science

Phytoalexin

Phytoalexin is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Phytoalexin rather than just read about it. In short: Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized de novo by plants that produce the compounds rapidly at sites of pathogen infection.

Key takeaways

Phytoalexin belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Phytoalexin to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Phytoalexin from memory before moving on to harder problems.

Reference excerpt

Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized de novo by plants that produce the compounds rapidly at sites of pathogen infection. In general phytoalexins are broad spectrum inhibitors; they are chemically diverse, and different chemical classes of compounds are characteristic of particular plant taxa. Phytoalexins tend to fall into several chemical classes, including terpenoids, glycosteroids, and alkaloids; however, the term applies to any phytochemicals that are induced by microbial infection.

Function Phytoalexins are produced in plants to act as toxins to the attacking organism. They may puncture the cell wall, delay maturation, disrupt metabolism or prevent reproduction of the pathogen in question. Their importance in plant defense is indicated by an increase in susceptibility of plant tissue to infection when phytoalexin biosynthesis is inhibited. Mutants incapable of phytoalexin production exhibit more extensive pathogen colonization as compared to wild types. As such, host-specific pathogens capable of degrading phytoalexins are more virulent than those unable to do so. When a plant cell recognizes particles from damaged cells or particles from the pathogen, the plant launches a two-pronged resistance: a general short-term response and a delayed long-term specific response. As part of the induced resistance, the 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, or programmed cell death, in order to prevent the spread of the pathogen to the rest of the plant. 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, or salicylic acid. The reception of the signal leads to global changes within the plant, which induce expression of genes that protect from further pathogen intrusion, including enzymes involved in the production of phytoalexins. Often, if jasmonates or ethylene (both gaseous hormones) are released from the wounded tissue, neighboring plants also manufacture phytoalexins in response. For herbivores, common vectors for plant diseases, these and other wound response aromatics seem to act as a warning that the plant is no longer edible. Also, in accordance with the old adage, "an enemy of my enemy is my friend", the aromatics may alert natural enemies of the plant invaders to the presence thereof.

Recent research Allixin (3-hydroxy-5-methoxy-6-methyl-2-pentyl-4H-pyran-4-one), a non-sulfur-containing compound having a γ-pyrone skeletal 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 to be useful compounds for cancer prevention or chemotherapy agents for other diseases.

Role of natural phenols in the plant defense against fungal pathogens Polyphenols, especially isoflavonoids and related substances, play a role in the plant defense against fungal and other microbial pathogens. 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. 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. 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. In Sorghum, the SbF3'H2 gene, encoding a flavonoid 3'-hydroxylase, seems to be expressed in pathogen-specific 3-deoxyanthocyanidin phytoalexin synthesis, for example in Sorghum-Colletotrichum interactions. 6-Methoxymellein is a dihydroisocoumarin and a phytoalexin induced in carrot slices by UV-C, that allows resistance to Botrytis cinerea and other microorganisms. Danielone is a phytoalexin found in the papaya fruit. This compound showed high antifungal activity against Colletotrichum gloesporioides, a pathogenic fungus of papaya. Stilbenes are produced in Eucalyptus sideroxylon in case of pathogen 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. Avenanthramides are phytoalexins produced by Avena sativa in its response to Puccinia coronata var. avenae f. sp. avenae, the oat crown rust. (Avenanthramides were formerly called avenalumins.)

See also Alexin (humoral immunity) Allicin Garlic Plant defense against herbivory Pterostilbene Salvestrol

References

Further reading Moriguchi, Toru; Matsuura, Hiromichi; Itakura, Yoichi; Katsuki, Hiroshi; Saito, Hiroshi; Nishiyama, Nobuyoshi (1997). "Allixin, a phytoalexin produced by garlic, and its analogues as novel exogenous substances with neurotrophic activity". Life Sciences. 61 (14): 1413–1420. doi:10.1016/S0024-3205(97)00687-5. PMID 9335231. Kodera, Yukihiro; Ichikawa, Makoto; Yoshida, Jiro; Kashimoto, Naoki; Uda, Naoto; Sumioka, Isao; Ide, Nagatoshi; Ono, Kazuhisa (2002). "Pharmacokinetic Study of Allixin, a Phytoalexin Produced by Garlic". Chemical and Pharmaceutical Bulletin. 50 (3): 354–363. doi:10.1248/cpb.50.354. PMID 11911198.

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Illustrations

Worked examples

Example 1 — a first encounter with Phytoalexin

Start with the simplest possible case. Write down what Phytoalexin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Phytoalexin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Phytoalexin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Phytoalexin

In research: Phytoalexin appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Phytoalexin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Phytoalexin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Phytoalexins, so understanding it makes those chapters shorter.
In everyday life: Look for Phytoalexin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Phytoalexin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phytoalexin means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Phytoalexin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phytoalexin in simple terms?

Why does Phytoalexin matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Phytoalexin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Phytoalexin.