Transcript No Slide Title

AGR2451 Lecture 14 (M. Raizada)
•Mid-term Exam coming up
•Review session (in class)coming up
•Reading for this week (Lecture 14 + 15): p.328-354
Plants, Genes and Agriculture (On Reserve)
•Review and completion of last lecture:
Lecture 14 Notes - Plants and the Environment II – “Chemical Ecology”
1. INTRODUCTION/OVERVIEW
Evolution created complex nutrient molecules, complex carbon
structures and storage tissues. Viruses, bacteria, fungi, insects,
herbivores want to get access to all of this.
How do pathogens gain access inside plant tissues?
From Biochemistry and Molecular Biology of Plants, page 1104
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
Slide 14.1
•first defense is waxy cuticle and cell wall
•plants are immobile but not defenseless
•plants are simple morphologically; instead they have invested in
complex biochemistry
•land plants use >50,000 chemicals (“secondary metabolites”) to interact
with their environments
•up to 10% of the dry mass of plants is made up of chemicals designed
for defense against predators
•toxic secondary metabolites are stored in the vacuole until needed
•worldwide preharvest crop losses are 14% from insects; 12% from
disease; 10% from weeds; additional losses occur post-harvest including
worms and insects that feed on seeds/storage organs
2. Overview of Plant Pathogens
•there has been co-evolution of plant pathogens (viruses, bacteria, fungi)
including herbivores (insects, etc.) and the toxic chemistry of land plants
•how do plant toxins damage pathogens?
• 1/6th of all food grown is consumed by insects
•insects are often specialists which only feed on one or a few species,
but there can also be generalists
•plants evolved toxic chemicals for all insects, but then a few insects
developed resistance, hence these are the specialists
•in farmer’s fields, the specialists thrive because of an abundant food
supply of a single species and hence these insects multiply
•crop rotation is very useful for combatting specialists (corn soybeans combat northern corn rootworm in the U.S. midwest).
Why?
2. Overview of Plant Pathogens
•nematodes – several hundred species feed on plants, usually roots – and
can grow outside or inside cells where they suck photosynthate
•viruses – their DNA can move through plasmodesmata, but do not
infect meristems.
•Why do viruses not infect meristems?
•if bacteria invade conductive tissues, then water cannot be transported,
so cause wilts
•if bacteria spread rapidly and kill cells, then called blights
•Agrobacterium also produces or mimics plant hormones to create
tumor galls
•of 100,000 fungal species, 8000 cause plant diseases – they produce
enzymes which can digest cell walls and then spread from cell to cell;
•plant rusts are among the worst diseases, caused by Basidiomycete fungi
From Biochemistry and Molecular Biology of Plants, page 1106
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
Slide 14.3
3. CLASSES OF SECONDARY METABOLITES (>50,000)
-3 main pathways (terpenes, alkaloids, phenylpropanoids)
A. Terpenes (>25,000 types)
•5C skeleton (isoprene) as 2,4,6, 8 or more polymers;
•the smaller ones (5C,10C) are volatile
What molecule does the 5C skeleton originate from?
•can be volatile, released by epidermal gland cells as in flowers (as
perfumes)
•terpenes are essential oils found in herbs and spices that give
peppermint, basil or sage their smells;
•they are the aromatic substance in the resin of wounded conifer trees
thyme
Gland
Hair of
spearmint
Lemon
Leaf
Secretory
cavity
Resin
Duct of
pine
Images from Biochemistry and Molecular Biology of Plants, pp. 1252 and 1255
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
Slide 14.4
A. Terpenes (continued)
why is volatility useful?
1) warn insects of plant toxicity
2) attract insect pollinators or herbivores to eat fruits
•terpenes include leaf feeding deterrants, allelopathic agents, insecticides,
pollinator attractants and anti-herbivore agents
From Biochemistry and Molecular Biology of Plants, page 1267
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
•40C version of terpene constitutes carotenoids such as lycopene which
gives tomato its red colour.
Why are carotenoids useful and how do they function?
antioxidants: ring structures absorb high energy electrons
Slide 14.5
B. Alkaloids (>12,000 types)
•synthesized from amino acids Tryptophan, Tyrosine, Phenylalanine,
Lysine and Arginine;
•all contain Nitrogen
Images from Biochemistry and Molecular Biology of Plants,
pp. 1288, 1269, 1271
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
•alkaloids affect the central nervous system, so used in medicines
(include morphine, codeins, ephedrine), as well as stimulants or sedatives
(caffeine, cocaine, and nicotine)
•Socrates drank hemlock, an alkaloid, to die (execution in 399 BC)
•Cleopatra used henbane to dilate her pupils to look more alluring to her
male political rivals
Slide 14.6
B. Alkaloids (continued)
•some larvae gather alkaloids from plants and convert them into
pheromones stored in abdominal scent organs
•nicotine: one of the first and most effective insecticides used by humans.
•Herbivory stimulates nicotine synthesis in wild tobacco
Images from Biochemistry and Molecular Biology of Plants, page 1273
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
•caffeine: at very low levels, caffeine kills the larvae of the tobacco
hornworn by inhibiting an essential enzymatic reaction
Slide 14.7
C. Phenylpropanoids (>8000 compounds)
•skeleton consists of C6 + C3: phenyl ring of 6 carbon with a hydroxyl
(OH) group attached to a 3 carbon tail
•they are derived from amino acids Phenylalanine or Tyrosine through
loss of the amino (NH2) group
•many compounds involved in cell wall structural role and hence were
essential for adaptation of plants onto land
•includes the phenolics = 6C ring with an OH (hydroxyl group); this
constitutes 40% of the organic carbon in the biosphere.
Why so much and in what form?
Images from Biochemistry and Molecular Biology of Plants, page 1287
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
Slide 14.8
C. Phenylpropanoids - lignin (cont’d)
•lignin is a complex polymer of phenol which is deposited into cell
walls of the vascular tissue of plants to give them strength
•lignin accounts for 20-30% of the dry weight of all vascular plant tissues
whereas cellulose permits water transport, it is lignin that blocks it;
•hence lignin required to transport water so essential for land plant
evolution
•lignin is often produced after pathogen attack to seal up the infected
cells and because it is toxic
Images from Biochemistry and Molecular Biology of Plants, page 1300
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
tannins are polymers of phenols which react with proteins to make them
less digestible; high levels of tannins in forage crops deter feeding by
cattle. Tannins may also give leaves an unpleasant taste
Slide 14.9
3. Phenylpropanoids - the flavonoids
•flavonoids (4500 compounds): these give flowers their colour as
pigments,
•flavonoids are often found inside plant vacuoles
•though they are also feeding deterrants and signalling molecules
(legume-Rhizobium signal);
Images from Biochemistry and Molecular Biology of Plants, page 1303 + 1305
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
Slide 14.10
C. Phenylpropanoids
•coumarin (a flavonoid) of which there are 1500 types can cause internal
bleeding in mammals, includes a rodenticide
•psoralen (a flavonoid) is derived from celery and is used to treat skin
disordersincludes anticancer drugs such as taxol
•plant phenolics contribute to fragrances and flavours
Images from Biochemistry and Molecular Biology of Plants, page 1315
B. Buchanan, W. Gruissem, and R. Jones
ASPP Publishing, Rockville, MD, 2000
Slide 14.11
D. Unusual amino acids
•plants produce up to 300 nonprotein amino acids within their seeds.
Why?
E. Inhibitors of Insect Digestive Enzymes
•plants produce proteins such as amylase inhibitors and protease
inhibitors which inhibit insect digestion of starch and protein in the
insect gut by direct binding to the enzyme
At the molecular level, how do insects (and other pathogens) gain
resistance to these secondary metabolites?
4. HERBICIDES/PESTICIDES and HUMAN HEALTH
Central Problem: need to breed for plants that have pesticides in leaves/
vegetative parts, but not in edible parts.
•Our ancestors bred away pesticides in seeds and other storage organs,
yet simultaneously increased the nutrients at these sinks, and so now we
compensate with artificial pesticides applied to the plant surface.
•example: wild lima beans have high leves of cyanogenic glucosides,
whereas cultivated lima beans have very low level, so cultivated plants
are more susceptible to attack by insects and pathogens
•often, modern agriculture has substituted good farming practices
for herbicides and pesticidesstressful (dense)
conditions
Slide 14.12
Metabolites and Human Health (continued)
•about half of the secondary compounds that have so far been tested
cause cancer in lab rodents at various doses, whereas others can
prevent cancer, but there is very limited research in this area
•what is natural? organic marketing makes it seem as if the only source
of pesticides are human
•what are the secondary metabolites found in herbs?
•In “natural/ancient” medicines? which are often leaves or roots
(vegetative structures).
•What are the problems associated with applied pesticides?
Slide 14.13