Download Rhizosphere AND PHYLLOSPHERE

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RHIZOSPHERE AND PHYLLOSPHERE
RHIZOSPHERE
The region in the vicinity of roots can be distinguished microhabitats as rhizosphere The term ‘rhizosphere’
was introduced in 1904 by the German scientist Hiltner to denote that region of that region which is
subject to the influence of plant roots. Rhizosphere is characterized by greater microbiological activity than
the soil away from the roots. The intensity of such activity depends on the distance to which exudations
from the root system can migrate. The term ‘rhizosphere- indicates the overall influence of plant roots on
soil microorganism. It is clearly established that greater number of bacteria, fungi, Actinomycetes are
present in the rhizosphere soil than in non-rhizosphere soil. Several factors such as soil type, its moisture,
pH and texture and the age and condition of plants are known to influence the rhizosphere effect. Apart
from the numerical preponderance of microorganisms in the rhizosphere, the rhizosphere effect is also
manifest in the occurrence and distribution of bacteria characterized by specific requirements like amino
acids, B-vitamins and specialized growth factors (nutritional groups). It has also been demonstrated that
the rates of metabolic activity of the rhizosphere microorganisms are different from those of the non
rhizosphere soil. A wide range of enzymes of plant and microorganisms present in the rhizosphere
catalyze the breakdown of or materials. These enzymes include oxidoreductases, hydrolases, transferases
besides cellulose, dehydrogenases and urease.
Rhizosphere soil can be separated and a soil suspension obtained by shaking roots in aliquots of sterile
water from which subsequent are made.
The rhizosphere to soil ratio (R: S) can be calculated by
dividing the number of microorganisms in the rhizosphere
soil by the number in the soil free from plant growth. Results
can also be expressed based on the weight of roots.
Greater rhizosphere effect is seen with bacteria (R : S values
ranging from 10 to 20 or sometimes more) than with
actinomycetes or fungi. Only negligible changes are noted
with regard to protozoa and algae. Several genera of
bacteria-Pseudomonas,
Arthrobacter,
Agrobacterium,
Azotobacter, Mycobacterium, Flavobacterium, Cellulomonas,
Micrococcus and others have been reported to be either
abundant or sparse in the rhizosphere. From the agronomic
point of view, the abundance of nitrogen fixing and
phosphate solubilizing bacteria in the rhizosphere of crop
plants assumes a natural significance. The preponderance of
amino acid requiring bacteria in the rhizosphere of crop
plants has been consistently observed by many workers. The
preferential stimulation of vitamin requiring bacteria
in the rhizosphere has also been observed.
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RHIZOSPHERE AND PHYLLOSPHERE
Factors Influencing Rhizosphere:
I.Microbial Characteristics:
1. Nutrient requirements, 2.Cellulase Production, 3. Early growth rate, 4.Antibiotic tolerance and
production, 5.Sideophore production, 6.Unique physiological attributes 7.Tolerance to fungicides and
other chemicals.
II. Plant Characteristics:
1. Plant species, 2.Plant age, 3.Altered plant genetics, and 4. Foliar treatments.
III. Environment
1. Soil type, 2. Soil texture, 3. Soil Moisture, 4. Soil temperature, 5.soil atmosphere, 6. Soil fertility, 7. Soil
applied pesticides.
Rhizosphere Effect
Greater rhizosphere effect is seen with bacteria (R: S values ranging from 10 to 20 or sometimes more)
than with actinomycetes or fungi. Only negligible changes are noted with regard to protozoa and algae.
Qualitative studies, however, reveal some distinct selective influence of the root system. An example of
this is the preferential stimulation of Gram-negative non-sporulating rod-shaped bacteria in the root
region of many plants. Several genera of bacteria—Pseudomonas, Arthrobacter, Agrobacterium,
Azotobacter, Mycobacterium, Flavobacterium, Cellulomonas, Micrococcus and others have been reported
to be either abundant or sparse in the rhizosphere. From the agronomic point of view, the abundance of
nitrogen fixing and phosphate solubilizing bacteria in the rhizosphere of crop plants assumes a natural
significance. The preponderance of amino acid requiring bacteria in the rhizosphere of crop plants has
been consistently observed. The preferential stimulation of vitamin requiring bacteria in the rhizosphere
has also been observed.
Alteration of Rhizosphere Microflora:
There are reports of changes in the rhizosphere microflora by (1) soil amendments; (2) foliar application of
nutrients; and (3) artificial inoculation of seed or soil with preparations containing live microorganisms,
especially bacteria (bacterization).
Many experiments have been done to find out the effects of N, P and K additions on rhizosphere micro
flora. Increase as well as decrease in R: S ratios have been reported as a sequel to fertilizer applications.
Translocation of photosynthates from leaves to roots is a well-known phenomenon as part of the normal
metabolic activity of plants.. Foliar sprays of urea are known to alter the number and nature of
microorganisms in the rhizosphere. Extensive studies have been done on induced changes in the
rhizosphere microflora by foliar sprays of antibiotics, growth regulators, pesticides and inorganic nutrients
in the hope that such an approach may serve as a new tool in biological control of root diseases. Microbial
seed inoculants such as Azotobacter, Beijerinckia, Rhizobium or P-solubilizing microorganisms may help in
the establishment of beneficial microorganisms in the rhizosphere or in the immediate vicinity of growing
roots.
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RHIZOSPHERE AND PHYLLOSPHERE
Nitrogen Fixation in the Rhizosphere:
It is now possible to measure nitrogenase activity in the rhizosphere of many non-legumes. In terms of
moles C2H4 per g. of dry root per hour, Brachiaria mutica showed nitrogenase activity of 150-750 in Brazil
and
maize
seedlings
showed
activity
in
France
in
the
range
of
100-3000.
Reports indicate that rhizosphere of grasses and weeds harbour nitrogen fixing bacteria of the families
Azotobacteriaceae,
Spirillaceae,
Enterobacteriaceae,
Bacillaceae,
Pseudomonadaceae
and
Achromobacteriaceae.
Colonization by Azotobacter has been shown to be limited in the rhizosphere and not on the root surface
(rhizoplane), due to acidity caused by root exudates. Colonization of root surface by Azospirillum has been
noticed with rather extensive intrusion within root tissues. Sporangia of Frankia have been observed in the
rhizosphere of Casuarina seedlings, indicating the variety of N 2 fixing microorganisms that are capable of
inhabiting the rhizosphere. Azotobacter chroococcum and A. paspali are known to elaborate gibberellins
and cytokin-like substances and these species of nitrogen fixing bacteria happen to be typical colonizers of
the
rhizosphere
of
grasses.
Epiphytic bacteria and algae colonize the surface of aquatic weeds..The bulk of symbiotic nitrogen fixation
in rice rhizosphere of flooded soil comes from root and stem nodules of leguminous green manure species
when they are ploughed into the soil.It is also now known that nitrogen fixing bacteria occur in the rhizosphere, stalks and phyllosphere of sugarcane plants and such bacteria have also been reported inside the
root
cells
Associative and Antagonistic Activities in the Rhizosphere:
The dependence of one microorganism upon another for extra cellular products, chiefly amino acids and
growth promoting factors, can be regarded as an associative effect. Many reports indicate that cellular
extracts of certain bacteria, fungi and algae increase the growth of other microorganisms in pure culture.
There is an increase in the exudation of organic acids, amino acids and monosaccharide by plant roots in
the presence of microorganisms. Microorganisms also influence root hair development, mucilage secretion
and lateral root development of several plants. The fungi inhabiting the surface of roots influence the
amount of substances absorbed into the root system. These attributes point out the existence of a twoway movement of metabolites between plants and microorganisms.
Secretion of antibiotics by microorganisms and the resultant biological inhibition of growth of other
susceptible microorganisms are demonstrable in soil as well as in pure cultures. Excessive inhibition of
Azotobacter or Rhizobium in the root region may lead to decreased nitrogen fixation or nodulation. On the
other hand, co inoculation of nitrogen fixing Azotobacter and Azospirillum isolates with Rhizobium appears
to have beneficial influence in increasing nodule number, nitrogen fixation and yield of soybean, pea and
clover.
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RHIZOSPHERE AND PHYLLOSPHERE
Root Exudates
Most of the studies on root exudates have been done in plants grown under aseptic conditions. One of the
most important factors responsible for rhizosphere effect is the great variety of organic substances
available at the root region by way of exudates from roots which directly or in directly influence the quality
and quantity of
Class of Organic Exudates components
microorganisms in
Compound
the root region. The
Sugars
Glucose, Fructose, Sucrose, Maltose, Oligosaccharides, Etc
substances exuded
by
plant
roots
Amino Compounds Asparagines, Alanine, Glutamine,Aspartic acid, Etc.
include
amino
Organic acids
Tartaric acids, Citric acids, Malic acids, Propionic acids,Etc
acids,
sugars,
organic
acids,
Fatty acids & Sterols Palmitic, Stearic, Oleic, Linoleic, Cholesterol.Etc.
vitamins,
Growth factors
Biotin, Thiamine, niacin, Pantothenate, Insitol, Nicotinic nucleotides
and
acid, Etc
many
other
Nucleotides,
Flavanone, Adenine, Guanine, Uridine, Cytidine, Invertase, unidentified
substances.
The
Favonones&
Amylase, Proteinase Etc
nature and amount
Enzymes
of
substances
Other Compounds
Auxins, Scopolein, mycelia growth inhibitors, Etc.
exuded
are
dependent on the species of the plant, age and environmental conditions under which they grow.
Novel Approaches (Biotechnology of the Rhizosphere)
An approach that merits consideration is the use of rhizosphere microorganisms in the control of weeds.
For example, the graminaceous downy brome is a weed in wheat cultivation that grows as fast as the main
crop. Several isolates of bacteria from the rhizosphere of the weed were found to inhibit the growth of the
weed and not of the wheat crop. In a field trial, the efficacy of a strain of Pseudomonas spp. from the
rhizosphere in controlling the growth of downy brome weed has been demonstrated.
Manipulation of the rhizosphere to enhance specific microorganisms to achieve in situ bioremediation of
pesticides has been attempted in some cases. In another study, the extent of degradation of several
polycyclic aromatic hydrocarbons was enhanced in soil planted with deep-rooted prairie grasses over that
of implanted soil. Similarly, the biodegradation of trichloroethylene has been shown to be significantly
higher in the rhizosphere soil in comparison with the non rhizosphere soil.
Beneficial Activities of Microorganisms in
Rhizosphere
4. Increasing plant nutrient availability P. Mn, Fe,
Zn, &Cu
1. Decomposition of Plant residue and organic
matter.
5. Symbiotic mycorrhizal association
2. Humus development
7. Oxidation –reduction reaction
3. Mineralization of organic nitrogen, Sulphur and
phosphorus.
8. Phosphorus solubilization
6. Production of organic chelating agents
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RHIZOSPHERE AND PHYLLOSPHERE
9. Helping biological Nitrogen fixation
Detrimental / Harmful Effects of Microflora on
Plants
10. Production of plant growth hormones
11. Protection against root pathogens and
Pseudo pathogens
12. Biodegrading synthetic pesticides or
contaminates
1.Aerobic bacteria remove oxygen or increase
carbon dioxide tension ,may reduce root
elongation and development.
2.Diminish the rate of nutrient and water uptake.
13. Enhancing drought tolerance
3.Root hair formation may be reduced.
14. Improving soil aggregation.
4. Mutual antagonism are more pronounced.
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THE PHYLLOSPHERE
Plant parts, especially leaves are exposed to dust and air currents resulting in the establishment of a typical
flora on their surface aided by the cuticle, waxes and appendages which help in the anchorage of
microorganisms.
These microorganisms may die, survive or proliferate on leaves depending on the extent of influence of the
materials in leaf diffusates or exudates. Leaf diffusates or leachates have been analysed for their chemical constituents. The principal nutritive factors are amino acids, glucose, fructose and sucrose.
If the catchment areas on leaves or leaf sheaths are significantly substantial, such specialized habitats may
provide niches for nitrogen fixation and secretion of substances capable of promoting the growth of plants.
The leaf surface has been termed 'phylloplane' and the zone on leaves inhabited by Microorganisms
as
'phyllosphere'.
The Dutch microbiologist Ruinen coined the word 'Phyllosphere' from her observations on Indonesian forest
vegetation where thick microbial epiphytic associations exist on leaves. The dominant and useful
microorganisms on the leaf surfaces in the forest vegetation in Indonesia happened to be nitrogen-fixing
bacteria such as Beijerinckia and Azotobacter.
In general, apart from nitrogen-fixing bacteria like Azotobacter, other genera such as Pseudomonas, Pseudo
bacterium, Phytomonas, Erwinia, Sarcina and other unidentified ones have been encountered on plant
surface, especially on leaf surface.
In Puerto Rico, nitrogen-fixing blue-green algae such as Anabaena, Calothrix, Nostoc, Scytonema and
Tolypothrix have been encountered on plant surfaces in moss forests. Some of the fungi and actinomycetes
recorded on the plant surface are: Cladosporium, Alternaria, Cercospora, Helminthosporium, Erysiphe,
Sphaerotheca, Podospora, Uncinula, Sporobolomyces, Bullera, Cryptococcus, Rhodotorula, Torula, Torulopsis,
Oidium, Puccinia, Melanospora, Saccharomyces, Candida, Tilletia, Tilletiopsis, Penicillium, Cephalosporium,
Fusarium, Periconia, Darluca, Rhynchosporium, Spermospora, Aureobasidium, Colletotrichum,.
Metarrhizium, Myrothecium, Verticillium, Pithomyces (potential skin infecting genus), Mucor,
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RHIZOSPHERE AND PHYLLOSPHERE
Cunninghamella, Fusarium, Aspergillus, Curvularia, Rhizopus, Trichoderma, Heterosporium, Stachybotrys,
Syncephalastrum, Actinomyces and Streptomyces.
Biochemical Reactions in the Phyllosphere
Alternatively, the phytoalexin may be normally present in plants and the concentration of such a substance
may rise markedly in response to microbial infection. The term phytoalexin is derived from Greek phyto
meaning plant and alexin meaning warding-off compound. Leaf surface microorganisms may perform an
effective function in controlling the spread of air-borne pathogens inciting plant diseases. The presence of
spores of a pathogen on the surface of leaves and pods results in the formation of substance referred to as
'Phytoalexin'.
The fungal spores produce a chemical substance or substances which are active in inducing the production of
phytoalexins by the host as a defence reaction. The fungal metabolite, even in the absence of spores, may also
independently induce phytoalexin formation.
Showing distribution of bacterial colonies on ventral (a) and dorsal (B) surface of mature (M), immature (I),
and young (Y) leaves of Lantana Camera
However, the concept of phytoalexin has been brought under a wide
coverage so as to include all chemical compounds contributing to disease
resistance in response to injury, physiological stimuli, the presence of
infectious agents and their products. In keeping with this view, some of the
known phytoalexins are summarized in. Resistance to disease causing
microorganisms has also been attributed to fungistatic compounds secreted
by leaves such as malic acid from leaves of Cicer arietinum, phenols from
apples and waxy materials (a-hexanol) from leaves of Ginko.
The name 'elicitor' has been commonly used to denote the compounds which
induce
the
synthesis
of
phytoalexins.
There are biotic elicitors which include complex polysaccharides from fungal
and plant cell walls, lipids, microbial enzymes and polypeptides. Others are
abiotic which range from heavy metal salts to detergents, autoclaved
ribonuclease, cold and U-V light.
Epiphytic microorganisms are known to synthesize indole acetic acid. A more
interesting and useful biological function is the fixation of nitrogen by microorganisms inhabiting the leaf
surface. Bacteria of the' genera Escherichia, Brevibacterium, Bacillus, Diplococcus, Pseudomonas,
Flexibacterium, Rhizobium, Beijerinckia, Azotobacter, Xanthomonas and Micrococcus have been isolated from
the phyllosphere of maize, cowpea, sugarcane and gram and some of them have proved to be potential
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RHIZOSPHERE AND PHYLLOSPHERE
nitrogen fixers.
Notwithstanding all the observations and implications of phyllosphere microorganisms in plant disease
manifestation, the exact role of these microorganisms in the nutrition of plants has remained largely
conjectural in spite of the demonstration that many epiphytic bacteria grow on nitrogen-free media and fix
atmospheric nitrogen to varying degrees.
Ex periments have also been done under laboratory conditions to demonstrate nitrogen fixation in the
phyllosphere of several plants by the use of 15N and the quantitative data obtained are so divergent that one
is led to believe that fixation of nitrogen is a very variable phenomenon on the plant surface.
Perhaps, the use of 15N in such studies with known bacteria on the leaf surface of aseptically grown plants
may be a rewarding exercise. Experiments have also to be designed to study the fate of biologically fixed
nitrogen in the phyllosphere. In recent years, spraying of leaves of crop plants with aqueous solutions of
sucrose or with bacterial suspensions has resulted in enhanced growth and yield of certain legumes and
cereals in pot trials.
Apparently, such sprays may have intensified the biochemical events on the phyllosphere towards the
beneficial side. These observations have to be necessarily evaluated under field conditions so as to exploit the
phyllosphere phenomenon towards improvement of agricultural out put (also See pages 358-360 on Frost
Control Biotechnology).
Some of the known phytoalexins, their incitants and specific hosts
Plant
Incitants
Compounds produced
(Phytoalexins)
Potato (Solanum
tuberosum)
Phytophthora infestans
Chlorogenic acid, Caffeic acid,
Scopolin, a-Solanine, a-Chaconine,
Solanidine, Rishitin and Phytuberin
Pea (Pisum sativum)
Aschochyta pisi, Penicillium expansum
Pisatin
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RHIZOSPHERE AND PHYLLOSPHERE
Nitrogen fixation by bacteria isolated from phyllosphere (Bhurat, 1969)
Crop
Bacteria
Wheat
Achromobhacter iophages
Pseudomonas atrofaciens
Cellulomonas galba Pseudomonas seminum
Cellumonas cellases
Pea
Achromobhacter iophages
Pseudomonas Calcis Achromobhacter xerosis
Cellulomonas uda
Bacillus licheniformis
* * * ** * * * * *
Pass word: rhizosphere