Phytoalexins Presented By Jadhav Murlidhar S Ph.D Biotechnology email@example.com
• Phytoalexins are defined as "low molecular weight, anti- microbial compounds that are both synthesized and accumulated in plants after exposure to microorganisms or abiotic agents“• The term phytoalexin is derived from Greek- phyto meaning plant and alexin means warding off compound
• The concept was formalized by Müller & Börger (1941)• Müller (1958) demonstrated the detection of a chemical entity as a phytoalexin while working with the hypersensitive response of bean tissue to the soft-fruit pathogen Monilinia fructicola
Concept of Phytoalexins• Mueller and Borger concept of Phytoalexins and their conclusions :• a) phytoalexin is formed only when the host cells come into contact with the parasite.• b) the defence reaction occurs only in the living cells.• c) the inhibitory material is a chemical substance & may be regarded as a product of necrobiosis of the host cell.
Contd………• d) phytoalexin is non-specific in its toxicity.• e) the resistant state is not inherited.• f) the defence reaction is confined to the tissue colonized by the fungus and its immediate neighbourhood.
Chemical Nature• They are broad spectrum inhibitors and are chemically diverse with different types characteristic of particular plant species.• Phytoalexins tend to fall into several classes including terpenoids, glycosteroids and alkaloids• Derivatives simple phenylpropanoid pathway, Shikimic acid pathway, Trp pathway and mevalonic acid pathway (Hammerschmidt, 1999)
• Derived from one or more primary biosynthetic pathway Capsidol-MVA pisatin- shikimic A and acetate -malonate pathway• Much diverse in chemical structure, phytoalexins produced by many plant families fall into the same class• Used to examine chemotaxonomic relationship
• Phytoalexin production is often associated with a widespread but poorly understood plant disease defense reaction called the hypersensitive reaction (HR).• observed after a few hours or a few days following infection by an incompatible race or species of plant pathogen as the death or d i s o r g a n i z a t i o n of the plant c e l l s immediately adjacent to the infection site concomitant with or preceeding the r e s t r i c t i o n of pathogen development
Phytoalexins in Health• Indole phytoalexins (Camalexin) have antioxidant, anticarcinogenic and cardiovascular protective activities of Brassica vegetables• Peanut (Arachis hypogea) phytoalexins have antidiabetic, anticancer and vasodilator effects• Glyceollin, a soybean (Glycine max) have antiproliferative and antitumor actions• The sorghum (Sorghum bicolor) phytoalexins, 3-deoxyanthocyanins, might be useful in helping to reduce incidence of gastrointestinal cancer• The phytoalexin resveratrol from grapevine (Vitis vinifera) has anti- aging, anticarcinogenic, anti-inflammatory and antioxidant properties (Ahuja et al, 2011)
• To evaluate the importance of phytoalexins in defence the following criteria are used:• 1. The restriction of the pathogen development must be associated it phytoalexin production,• 2. Phytoalexins must accumulate to antimicrobial levels at the infection site in resistant plants or cultivars that could result the cessation of the pathogen growth• 3. There must be strong evidence that the phytoalexins have vital importance in resistance, and absence of these compounds would result enhanced susceptibility (Merk-Turk, 2002)
TYPES OF PHYTOALEXINS:• Ipomoeamarone:• It is an abnormal sesquiterpinoid induced in sweet potato tissue infected with black rot fungus Ceratocystis fimbriata. It has a striking inhibitory effect on the fungus even in 0.1% concentrations. More phytoalexin is produced in the resistant varieties than in susceptible ones. (Ahuja et al, 2012)
• Pisatin: ▫ It has the chromocoumarin ring system and is a phenolic ether. produced in pea in response to inoculation with many fungi or injury. ▫ Production of pisatin by peapods inoculated with Monilia fructicola , a non pathogen is reduced at high temperature &on anaerobic storage. It is a weak antibiotic with broad spectrum
contd….• Phaseollin: ▫ It is similar to pisatin in chemistry and function. It is fungicidal at high concentrations and fungistatic at low concentrations against S. fructigena. ▫ A no. of compounds such as phaseollidin , phaseollinisoflavan and kievitone which are structurally similar to phaseollin have been identified.
contd….Glyceollin: produced in soybean plants infected with the fungus Phytophthora megasperma f.sp.glycinea. Inoculation of fungal races resulted in higher concentrations in incompatible host cultivars than in inoculations of fungal races on compatible cultivars. Due to reduced biodegradation rather than increased biosynthesis.
contd….• Isocoumarin: ▫ isolated from carrot root tissues inoculated with a fungus non-pathogenic to carrot, Ceratocystis fimbriata. ▫ It can also be produced in response to a no.of non- pathogenic microorganisms such as C.ulmi, Helminthosporium carbonum, Fusarium oxysporum f.sp.lycopersici & Thielaviopsis basicola. ▫ chemically related to the pterocarpan phaseollin.
contd….▫ Trifolirhizin: It is a new glucoside which has been isolated from the roots of red cloves. Its structure indicates that it is chemically closely related to pisatin.▫ Rishitin: Muller and Boerger(1940) were the first to show that the potato tubers carying the gene R1 for late blight resistance responded when inoculated with avirulent race of P.infestans by producing a phytoalexin that inhibited the development of a virulent race.▫ It is a bicyclic non-sesquiterpine alcohol
• Gossypol: ▫ It is an ether soluble phenol . It is produced in diseases like black spot of rose (Diplocarpon rosa),leaf spot of wheat (Septoria tritici).• Xanthotoxin: ▫ Isolated from parsnip root discs inoculated with C. fimbriata Inoculation with other non pathogens resulted in production of xanthotoxin
Capsidiol: it is a sequisterpene phytoalexin produced in pepper fruits inoculated with a non – pathogenic fungi. Produced concentrations are sufficient to inhibit these fungi in vitro.Medicarpin: Alfalfa (Medicago sativa) inoculated with a series of pathogens and non pathogens have been studied. The antifungal compound was isolated and identified as Medicarpin
• Camalexin: ▫ an indolic secondary metabolite, is a major phytoalexin in Arabidopsis thaliana. Its synthesis is stimulated by a variety of microorganisms ▫ including Pseudomonas syringae, Alternaria brassicicola, and Botrytis cinerea and by ▫ some abiotic stresses, such as AgNO3 and amino acid starvation, and it has been shown to inhibit the growth of fungal pathogens. ▫ However, the signaling pathway connecting pathogen infection to camalexin biosynthesis is not completely known
Induction of phytoalexin biosynthesis• induction of a mitogen-activated protein kinase (MAPK) cascade involving MPK3 and MPK6.• Camalexin induction in Arabidopsis infected with P. syringae is dependent on the transcription factor WRKY33, which binds directly to the promoter of the camalexin biosynthesis gene PAD3 (Qiu et al., 2008).• MPK3/ MPK6 signaling leads directly to phosphorylation of WRKY33, and this drives camalexin production in Arabidopsis challenged by pathogens. (Kishi-Kobashi, 2010)
Nancy A. Eckardt• This work establishes a direct link between MPK3/MPK6 and WRKY33, demonstrating that WRKY33 is a target of MPK3/ MPK6 signaling• and is necessary for the induction of camalexin biosynthesis in Arabidopsis following infection by the necrotrophic fungus B. cinerea.
Conclusion• Phytoalexins are only one components of the complex mechanisms for disease resistance in plants• Most of them regulated through MAP kinase signalling pathway• Health promoting effect• Challenge is to decipher and identify the complete biosynthetic pathway and the key enzyme to employ transgenic strategy in disease resistance