Interactions between microorganisms and plants

1. INTERACTIONS
BETWEEN
MICROORGANISMS
AND PLANTS
By:yusur alani
2.5.2016

2. Outline
 Introduction
 Symbiotic Associations with Cyanobacterial
 Interactions in the Rhizosphere
 Mycorrhizae
 Nitrogen-Fixing Bacteria and Higher Plants
 Bacteria Supporting Plant Growth
 Leaf Surfaces and Microorganisms
 Detrimental Activities of Microorganisms on Plants
 Fungi Promoting Increased Heat Tolerance in
Plants
 Biocontrol of Pests and Pathogens
 Summary

3. Introduction
 Cyanobacteria have symbiotic relationships with
several different cellular systems. Due to cyanobacteria
provide carbon materials resulting from photosynthesis
to the other partner.
 The rhizosphere is an environment where microbial
activities contribute to plant growth. Bacteria and fungi
grow on organic compounds released from the plant
root and produce plant-line hormones
 Atmospheric nitrogen may be reduced to ammonia by
cyanobacteria and other freeliving prokaryotes
 Gall production on plants is attributed to infection by
Agrobacterium tumefaciens that contains the Ti
plasmid
 Microorganisms can be used as biocontrol agents to
control agricultural pests and pathogens.

4. • Earth is richly populated with plants ,
microorganisms are also beneficial to
plants,increased plant growth.

5. plants benefit from two types of
microbe–plant associations:
(1) highly specialized interaction, where there is
considerable
specificity found in mutalistic activities.
(2) commensalism resulting from nutrient secretion
from plants when bacteria and fungi grow in close
proximity to the roots providing no apparent benefit
to the plant.
 In marine environments, the number of bacteria
may reach a million/mL along the surface of red
algal filaments. Because terrestrial plants do not
synthesize vitamin B12.
 A disease results when the natural defense
systems of the plant are ineffective.

6. Model indicating various plant–microbe
interactions

7. SYMBIOTIC ASSOCIATIONS WITH
CYANOBACTERIA:
 Cyanbobacteria are broadly distributed in nature and form
symbiotic relationships with
many different organisms.
 cyanobacteria enhance their survival by establishing an
association with a biological partner.

8. Azolla: is an aquatic fern that contains bilobed leaves
attached to a stem and is found
floating in freshwater.
 The cyanobacteria are found in a cavity
between the ventral and dorsal epidermal
layers of the leaf.

9. .Azolla has long been used to enrich the nitrogen
level of rice fields and is often used as
a fertilizer known as “green manure.
INTERACTIONS IN THE RHIZOSPHERE:
 The soil layer that is within a few millimeters
from the root system is the rhizosphere,which
consists of soil,and all the biological agents
present in the soil.
 This term was introduced by Lorenz Hiltner
1904 , and this designation includes bacteria,
fungi, algae, protozoans, and soil animals .

10. Several different activities are found along the
region of a growing plant root.
 Elongation occurs at the root tip where
mucilage is secreted and entraps materials
released from the root.
 The amount of carbon loss from roots is often
expressed as a percentage of net carbon fixed
and can be considerable
 For example the carbon loss by tomato, pea,
barley, and wheat has been estimated to be 35–
40% of total plant carbon.

12.  In comparing the rhizosphere to nonrhizosphere
environment, the pH of the rhizosphere generally is
acidic as a result of proton secretion and can be as
much as 2 pH units lower than in the soil.
 Not all plant systems are identical because the
gene content of a cultivar selects for specific
bacteria and fungi in the rhizosphere.
 Mycorrhizae:
 Rhizospheric interactions of terrestrial fungi are
influenced by the number of microorganisms, type
of microorganisms, and specific plant root
exudates, some fungi in the rhizoplane may
establish a mutualistic relationship with the root.

13. When plants are introduced into low-nutrient soil, there is an
obvious benefit to plants inoculated with mycorrhizal fungi,
as can be observed from growth
 A significant benefit to the plant with either
endomycorrhizae or ectomycorrhizae fungi is the
enhanced uptake of nutrients from the soil.
 The zone of nutrient absorption by root hairs is only a
few millimeters, while the ectomycorrhizae may extend
20 cm or more from the root while the endomycorrhizae
may scavenge for nutrients for about 8 cm from the
root.
 The terms mycorrhizae and mycorrhizas are plural
expressions, while mycorrhiza is singular and refers to
a specific root–fungus association

14. Ectomycorrhizae
 are commonly found on tree roots of
gymnosperms or woody angiosperms.
 Approximately6000 fungal species form
ectomycorrhizae, and ∼75% of these will
produce aboveground reproductive structures
commonly called “mushrooms.”

15. Most ectomycorrhizae
fungi are basidiomycetes; however, a few are
ascomycetes, and these form
subterranean reproductive structures known
as “truffles.”

16. • Conifers and deciduous broadleaf trees (especially
oak, beech, eucalyptus, and hickory)
are hosts for the mycorrhizal fungi, and plants placed
in nutrient-deficient soils will not
grow unless appropriate mycorrhizal fungi are added.

17. . roots restricted to the penetration between the
epidermal cells forming an intercellular
association with the root cortex and rarely into the root
cells .Infection
by fungi into the surface layers of the root produces a
compact mass that is referred to
as the Hartig net.

18. Fungal hyphae grow extensively on the young root
surface, producing
a web-like mantle or sheath that may be 40 μm thick and
account for 40% of the dry
mass of the root. The surface of a plant root covered
with ectomycorrhizae.

19.  An additional benefit of a plant having mycorrhizal
fungus is the trapping of nutrients. This is
observed during periods of rain when nutrients are
washed from the leaf litter into the soil but are
immobilized in the mycorrhizal mantle before they
can be carried out of the root zone.
 Generally, ectomycorrhizal fungi are incapable of
digesting complex carbohydrates such as lignin or
cellulose and require
sugars, organic acids, amino acids, and vitamins
(especially thiamine) for growth

20. Endomycorrhizae :
 are found on most herbaceous plants, including lower
vascular plants such as ferns and bryophytes.
 are the most common type of mycorrhizae because
they have been demonstrated in almost every plant
family.
 The spores of endomycorrhizal fungi have a thick
wall, and under appropriate conditions germination
will produce a germ tube with an emerging fungal cell.
 The germ tube must associate with a root because
endomycorrhizal fungi have limited saprophytic
metabolism. The fungi penetrate the root with growth
between epidermal cells or through root hairs and
invade cortex cells.

22. Other Mycorrhizal Associations
 there are several other morphological
types of mycorrhizae.Ectendomycorrhizae
are characterized as having a relatively
small fungal sheath, an extensive Hartig
net, and fungal penetration into root cells.
 This intracellular activity is especially
obvious in older roots.
 In one environment ectomycorrhizae may
be produced, but in another environment
the same two partners may produce
ectendomycorrhizae.

23.  In the plants of the Ericales, the symbiotic
fungal association is called ericoid
mycorrhizae. The mycorrhizal fungus is found
in the root hairs that consist of only a few
layers of cells.
 The ericoid
mycorrhizae are
commonly
ascomycetes or
basidiomycetes,
and their presence
stimulates plant growth
and promotes fruiting
by the plant.

24. NITROGEN-FIXING BACTERIA AND HIGHER
PLANTS
 Examples of nitrogen-fixing bacteria that grow
symbiotically with plants are given in this Table:

25.  The enzyme system for nitrogen fixation is found
only in the prokaryotes,
and in the case of symbiotic nitrogen fixation in
plants,
 there is considerable specificity between the
legume symbiont and bacteria for the stable
association.
 The plant provides the carbon and energy source
for the bacteria to grow, while the bacteria fix
nitrogen with the production of amino acids for
plant growth.

26. Root Associations
 Legume Nodules. The association of nodules with roots of leguminous
plants is one of the oldest reports in microbial ecology. In the late 1600s,
Marcelo Malpighi made extensive drawings of legumes showing roots with
well-developed nodules.
 MartinusBeijerinck isolated bacteria from the root nodules of legume plants
and established the nitrogen-fixing capability of these bacteria when
associated with plants. The bacteria that grow symbiotically with roots of
legumes are frequently referred to as rhizobia.
 The establishment of root nodules requires bacteria that can
compete in the rhizosphere as well as establish a symbiotic
relationship with roots.
 An important aspect for rhizospheric and symbiotic bacteria is iron
nutrition, and Michael O’Connell is one of the leading experts
addressing siderophore production by rhizobia

27. Bacteria are specific for a legume species, and formation of the root nodule
is the result
of a unique differentiation process. A model representing some of the steps
involved in
the formation of a nitrogen-fixing nodule are presented later
 the individual steps are summarized as follows:
1. The legume root secretes specific chemicals known as flavonoids ,
and these signaling molecules attract rhizobia growing in the rhizosphere.
2. The flavonoids also induce transcription of nod genes in the rhizobial
genome to produce lipochitin oligosaccharides, called Nod factors.
3. The plant root recognizes the chemical structure of the Nod factors, and these
lipochitin oligosaccharide molecules are taken up by legume receptor kinases. The
Nod factors activate the plant hair roots, and this recognition is responsible in part
for the specificity between the host and legumes.
4. The symbiotic bacteria attach onto the root hairs and enter into the root by a
process
known as root infection.
.

28. 5. The bacteria produce a lipopolysaccharide capsule that enables
the rhizobia to evade plant defense systems and enter the root by
a structure designated as the
infection thread

30. 6. Phytohormones are produced by bacteria to stimulate root cell division,
and the plant root expands quickly to produce a nodule.
7. Bacteria in the root nodule adjust to the low-nitrogen environment and the
reduced oxygen level in the nodule. In some cases the bacteria differentiate
to produce rounded cells or cells with a “Y” shape . These differentiated
bacteria
are bacteroids, and these cellular units are important in fixing nitrogen.

31. A photograph of soybean roots with nodules present on
the root hairs is shown in

32. o Nitrogen fixation in the soybean nodule is attributed to the
bacteroids,
differentiated rhizobial cells, present in the symbiosomes
 Because the nitrogenase enzyme is oxygen-sensitive, leghemoglobin
in the nodule reduces the level of free O2 to enable nitrogen fixation.
The product of nitrogen fixation is the production of glutaminethis
amino acid is transported by way of the root hairs to regions of protein
synthesis

33. BACTERIA SUPPORTING PLANT
GROWTH
 Production of Hormones:Many rhizospheric microorganisms produce
chemicals that stimulate plant growth, andthese chemicals have a
molecular structure similar to that of plant hormones. many bacteria
produce plant-stimulating compounds, including auxins, gibberellins,
cytokinins, ethylene, and abscisic acid.
 These plant-like hormones are produced by free-living, symbiotic, and
pathogenic strains of rhizospheric microorganisms.

34. Growth-Promoting Rhizobacterian:
The most numerous microorganisms in the soil are bacteria,
and these organisms may influence mycorrhizal growth.
Scientists recognize the importance of soil bacteria and have
called them plant-growth-promoting rhizobacteria (PGPR) or
simply growth-promoting bacteria.
 A mixture of several species of microorganisms is more
effective in stimulating plant growth than is a monoculture of
bacteria or fungi.
Cactus Symbiosis:
 bacteria enable cactus to grow. In the harshest of terrestrial
environments.
 The bacteria are found as endophytes of the plant and also are
present in the cactus seeds.

35. o When the seeds fall on animal dung, the bacteria grow
with the production of organic acids that convert rock
particles to mineral soil.
 These endophytic bacteria continue to grow in the roots
of the cactus with the continuous secretion of organic
acids and fixation of nitrogen.

36. DETRIMENTAL ACTIVITIES OF
MICROORGANISMS ON PLANTS:
 Fungal Parasites:
Several fungi and a small number of bacteria are pathogenic for
plants. Fungi of all taxonomic classes are capable of causing
epidemic plant diseases that produce considerable economic loss
and hardship for humans

37. The pattern followed for disease formation in plants is
generally as follows:
1. Spores or vegetative cells of fungi come in contact
with the plant.
2. The fungi penetrates the surface through wounds,
stomata, and other openings in the plant cell.
3. An infection occurs with the fungi growing in either the
intracellular or intercellular region.
4. Other tissues are invaded, and the pathogen is
spread throughout the plant.
5. Depending on the pathogen, death of the plant may
be observed in a few days or a few weeks.

38. Bacterial Pathogens
 The bacteria-producing diseases in plants generally
display a number of hydrolyticenzymes for penetration
of the plant surface, and growth in the plant is generally
intercellular between the parenchyma cells.
 As the disease progresses in the plant, bacterial
distribution may become systemic as a result of migration
into the vascular tissue.
 In some cases, the bacteria release pectinase that
hydrolyzes the plant cell walls and the plant collapses.

39. .
.The bacterium Agrobacterium
tumefaciens produces tumors in a
diverse group of dicotyledonous plants
at the root–stem interface that is called
the “crown” of the plant.
Thus, the disease has been called
“crown gall.” The microorganism is an
aerobic Gram negative
bacterium that is widely distributed in
soils where it grows on a variety of
sugars and organic acids

40. FUNGI PROMOTING INCREASED HEAT
TOLERANCE IN PLANTS
 When the fungus Curvularia protuberate
infects Dichantheilum lanuginosum, a tropical
grass, the plant is capable of growing in soil
that has a temperature of 65◦C. However, for
this heat tolerance,
 the fungus must also be infected with a
dsRNA virus consisting of two segments, and
this virus has been designated as Curvularia
thermal tolerance virus (CThTV)

41. BIOCONTROL OF PESTS AND PATHOGENS:
 the control of insects and agents responsible for
disease production in plants has been achieved
through the use of chemicals.
 However, an increased concern for the addition of
chemicals into the environment has prompted scientists
to explore theuse of biological agents to control or
prevent the growth of pathogens
 Plant-growth-promoting rhizobacteria (PGPR) produce
a variety of antibiotic compounds that inhibit a variety of
Gram-negative and Gram-positive soil bacteria,
 Nematodes found in the soil are roundworms of a few
millimeters in length, and some of these are parasitic
for plants

42.  As a result of nematodes attacking roots and
underground parts of the plant, death of the plant may
be due to direct damage by the nematode or to
secondary infection by bacteria, fungi, or viruses

43.  Certain strains of Bacillus thuringiensis produce a
protein that has been used to
 control insect populations and some of the insects
controlled are listed in Table
 Bacillus thuringiensis is an aerobic, Gram-positive
bacterium found broadly distributed in soil.

45. SUMMARY
 A diverse group of microorganisms are found in the root
zone rhizosphere, and bacteria present in the rhizoplane
have considerable influence on the plant
 specific bacteria may enhance plant growth, due to the
development of mycorrhizae, or prevent growth of
phytopathogenic fungi
 Fungi may be found in symbiotic association with plant
roots, where the plant provides sugars and organic
acids while the fungus partner enhances mineral uptake
by plants
 The two principal types of fungal association are
endomycorrhizae found commonly on herbaceous plants,
and ectomycorrhizae, which are generally associated with
woody plants.

46.  Symbiotic nitrogen-fixing plants have specific
bacteria as partners for the conversion of
atmospheric N2 to ammonia.
 Establishing the plant–bacteria activity results
from signal responses on the part of both
partners in this symbiosis
 A few of the fungi and bacteria are plant
pathogens producing highly distinctive plant
pathologies.
 An important bacterial disease in plants is crown
gall attributed to Agrobacteriu tumefaciens,

47.  Unique in this infection is the presence of
a Ti plasmid in the bacterium, and this
specific plasmid carries genes for plant
hormones to stimulate tissue proliferation.

48. Thank you