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PLANT DEFENSE SYSTEMS
Chapter 37
Plants, like animals, are subject to diseases and a type of predation called herbivory.
Plants cannot escape threat like animals do; plants are sessile.
There is great deal of research going in the area of plant defenses.
BARRIERS TO ENTRY
There are morphological and anatomical adaptations that protect plants against pathogens and
herbivores.
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Pathogen: a disease causing organism.
Herbivores: plant eaters; this definition includes animals that feed on nectar, fruit,
seeds, etc. and not only that feed on leaves.
Cuticle and cutinase.
The cuticle of plants is made of wax.
Epidermal cells secrete the cuticle.
Cuticular waxes consist of a matrix of long lipid chains in which cutin, another lipid, is
embedded.
The lipid chains are cross-linked.
Waxes are hydrophobic and protect plants against loss or entry of water through the waxy
area.
The waxy cuticle forms the first line of defenses against the entry of pathogens. It prevents
viruses, bacteria, spores, fungi, etc. from penetrating into the body of the plant.
Wounds in the plant allow the entry of pathogens.
Some fungi have cutinase enzymes that disrupt the cross-links in the lipid chains and penetrate
into the plant.
There are different types of cutinase enzymes with different ability to disrupt the cuticle matrix.
The ability to disrupt the matrix accounts for the virulence of the pathogen.
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Virulence: ability to cause disease.
In some fungi, virulence depends on the number of genes for cutinase present in the strain.
MORPHOLOGICAL ADAPTATIONS (WEAPONS)
Spines, thorns, prickles, and hairs are adaptations to discourage herbivores from eating plant
parts.
Associations with animals also deter herbivores, e.g.
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Ants and acacias: the acacia has a swollen and hollow thorn base provides a home
for the protective ants.
Some ferns have swollen rhizomes in which ants live.
PLANT POISONS
Chemical defenses include the accumulation of compounds that make the plant parts difficult
to eat, hard to digest, unpalatable, repugnant or toxic.
Plant defense compounds resemble molecules required by metabolic functions, e.g. amino
acids. It is hypothesized that mutations caused chemicals that were not functional in metabolism
but had a repellent property that allowed individuals with the mutation to survive and reproduce
more successfully than those without the mutations.
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E. g. The secondary metabolite DIMBOA has been shown to be an offshoot of tryptophan
synthesis involving five different genes that synthesize an intermediate product that led to
the final production of DIMBOA.
Protective chemicals are called secondary compounds since they are not essential for the
metabolic processes of the plant.
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Substances not produced as part of primary metabolism in plant; frequently with an
uncertain function.
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Plant poisons or allelochemics are constantly produced in plants. There is no need for an
stimulus.
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Allelochemics are secondary substances capable of modifying the growth, behavior or
population dynamics of other species through inhibitory or regulatory processes.
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These compounds cover a wide range of organic chemicals: toxic proteins, tannins,
terpenes, alkaloids, phenolics, resins, steroidal, cyanogenic and mustard oil glycosides and
tannins (contain aromatic rings, some are glycosides).
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Tannins bind to the digestive enzymes of insects that sicken the insect. They also interfere
with protein break down.
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Phenolics are very common amino acid derivatives found in seed-producing plants; they are
the burning substances in poison ivy and poison oak. Alkaloids are also amino acid
derivatives found thousands of species of plants. Cyanogenic glycosides are found in a few
hundreds of species.
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Glycosides are oligosaccharides bound to alcohols, phenols or amino groups. They usually
interfere with the formation of ATP.
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Nicotine, caffeine, cocaine and morphine are alkaloids. Alkaloids are found in about 20% of
the plant species. Alkaloids are highly toxic to herbivores and parasites; disrupt several cell
mechanisms: enzyme poisoning, inhibition of protein synthesis, disruption of a membrane
transport system, etc.
THE COST OF DEFENSE
Plants spend energy resources in making secondary compounds, e. g. large amount of ATP.
The number of parasites and herbivores play a selective role in the plant population, e. g...
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Abundant herbivores will eliminate poorly defended plants and well-defended plants will
survive and produce offspring that synthesize large amount of secondary metabolites.
Well-defended plants do better when pest pressure is high.
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Few herbivores will allow poorly defended plant to survive in large numbers and the
population will have many individuals that synthesize small amounts of poison.
Producing secondary metabolites reduces the amount of energy available for growth and
reproduction.
Responding to pathogens.
Plants can respond to pathogens and herbivores after they are attacked.
Infected cells respond by dying. This is called a hypersensitive response or HR.
Gene-for-gene hypothesis
When gene products from the plant and the pathogen match and interact.
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Pathogens infect the plant via a wound or some other means.
Pathogens release their own proteins in the plant tissues.
These proteins cause the plant to react and produce their own proteins that may or may
not inactivate the pathogen's proteins.
Binding between the plant and pathogen proteins causes the hypersensitive response
and the plant cell dies and the pathogen with it.
Not binding (no match) between the plant and the pathogen proteins causes nor HR
reaction and the plant becomes seriously infected and eventually succumbs to disease.
Experiments from around the world have confirmed the gene-for-gene hypothesis through the
synthesis of R (plant gene) and avr (virulent/avirulent pathogen gene) gene products that
interact.
These experiments were confirmed in 1996 by an experiment designed and carried out by
Scofield and colleagues.
Resistance loci.
When similar genes are found clustered together in a chromosome they are said to form a gene
family.
Gene families originated in all probability by incorrect crossing over or some other geneduplicating event.
Diploid plants have alleles for the same locus, one in each chromosome. If there are many loci,
there are also many alleles, which are probably different from one another.
The different alleles allow to recognize different protein from the same pathogen. This is
important because avr products (virulent) arise continuously in the pathogen population via
mutations.
This large number of R alleles allows the plant to react to a wide variety of pathogenic products
and therefore create resistance.
Reactive oxygen intermediates (ROI)
The interaction between R proteins in plants a pathogen's proteins initiate a series of plant
responses that include the formation of hydrogen peroxide, H2O2, and O2-, superoxide. These
molecular responses are collectively called ROI or reactive oxygen intermediates.
ROI create reactions that strengthens the cell wall, trigger plant cell death or that of the
pathogen.
Researchers have found that ROI molecules and nitric oxide, NO, must be present in order to
have the typical HR.
Phytoalexin production
Plants can produce certain antibiotic compounds called phytoalexins.
A phytoalexin is small molecule that is induced by infection and that poisons the pathogen.
Plants make these antibiotics when infected by a pathogen, e. g. chickpeas.
Phytoalexins occur at the point of infection but a slower and more widespread reaction occurs,
the systemic acquired resistance (SAR).
Salicylic acid concentration increases dramatically in infected plants. Experiments have shown
that addition of SA triggers an SAR response.
It is not clear if SA is the hormone that causes SAR or is only a local signal that causes the
expression of genes involved in the SAR response.
Responding to herbivores
Most species of insects feed on leaves, stem and root parts, pollen, etc. They are herbivores.
Proteinase inhibitors.
Proteinase inhibitors inhibit the enzymes responsible for the digestion of proteins.
Herbivores detect proteinase inhibitors by taste and avoid plants with large concentration these
substances.
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The hormone systemin is a polypeptide 18 AA long.
Damaged cells in potato and tomato plants produce systemin, which binds to receptors of
undamaged cells.
This activated receptors produce a series of reactions that eventually produce jasmonic
acid.
Jasmonic acid triggers the transcription of more than 15 gene products including proteinase
inhibitors that will discourage herbivores from attacking this plant.
Parasitoids
Parasitoids lay their eggs in the larvae of insects and devour the larva slowly as they grow and
develop. By the time larva dies, the parasitoid larvae is ready to emerge as an adult.
Caterpillar saliva has a substance called volicitin that induces damaged leaves to produce
volatile substances that attract wasps.
These wasps are parasitoids and lay their eggs in the caterpillars that have damaged the plant.
In this way plants recruit parasitoids to infect the herbivores that are eating them.