PHYTOALEXINS

The isolation, structure determination, total synthesis and antifungal activity of erucalexin, a novel

phytoalexin produced by the wild crucifer dog mustard are described. Erucalexin is a structurally unique plant

alkaloid, representing the first example of a spiro[2H-indole-2,5′(4′H)-thiazol]-3-one, likely derived from a C-

3–C-2 carbon migration in a 3-substituted indolyl nucleus.

Natural Peanut Resistence to Pests

Stilbene phytoalexin (in peanuts)

A dissected peanut kernel infected by a soil fungus (Aspergillus niger). Yellow-colored

phytoalexin is locally produced by the kernel tissues (arrow). The phytoalexin (termed SB-

1, the structure is shown) was first discovered at the NPRL.

Photomicrograph of peanut root tip showing the yellow mucilage layer. Seven new

hydrophobic prenylated stilbenoids, including mucilagin A, were isolated from the mucilage

for the first time; their structures were elucidated. These compounds were restricted to

mucilage, which suggests their special role in regulating root-soil pathogen interactions.

Chemical structures of stilbene phytoalexins. Glc: glucosyl (C6H11O5)

Interrelationship of stilbene phytoalexin production and resistance of several peanut genotypes to major

peanut diseases, including TSWV was also demonstrated for the first time.

Flavonoids and spermidines. Sound seed yield depends heavily on high peanut resistance to pests and

uncompromised plant fertility. Peanut flowers have been considered resistant to pests. Compounds that may

be responsible for such resistance have not been investigated.

The study revealed the production of flavonoid and spermidine conjugates in the peanut flower at high levels.

All major metabolites have been detected in the flowers for the first time. One of the metabolites,

acetyldicoumaroylspermidine (structure is shown below), has not been previously reported in plants. Both

flavonoid and spermidine conjugates may play a protective role against pests as it has been shown for similar

compounds in other plants. The new spermidine conjugate may be involved in regulating the plant

reproduction process.

Shown are the peanut flower and the structure of a new spermidine derivative. 1 –

standard; 2 – one of two symmetrical wings (one of the wings is removed for a better

observation of the keel); 3 – keel (2 fused petals that enclose pistil and stamens); 4 –

hypanthium.

Spermidine compounds, similar to the one above, have been reported to appreciably inhibit HIV-1 protease

that is essential for the life-cycle of HIV. One of the compounds also demonstrated high activity against

Helicobacter pylori, a major etiological agent in gastroduodenal disorders. Study of biological activity of the

new peanut spermidine derivative is anticipated.

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ARABIDOPSIS

Secondary products derived from aromatic amino acids Camalexin

Camalexin (3-thiazol-2'yl-indole), an indolic secondary metabolite, is a major phytoalexin in Arabidopsis. Camalexin accumulation caused by infection with plant pathogens and by abiotic elicitors inhibits the growth of pathogenic fungi and bacteria.

The indole ring of camalexin is proposed to be derived from indole-3-glycerol phosphate, an intermediate in tryptophan biosynthesis. The accumulation of this phytoalexin is accompanied by the induction of mRNAs and proteins of all of the tryptophan biosynthetic enzymes. Salicylic acid is required, but is not sufficient for this co-ordinated induction. Surprisingly, however, camalexin induction was only modestly affected in the trp5, trp4, trp1 and trp4:trp1 double mutants . This suggests either that the indole ring of camalexin may be derived from biosynthetic pathways other than tryptophan, or that there may be other isoenzymes important for the production of this secondary metabolite (cf. Ruta graveolens, where fungal elicitors induce a tryptophan-insensitive ASa gene. The latter explanation seems unlikely because trp1 is defective in what appears to be the only gene encoding PAT in Arabidopsis . The tryptophan pathway enzymes and camalexin accumulation are induced in response to amino acid starvation in Arabidopsis.

Camalexin incorporates label from anthranilate but not from tryptophan in Arabidopsis, suggesting that the camalexin biosynthetic pathway originates from an intermediate of the Trp pathway between anthranilate and indole. Zook It has been postulated that indole destined for camalexin synthesis is produced by a separate enzymatic reaction that does not involve tryptophan synthase.

Cysteine serves as the precursor of the thiazole ring of camalexin. Condensation of indole-3-carboxaldehyde with cysteine is proposed as the first step in this pathway

KNOWN RICE PHYTOALEXINS

OTHER PHYTOALEXINS

Ipomeamarone

A sesquiterpenoid with a striking inhibitory effect of the

fungus even in 0,1% concentrations.

Causal organism: Ceratocystis fimbriata Found on moldy sweet potatoes (eg Ipomea batatas) infected with

black rot. Remarkable silence in the medical literature in early 1960's but then noted to be hepatotoxic. There

is a toxic derivative called ipomeamaronol.

Causal organism : Alternaria alternata or black rot found in a wide variety of vegetables,

here in peas .

Pisatin has the chromocoumarin ring system and is a phenolic ether .Produced by

the exposed endocarp of the detached pea in response to inoculation with many funi or injury.

Causal organism : Pseudomonas syringae pv. Phaseolicola (bacterium) which forms a halo blight.

This induces the production of phytoalexin phaseollin. It is similar to pisatin

in chemistry and function.

Causal organism : Phytopthora meagasperma f.sp. glycinea in soybean

Glyceollin I is a glyceollin, a type of prenylated pterocarpan. It is a phytoalexin found in the soybean.

Glyceollin synthase is an enzyme responsible for the production of glyceollin.[2]

The five substrates of this

enzyme are 2-dimethylallyl-(6aS,11aS)-3,6a,9-trihydroxypterocarpan, 4-dimethylallyl-(6aS,11aS)-3,6a,9-

trihydroxypterocarpan, NADPH, H+, and O2, whereas its three products are glyceollin, NADP

+, and H2O.

This molecule exhibits antiestrogenic properties.

BIOSYNTHESIS OF GLYCEOLLIN IN SOYBEAN (Part1)

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aBIOSYNTHESIS OF GLYCEOLLIN IN SOYBEAN (Part2)

Isocoumarin is a lactone, a type of natural organic compound produced from carrot culture tissues inoculated with a

fungus non pathogenic to carrot, Ceratocystis fimbriata and others such C.ulmi, Helminthosporium carbonum,

Fusarium oxysporum f.sp. lycopersici, Thielaviopsis basicola. It is chemically related to phaseollin. Isocoumarin is

studied as antimicrobial, anticoagulant, antioxidant and anticancer.

Rhisitin It is a bicyclic non – sesquiterpene alcohol. Potato tubers carrying R1

resistance gene for late blight (Phytopthora infestans) responded when inoculate

with a virulent race of P.infestans by producing a phytoalexin that inhibited the

development of virulent race.

Gossypol

Gossypol is a natural phenol derived from the cotton plant (genus Gossypium). Gossypol is a phenolic

aldehyde that permeates cells and acts as an inhibitor for several dehydrogenase enzymes. It is a yellow

pigment.

Among other things, it has been tested as a male oral contraceptive but stopped because the contraceptive

dose was 10 times the toxic dose for primates. In addition to its contraceptive properties, gossypol has also

long been known to possess antimalarial properties. Other researchers are investigating the anticancer

properties of gossypol.

Biological properties

It has proapoptotic properties, probably due to the regulation of the Bax and Bcl2. It also reversibly inhibits

calcineurin and binds to calmodulin. It inhibits replication of the HIV-1 virus.[1]

It is an effective protein

kinase C inhibitor.[2]

It also causes low potassium levels, and thus causes temporary to permanent paralysis.

Gossypol is toxic to erythrocytes in vitro by stimulating cell death contributing to the side effect of hemolytic

anemia.

Biosynthesis

Gossypol is a ether soluble alcoho/terpenoid aldehyde, which is formed metabolically through acetate via the

isoprenoid pathway.[3]

Sesquiterpene dimer undergoes a radical coupling reaction to form gossypol.[4]

Geranyl

pyrophosphate (GPP) and IPP make sesquiterpene precursor, farnesyl diphosphate (FPP), for gossypol. The

biosynthesis of gossypol is summarized in figure below. The biosynthesis begins with the 0 compound

derived from GPP and IPP. Cadinyl cation (1) is oxidized to 2 by (+)-d-cadinene synthase. The (+)-d-cadinene

(2) is involved in making the basic aromatic sesquiterpene unit, homigossypol, by oxidation, which generates

the 3 (8-hydroxy-d-cadinene) with the help of (+)-d-cadinene 8-hyroxylase. At 5, the 3 goes through various

oxidative processes to make 4 (deoxyhemigossypol), which is oxidized by one electron into hemigossypol

(5,6,7) and then undergoes a phenolic oxidative coupling, ortho to the phenol groups, to form 8 (gossypol).[5]

The coupling is catalyzed by a hydrogen peroxide-dependent peroxidise enzyme, which results in the final

product.[5]

S.tritici D.rosa

Gossypol is produced as a phytoalexin in diseases like black spot of rose (Diplocarpon rosa) , leaf spot of weat

(Septoria tritici).

Methoxsalen (also called xanthotoxin, marketed under the trade names Oxsoralen, Deltasoralen, Meladinine) is a

drug used to treat psoriasis, eczema, vitiligo, and some cutaneous lymphomas in conjunction with exposing the skin to

sunlight. Xanthotoxin is isolated from parsnip root (Pastinaca sativa) disks when inoculated with C.fibriata.

Capsidiol is produced in the pepper fruit Capsicum annuum or tobacco Nicotiana tabacum after infection by the oomycete water-mold Phytophthora capsici. In pepper or tobacco fields, the fungus is soil-borne and initially infects roots, collars and lower leaves. Sporangia are moved within fields by contact with field equipment, clothing, gloves, tools etc. and initial infections spread zoospores through splashing of water from irrigation or rain.[3] Initial responses to infection include production of radical oxygen species, including H2O2.

[4] Exogenous treatment with H2O2 alone has been shown to induce capsidiol production.[4] Capsidiol production is then increased in response to radical oxygen

species production.

Visualisation of capsidiol, a phytoalexin produced

during the tobacco defense response, following

inoculation with Phytophthora infestans

(right hand image shows leaf viewed under UV light)

MECHANISM THAT TRIGGERS PHYTOALEXIN PRODUCTION

PHYTOALEXINS IN VARIOUS BOTANIC GROUPS