Introduction to concepts of plant disease and its effect on food security. Plant defense mechanisms are also explored.

1. CRS 7109: Molecular Plant Microbe
Interactions
M. Ochwo-Ssemakula

2.  Make their own food by photosynthesis and are a source of
energy for the ecosystem
 Provide nutrition for both herbivores and omnivores
 Herbivores: Direct benefit from plants
 Omnivores: Benefit both from plants and herbivores
 Feeders broadly grouped as : Protists (Oomycetes), fungi,
bacteria, viruses, phytoplasma, viroids, nematodes, parasitic
plants & insects
 Thus exposed to exfoliation, defoliation & disease
 Lack an immune system so have developed defense
mechanisms against biotic & abiotic stresses

3.  Broadly classified as:
 Constitutive: Always present in the plant
 Induced: Produced or mobilized to the site of attack
 Constitutive defenses: Range from mechanical defenses to
digestibility reducers and toxins. Many external mechanical
defenses and large quantitative defenses are constitutive, as
they require large amounts of resources to produce and difficult
to mobilize.
 Induced defenses: include secondary metabolic
products (metabolites), morphological and physiological
changes

4.  Three types: Chemical, biological & mechanical
 CHEMICAL DEFENSES:
 Involve secondary metabolites (SM) (allelochemicals) that are organic
compounds not directly involved in growth, development &
reproduction
 Thought to play major role in defense against herbivores
 Characterized as qualitative or quantitative
 Quantitative metabolites: Present in high concentrations (5-
40% dry wt), effective against specialist and generalist
herbivores, effects are dosage-dependent and generally
reduce digestibility. Typically large molecules and
metabolically expensive to produce and maintain e.g.
Geranium: Japanese beetle

5.  Qualitative metabolites: Present at low concentrations (<5%
dry wt), are NOT dosage-dependent and interfere with
herbivore metabolism (block specific biochemical reactions).
Small molecules that are rapidly produced and transported
 Sites of SM production & utilization may be specific e.g.
flavonoids & anthocyanins in fruits and flowers BUT long
distance transport is possible e.g. lupine alkaloids formed in
green aerial parts are synthesized in chloroplast stroma
 Classes of SMs: Nitrogen cpds (alkaloids, cyanogenic glycosides,
glucosinolates and benzoxazinoids), terpenoids, phenolics,
amino acids, peptides and fatty acid derivates

6. 1. Alkaloids: Over 3000, typically invoke an aversively bitter taste
and are derived from amino acids e.g. nicotine, caffeine,
morphine, colchicine, ergolines, strychnine, and quinine. Modes
of action vary (inhibit or activate enzymes, alter carbohydrate
and fat storage by inhibiting the formation phosphodiester
bonds involved in their breakdown, bind to nucleic acids and
inhibit synthesis of proteins and affect DNA repair mechanisms,
affect cell membrane and cytoskeletal structure causing the cells
to weaken, collapse, or leak; and can affect nerve transmission.
2. Cyanogenic glycosides are stored in inactive forms in plant
vacuoles. Become toxic when herbivores eat the plant and break
cell membranes, allowing the glycosides to come into contact
with enzymes in the cytoplasm releasing hydrogen cyanide
which blocks cellular respiration e.g. linamarin (cassava), dhurrin
(sorghum)

7. 3. Glucosinolates: Activated as for cyanogenic glucosides, and the
products can cause gastroenteritis, salivation, diarrhea, and
irritation of the mouth
4. Benzoxazinoids: Characteristic for grasses, are stored as inactive
glucosides in plant vacuoles. Upon tissue disruption they get into
contact with β-glucosidases from the chloroplasts, which
enzymatically release the toxic aglucones. Whereas some
benzoxazinoids are constitutively present, others are only
synthesized following herbivore infestation, and thus, considered
inducible plant defenses against herbivory
5. Terpenoids (isoprenoids): Organic chemicals similar to terpenes,
derived from five-carbon isoprene units. Are over 10,000 known
types of terpenoids. Most are multi-cyclic structures which differ
from one another in both functional groups, and in basic carbon
skeletons.

8. Monoterpenoids (2 isoprene units) are volatile essential oils such
as citronella, limonene, menthol, camphor, and pinene.
Diterpenoids (4 isoprene units) are widely distributed in latex and
resins. Are responsible for making Rhododendron leaves
poisonous. Plant steroids and sterols are also produced from
terpenoid precursors, including vitamin D, glycosides (such as
digitalis) and saponins (which lyse red blood cells of herbivores)
6. Phenolics (phenols): Consist of a 6-carbon ring bonded to a
hydroxy group e.g. lignin, silymarin & cannabinoids. Action:
Antiseptic properties & disrupt endocrine activity. Range from
simple tannins to more complex flavonoids that give plants their
red, blue, yellow and white pigments. Condensed tannins
(composed of 2 - >50 flavonoid molecules), inhibit herbivore
digestion by binding to consumed plant proteins and making them
more difficult to digest, and by interfering with protein absorption
and digestive enzymes. Silica and lignins, which are completely
indigestible to animals, grind down insect mandibles

9.  External plant structural features on stems and leaves can
deter, injure, or kill the herbivore
 Produced internally and released to the surface e.g. resins,
lignins, silica, and wax
 Others e.g. prickles, spines & trichomes restrict feeding by
large herbivores and contain irritants
 Types: Thigmonasty, mimicry & camouflage
 Thigmonasty: Movements of plant parts in a way to portray wilting due
to changes in turgor pressure e.g. Mimosa
 Mimicry: Passiflora contain physical structures resembling yellow eggs
of Heliconius butterflies, preventing oviposition

10.  Features that indirectly protect the plant by enhancing the
probability of attracting the natural enemies of herbivores
 Types: Semiochemicals, biotic mechanisms, endophytic fungi
& plant immunity
 Semiochemicals: Volatile organic cpds involved in interactions i.e.
communication & locating food e.g. allelochemicals. Attract predators
of the herbivores attacking plants e.g. peptide inceptin produced by
cowpea under attack by armyworms
 Biotic mechanisms: Plants provide food and “housing” for natural
enemies e.g. Macaranga adapt their thin stem walls to create ideal
habitats for ant species (genus Crematogaster), which protect the plant
from herbivores

11.  Endophytic fungi: Endophytes live within the plant & produce
toxins harmful to organisms that would attack the plant e.g.
loline alkaloid produced by fungi which are common in grasses
such as tall fescue, Festuca arundinacea
 Plant immunity: Plants have innate immune systems with
receptor proteins inside and outside cell to recognize specific
molecular features of invading microbes during infection e.g.
RIN4 in Arabidopsis plasma membrane regulates AHA1 that
controls opening and closing of stomata.

12.  Successful host defenses disrupt the disease cycle, primarily in
the pre-penetration, penetration, or infection phases.
 In general, passive defenses against pathogen attack are more
prevalent in the pre-penetration and penetration phases, and
active defenses are more important in the infection phase.
 PRE-PENETRATION: Pathogen colonization is affected by the
physical and chemical attributes of a plant, which further
determine the level of competition. At this stage, appendages
for attachment & penetration are formed
 PENETRATION: Passive defenses include mechanical defenses
i.e. cuticle (made of cutin that is waxy and contains fatty
acids) & cell walls. Active defenses are based on host
recognition of the pathogen

13.  PENETRATION: A series of events occur i.e. signal recognition,
signal transduction and gene activation. Gene activation results
in products that contain, inhibit or kill the pathogen. Active
defenses are categorized as primary, secondary or systemic.
 Primary: Limited to cell in contact with pathogen and characterized by
transduction pathway that involves ion fluxes (Cl− , K+ & H+ , Ca++ in),
oxidative bursts (rapid H2O2 pdn to signal cell death, overwhelm
antioxidant cell protection, promote rapid cell wall reinforcement &
induce gene expression), protein phosphorylation (primary means to
regulate transcription), and signaling molecules induced by SA, JA &
ethylene (Salicylic acid-systemic resistance, jasmonic acid-fungi and
nematodes & ethylene-necrotrophic fungi and non-host resistance)
 Secondary: Induced in cells next to the invaded cell. Include papillae
(composed of phenolics, cellulose & lignin) that is deposited btn plasma
membrane and cell wall
 Systemic: Induced throughout the plant

14.  INFECTION:
 Passive- Antimicrobial cpds can be grouped as:
Phytoanticipins & phytoalexins. Phytoanticipins are pre-
established low-molecular weight compounds, stored within
vesicles or in glycosylated form until cells are damaged.
Glycosylation (chemical bonding to a glucose molecule)
converts a reactive and toxic phytoanticipin to a stable,
nonreactive storage form that is more likely to be water
soluble.
 Active: Phytoalexins are antimicrobial products that are
produced after infection or elicitation by abiotic agents, both
as primary & secondary responses e.g. Jasmonic acid in early
infection for bacterial, viral, some fungal and nematode
diseases.

15.  INFECTION:
 Active: Antimicrobial proteins such as chitinases and β-glucanases also
serve a protective role. A hypersensitive reaction (HR) is more specific
than the above two biochemical responses because it is associated with
response to avirulence genes produced by the disease agent. HR
includes signal transduction, programmed cell death, increased
activation of defense related genes (e.g. synthesis of phytoalexins,
salicylic acid and antimicrobial proteins), and a distant induction of
general defense mechanisms that serve to protect the plant [i.e.,
systemic acquired resistance (SAR)]. Systemic resistance: Although
plant systems do not truly mimic the immune system of mammals, they
do have the ability to better resist pathogens after exposure to other
organisms. Infection or colonization of the plant by one organism can
induce host resistance to other pathogens.

16.  One concept that must be made clear at the outset is that whatever
the kind of defense or resistance a host plant employs against a
pathogen or against an abiotic agent, it is ultimately controlled,
directly or indirectly, by the genetic material (genes) of the host
plant and of the pathogen.
 Most plants are resistant to most pathogens.
 Resistant plants produce passive barriers that the pathogen cannot
overcome or they must be able to activate successful defense(s) that
arrest pathogen development.
 Although both active and passive defenses can be important at any
stage of the disease cycle, passive defense is usually more important
during prepenetration and penetration, and active defense is usually
most important in the infection stage.
 Active disease responses require signal recognition/transduction,
followed by gene activation.