Rhizosphere in relation to Plant Pathogens
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. For example 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 antagonists 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.