The document discusses rhizosphere and rhizoplane microflora. It defines the rhizosphere microbiome as the microbial population in the area surrounding plant roots, which is much higher than in bulk soil. It provides examples of common microorganisms found in the rhizosphere like bacteria, fungi, protozoa. Rhizosphere microbes can positively impact plants by increasing nutrient supply, stimulating growth, and preventing pathogens. However, they can also negatively affect plants through competition for resources and by acting as pathogens themselves. The document also discusses actinorhizal bacteria like Frankia that can form symbiotic nitrogen-fixing nodules on roots of certain plants.

1. Rhizosphere and Rhizoplane microflora
MUSKAN
M.SC MICROBIOLOGY

10. Microorganisms found in Rhizosphere (Rhizosphere microbiome)
 The microbial population in the rhizosphere consists of different groups of microorganisms like bacteria, fungi, parasites,
viruses, and algae.
 The microbial population in the rhizosphere is known as the rhizosphere microbiome and the microbial population in such an
area much higher than the bulk soil.
 In the rhizosphere, there is a microbial population distinct from the rest of the soil.
 Bacteria in the rhizosphere are larger and have higher proportions of Gram-negative and denitrifying bacteria than those in the
bulk soil.
 Rhizosphere fungal populations, abundant in both pathogenic and mycorrhizal species, can be 10 to 20 times higher than those
in the non-rhizosphere.
 Protozoa and other microfauna also thrive in the rhizosphere because that is where food is most plentiful.
 The type and population of microorganisms in the rhizosphere are highly influenced by the type of plant grown on the soil.
 Microbes in the bulk soil often experience long periods of nutrient deprivation; they have different survival strategies in
dealing with starvation and stress.
 The rhizosphere bacterial community is recruited from the main reservoir of microorganisms present in the soil.
 Next to the recruitment of specific soil microbes into the rhizosphere microbiome, plant roots also influence specific functions
of the microbiome.
 Some of the examples of microorganisms found in the rhizosphere region include Bacillus, Arthrobacter, Pseudomonas,
Agrobacterium, Alcaligenes, Clostridium, Flavobacterium, Corynebacterium, Micrococcus, Xanthomonas, Amanita,
Tricholoma, Torrendia, Descomyces, Thelephora, Verticillium, Phytophthora, Rhizoctonia, Micromonospora,
Thermoactinomycetes, Amycolaptosis, Actinomadura, etc.

11. Positive effect of Rhizospheric microorganisms on Plants
 Rhizospheric microorganisms play an important role in the ecological fitness of the plant and the
soil.
 Important microbial processes like plant protection, growth promotion, production of antibiotics,
geochemical cycling, and plant colonization take place in the rhizosphere.
 Rhizospheric microorganisms increase the supply of mineral nutrients from the soil to the plant.
 Another group of microorganisms in the rhizosphere stimulate plant growth indirectly by
preventing the growth or activity of plant pathogens.
 These microorganisms are responsible for direct growth promotion by the production of
phytohormones.
 Plant growth-promoting rhizobacteria act as biofertilizers by enhancing phytochrome production,
phosphate solubilization, and siderophore production.
 The capacity of rhizospheric organisms to synthesize anti-fungal metabolites such as antibiotics,
fungal cell wall-lysing enzymes, or hydrogen cyanide suppresses the growth of fungal pathogens.

12. Negative effect of Rhizospheric microorganisms on Plants
 One of the most important negative effects of rhizosphere microorganisms is a competition where
the microorganisms compete with the plants for water, nutrient, and space.
 Some of the members of the rhizosphere microbiome might act as plant pathogens, resulting in
different forms of plant diseases.
 Competition between the microorganisms in the microbial community in the rhizosphere can even
result in the loss of beneficial microorganisms.

14. Estimation
 Microorganisms are utilized in agriculture for various purposes; as important components of
organic amendments and composts, as inoculants for biological nitrogen fixation, phosphorous
solubilization and indole acetic acid (IAA), to improve crop quality and yields.
 One of the most common strategies to increase agricultural production is through the
improvement of soil fertility. Nitrogen (N) and phosphorus (P) are the two most limiting nutrients
in soil. Indole acetic acid is an essential natural growth promoter that extensively affects plant
growth and development.
 Reports have indicated the ability of many bacterial microorganisms to produce phytohormones
that can enhance the plant root contact surface with soil and subsequently the increase of nutrient
uptake via root elongation. Due to this ability, microorganism inoculants can be used as a
substitute for chemical fertilizers in partially fertile soils and/or at least as a supplement for
chemical fertilizers in infertile soils.
 Therefore, the estimation was undertaken to screen the rhizosphere, rhizoplane and phyllosphere
bacteria and fungi isolated from rice growing regions of Kenya for their physiological
characteristics, including P-solubilization, N-fixation and IAA production.

15. .
 Most of the bacterial isolates from the rhizosphere, rhizoplane as well as the phyllosphere had
ARA though at low levels. Bacterial isolates from the rhizosphere and rhizoplane were found to
be efficient in P solubilization whereas the fungal isolates were mostly non-solubilizers. Although
the percentage of IAA production by the tested isolates was not high, the bacterial isolates
performed better than the fungal isolates.
 The results therefore suggest that these microorganisms have the potential to be utilized as
microbial inoculants to replace chemical fertilizers for sustainable rice cultivation in the Kenyan
rice growing regions. Thus the future of biofertilizers based on nitrogen fixing, phosphate
solubilizing and IAA producing bacteria and to some extent fungi seems very promising.

34. Introduction
 An actinomycete (filamentous, branching, gram-positive bacteria) that forms nodules and fixes
dinitrogen in a fashion analogous to that used by rhizobia.
 The dominant actinorhizal genus is Frankia, occurring on roots of 8 plant families, encompassing
24 genera and some 230 species of dicotyledons.
 Prominent actinorhizal plant families and genera are: Betulaceae, on 47 Alnus species;
Casuarinaceae, on 16 spp. of Casuarina, and 54 spp. of Allocasuarina; Myricaceae, 28 spp. of
Myrica, and Rhamnaceae, 31 spp. of Ceanothus.
 Frankia is a genus of soil actinomycetes famous for its ability to form N2-fixing root nodule
symbioses with actinorhizal plants.
 Although Frankia strains display a high diversity in terms of ecological niches in soil, current
knowledge about Frankia is dominated by its life as an endophyte in root nodules. Increased use
of molecular methods has refined and expanded insights into endophyte-host specificities and
Frankia phylogeny.
 Frankia is a genus of soil actinomycetes in the family Frankiaceae occurring also in symbiosis
with certain angiosperms. The actinomycete Frankia is defined as the N2-fixing microsymbiont of
actinorhizal plants.

35. .
Actinorhizal symbioses as examplified from Patagonia,
Argentina. a Discaria chacaye shrubs growing along a
river in northwestern Patagonia; b Discaria trinervis
shoot with mature fruits; c Discaria trinervis multilobed
nodule; d Discaria trinervis mature nodule in
longitudinal section showing characteristic central
vascular tissue (vt), apical meristem (m) and infected
cells full of vesicle clusters stained blue ; e liquid
culture of Frankia strain DcI45 isolated from Discaria
chacaye root nodules; f Frankia strain DaI1 isolated
form Dicsaria articulata showing characteristic hyphae,
multilocular sporangia, spores and a spherical vesicle
(inset)

36. Overview
 Actinorhizal plants are classified into four subclasses, eight families, and 25 genera comprising
more than220 species.
 There are two main types of symbioses between nitrogen-fixing bacteria and vascular plants: one
between Rhizobium and leguminous plants, and the other between Frankia and actinorhizal
plants.
 Actinorhizal plants comprise more than 220 species symbiotically associated with the
filamentous actinomycete Frankia
 Gunnera, which establishes a symbiosis with cyanobacteria in a specialized stem structure,
represents a third type of nitrogen-fixing symbiosis. A fourth type is that which occurs between
cyanobacteria and cycads.
 Frankia-actinorhizal plant and Rhizobium- legume symbioses have been known for many years to
benefit soil fertility.
 A fourth type is that which occurs between cyanobacteria and cycads. Other diverse diazotrophs,
such as Azospirillum, Herbaspirillum, and Acetobacter, have been isolated and identified from the
rhizosphere or from roots of many other plants, generally grasses, but are not symbiotically
associated in root nodules.

38. The bacteria
 The microsymbiont of actinorhizal plants was first referred to as Frankia in 1888 by Brunchorst and was
later classified as an actinomycete after studies by Krebber in 1932 (Quispel 1990).
 The genusnFrankia is comprised of gram-positive and gram- variable actinomycetes (Lechevalier and
Lechevalier 1990). The first cultured Frankia, isolated from Alnus root nodules was reported by Pommer
(1959), but unfortunately the culture was lost. In 1978, the first successful isolation of Frankia was reported
from Comptonia peregrina root nodules (Callaham and others 1978), beginning a new era in actinorhizal
symbiosis research (Quispel 1990).
 Filaments, vesicles, and sporangia have the potential for being infective particles although they must
germinate and grow as new filaments to infect the root. Spores are probably a major means of Frankia
propagation in nature. It has been shown that Frankia cells are distributed through air by birds and also
accumulate in river and lake sediments. All three cell types can be found in the symbiotic state, although
there are some exceptions.
 Cultivated Frankia cells behave as heterotrophic aerobic bacteria with doubling times of 15 h, com- pared
with 3 h for rhizobia.
 Frankia comprises not only symbiotic bacteria but also free-living actinomycetes in the soil. Frankia cell
wall and cell envelope composition is distinct from that of other bacteria, in particular, because of the
presence of hopanoids in the multi- layer envelope of the vesicle (Harriot and others 1991). This lipid
envelope acts as a gas diffusion barrier to prevent high oxygen tension within vesicles, thereby permitting
nitrogenase expression and activity, both in culture and in symbiotic state.

39. .
 It is worth noting that Frankia is not the only microorganism that can be isolated from actinorhizal
root nodules. For instance, a previously unrecognized actinomycete was isolated from root
nodules of Casuarina trees growing in Mexico. This microorganism appears to fix nitrogen, on the
basis of acetylene reduction assays, but does not develop vesicles or sporangia in culture, and
moreover, it is unable to reinfect its original host.


40. THE ACTINORHIZAL PLANT
 All the actinorhizal plants are trees or shrubs, except for the genus Datisca, which is herbaceous.
Some species are very well adapted to flooded lands, warm arid and semiarid regions, and areas
of devastation (for example, rock slides).
 Actinorhizal plants have numerous uses: soil restoration, fuel wood, production of wood and
derivatives, agroforestry, coastal restoration, and the prevention of desertification.
 This tripartite symbiosis gives a high degree of autotrophy to these plant-microorganism
associations. Thus, actinorhizal plants are natural pioneers in succession on land, and they are
frequently the first species colonizing disturbed areas.
 Nitrogen fixation by actinorhizal plants in nature seems to be of similar magnitude as that of the
legumes showing diurnal and seasonal variation with an estimated annual rate of 240—350 kg
ha−1 y−1.
 Actinorhizal plants are perennial, so their contribution to N cycle through litter fall and soil
decomposition is ecologically relevant.

41. THE ROOT NODULE
 Actinorhizal nodules resemble modified lateral roots, having a central vascular bundle. Because of its
indeterminate structure, the development of the symbiotic association is recapitulated longitudinally in a
mature nodule.
 At the nodule tip is the uninfected apical meristem from which nodule parenchyma develops. Adjoining the
meristem in a basipetal direction is a region of uninfected cells followed by a region of recently infected
cells without vesicle differentiation, known as the infection zone.
 The central nodule tissue, or fixation zone, contains two types of cells: mature infected cells with
differentiated vesicles, where nitrogen fixation takes place, and uninfected cells, which are probably
involved in assimilation of the fixed N and exchange of C.
 The distribution of infected and uninfected cells in the fixation zone differ depending on the actinorhizal
plant genus. The different arrangements are attributed to differences in oxygen protection mechanisms. At
the nodule base, the senescent zone is present. Because actinorhizal nodules are perennial, they show
seasonal variations in the proportion of these zones.

42. Applications
 Actinorhizal plants could be useful tools to develop a sustainable economy. If there is interest in
extending the ability to establish a nitrogen-fixing symbiosis to a plant species with economic
potential, efforts should concentrate first on close relatives of well known actinorhizal plants.
 Beneficial role of the symbiotic state there may be greater pathogenic resistance induced after
Frankia nodulation.
 More basic studies on the complex interactions between Frankia and the actinorhizal plants will
help us achieve a better understanding not only of symbiosis but also of plant growth regulation.

43. Mycorrhizal fungi and its effect on plants.

54. THANK- YOU