Role and Function of Phyto-Alexins in Plant Disease Resistance

What are Phyto-Alexins?

Phyto-alexins are antimicrobial compounds produced by plants as a defense response to pathogen infection. The term "phyto-alexin" combines the Greek words "phyto" meaning plant and "alexin" meaning to ward off or repel. Phyto-alexins are part of the induced defenses of plants, meaning their production is triggered by the presence of a pathogen rather than being constitutively expressed.

Plants have evolved intricate mechanisms to detect pathogens and activate defense responses. When a plant is infected by a bacteria, fungus, virus, or nematode, it recognizes conserved molecules called microbe-associated molecular patterns (MAMPs) that are specific to different classes of pathogens. Recognition of MAMPs leads to MAMP-triggered immunity (MTI) which involves pathogenesis-related (PR) protein production, cell wall fortification, and phyto-alexin synthesis.

Functions of Phyto-Alexins

Phyto-alexins have antimicrobial activities that inhibit the growth and spread of pathogens in plant tissues. The main functions include:

• Direct toxicity against plant pathogens due to their ability to disrupt pathogen cell membranes, inhibit pathogen enzymes, and interfere with pathogen metabolism.
• Inhibition of pathogen growth, restricting the pathogen to the initial infection site.
• Contribution to the formation of structural barriers like papillae that wall off infection sites.
• Signaling molecules to induce systemic acquired resistance (SAR) against a broad spectrum of pathogens.

By impeding pathogen growth and colonization, phyto-alexins play a crucial role in disease resistance and limiting the severity of infections in plants.

Major Types of Phyto-Alexins

Plants produce a diversity of phyto-alexins that belong to various chemical classes including:

• Phenolics: Flavonoids, stilbenes, coumarins
• Terpenoids: Sesquiterpenes, diterpenes
• Nitrogen-containing compounds: Alkaloids, glucosinolates, indoles
• Steroids
• Glycosides

Well-studied examples of important phyto-alexins include:

• Resveratrol - a stilbene produced by grapevines as a defense against fungal pathogens like Botrytis cinerea.
• Pisatin - an isoflavonoid phyto-alexin induced in pea plants after fungal infection.
• Camalexin - an indole phyto-alexin produced by Arabidopsis thaliana providing resistance against oomycetes and fungi.
• Rishitin - a sesquiterpenoid phyto-alexin synthesized by potato tubers in response to fungal infection.
• Glyceollin - an isoflavonoid phyto-alexin derived from soybeans with activity against Phytophthora sojae.

Biosynthesis of Phyto-Alexins

The biosynthesis of phyto-alexins involves complex metabolic pathways that are induced in response to biotic stress and pathogen detection. These pathways utilize primary metabolism intermediates as precursors which are modified through diverse reactions such as oxidations, reductions, methylations, and prenylations to generate active phyto-alexins.

While the biosynthetic steps vary for different classes of phyto-alexins, some common features include:

• Use of phenylpropanoid, isoprenoid, fatty acid, and amino acid metabolic pathways for precursor supply.
• Key branching points after inducible enzymes that divert metabolites towards phyto-alexin synthesis.
• Multiple enzymatic steps involving cytochrome P450s, methyltransferases, oxidases, reductases, and synthases.
• Compartmentalization of biosynthetic reactions in different organelles like the endoplasmic reticulum and chloroplast.
• Transport of phyto-alexin precursors and end products by membrane transporters.

The genes encoding phyto-alexin biosynthetic enzymes are upregulated during pathogen attack via signaling cascades initiated after MAMP recognition. Phytohormones like salicylic acid, jasmonic acid, and ethylene mediate the signal transduction leading to phyto-alexin production.

Key Enzymes in Phyto-Alexin Biosynthesis

Some examples of key inducible enzymes involved in phyto-alexin biosynthesis include:

• Phenylalanine ammonia lyases (PAL) - deaminate phenylalanine to cinnamic acid, providing precursors for phenolic phyto-alexins.
• Terpene synthases (TPS) - catalyze committed steps in terpenoid phyto-alexin synthesis.
• Cytochrome P450 monoxygenases - introduce oxygen to form secondary metabolite backbones.
• O-methyltransferases - catalyze methylation of precursors.
• UDP-glucosyltransferases - attach glucose moieties to aglycones.
• Prenyltransferases - catalyze attachment of prenyl groups to aromatic substrates.

Manipulating the expression of genes encoding these enzymes using transgenic approaches can lead to increased phyto-alexin production and enhanced disease resistance.

Subcellular Compartmentalization

Phyto-alexins are synthesized in different organelles and cell types:

• Phenolic phyto-alexins like isoflavonoids form in the cytoplasm.
• Terpenoid phyto-alexins are made in plastids.
• Alkaloid phyto-alexins derive from amino acid precursors in multiple sites.
• Biosynthesis occurs mainly in parenchyma cells surrounding infection sites.

Intermediate precursors and end products are transported between cellular compartments by ATP-binding cassette (ABC) transporters and multidrug and toxic compound extrusion (MATE) transporters.

Role of Phyto-Alexins in Plant Disease Resistance

Phyto-alexins contribute to disease resistance by inhibiting pathogen growth at infection sites. The importance of phyto-alexins is demonstrated by increased susceptibility of plants impaired in phyto-alexin production due to mutations or inhibitor treatments. For example:

• Arabidopsis camalexin-deficient mutants exhibit enhanced susceptibility to fungal and bacterial pathogens.
• Inhibition of stilbene synthase in grapes leads to elevated sensitivity to Botrytis cinerea.
• Soybean cultivars that produce lower levels of glyceollin suffer more severe Phytophthora sojae infections.

However, the relationship between phyto-alexins and disease resistance depends on multiple factors:

• Pathogen lifestyles - Phyto-alexins are more effective against biotrophic pathogens restricted to host cells versus necrotrophic pathogens that kill cells.
• Phyto-alexin diversity - Plants producing several phyto-alexins tend to have greater broad-spectrum resistance.
• Pathogen detoxification - Some pathogens can detoxify or tolerate certain phyto-alexins.
• Accumulation speed and levels - Rapid and abundant accumulation confers better resistance.

Overall, phyto-alexins serve as an integral barrier against pathogens, acting in concert with other induced defenses in determining plant disease resistance capabilities.

Applications for Disease Control

The pathogen-inhibitory properties of phyto-alexins can be harnessed to reduce crop losses from diseases. Potential strategies include:

• Breeding/engineering plants to increase phyto-alexin production through overexpression of biosynthetic genes.
• Exogenous application of purified phyto-alexins directly onto crops.
• Use of elicitors that stimulate phyto-alexin accumulation in plants.
• Inhibition of pathogen enzymes involved in phyto-alexin detoxification.
• Combining phyto-alexins with other antimicrobial compounds for synergistic effects.

Further research on phyto-alexin biosynthesis regulation and field-testing of these approaches is needed. But exploiting phyto-alexins holds promise for developing integrated and sustainable disease management solutions.

FAQs

What are the main functions of phyto-alexins in plant defense?

The main functions of phyto-alexins include: direct toxicity against pathogens, restricting pathogen growth, contributing to structural barriers like papillae, and signaling to induce systemic acquired resistance.

What are the major types of phyto-alexins produced by plants?

Major types of phyto-alexins include: phenolics like flavonoids and stilbenes; terpenoids like sesquiterpenes; nitrogen-containing compounds like alkaloids and indoles; steroids; and glycosides.

Where does the biosynthesis of phyto-alexins take place in plant cells?

Phyto-alexins are synthesized in different organelles and cell types. Phenolics form in the cytoplasm, terpenoids in plastids, alkaloids at multiple sites, and production mainly occurs in parenchyma cells near infection sites.

How do phyto-alexins contribute to disease resistance?

Phyto-alexins inhibit pathogen growth at infection sites. Plants impaired in phyto-alexin production are more susceptible. Their effectiveness depends on pathogen lifestyles, phyto-alexin diversity, pathogen detoxification abilities, and accumulation levels.

How can we exploit phyto-alexins for disease control in crops?

Strategies include: breeding/engineering plants to increase phyto-alexin production, exogenous application of phyto-alexins, use of elicitors that stimulate phyto-alexin accumulation, and inhibiting pathogen detoxification enzymes.