plants and potential pathogens determines the ultimate outcome of their interaction. The directed delivery of microbial molecules into and around the host cell, and the subsequent perception of these by the invaded plant tissue (or lack thereof), determines the difference between disease and disease resistance. In theory, any foreign molecule produced by an invading pathogen could act as an elicitor of the broad physiological and transcriptional re-programming indicative of a plant defense response. The diversity of elicitors recognized by plants seems to support this hypothesis. Additionally, these elicitors are often virulence factors from the pathogen recognized by the host. This recognition, though genetically as simple as a ligand-receptor interaction, may require additional host proteins that are the nominal targets of virulence factor action. Transduction of recognition probably requires regulated protein degradation and results in massive changes in cellular homeostasis, including a programmed cell death known as the hypersensitive response that indicates a successful, if perhaps over-zealous, disease resistance response.

Recognition and Response in the Plant Immune System

The triterpenes are one of the most numerous and diverse groups of plant natural products. They are complex molecules that are, for the most part, beyond the reach of chemical synthesis. Simple triterpenes are components of surface waxes and specialized membranes and may potentially act as signaling molecules, whereas complex glycosylated triterpenes (saponins) provide protection against pathogens and pests. Simple and conjugated triterpenes have a wide range of applications in the food, health, and industrial biotechnology sectors. Here, we review recent developments in the field of triterpene biosynthesis, give an overview of the genes and enzymes that have been identified to date, and discuss strategies for discovering new triterpene biosynthetic pathways.

Triterpene Biosynthesis in Plants

With tens of thousands of plant species on earth, we are endowed with an enormous wealth of medicinal remedies from Mother Nature. Natural products and their derivatives represent more than 50% of all the drugs in modern therapeutics. Because of the low success rate and huge capital investment need, the research and development of conventional drugs are very costly and difficult. Over the past few decades, researchers have focused on drug discovery from herbal medicines or botanical sources, an important group of complementary and alternative medicine (CAM) therapy. With a long history of herbal usage for the clinical management of a variety of diseases in indigenous cultures, the success rate of developing a new drug from herbal medicinal preparations should, in theory, be higher than that from chemical synthesis. While the endeavor for drug discovery from herbal medicines is “experience driven,” the search for a therapeutically useful synthetic drug, like “looking for a needle in a haystack,” is a daunting task. In this paper, we first illustrated various approaches of drug discovery from herbal medicines. Typical examples of successful drug discovery from botanical sources were given. In addition, problems in drug discovery from herbal medicines were described and possible solutions were proposed. The prospect of drug discovery from herbal medicines in the postgenomic era was made with the provision of future directions in this area of drug development.

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New Perspectives on How to Discover Drugs from Herbal Medicines: CAM's Outstanding Contribution to Modern Therapeutics.

In order to further increase the shoot regeneration frequency of Artemisia annua L., the effects of silver nitrate on this process was investigated in this study. Different concentration of silver nitrate was added to the shoot induction medium, which was MS basic medium containing 1.0 mg l−1 6-benzyladenine (6-BA) and 0.05 mg l−1 α-napthaleneacetic acid (NAA). When 2 mg l−1 silver nitrate was added to the shoot induction medium, the shoot induction frequency and shoot number per explants was significantly higher than that of the control (without silver nitrate). In addition, silver nitrate at all the tested concentrations could significantly reduce callus formation of the explants. Silver nitrate had also positive influence on shoot elongation in the first 20 days. Furthermore, silver nitrate did not affect the sensitivity of A. annua shoots to Kanamycin (KM); therefore, silver nitrate could be used to improve shoot regeneration capacity and frequency in A. annua genetic transformation.

Effects of silver nitrate on shoot regeneration of Artemisia annua L.

Advances in molecular regulation of artemisinin biosynthesis

Artemisinin, a sesquiterpene lactone endoperoxide, is a valuable and powerful antimalarial drug obtained from the aerial parts of a Chinese herb, Artemisia annua (Liu et al., 1979). Artemisinin and its derivatives show few or no side effects with the existing antimalarial drugs, it has consequently been regarded as the next generation of antimalarial drugs (Looaresuwan, 1994). Currently, commercial production of artemisinin mainly based on its extraction and purification from plant material, however, the endogenous production of artemisinin is very low (0.01%-0.8% dry weight) (Wallaart et al., 1999). In view of the limited availability of artemisinin and the increased demand, the synthetic preparation of artemisinin becomes an attractive proposition. However due to its complex structure, the complete chemical synthesis is very difficult (Schmid & Hofheinz, 1983). Artemisia annua as the only valid source, many research groups have directed their investigations toward the enhancement of artemisinin production in A. annua cell cultures or whole plants by biotechnological approaches. However these approaches were still proved to be not successful (Ghingra et al., 2000). Recently, several genes in artemisinin biosynthesis have been cloned, and important advances in artemisinin biosynthesis have been achieved, which makes it possible to regulate artemisinin biosynthesis in a direct way, for example, by metabolic engineering (Abdin et al., 2003).

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Effect of Antisense Squalene Synthase Gene Expression on the Increase of Artemisinin Content in Artemisia anuua

By using the TAIL-PCR and over-hang extension PCR methods, we successfully isolated a three-intron type III PKS gene and cDNA clone from the Polygonum cuspidatum Sieb. et Zucc, and named it PcPKS4. The phylogenetic analysis showed that PcPKS4 is located in a cluster containing all functionally divergent CHS-like enzymes from angiosperms. Interestingly, the CHS “gatekeeper” phenylalanine, Phe265 is uniquely replaced by Leu in PcPKS4. Different from our previously results, RNA gel–blot analysis revealed that the PcPKS4 transcripts were present at a low level in roots.

Cloning of a novel type III polyketide synthase encoded by a three-intron gene from Polygonum cuspidatum

Artemisia annua L. is a traditional Chinese medicinal plant. In the 1970s, Chinese scientists first isolated colorless needle-like crystals with antimalarial activity from A. annua and named artemisinin. The chemical structure of artemisinin was confirmed as a sesquiterpene lactone with an endoperoxide bridge, of melting point 156–157 °C and molecular formula C15H22O5.

Benye Liu

Papers

Effects of silver nitrate on shoot regeneration of Artemisia annua L.

Advances in molecular regulation of artemisinin biosynthesis

Effect of Antisense Squalene Synthase Gene Expression on the Increase of Artemisinin Content in Artemisia anuua

Cloning of a novel type III polyketide synthase encoded by a three-intron gene from Polygonum cuspidatum

Metabolomics-Based Studies on Artemisinin Biosynthesis