MHB are bacteria that stimulate the formation of mycorrhizal symbiosis between fungi and plant roots. They indirectly promote plant growth through mechanisms like nutrient mobilization, pathogen protection, and stimulating mycorrhizal spore germination and mycelial growth. While many studies have focused on temperate plants, more research is needed on tropical species. MHB can benefit agriculture by improving nutrient uptake and decreasing fertilizer needs. Further research should identify specific genes and traits of MHB, localize their activities, and explore their roles in different mycorrhizal systems.

4. MHB is a
generic name
given to
bacteria
which
stimulate the
formation of
mycorrhizal
symbiosis
Some
bacterial
strains that
positively
impact the
functioning of
mycorrhizal
symbiosis
mentioned as
Mycorrhizatio
n Helper
Bacteria.
1

5. Indirect stimulation of plant growth
Protection against pathogens or
against adverse conditions
Use them to decrease fertilizer use
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6. 3

7. Many plant models have been used to study
the MHB effect, including herbaceous and
woody plant species, mainly from temperate
ecosystems. Only a few studies have focused
on tropical plant species.
It depends neither on the type of
the mycorrhizal symbiosis nor on
the taxonomy of the MHB strains
MHB concept is generic
4

8. In the case of arbuscular
mycorrhizal fungi, many
examples of MHB have been
described in the genus
Glomus.
In ectomycorrhizal fungi,
only Basidiomycetes have
been reported to
positively interact with
MHB.
Exception: In the case of the
ectomycorrhizal Ascomycete
Tuber melanosporum, an
indirect helper effect of soil
Pseudomonas on the T.
melanosporum symbiosis.
5

9. 6

10. Natural
colonization of
mycorhizosphe
re also
occurred by
MHB
e.g; Pinus
radiata
They are
generalist as
associate with
both
herbaceous
and woody
mycorrhizal
plants
MHB are not
restricted to
a specific
type of
ecosystem
7

11. Those which
are isolated
from fungal
environments
(i.e.,
hyphosphere,
mycorrhizosp
here,
sporocarps)
Those
isolated from
elsewhere
(e.g., plant
endosphere,
soil, root
nodules,
rhizosphere).
8

12. 9

13. This characteristic is of
importance for field
application
MHB stimulate mycorrhiza
formation of a large number of
hosts
Example: Paenibacillus
enhances mycorrhiza formation in
Pinus sylvestris (ECM) and in
Glomus mosseae (AM)
Drawback in field application
Some MHB could have unexpected side effects on the
composition and the functioning of the natural
microbiota
10

14. • Some MHB also
behave as Plant
Growth
Promoting
Rhizobacteria
(PGPR)
Plant growth
promoters
• Some MHB
also possess
nitrogen‐fixing
abilities that
could benefit
plant nutrition
Nitrogen fixing
abilities
• Some MHB
have also been
reported to
protect their host
plant against
pathogens
Protection against
pathogens
11

15. Mechanisms of MHB
effect
12

16. 1.Spore germination
• Some MHB able to stimulate arbuscular
mycorrhizal fungal spore germination of G.
mosseae.
• Volatile compounds produced by different
species of Streptomyces were proved to
promote the germination of G. mosseae
spores
2.Mycelial growth
• A significant correlation has been shown
to exist that increases mycelial biomass
and promotion of mycorrhizal
establishment.
• The MHB P. monteilii produces currently
unknown gaseous compounds that
increase the growth rate of Petasites albus
13

17. Pseudomonas
• The only MHB that
simultaneously
enhances significantly
the growth, the
branching angle and
the branching density
of the mycelium
• Also the number of
apices
Streptomyces
• It promotes mycelial
extension
• It sharply reduces
hyphal biomass as a
result of a reduction
in mycelial density
• Also reduces the
thickness of the
fungal hyphae
14

18. MHB detoxify the soil, restoring soil
conduciveness.
A Bacillus sp. strain had a stronger
positive effect on the intensity of root
cortex colonization
MHB also reduced the concentrations of
phenolic antagonistic substances
produced by mycorrhizal fungi
15

19. MHB could
indirectly
facilitate root
colonization by
inducing the
release of plant
flavonoids
Burkholderia and
Rhodococcus strains
increased the
formation of only
second-order
ectomycorrhizal roots
Bacillus strain
increased the
formation of only
first-order
ectomycorrhizal
roots
Stimulation of
lateral root
formation is a
frequently
observed
characteristic
of MHB.
16

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21. 18

22. In the case of
arbuscular
mycorrhizas, two
MHB strains of
Enterobacter sp.
and Bacillus
subtilis, when
inoculated
together with G.
intraradices,
enhanced
phosphorus
uptake.
This is consistent
with the recent
demonstration that
the solubilization of
rock phosphate is
enhanced by
formation of mixed
biofilms between
phosphate-
solubilizing
saprotrophic fungi
and a
Bradyrhizobium
elkanii strain.
These recent
findings strongly
suggest that
ectomycorrhiza
associated
bacteria
complement the
roles of the
external
mycelium by
mobilizing
nutrients from
minerals
19

23. Potential
nitrogen
fixation by
bacteria
associated
with
ectomycorrhiz
as, the
dominant
mycorrhizal
type of trees
in mostly
nitrogen-poor
temperate
and boreal
forests.
The
presence of
nitrogen-
fixing
bacteria in
diverse
ectomycorrh
izal types
clearly
supports
their
potential for
improving
plant
nutrition.
All these
results
suggest that
diazotrophic
bacteria
embedded in
ectomycorrhi
zal tissues
directly
providing
nitrogen of
atmospheric
origin to the
two partners
of the
symbiosis
20

24. Mycorrhiza-
associated bacteria
also contribute,
together with the
fungal symbiont, to
protection against
root pathogens
A significantly higher
proportion of
fluorescent
Pseudomonas
inhibiting the growth
of seven root-
pathogenic fungi
belonging to the
genera Rhizoctonia,
Fusarium,
Phytophthora and
Heterobasidion
In vitro
antagonism
against
phytopathogens
by mycorrhiza-
associated
bacteria has been
frequently
observed
21

25. Solid arrows (1–3) represent specific
helper functions. (1) The bacterium
contributes to nutrient mobilization
from soil minerals and organic matter,
and to detoxification of the
ectomycorrhizospheric soil in terms
of removal and/or degradation of
allelopathics/antagonistic metabolites
or xenobiotics; (2) the bacterium has
an impact on root architecture
through the production of growth
factors and protection of plants
against phytopathogens; (3) the
bacterium improves fungal nutrition
by, for example, the provision of
nitrogen in the case of diazotrophs
and enhances mycelial extension by
the production of growth factors; (4)
fungal exudates serve as nutrients for
the bacteria; (5) the fungus mobilizes
nutrients from soil minerals and
organic matter; (6) the soil provides
the plant with water and solutes; (7)
the root contributes to mobilization of
nutrients from soil minerals and
organic matter; (8) the fungus
transfers water and mineral nutrients
to the roots and protects the plant
against pathogens; (9) the root
provides the fungus with
photosynthates.
22

26. 23

27. Identification of
marker traits and
genes specific for
MHB functions
should be
performed both in
fungi and in
bacteria
Imaging
techniques
should be used to
specifically
localize bacterial
cells and their
activities related
to the helper
effect
The
contribution of
mycorrhiza-
associated
bacteria to
mycorrhizal
functions
should be
investigated.
The principles
and practices of
controlled
mycorrhization in
agriculture,
horticulture and
forestry should
be revisited
Searches for
MHB in a wider
range of
mycorrhizal
systems should
be carried out in
order to better
explore the
question of their
specificity
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