Phytoalexin

Phytoalexin

1. Phytoalexins Vaishali S.Patil Professor, Department of Botany Shri Shivaji College of Arts, Commerce & Science Akola

2. Introduction Phyto =plant, alexin =warding off compound Phytoalexins are toxic antimicrobial and often  antioxidative substances synthesized ‘de novo ’(in the begining ) in the plants in response to injury, infectious agents. The term phytoalexin was first used by the two phytopathologists Muller and Borger (1940 ) for fungi static compounds produced by plants in response to mechanical or chemical injury or infection . Cruickshank and Perrin (1960)  crystallized and chemically characterized the first phytoalexin ; this was pisatin , a pterocarpan derivative produced by pods. Other examples are Phaseolus vulgaris , pterocarpans (e.g. phaseollin ) on fungal inoculation; Convolvulaceae  ( ipomeamarone from infected sweet potato) and of the  Orchidaceae  ( orchinol from

3. orchid tubers). All phytoalexins are lipophilic compounds and were first detected after a study of late blight of potato caused by Phytophthora infestans . It is a compound which inhibits the development of the fungus in hypersensitive tissues and is formed or activated  only when   the host plants come in contact with the parasite. Some fungi have the capacity to further metabolize and detoxify phytoalexins . Phytoalexins are believed to be synthesized in living cells but surprisingly necrosis(death of tissue) follows very quickly. A metabolite of the host plant interacts with specific receptor on the pathogen’s membrane resulting in the secretion of “ phytoalexin elicitor” which enters the host plant cells and stimulates the phytoalexin synthesis.

4. Phytoalexins are considered to stop the growth of pathogens by altering the plasma membrane and inhibiting the oxidative phosphorylation . Phytoalexins have been identified in a wide variety of species of plants such as Soyabean , Potato, sweet potato, barley, carrot, cotton etc. Classes are   terpenoids ,  glycosteroids  and alkaloids. Some common phytoalexins are Ipomeamarone , Orchinol , Pistatin , Phaseolin , Medicarpin , Rishitin , Isocoumarin , ‘Gossypol’ Cicerin , Glyceolin , Capisidiol etc. The inhibitory agent is a discrete chemical substance, a product of the host cell. They are non-specific in its toxicity towards fungi; however, fungal species may be differentially sensitive to it.

5. The basic response in both resistant and susceptible cells is the same, the basis of differentiation between resistant and susceptible hosts being the speed of formation of the phytoalexin . The defence reaction is confined to the tissue colonized by the fungus and its immediate neighbourhood. The resistant state is not inherited; it is developed after the fungus has attempted infection. The sensitivity of the host cell which determines the speed of the host reaction is specific and genotypically determined. In all phytoalexin is a substance that is produced by plant tissues in response to contact with a parasite and specifically inhibits the growth of that parasite.

6. Types-The following Table gives a list of phytoalexins , chemical nature, the host and the pathogens in response to which these are produced:

7. 1. Ipomeamarone : Ipomeamarone is a furanosesquiterpene ketone ( tetrahydrofuran , C 15 H 22 O 3 ; mel.wt . 230) and is synthesized in the roots of sweet potato (Ipomoea batata ) infected with the fungus Ceratocystis funbriata causing black rot disease. Biosynthesis of ipomeamarone may involve mevalonic acid as a precursor and could perhaps also be induced by poisonous chemicals and pectinase enzyme. It functions as an inhibitor of electron transport and energy transfer reactions and thus prevents mycelial growth, sporulation , and protein synthesis of the pathogen. Ipomeamarone is also produced in roots of sweet potato in response to infection by violet root fungus Helicobasidium mompa , and in presence of toxic chemicals. However, ipomeamarone is toxic to both non­pathogenic and pathogenic fungi and to some bacteria, and there seems to be no correlation between its inhibitory effect and pathogenecity .

8. 2. Pisatin : Pisatin is a weak phytoalexin with broad spectrum. It possesses chromocoumarin ring system and is phenolic ether (C 17 H 14 O 6 , mol. wt. 314). It has been reported produced by the exposed endocarp of detached pea pods in response to inoculation of many fungi (e.g., Monilinia fructicola , Ascochyta pisi ) or injury. When high concentrations of pisatin are accumulated, as happens when the storage temperature is normal and conditions are aerobic, the pea pods show resistance to these fungi. Studies reveal that plants produce high concentrations of pisatin as found in host against pathogenic fungi. 3. Phaseollin : Phaseollin is similar to pisatin in chemistry and function. It was isolated from detached, opened bean pods following inoculation with non-pathogenic fungus Monilinia fructicola . It has also been detected in bean leaves inoculated with the bacterium Pseudomonas phaseolicola . More recently, a number of compounds such as ‘ phaseollidin ’, ‘ phaseollinisoflavan ’ and ‘ kievitone ’, which are structurally similar to phaseollin , have been identified from diffusates or tissues inoculated

9. with fungi, bacteria, and viruses. Phaseollin has been found important in determining resistance at variety level to bean anthracnose caused by Colletotrichum lindemuthianum . It accumulates more rapidly in bean hypocotyl inoculated with an avirulent race of the pathogen as against much lower accumulation when inoculated with a virulent race. 4. Medicarpin : This antifungal compound was isolated from diffusate solutions of leaves of Medicago spp. (alfalfa) when inoculated with many pathogenic and non-pathogenic fungi such as Colletotrichum phomoides , Helminthosporium turcicum . This antifungal compound has been identified as (-) dimethyl-homopterocarpin and named ‘ medicarpin ’. Medicarpin occurs very commonly in legumes and is subject to de- methylation . Possibly significantly, demethylmedicarpin was found in leaves of groundnut susceptible to leaf rust caused by Puccinia arachidis but not in cultivars which were resistant.

10. 5. Rishitin : Rishitin is a nonsesquiterpene alcohol and is produced by potato tubers of Solatium tuberosum var. rishiri inoculated with zoospores of an incompatible race of Phytophthora infestans ; produced by potato tubers inoculated with Fusarium solani f. sp. phaseoli . 6. Glyceollin I : Glyceollin I have been isolated from soybean roots inoculated with the fungus Phytophthora megasperma . f.sp . glycinea . Inoculation of fungal races on susceptible host cultivars resulted in lower concentrations of glyceollin I than in inoculations of fungal races on resistant cultivars in which the concentrations were higher. Plant pathologists suggest that the higher concentrations of the phytoalexin in resistant cultivars are the result of reduced biodegradation rather than increased biosynthesis. Recently in 1991, when a resistant cultivar and a susceptible cultivar of soybean were inoculated with the soybean cyst nematode, Heterodera glycines , it was found that glyceollin I accumulated in the head region of the nematode 8 hour after inoculation in the resistant cultivar, whereas

11. none was found in the nematode inoculated in the susceptible cultivar. 7 . Orchinol : Orchinol is a phenanthrene produced by tubers of orchids (e.g., O. malitaris , O. mario ) when infected with fungus Rhizoctonia repens . It is a strong fungistatic and its production is extended over the whole tuber. Antimicrobial spectrum of orchinol is broad and hardly specific. 8. Gossypol: Gossypol is an ether soluble phenol and has been first isolated in 1967 by Bell from xylem vessels of the excised stems of Gossypium species and several cultivars of cotton inoculated with conidia of Verticillium alboatrum and Rhizopus nigricans , heavy metal ions and various metabolic inhibitors. He related gossypol and gossypol-like compounds to host resistance and inversely to the virulence of the pathogen. 9. Cicerin : Cicerin is composed of two fluorescent phenols or phenolic compounds and has been isolated from Cicer (gram) diseased by Ascochyta blight. This compound diffuses into inoculation droplets during the course of interaction between pod tissues of Cicer arientinum and the pathogen .

12. 10. Isocoumarin : Isocoumarin is produced in the tap root tissues of carrot ( Daucus carota ) inoculated with Cyratocystis fimbriata , a non-pathogen of carrot roots. This compound is also produced in some quantity when tap root tissues of carrot are inoculated with fungal pathogens such as Fusarium oxysporum f.sp . lycopersici , Rhizopus stolonifer , Helminthosporium carbonum . Besides the above described, some other phytoalexins have also been isolated in recent years. Such phytoalexins are – Trifolirhizin (isolated from the roots of red clover); 11.In  Vitis vinifera  grape,  trans - resveratrol   is a phytoalexin produced against the growth of fungal pathogens such as  Botrytis cinerea and  delta- viniferin  is another grapevine phytoalexin produced following fungal infection by  Plasmopara viticola . 12. Pinosylvin is a pre-infectious  stilbenoid  toxin (i.e. synthesized prior to infection), contrary to phytoalexins which are synthesized during infection. It is present in the heartwood of  Pinaceae. It is a fungitoxin protecting the wood from fungal infection.

13. 13. Sakuranetin  is a  flavanone , a type of flavonoid . It can be found in  Polymnia fruticosa and rice , where it acts as a phytoalexin against spore germination of  Pyricularia oryzae . 14.I n  Sorghum , the  SbF3'H2  gene, encoding a  flavonoid 3'-hydroxylase , seems to be expressed in pathogen-specific 3-deoxyanthocyanidin phytoalexins synthesis, for example in  Sorghum- Colletotrichum interactions. 15. 6-Methoxymellein is a  dihydroisocoumarin  and a phytoalexin induced in carrot slices by UV-C, that allows resistance to  Botrytis cinerea  and other microorganisms. 16. Danielone  is a phytoalexin found in the  papaya  fruit. This compound showed high antifungal activity against  Colletotrichum gloesporioides , a pathogenic fungus of papaya. 17. Stilbenes  are produced in  Eucalyptus sideroxylon  in case of pathogens attacks. Such compounds can be implied in the hypersensitive response of plants. High levels of  polyphenols  in some woods can explain their natural preservation against rot.

14. 18. Allixin  (3-hydroxy-5-methoxy-6-methyl-2-pentyl-4 H -pyran-4-one), a non- sulfur -containing compound having a γ- pyrone  skeleton structure, was the first compound isolated from  garlic  as a phytoalexin , a product induced in plants by continuous stress. This compound has been shown to have unique biological properties, such as anti-oxidative effects, anti-microbial effects, anti- tumor promoting effects, inhibition of  aflatoxin  B2 DNA binding, and neurotrophic effects.  Allixin showed an anti- tumor promoting effect in vivo, inhibiting skin  tumor  formation by TPA in DMBA initiated mice. Herein, allixin and/or its analogs may be expected useful compounds for cancer prevention or chemotherapy agents for other diseases Other types- Wyerone acid (isolated from the leaves of broad bean infected with fungus Botrytis fabae ); hircinol (isolated from Loroglassum hircinum against outside stimulus); capsidiol (isolated from diffusates from pepper, Capsicum frutescens , inoculated with number of non-pathogenic fungi); xanthotoxin (isolated from the top millimeter of parsnip root discs inoculated with Ceratocystis fimbriata , a non-pathogen); camalexin from Arobidopsis thaliana ; fungus Ascochyta rabie . casbene from Ricinus communis ; phytuberin from Solatium tuberosum ; etc.

15. Synthesis of Phytoalexins Host plant surface +water Diffusion of host nutrients Host lechates +Fungal spores Germination & growth of fungus Mycoantigen+viable host cells Stimulation of host metabolism Production of Phytoalexin

16. Function 1.Phytoalexins produced in plants act as toxins to the attacking organism. 2.They may puncture the cell wall, delay maturation, disrupt metabolism or prevent reproduction of the pathogen in question. 3.They  increase in susceptibility of plant tissue to infection. 4.When a plant cell recognizes particles from damaged cells or particles from the pathogen, the plant launches a two-pronged resistance i ) a general short-term response ii) a delayed long-term specific response . i ) short-term response, the plant deploys reactive oxygen species such as superoxide and hydrogen peroxide to kill invading cells. In pathogen interactions, the common short-term response is the hypersensitive response, in which cells surrounding the site of infection are signaled to undergo apoptosis ,( cell suicide) or programmed cell death, in order to prevent the spread of the pathogen to the rest of the plant . ii) Long-term resistance, or systemic acquired resistance (SAR), involves communication of the damaged tissue with the rest of the plant using plant hormones such as  jasmonic acid , ethylene,  abscisic acid   

17. or salicylic acid. The reception of the signal leads to global changes within the plant, which induce genes that protect from further pathogen intrusion, including enzymes involved in the production of phytoalexins . Often, if jasmonates or ethylene (both gaseous hormones) is released from the wounded tissue, neighbouring plants also manufacture phytoalexins in response. For herbivores, common vectors for disease, these and other wound response aromatics seem to act as a warning that the plant is no longer edible.