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V. The Kinds of Microorganisms Found on Plant Roots

V. The Kinds of Microorganisms Found on Plant Roots

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SOIL MICROORGANISMS AND PLANT ROOTS



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given to many observations, and some isolated and unconfirmed reports

will be ignored altogether.

The increased microbial flora within the rhizosphere is predominantly

bacterial. Starkey’s (1931a) data on sweet clover show that bacteria

are 200 times more numerous in the rhizoplane than in root-free soil, and

that fungi and actinomycetes are 10 times more numerous. Thom and

Humfeld (1932) found that the roots of alfalfa, rye, and vetch stimulated

bacteria, fungi, and actinomycetes in the order named. Adati (1939)

noted increases in these groups in the same relative order. Frequently,

t.he rhizosphere effect on fungi and actinomycetes is reported as negligible.

There is direct microscopic evidence that protozoa and microphagow

nematodes are more numerous within the root zone. By cultural means,

Katznelson (1946) has found a twenty-fold increase of protozoa on roots

of mangels grown in manured soil. Further attempt6 to determine

whether other plants exerted a favorable influence on protozoa were not

successful (Katznelson et al., 1948). The fact that protozoa quite frequently increase in numbers following increases in bacterial populations

makes i t probable that the accumulation of bacteria on roots is aceompanied by increased numbers of protozoa.

6. The Fungal Flora of the Rhizosphere



It remains unsettled whether certain species of fungi are preferentially

encouraged by plant roots. Timonin (1940a) failed to find significant

differences in the fungal flora of the rhizosphere of seedling wheat, oats,

alfalfa, and clover. Later, he (1941) reported that flax varieties susceptible to disease preferentially encouraged certain genera of fungi.

West and Hildebrand (1941) also noted qualitative differences in the

fungal flora of strawberry roots grown under different treatments for

root rot control, and Clark (1942) found that certain groups of fungi

responded t o cotton roots variously treated. Inasmuch as these observations are complicated by plant treatment and by the factor of disease,

there does not yet appear sufficient evidence to name individual species

of fungi as rhizophilic in habit. Even less is known about t,he response

of individual species of actinomycetes, although the direct microscopic

as well as the cultural evidence indicates some increased growth of this

group about plant roots.

3. T h e Bacterial Flora of the Rhizosphere

a. Physiological and Morphological Characterization. According to

Thom (1935), the microorganisms associated with root surfaces belong

to species active in the decomposition of fresh organic matter in soil, and

not to species associated with the breakdown of humus residues. The



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former type of flora is considered to consist, of rapidly growing organisms

with specialized saccharolytic or proteolytic abilities, the latter, of slower

growing forms less specialized in their feeding habits. Norman (1947)

has stated that the soil populat.ion consists almost exclusively of organisms that have a wide range of activities and which possess a great

diversity of enzyme systems.

Lochhead (1940), following single-culture studies of a number of soil

and rhizosphere isolates, concluded that isolat.es from the rhizosphere

showed greater physiological activity than those from soil in that a

greater percentage of them liquefied gelatin and fermented glucose.

Krassilnikov et al. (1936a) similarly reported that the predominant

organisms in the rhizosphere consisted of rods with pronounced ammonifying capacity ; cellulolytic types also were numerous. West and Lochhead (1940a) reported that the bacteria of the rhizosphere of flax and

tobacco possessed more complex nutribive requirements than did those of

the corresponding control soils. Differing requirements for thiamine,

biotin, and amino nitrogen were evident. These authors (1940b) subsequently proposed that the equilibrium existing between nutritional groups

in soil or about roots be expressed as the “Bacterial Balance Index,”

which was calculated by assigning a negative value to the percentage of

gram-negative bacteria growing readily in a simple inorganic medium,

and a positive value to the bacteria requiring amino acids and growth

factors. The sum of the two values represented the “Bacterial Balance

Index.” Hildebrand and West (1941) used such an index in the characterization of the rhizosphere flora established in certain root rot disease

treatmenh. Lochhead and Chase (1943) and Lochhead and Thexton

(1947) have used such an index in soil bacteriology studies, but investigators other than these Canadians have not adopted this index as a

means of characterizing the rhizosphere flora.

Gram-negative bacteria of simple morphology are either predominant

in the rhizosphere or relatively more abundant therein than in soil. Both

cultural and direct microscopic studies have shown spore-forming rods

generally less abundant in the rhizosphere than in soil. The relative

numbers of different morphological types on roots as compared to those

in soil have been shown by a study (Clark, 1940) of four hundred isolates

from cotton roots and from the surrounding soil. Gram-negative rods

accounted for 67 per cent of the root isolates, but for only 20 per cent

of the soil isolates. Gram-poskive coccoids, on the contrary, accounted

for only 12 per cent of the root isolates, and for 40 per cent of the soil

isolates. Spore-forming rods were even more sharply depressed in number by the presence of roots, as they accounted for only 1 per cent of the

root isolates, but for 30 per cent of the soil isolates.



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b. The Incidence of Individual Genera and Species. The bacterial

genera most frequently named as responding favorably to root development have been Radiobacter (now called Agrobacterium, Breed et al.,

1948) and Pseudomonas. Some very marked increases in numbers have

been reported for the Radiobacter group in the presence of plant roots;

Starkey (1931a), for example, stated that this group is 112 times more

numerous on roots of legumes than in soil. He pointed out, however, that

in dilution plate cultures, Radiobacter is indistinguishable from Rhizobium, and therefore counts, as commonly recorded, undoubtedly include

the Rhizobium population also. Although Radiobacter and Rhizobium

are markedly increased in number in the presence of leguminous plants,

knowledge concerning the response of this group to nonlegumes is meagre.

Fluorescent gram-negative rods of the genera Pseudomonas and

Xunthomonas are commonly encountered in greater abundance on roots

rather than in soil (Clark, 1940; Starc, 1943). The genus Mycoplanu is

considered to be more numerous in the rhizosphere than in surrounding

soil (Lochhead, 1940). Clostridium is also reported to be encouraged

by root development (Katznelson, 1946; Sen, 1929; Velich, 1903).

The incidence of Azotobacter, and of Nitrosomonas and Nitrobacter,

upon the surfaces of plant roots has been discussed in preceding sections

(III-1-a and III-1-b), and it will suffice here to state summarily that

these genera fail to show a positive response to the presence of plant

roots. Proactinomyces, and soil corynebacteria, which Conn and Dimick

(1947) recently proposed be placed in the genus Arthrobacter, are depressed in number by root development,. The genus Bacillus is also less

frequently encountered in the rhizosphere than in soil apart. from roots

(Clark, 1940; Krassilnikov, 1934; Krassilnikov et al., 1936a; Lochhead,

1940; Lochhead et al., 1940).

Very few studies have been made of the occurrence of individual

species of bacteria within the rhizosphere. Although the literature is

quite fragmentary, it does suggest that certain species, as well as certain

genera, are more frequently encountered on root surfaces than in soil

generally. Within the genus Arthrobacter, Lochhead (1948) has noted

a greater incidence of chromogenic types in the rhizosphere than in

adjacent soil. Within the genus Bacillus, Clark (1940) has noted that

B. brevis, B . circulans, and B. polymyxa constitute more important fractions of the bacillus population in the rhizoplane than they do in soil.

Within the fluorescent Pseudomonas group, single culture studies have

indicated that the types encountered in soil are different from those

associated with root surfaces. The majority of the soil isolates did not

utilize sucrose or st.arch, whereas the majority of the root isolates fermented these materials.



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Taxonomic studies, although limited, are sufficient to indicate qualitative differences in the bacterial floras of the rhizosphere and of the soil,

even though for the most part genera and species of bacteria cannot

yet be catalogued as to whether they are encouraged by, indifferent to,

or inhibited by root development.

VI. MODIFICATION

OF THE ROOTSURFACE

MICROFLORA

1. B y Seed or Soil Inoculations

Soil bacteriologists long have attempted to control the root surface

microflora by seed or soil inoculations. The most notable success has

been in the inoculation of legumes with rhizobia, a practice which dates

from the work of Hellriegel and Wilfarth (1888). Subsequent investigators of the symbiotic nitrogen-fixing organisms have been more concerned

with the selection and maintenance of effective strains and with consideration of factors which affect the persistence of strains of rhizobia

in soil after being introduced therein than with the feasibility of inoculation. Problems of soil reaction, of bacteriophage, of the effect of various

intervening crops, and of the applications of fertilizers and lime have

been studied.

Inoculation of nonlegumes, as recommended initially by Caron (1895)

and Stoklasa (1908) generally has not been found beneficial, although

in this connection possibly exception should be made for the Russian

claims on Azotogen and allied preparations. This controversy has been

discussed in Section III-1-b above. Apart from seed and soil treatments

with Azotobacter, certain inoculation attempts have been made in connection with root rot studies that are pertinent here. Morrow et al.

(1938) were of the opinion that both molds and bacteria could be successfully established on cotton roots by use of proper inocula on seeds or

seedlings. Recovery of an inoculated microorganism from the rhizosphere, wit.hout demonstration that it is of more than chance occurrence

therein or simply capable of a passive survival for a period of time, may

be misleading. The writer remains in agreement with Greaves (1918)

that a few organisms placed in a new environment seldom gain ascendency over the organisms naturally present and struggling for countless

generations to adapt themselves to that environment. Exception of

course must be made for phytopathogens and for the nodule bacteria,

which are capable of invading the root tissue or histosphere, and in consequence, enjoy a special advantage over the noninvading types. Even

the plant pathogens, however, are not always easily established upon

roots when subject to competition from the soil microflora, as is shown

by several comparatively recent experiments on the take-all disease of



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275



wheat, wherein, in order to establish experimental infection, the fungus

(Ophiobolus graminis Sacc.) was introduced into the soil along with the

organic substrate on which it had been grown. With such procedure,

there frequently was failure to establish infection. Stimulation of growth

of the antagonistic flora in the soil by the added organic material has

been considered responsible for the rapid disappearance from soil of t,he

fungal parasite introduced (Garrett, 1944).

2. B y Soil Treatment



Inasmuch as organic manures are known to affect the incidence of

certain root-infecting fungi, numerous studies have been made of the

effect of such treatments on the rhizosphere microflora. I n experiments

in which manured, untreated, and steam-sterilized soil samples were

established in containers cropped to wheat, Clark (1939) found that the

root flora was largely independent of the soil flora. Clark and Thom

(1939) believed that the effects of organic manuring were primarily

evident on the soil microflora, and that the root microflora of the plants

themselves was relatively little affected. In view of an observed independence of the root microflora from manurial treatments capable of

affecting differences in disease incidence, Stumbo et al. (1942) expressed

the opinion that factors of host nutrition were of greater importance than

microbial antagonisms in the rhizosphere in the control of take-all

disease of wheat. Timonin (1940b), Katznelson and Richardson (1943)

and Mitchell et al. (1941) also showed that organic manures did not

greatly affect the rhizosphere microflora. In some instances, however,

there do exist indirect influences of manurial treatments. Katznelson

(1946) has pointed out that manuring may affect the growth rate, vigor,

and maturity of the plant, and that stage of plant development is known

to influence t.he flora of the rhizosphere. Hildebrand and West (1941)

found that manurial treatment affecting the incidence of Ontario rot of

strawberries also affected the relative incidence of nutritional groups

within the rhizosphere.

Differences in host plant nutrition following organic fertilization may

induce changes in the rhizosphere microflora, which in turn may affect

disease incidence. Differences in plant nutrition may also affect incidence

of disease. Following primary invasion by root parasites, there typically

is secondary invasion by members of the soil population, and in such

circumstance there is abrupt disturbance of the rhizosphere flora. Frequently it is difficult properly to separate cause from effect. There appear instances in the root rot literature where this has been attempted

without sufficient information on the many factors involved in incidence

of root diseases.



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