Plant Secondary Metabolites and Defenses

Plant Secondary Metabolites

• Plants produce >60,000 secondary metabolites.
• These metabolites are involved in plant defense.
• Up to 10% of a plant's dry mass might be made up of these secondary metabolites.
• Secondary metabolites are stored in vacuoles, secreted, or part of surface structures (e.g., trichomes) or internal organs (e.g., resin ducts).
• Plant secondary metabolites are divided into three main groups: terpenes, alkaloids, and phenolics.

Plant Chemical Defenses

• Worldwide pre-harvest crop losses include 14% from insects, 12% from diseases, and 10% from weeds.
• Post-harvest losses involve worms and insects damaging seeds & storage organs.
• Fungal and bacterial attacks (e.g., Erwinia) also cause crop loss.
• Secondary metabolites protect plants from herbivores (insects and nematodes) and pathogens (fungi, bacteria, and viruses).

Plant Secondary Metabolites: Functions

• Carry out diverse ecological functions, such as attracting pollinators and modulating soil microbiota (plant growth-promoting bacteria).
• Plants use secondary metabolites for UV protection, defense (direct and indirect), immunity, warning signals, pollinator recruitment, allelopathy, and pathogen suppression.

Human Uses of Plant Metabolites

• Humans use plant metabolites for therapeutic drugs.
• Also for pesticides, microbicides, herbicides, poisons.
• Flavors, pigments, dyes, construction, religious and cultural roles, as well as perfumes and scents.

Lecture Overview

• Where do chemical defenses come from and how are they made?
• How are they grouped?
• What uses have humans put them to?
• What uses may humans put them to in the future?

Plant Defenses - Arrangement

• Plants are immobile, but possess a variety of defenses.
• Constitutive defenses are present all the time.
• Inducible defenses are activated when a threat is detected.
• The first line of defense often involves waxy cuticles and cell walls.

Secondary Metabolites - Details

• Some morphological defenses against herbivores and insects exist (e.g., thorns).
• Most defenses involve complex biochemistry.

• 40,000 terpenes are based on the 5-carbon isoprene unit linked in various ways as polymers.

• Smaller terpenes can be volatile (e.g., essential oils) and are often released by epidermal gland cells in flowers.
• Examples of 'evil' terpenes are capsaicin and resiniferatoxin.

Terpenes

• Terpenes include essential oils found in herbs and spices, many have strong flavors.
• Medicinal functions include the sesquiterpene lactone Artemisinin is an anti-malaria drug.
• Some terpenes function as allelopathic agents, insecticides, or ways to deter herbivores/insect attack.

Terpenes: Carotenoids

• Carotenoids, a type of terpene, are responsible for the red color of tomatoes.
• Other carotenoids contribute color to fruits and vegetables, with carrots having the highest concentration.
• Carotenoids are beneficial as antioxidants, absorbing high-energy electrons and capable of synthesizing Vitamin A.
• Lycopene, a carotenoid, is shown to be effective at preventing stroke and some cancers.

Alkaloids

• More than 12,000 types of alkaloids exist. These are synthesized from amino acids like tryptophan, tyrosine, phenylalanine, lysine, and arginine
• All alkaloids contain nitrogen.
• Many alkaloids affect the nervous system and have medicinal uses as stimulants and sedatives.
• Many plants produce toxic alkaloids like solanine found in potatoes.

Example Alkaloids

• Codeine and morphine (opium poppy)
• Atropine and scopolamine (Belladonna and Datura – eye surgeries, anti-parkinson)
• Caffeine (coffee)
• Ephedrine (Ephedra)
• Quinine (Chincona spp – malaria)
• Aconitine (Aconitum spp – analgesic, anti-inflammatory, and cardiotonic)
• Cocaine (coca)
• Nicotine (tobacco)
• Piperine (black pepper)
• Berberine (barberry)

Alkaloids: Toxicity and Use

• Nicotine functions as a toxic insecticide.
• Nicotine affects the production of tobacco hornworm larvae and is also allelopathic.

Phenolics

• More than 8,000 types of phenolics exist.
• Phenolics have a phenyl ring and a 3-carbon tail, often with an -OH group.
• Phenolics are derived from phenylalanine or tyrosine.
• Plant use and human use is discussed in the text to categorize in different types like Lignins, Tannins and Flavonoids

Phenolics: Lignins

• Lignins are a component of cell walls.
• They have complex phenol polymer structure and provides structural support to the plant.
• Lignins are toxic to microbes and are important for water resistance and strength.
• Lignin production increases in response to pathogens or wounding, for defense purposes.

Phenolics: Tannins

• Tannins are phenolic polymers.
• They cross-link with proteins, making them less digestible, and are often found in high concentrations in plants to deter herbivores.
• Humans use tannins to process animal skins and make leather, also have other medicinal uses.

Phenolics: Flavonoids

• Flavonoids are a group of phenolic compounds.
• Flavonoids add color to flowers as pigments.
• Flavonoids act as feeding deterrents, signalling molecules in plants, and help with plant-microbe communication.
• Flavonoids are associated with health benefits, like anti-inflammatory and anti-cancer effects.

The Future of Plant Secondary Metabolites

• There are many potential uses for these metabolites in food crops, like defense chemicals or flavor production in agricultural systems.

Human Health

• Some naturally occurring secondary metabolites may cause cancer. Others may prevent cancer.
• Herbal remedies often have serious side effects.
• Many modern drugs have plant origins.

Agriculture

• Historical pesticides often have untested side effects on human health and the wider environment.
• Synthetic pesticides can damage other beneficial plants and animals
• Synthetic pesticides persist for a long time and leave traces in the environment.
• Some techniques exist to control/regulate how much secondary metabolites are produced

Plant Metabolomics and Further Studies (Other topics presented in the documents)

• Metabolomics, a study into the chemicals in living organisms.
• Terpene metabolism is an active area of study.
• The use of mass spectrometry (GCMS and LCMS/MS) in plant research.
• Further topics presented in the documents include biochemistry of plants, medicinal plants, and human health.