Transcript المحاضرة السادسة

Plant Defense
1- Physical Defense
6th Lecture
Plant-insect Interactions
Plant Defense
It seems that plants are unmovable
and silent. Thus, it thoughts that no
quick responses have been expected,
when plants are under attack.
Introduction
Plant defense against herbivory or
host-plant resistance (HPR) includes
a range of adaptations evolved by
plants that improve their survival and
reproduction by reducing the impact of
herbivores.
Introduction
Plant defenses can be classified
generally as induced or constitutive.
Constitutive defenses are always
present in the plant species, while
induced defenses are synthesized or
mobilized to the site where a plant is
injured
Introduction
Most external mechanical defenses
and large quantitative defenses are
constitutive
Induced defenses include secondary
metabolic products, as well as
morphological and physiological
changes.
Quantitative and qualitative
Quantitative defenses are dose dependent
compounds which are considered to be costly to
produce. Feeding deterrents such as tannins,
terpenes, and flavanoids are quantitative defenses.
These defenses are termed "immobile" defenses in
the RAH vernacular. Conversely, qualitative
defenses are the toxins which are lethal in small
concentrations and are thus considered to be less
costly to produce than quantitative defenses. These
types of defenses are known as "mobile" defenses
in the RAH.
RAH = The resource availability hypothesis
HM what is RAH
Facts
Plant defense responses to herbivore
attack are rapid and highly dynamic.
Introduction
Plant-insect interaction is a dynamic
system, subjected to continual
variation and change. In order to
reduce insect attack, plants developed
different defense mechanisms
including chemical and physical
barriers
Physical Defense
Plant Structure
Morphological or physical
resistances
Plant structures that interfere
physically with the insect's
locomotive, feeding, or reproductive
functions.
These functions may involve plant
color and shape or more specialized
defensive adaptations such as
trichomes, tough tissues, surface
waxes, or cell silication
1- Trichomes
Trichomes are cellular, hair-like outgrwoths of the plant epidermis, which may
occur on leaves, shoots, or roots.
– Water conservation
– Probably the most important morphological
defense against insects attack
Insects vary in their response to trichomes
Trichomes may interfere with insect oviposition,
attachment of the insect to plant, feeding, or
ingestion
Trichomes
The mechanical effects of trichomes
depends on four main characteristics:–Density
–Erectness
–Length
–Shape
Trichomes
Some trichomes possess glands that
exude secondary plant products
Physical and chemical defense in one
structure
Trichomes
In other cases, the exudates is sticky
material that acts physically to glue the
insect's legs or other body parts
together and thus reduce locomotion.
Trichomes
Insect of piercing-sucking mouth
parts generally unable to feed on
plants that are pubescent
Xylem and phloem feeders
Mesophyll feeders
Short trichomes an effective barrier
to first group (Xylem and phloem
feeders)
Trichomes
Trichomes on
soybean
Potato leafhopper
1 mm in length at
density of 8
trichomes per mm
Proboscis length
0.2-0.4 mm
Conclusion
Trichomes
barrier to feeding
Trichomes
Chewing mouth-parts
Cereal leaf beetle
– Egg viability, larval growth and survival
were reduced on varieties of wheat with
having densely pubescent leaves.
– BUT!
– Table 3.2
Trichomes
Trichomes may slow the searching
rate of predators and parasitoids, or
make the herbivore inaccessible to
the point where enemies become
ineffective.
Example
– Whitefly on cucumber plant and
Encarsia formosa
Fig 3.6
Trichomes
Some insects species have been able
to counter the problem that may
pose. The aphid Myzocallis schreiberi
has a pair of claws and a pair of
flexible empodia that help it to get a
good grip on the short woolly
trichomes
Trichomes
Trichomes density may vary not only with
abiotic growth conditions, but also with
plant damage caused by herbivory.
Example
After larvae of pieris rapae had consumed
parts of young black mustard, trichomes
densities on some newly expanded leaves
increased.
2- Epicuticular waxes
The cuticle of most plants are
covered by a thin layer of wax
Functions of wax layers
– Maintaining the water balance of plants
– Reduce attacks by some pathogens and
insects
fig 3.3
Wax layers
Brassica oleracea plants are more
resistances to attack by the cabbage flea
beetle on normal plant than on glossy-leaf
plant.
Wax layers cause leaf slipperiness. As
results the survival rate decreased due to
starvation.
In contrast to, the cabbage aphid and the
whitefly developed a large colonies on the
waxy leaves of broccoli plants.
As indirect effect upon herbivores,
wax may impair the adhesion,
mobility, and effectiveness of
predatory insects in an increasing of
herbivore populations.
Larvae of the lacewing reduced
population of its prey on glossy than
normal waxbloom cabbage plant
Fig
3.4 & table 3.1
3- Leaf toughness
Plant cell walls strengthened by
cellulose, ligin, … make a plant
resistant to mechanical injury ,
tearing actions by mandibles,
penetration of piercing-sucking
mouthparts, and oviposition.
Leaf toughness acts as an effective
factor reducing herbivory.
Leaf toughness
Insect species vary in strength of
mandibles.
Plant leaves vary in toughness. Grass
are three times tougher than herbs.
Table 3.3
There are also differences in leaf
toughness between different climatic
zone. Thus, leave toughness of
tropical forest shows 3-fold greater
value than those in temperate zones.
Leaf toughness
Differences in leaf toughness affect
insect feeding and growth. Fig. 3.7
Leaf toughness
The size of leaf fragments swallowed by
chewing insect species varies with size
(instar) of the insect and hardness of the
food. Thus, the alimentary tract of
saturniid caterpillars feeding on tough
leaves contain leaf particles that are large
and very regular in size, whereas,
sphingid larvae generally feeding on soft
leaves of herbaceous host bite off small
leaf particles independent of caterpillar
size. Fig 3.8
Leaf toughness
Saturniid
Leaf toughness
Sphingids
Leaf toughness
As most insect digest cell wall only to
very limits degree. The frass of
lepidopteran (Paratrytone melane)
contains leaf pieces with 76-86%
uncrushed cells.
Leaf toughness
Within certain limits an insect may
adapt its head morphometric to the
toughness of its food. The larvae of
Pseudaletia unipunctata, in which g
the head is twice as large when fed
on hard grass than on soft artifcial
food.
Leaf toughness
Pseudaletia unipunctata
Leaf toughness
Beside cellulose, silicon at high
concentrations, such as in Poaceae
and Palmae, enrich the toughness of
the plants
Fig 3.10 A and B
4- Structure involved in mutualistic relations
Domatia and plant extrafloral nectaries
Domatia and extrafloral nectaries are
plant structures that provide shelter
and food to predaceous arthropods
and thus affect herbivorous insects
only indirectly.
Domatia occur in plant species
belonging to over 90 families
Domatia
Extrafloral nectaries
Plants can provide carnivores with
extrafloral nectar (Fig. 10.8).
Its production can be induced by
herbivory or mechanical wounding.