Plant root exudates influence
pathogenic fungi, bacteria and nematodes in various ways. The effect may be in
the form of attraction of fungal zoospores, or bacterial cells towards the
roots; stimulation of germination of dormant spores and hatching of cysts of
nematodes. Root exudates may contain inhibitory substances preventing the
establishment of pathogens. The balance between the rhizosphere microflora and
plant pathogens and soil microflora and plant pathogens is important in
host-pathogenic relationship. In this context, the biochemical qualities of
root exudates and the presence of antagonistic micro-organisms plays an
important role in the proliferation and survival of root infecting pathogens in
soil either through soil fungi stasis, inhibition or antibiosis of pathogens in
the rhizosphere.
Some of the most common interactions
between plant roots and plant pathogenic microorganisms in the rhizosphere are
discussed herewith.
A.
Zoospore attraction: Amino acids, organic acids and
sugars in the root exudates stimulate the movement and attraction of zoospores
towards root of the plants.
Eg. Attraction of zoospores has been reported in Phytophthora citrophthora (Citrus
roots), P. parasitica
(tobacco roots) and Pythium
aphanidermatum (pea root).
B.
Spore germination: The
spores or conidia of many pathogenic fungi such as Rhizoctonia, Fusarium, Sclerotium, Pythium,
Phytophthora etc. have been stimulated to germinate by the root
exudates of susceptible cultivars of the host plants. There are some reports on
the selective stimulation of Fusarium,
Pseudomonas and root infecting nematodes in the rhizosphere region
of the respective susceptible hosts. This stimulus to germination is especially
important to those plant pathogens which are not vigorous competitors and
remain in resting stage due to shortage of nutrients or fungistasis.
As a
rule, germination and subsequent hyphal development are promoted by non host
species and also by both susceptible and resistant cultivars of the host
plants. The quantity and quality of microorganisms present in the rhizosphere
of disease resistant crop varieties are significantly different from those of
susceptible varieties.
C. Changes in morphology and
physiology of host plant: Changes
in the physiology and morphology of host plant influence the rhizosphere
microflora through root exudations. Hence, significant changes in the
rhizosphere microflora of diseased plants were reported which are attributed to
the nature and severity of the disease. Systemic virus diseases cause marked
changes in the plant morphology and physiology to drastically alter the
rhizosphere microflora.
D. Increase in antagonist’s
activity: Root
exudates provide a food base for the growth of antagonistic organisms which
plays an important role in controlling / suppressing some of the soil borne
plant pathogens. Generally, rhizosphere of the resistant plant varieties
harboure moer number of Streptomyces
and Trichoderma
than that of susceptible varieties. For example in the rhizosphere
of pigeon pea varieties resistant to Fusarium
udum, the population of Streptomyces
was found more which inhibited the growth of the pathogen. High
density of Trichoderma
viride in the rhizosphere of Tomato varieties resistant to Verticillium wilt
has been reported with its ability to reduce the severity of wilt in
susceptible plants.
E.
Inhibition of pathogen: Root
exudates containing toxic substances such as glycosides and hydrocyanic acid
may inhibit the growth of pathogens in the rhizosphere. It has been reported
that root exudates from resistant varieties of Flax (eg. Bison) excrete a
glucoside which on hydrolysis produces hydrocyanic acid that inhibits Fusarium
oxysporum, the flax root pathogen. Exudates of resistant pea reduce the
germination of spores of Fusarium oxysporum.
In this
light, the rhizosphere may be considered as a microbiological buffer zone in
which the microflora serves to protect the plants against the attack of the
pathogens.
F.
Attraction of bacteria and nematodes: Root exudates attracts phytopathogenic bacteria
and fungi in the rhizosphere for example Agrobacterium
tumefaciens have been reported to be attracted to the roots of the
host plants like peas, maize, onion, tobacco, tomato and cucumber.
Host
root exudates also influence phytopathogenic nematodes in two ways: (i) though
stimulation of egg-hatching process and (ii) attraction of larvae towards plant
roots.
Article
compiled by Mr. Amol Vijay Shitole (Ph.D. Scholar)
Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola (M.S.)
No comments:
Post a Comment