PLANT DEFENSE & HUMAN NUTRITION: PHENYLPROPANOIDS ON THE MENU

Marcello Iriti and Franco Faoro University of milano, Italy. Current topics in nutraceutical research (Dec, 2003) ABSTRACT y Phenylpropanoid pathway - defense mechanism against environmental fluctuations and pathogen attacks. y Phenylpropanoid compounds - plant food . y Traditional breeding - modification in plant 2º metabolite profiles by selecting fruits, flowers and vegetables based on x Ease of cultivation x Edibility x Digestibility x Nutritional value x Colors & scents y Phenylpropanoids - nutraceuticals or pharmaconutrients y Pharmaconutrients - not drugs but pharmacologically active compounds that will Ń Antagonize, modify some physiological & metabolic functions providing health benefits Phenylpropanoids in functional foods y y y Flavanoids y Stilbens INTRODUCTION y Sessile status of land plants - biotic & abiotic stresses reduce crop yield. y Daily environmental fluctuations around plants. y Unique traits for plant fitness - STRESS AVOIDANCE & EFFECT TOLERANCE. y Stress - altered physiological condition caused by factors that modify homeostasis. y Plant stress has induced plant 2º metabolism, thereby, allowing plants to deal with their constantly fluctuating environment. y Secondary metabolites - cpds involved in plant communication & defense - not involved in plant development & growth y Natural plant products - interaction with pathogens, herbivores & other plants - protection from UV radiation - attract beneficial insects or microbes. PHYTOCHEMICALS IN HUMAN DIETARY SELECTION y Plant foodstuffs - toxins & digestibility reducing substances. (Rhoades & Cates, 1976). y DRS - act within the gastrointestinal tract & reduce the availability of other nutrients. (Liener, 1980). y These include - protease inhibitors, celluose, starch, & tanins. y In this view, the herbivore selection behaviour depends on these 2º cpds (Glander, 1982). y Primates feeding strategies - & tanins. y Nonhuman primates - leaf selection is affected by presence of phenolic cpds (Freeland & Jenson, 1974) y Tanins are found in the bark, leaves & fruits - content increases with maturity and hence the herbivores opted for younger leaves than the older ones (Swain, 1979) y Non-fire-using hominids - cellulose & starch are not readily digestible in raw state. y Hence before using fire for cooking, the hominids should have opted for mechanical means of deactivating these digestion inhibitors. y Crushing, soaking, fermenting, drying, eating with clay. DIETARY BEHAVIOURAL CHANGES IN HUMAN POPULATIONS WITH AGRICULTURE y Neolithic period - humans made a transition from foraging to farming. y Emergence of agriculture was a gradual process that affected the dietary behaviour of various human groups. y This shift from foraging to farming i.e., from food collection to food production caused significant biological changes in human populations. y For e.g., a decline in the nutritional quality, consumption of a narrower range of foods. y This diet narrowing lead to reduced availability of animal proteins, caused dependence on a limited number of domesticated plants (Johnston, 1987). y Wild fruits & their cultivated counterparts - decreased nutritional value (marginal or poor amts of amino acids, fibers, pectins) (Milton, 1999). y This in turn had affected the plant defense mechanism which involves these secondary cpds. y Result : Increased vulnerability to microbial & herbivore attacks (Swain, 1977). y Holocene period: population size, in areas adopting agriculture, increased and mortality decreased. y Our diet : adapted the diet of our pleistocene ancestors - reduced the frequency of many chronic diseases due to high salt, fat & sugar intake. y Ancestral diet includes - adequate minerals & vitamins, less fat & sodium, more fibers, flavanoids & phenolic cpds. y But, current ´Westernµ diet practices lead to insufficiency of fruits & vegetables, high intake of sugar-rich foods & saturated animal fats ( Gliebermann, 1973) PLANT IMMUNITY y 2 main strategies of defense mechanisms : x Preformed physical & chemical defenses x Induced resistance mechanism. y Induced resistance mechanism includes - enhancement of mechanical barriers, defense compound synthesis. y Both, preformed & synthesized natural products are divided into 3 main groups : x Phenylpropanoids x Isoprenoids x Alkaloids PRIMARY & SECONDARY PLANT METABOLISM y Glycolysis & pentose phosphate pathway - major metabolic pathways. y Glycolysis : Ń ATP - ready to use highly energetic cpd. Ń NADH -carrier of chemical energy. Ń Pyruvate - precursor of Kreb·s cycle. Ń Phosphoenolpyruvate - intermediate y Pentose phosphate pathway : Ń NAD(P)H Ń Erythrose 4- phosphate - intermediate y Under stress, the glycolytic display lower activity than those of pentose phosphate pathway. y Rxn b/w intermediates * shikimic acid * aromatic biosyn y , & - serve as a source of phenylpropanoids. AROMATIC A.A BIOSYNTHESIS PEP + E-4-P

Shikimate

Chorismate

Trp Phe Tyr

Alkaloids Flavanoids Cyanogenic glycosides , Coumarins PHENYLPROPANOID PATHWAY

Shikimate pathway Phe

deamination PAL ( Phe ammonia lyase)

Cinnamic acid

Lignans Simple phenols Chalcone

Lignin Stilbenes , cutin Flavanoids

Tannins & BENZOIC ACID DERIVATIVES

Cinnamic acid

p- Coumaric acid

hydroxylase

Caffeic acid

O - methyl transferase

Ferulic acid Hydroxylase / o - methyl transferase Sinapic acid y p - coumaric acids, , , Sinapic acid - CINNAMATES y Cinnamates - free radical scavengers - preventing the oxidative modification of LDL. y Present in various food products - Ń Pome fruits - apples & pears Ń Stone fruits - apricot, cherry Ń Berry fruits - berries Ń Citrus fruits, grapes & wine. y - derivative of benzoic acid - key cpd in molecular signalling pathways in plant defense against biotic & abiotic stresses. COUMARINS y Coumarins - , antimicrobial compounds (Dravill, 1984). y Furanocoumarins - psoralen Uv illuminated Psoralen + pyrimidine DNA bases

transcription, DNA repair mechanism (induces cell death) y These chemicals - used in skin photochemotherapy against x Mycosis x Psoriasis x Vitiligo y Side effects - Skin cancer, somatic gene mutations, chromosomal abberations, erythema & skin photodermatitis. LIGNANS y Carboxylic group reduction of hydroxycinnamates * * dimerize to form lignans y Eg., podophyllotoxin - from th resin of Podophyllum hexandrum. x A mitosis toxin - binds to tubulin & colchicine x Inhibits microtubule assembly & mitotic spindle formation x Interacts with topoisomerase II - arrests cell division (Damayanthi, 1998). y Podophyllotoxin - precursor for synthesis of anticancer drugs. - exhibits antiviral activity (Charlton, 1998) PLANT CELL WALL CONSTITUENTS & y Suberin, cutin & lignin - play structural & defense role in impermeable layers & cell wall reinforcement in opposition to pathogen penetrating attempts (Davin & Lewis, 1992). y Dietary fiber - group of chemically different compounds - resistant to human digestive enzymes. y Epidemiological studies - inverse relationship exists between fiber intake and colon cancer risk (Trock et al., 1990). y The inhibitory effect of dietary fibers on colon cancer depends on its fermentability. y Less / non-fermentable fibers are more protective than highly fermentable ones (Lupton, 2000).