EFB530 Plant Physiology

Biotic stress

Preformed defenses:

Structural=cuticle, trichomes, spines (modified leaves), bark

Chemical=secondary metabolites

Secondary metabolism

Isoprene as a building block for terpenoids

• isoprene is a common biosynthetic building block in plant metabolism
• assembly of 2 isoprenes=monoterpene, defense compounds like pinene, pyrethroids; also essential oils like menthol, peppermint
• ABA is synthesized from 3 isoprenes=sesquiterpene (also gossypol, defense compound in cotton seeds)
• assembly of 6 isoprenes=triterpenes = steroids (analogs of animal/insect steroid hormones, like phytoecdysone, digitoxigenin from foxglove)
• assembly of 8 isoprenes=tetraterpenes=carotenoids
• isoprene polymer (>1500 isoprenes)=rubber

Phenolic compounds are made in the shikimic acid pathway

• PEP + erythrose-4-P combine to make shikimic acid, which is a precursor to Phe, Trp, Tyr
• Phe is a precursor to many phenolic compounds, which go on to make lignin and tannins
• flavonoids and anthocyanins are also made from this pathway

Alkaloids are complex, nitrogen-containing compounds made from amino acid precursors

• includes nicotine, cocaine, morphine, codeine, strychnine, caffeine

Inducible signalling of plant defense responses

Systemin and wound signal transduction

Many plants respond to wounding (such as from insect attack) by producing defense proteins

• some of these defense proteins are proteinase inhibitor (PIN) proteins
• when ingested by insects, they inhibit proteinases in the insect gut, the insect suffers from malnutrition, can die
• these proteins accumulate systemically when one leaf on a plant is wounded, the proteinase inhibitors are expressed in unwounded leaves also
• peak accumulation of proteinase inhibitor in the unwounded leaves occurs 4-5 days after wounding
• phenomenon observed in tomato, potato, poplar, maize, alfalfa, melon

The compound that travels through the plant (in tomatoes and potatoes) as a signal is an 18 amino acid peptide = systemin

• systemin in produced at the site of wounding
• systemin is cleaved from a larger protein, called prosystemin, by a protease
• the regulation of this protease is unknown, but expression of prosystemin is higher after wounding

Systemin (and the cell wall fragments) are perceived by target cells at the plasma membrane by receptor proteins

• the receptor activates the release and chemical modification of particular membrane lipids, starting with linolenic acid (18:3 fatty acid)
• this initiates a cascade of modifications of these lipids, including a step catalyzed by lipoxygenase, leading to the production of jasmonic acid
• both wounding and application of systemin cause a transient increase in jasmonic acid
• jasmonic acid is the last step known, leading to activation of the proteinase inhibitor genes

Jasmonic acid can readily convert to methyl jasmonate, which is volatile

• wounding of a plant can lead to the production of methyl jasmonate
• this can diffuse through the air to a nearby plant and trigger expression of defense genes, including proteinase inhibitor proteins
• example of interplant signalling, could even occur between two species
• this has been demonstrated in the lab, but there is no evidence that it occurs in the field

Defense responses induced by pathogen attack

Upon invasion by a weak pathogen or a non-pathogen, there is often a process of programmed cell death, which contains the microbe and stops further replication = hypersensitive response (HR)

• this response often also includes a rise in salicylic acid (SA) and production of an unknown systemic, phloem-mobile signal
• SA then induces a number of proteins that combine to provide improved defense to later pathogen attack = systemic acquired resistance (SAR)
• salicylic acid can be converted to methyl salicylate, which is volatile and may diffuse to another plant to induce PR proteins even in the absence of a direct pathogen attack on that plant
• some of these proteins are called pathogenesis-related (PR) proteins and include chitinase (degrades fungal cell walls), peroxidase (cross-links plant cell walls to strengthen them), phytoalexins (antimicrobial compounds), other antimicrobial proteins
• defense proteins can also be induced by cell wall fragments produced during fungal/bacterial invasion (some pathogens make cell wall-degrading enzymes)

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