Biotic Interactions PDF

BIOTIC INTERACTIONS Chapter 23 INTRODUCTION Plants do not exist in isolation but interact with many other species in their communities. • Some of these interspecific interactions, e.g., associations with mycorrhizae or with insect pollinators are mutually beneficial. • Most interactions are not...

BIOTIC INTERACTIONS Chapter 23 INTRODUCTION Plants do not exist in isolation but interact with many other species in their communities. • Some of these interspecific interactions, e.g., associations with mycorrhizae or with insect pollinators are mutually beneficial. • Most interactions are not beneficial to plants – As primary producers, plants are at the base of most food webs and are subject to attack by a wide variety herbivorous animals • Plants are also subject to attacks by pathogenic viruses, bacteria, and fungi Plant Defense: Multitrophic Interactions Pollinators Competitor plants Secondary carnivores Carnivores Herbivores Shoots and flowers Roots Parasitic plants Symbionts Modified from Bruinsima & Dicke 2008 Herbivores Carnivores Pathogens Pathogens Aboveground Belowground Cotton boll weevil Agricultural pests – Economic damage – Cost of prevention & eradication HARMFUL INTERACTIONS • Plant pathology: study of plant disease – Fungi, oomycetes, bacteria, virus • Oomycetes: one of the most destructive pathogens in history • Fungi, oomycetes, virus à most harmful • Bacteria: in general, less devastating • Herbivory: ~1 million insects feed on plants! – 90% restricted to single plant family/closely related spp – 10% generalists Coevolution LINES OF DEFENSE • Herbivory is a stress that plants face in any ecosystem • Plants counter it with – Physical defenses: thorns, cuticle, periderm – Biochemical defenses: production of distasteful or toxic compounds • Constitutive defenses: always present • Inducible defenses: triggered in response to attack Constitutive defense always present Costly: energy investment. Pests and pathogens can adapt. Induced defense synthesized in response to challenge. Most plants have evolved these. Response is more flexible Classes of plant defenses 1. PHYSICAL/MECHANICAL DEFENSES Spines, thorns, trichomes, prickles Cutins , waxes, suberins, phyloliths, crystals/raphides 2. BIOCHEMICAL DEFENSES - SECONDARY METABOLITES** Phenolics Defense-related proteins phenolic glycosides peroxidases bound phenolics polyphenol oxidase lignin? PAL condensed tannins hydrolysable tannins Terpenes N-containing monoterpenes Alkaloids diterpene acids Mustard oils PHYSICAL/MECHANICAL • First line of defense • Thorns, spines, prickles, trichomes Stem spines Colletia paradoxa Leaf spines- Opuntia invicta Shoot spines- Dovyalis caffra A closer look… RAPHIDES PHYTOLITHS • Trichomes/hairs: many are glandular -> pockets that burst and release contents (secondary metabolites). They can also send messages to surrounding cells when bent Urtica dioica Cutin, Waxes, Suberins All plant parts exposed to the atmosphere are coated with layers of lipid material that reduce water loss and help block the entry of pathogen fungi and bacteria. Hydrophobic compounds. The principal types of coating are cutin, suberin and waxes 1. CUTIN • Major component of plant cuticle, a multilayered secreted structure that coats the outer cell wall of epidermis on the areal parts • Plants’ cuticles is composed of a top coating of wax, often vary with the climate in which they live 2. Suberin • • • • • Fatty acids but has a different structure from cutin Often within roots It can protect against pathogens and other damage A cell wall constituent Endodermis has suberin side walls 3. Waxes • Complex mixtures of long-chain lipids that are extremely hydrophobic • They exuded through pores in the epidermal cell wall by an unknown mechanism Plants use multiple lines of defense against pathogens • A plant’s first line of defense against infection is the physical barrier However, viruses, bacteria, and the spores and hyphae of fungi can enter the plant through injuries or through the epidermis (stomata) Once a pathogen invades, the plant mounts a chemical attack as a second line of defense Plant cell Plant cell membrane Nutrient transfer Haustorium Adhesion pad Germinating fungal spore Fungal hypha Plant epidermal cell Fungus entering stoma SECONDARY METABOLITES • Chemical defense: primary and secondary metabolites – Primary: all plants produce, involved in growth and development • Hormones: considered primary, although pathways of secondary – Secondary: species-specific compounds. Terpenoids, phenolics, N-containing compounds Thakur et al. 2019. https://www.sciencedirect.com/science/article/pii/S2214786118303577 GLIFOSATO! SECONDARY METABOLITES • • • • Protect primary metabolism by deterring herbivores, reduce tissue loss Also attract pollinators and seed-dispersing animals Formed from the byproducts or intermediates of primary metabolism They could be toxic to the plant à isolate (vacuoles, organelles, specialized structures, glandular trichomes, laticifers, resin canals) Terpenes • They function as herbivore deterrents: can be produced in response to herbivore feeding, and to attract predatory insects and parasites of the feeding herbivore • They are constituents of essential oils Examples: 1. Resins of conifers are monoterpenes 2. Essential oils - peppermint, lemon, eucalypt, tea tree Prior to divergence of gymnosperms and angiosperms, during the carboniferous, the duplication of an ancestral terpene synthase gene occurred. One copy of the duplicated ancestral gene remained highly conserved in structure and function, and this gene may have contemporary descendants in the terpene synthases involved in giberellins biosynthesis. The second ancestral gene copy diverged in structure and function, by adaptive evolutionary processes, to yield a large superfamily of terpene synthases involved in secondary metabolic pathways. Phenolic Compounds • • Secondary metabolites containing a hydroxyl functional group on an aromatic ring Many serves as defense compounds against herbivores and pathogens. Other function in attracting pollinators and fruit dispensers Phenolic substances are the most resistant metabolites produced by plants. Better understanding of plant phenolics is essential, due to its wide array of functions in the plant development, and its practical applications in many streams such as agriculture, medicine, nutrition, pesticide management, and industry. During abiotic stress also, the plants can produce phenols as tolerance mechanism to cope with the unfavorable conditions. 10.5772/intechopen.102873 N-containing secondary compounds • Those are encountered less commonly in plants than the phenolics and terpenoids • Bioactivity as drugs and toxins Examples: Alkaloids, cyanogenic glycosides, glucosinolates*, nonprotein amino acids ALKALOIDS • The most important N-containing secondary compounds. Exclusive to plants? • More than 20 different classes (pyrrolidines, pyrrolizidine, quinolizidine, etc.) • 4000 known compounds; 300 plant families, ~7500 spp; ca. 20% vascular plants • Leaves, fruits, flowers, seeds, shoots, roots • Alkaloids may act to toughen leaves (discourage herbivory) as well as to store carbon and nitrogen • Important medicines N is usually part of a heterocyclic ring with C atoms Ricin (Ricinus communis) Six times more lethal than cyanide and twice as lethal as cobra venom. A single seed can kill a small child. • Alkaloids have been used by man more than 3000 years ago (we know of**) for many purposes. In Mesopotamia since 2000 BCE, medicinal plants Papaver somniferum and Atropa belladonna à therapeutic purposes • Opium alkaloids: isolated first time in 1803; three years later alkaline nature recognized and after ten years named as morphine. • From 1817 to 1820, Pelletier and Caventou discovered an exciting series of active compounds, including caffeine from coffee, strychnine from nux-vomica, emetine from ipecac, quinine and cinchonine from cinchona bark, shortly after that followed by coniine. • The main toxic effects of alkaloids result in disturbances of the central nervous system, digestive processes, reproduction, and the immune system. Night shade Mandrágora Atropa belladona Poison, cosmetic, medicine, hallucinogenic Datura CYANOGENIC GLYCOSIDES • Release the toxic gas hydrogen cyanide • Plants must have enzymes to break down the compounds and release a sugar molecule yielding a compound that can decompose to form HCN • Glycosides and enzymes which break them down are usually spatially separated (in different cellular compartments or different tissues) Many pits and seeds: apples, peaches, apricots, plums, elderberries à protect seed Cassava (Manihot esculenta) is a native of South America and sub-Saharan Africa, which is widely grown in the tropics to produce flour and tapioca. The grated roots must be thoroughly washed to remove the toxic material. Badly prepared cassava causes signs of hydrocyanic acid poisoning: nausea, vomiting, abdominal distension and respiratory difficulty. Chronic cassava ingestion can cause an ataxic neuropathy, with bilateral primary optic atrophy, bilateral perceptive deafness, myelopathy and peripheral neuropathy. Konzo (‘tired legs’), a symmetrical, non-progressive, nonremitting spastic paraparesis, which occurs in epidemic and endemic forms in several African countries and is invariably associated with consumption of inadequately processed bitter cassava roots and minimal protein; it may be associated with thiamine deficiency. cyano groups: direct neurotoxic actions not mediated by systemic cyanide release https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/cyanogenic-glycosides High levels of cyanogenic glycosides. Chronic cyanide poisoning are not uncommon. GLUCOSINOLATES • These compounds release volatile defensive substances, “mustard oils”, (often herbivore repellents) • Plants like cabbage, broccoli, and radishes (Brassicaceae family) have these. OPTIMAL DEFENSE HYPOTHESIS • Compounds are concentrated in the most needed areas = maximize fitness • Higher in young leaves – This has been shown in a wide number of studies • Young tissues contain highest levels of constitutive and inducible secondary compounds • Controversial* Hunziker et al., 2021. PNAS Plants are generally resistant to most pathogens – Plants have an ability to recognize invading pathogens and to mount successful defenses – In a converse manner, successful pathogens cause disease because they can evade recognition or suppress host defense mechanisms • Those few pathogens against which a plant has little specific defense à virulent – A kind of “compromise” has coevolved between plants and most of their pathogens • Avirulent pathogens gain enough access to its host to perpetuate itself without severely damaging or killing the plant A wound response occurs when a leaf is chewed or injured • ELICITORS: trigger defense responses – Plants can recognize saliva – Leads to rapid production of proteinase inhibitors throughout the plant – Bind to digestive enzymes in the gut of the herbivore – Concerted action of signaling compounds is needed for full activation of induced defense responses – Signaling pathway involves • • • • Jasmonic acid Salicylic acid Ethylene Cell fragments When elicitors are recognized à more cytosolic Ca2+ à activates many target proteins or jasmonic acid -> signaling pathway • Jasmonic acid: levels rise steeply in response to herbivore damage à triggers proteins involved in defenses – It also allows reallocation of resources to defense JA accumulates within minutes à there’s electric signaling in the spread of JA to undamaged leaves. 9 cm/min. GLR genes: generate the signal. Responsible of longdistance signals in response to herbivory. Induction and release of volatiles organic compounds (VOCs) à complex functions of 2ary compounds. Combination of molecules often specific for each insect (plants can distinguish). All plants emit lipid-derived, green-leaf volatiles à attract natural enemies Resistance Traits One of the greatest problems with nonnative invasive species, such as the emerald ash borer, is the lack of natural predators in the new environment Modified from the original (Amanda Accamando) • Volatiles can also signal neighboring plants to initiate their own defense!! They also signal other parts of the plant – Terpenoids and green-leaf volatiles Circadian Rhytms 1/3 plant genes exhibit CR regulation JA vs SA: JA accumulates middle of the day SA: middle of the night à resistance against bacteria that infects early morning Defenses against pathogens • Plants don’t have immune system per se BUT they’re very resistant to diseases! – Systemic acquired resistance (SAR) – Induced systemic resistance (ISR) • Pathogens have evolved numerous ways to infect plants: lytic enzymes, stomata, wounds, transferred by herbivores (vectors) • Disease epidemics are rare in nature à plants are effective in their defenses! MAMP: microbe-associated molecular pattern DAMP: damage-associated molecular pattern RLK: receptor-like kinase RLP: receptor-like protein Once inside, no detection at the membrane level. This put evolutionary pressure on plants! Second line of defense: R (resistance) genes: recognize intracellular effectors and trigger defense The Hypersensitive Response • The hypersensitive response – Induces production of phytoalexins and PR proteins (pathogen-related), which attack the pathogen – Stimulates changes in the cell wall that confine the pathogen – Causes cell and tissue death near the infection site – Some of these are antimicrobial and attack bacterial cell walls & “news” spread of the infection to nearby cells • Infection also stimulates cross-linking of molecules in the cell wall and deposition of lignins – This sets up a local barricade that slows spread of the pathogen to other parts of the plant 4 3 Signal Hypersensitive response à creates systemic acquired resistance (SAR). Nonspecific, providing protection against a diversity of pathogens for days. 5 Hypersensitive response 2 Signal transduction pathway 6 Signal transduction pathway 7 Acquired resistance 1 R protein Avirulent pathogen Avr effector protein R-Avr recognition and hypersensitive response Systemic acquired resistance (SAR) – Long-distance inducer is likely salicylic acid – Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance – At the cellular level, jasmonic acid is involved in SAR signaling – SAR allows the plant to respond more quickly to a second attack • Nematodes and parasitic plants also present problems Nematodes: can invade all parts of a plant! – Syncytium: feeding site – Roots infected: knots or galls Plants against plants Allelopathic plants • Secrete chemicals to block seed germination or inhibit growth of nearby plants • This strategy minimizes competition for resources • Root exudates