Common information about Agrobacterium tumefaciens, Rhizobium rhizogenes and plant diseases caused by them.

2. Diseases caused by Agrobacterium
There are two major disease caused by Agrobacterium in
plants.
1. Crown Gall
2. Hairy Root

3. Crown gall
Symptoms :
 Crown gall is identified by overgrowths appearing as galls on
roots and at the base or "crown" of woody plants .
 These growth first appears as whitish pustules and letter
starts to increase in size.
 These galls becomes hard and woody as it matures.
 Plant growth is retarded and gets slow
 Plant becomes weak and may effect on yield of crop.

4. Symptoms of crown gall

5. Causal organism :
 Crown gall disease is caused by the
 bacterium Rhizobium radiobacter
 (previously Agrobacterium tumefaciens)
Host :
 It has a wide range of host
 Members of 93 families of plants.
 It causes "galls" of woody plants such as e.g., apple,
pear, cherry, apricot, almond, walnut trees . Crown
galls are also formed on ornamental woody crops such
as roses, Marguerite daisies, and Chrysanthemum spp.
as well as on vines and canes such as grapevines and
raspberries.

6. Etiology :
 It is Gram-negative, bacilliform bacterium that is normally
associated with the roots of different plants in the field.
 This bacterium can survive in the free-living state in many
soils with good aeration where crown gall diseased plants
have grown. The bacterium can also survive on the surface
of roots of many orchard weeds.
 characteristics are Gram-negative, strictly aerobic,
bacilliform rods measuring 1 x 3 µm, and whose
nutritional requirements are non-fastidious. The rods
bear flagella that are arranged subpolarly around the
cylindrical circumference of the cell, referred as
circumthecal flagellation.

7.  when cells perceive plant phenolic compounds, the
virulence genes that are located in the resident Ti (tumour-
inducing) plasmid are expressed, resulting in the formation
of a long flexuous filament called the T pilus.
 Bacterium gets attached to the plant cell and transfer its ti
plamid into the plant cell.

8. Rhizobium radiobacter

9. Spread and dissimilation :
 Bacterium is passive invader
 Entry of pathogen through wound by insect injury, pruning,
wind damage, etc.
 Bacteria multiply in galls in large number then insect, fungi or
other organism breaks the gall and peaces of gall fall on
ground which have millions of bacterial cells in them.
 Bacteria can move from field to field through runoff water.
 R.radiobactor can survive up to two years in soil living on
dead matter of plants.

12. Management :
 Planting stocks. Visual examination for crown gall tumours has been the
conventional primary screen for diseased material. The method is limited for
complete disease control because R. radio bacteria can reside on the rhizoplane
and systemically in certain host plants such as grapevines, rose,
chrysanthemums, and marguerite daisy.
 Site selection. Fields that have grown cereal crops for a long period are
favoured as crown gall-free sites. Fields previously used for growing fruit and
nut crops can remain infested with R. radiobacter . Certain weeds such as
morning glory (Ipomoea leptphylla) can serve as natural hosts of R.
radiobacter and therefore perpetuate the survival of this pathogen in field
soils.
 Chemical eradicants. Eradication of crown gall using creosote-based
compounds, copper-based solutions, and strong oxidants such as sodium
hypochlorite are transiently effective. The chemical eradicant application
procedure is labour intensive and therefore costly both monetarily and to the
environment. The superficial treatments are ineffective against systemically
infected plants. Generally, chemicals are rarely used for control of crown gall.

13.  Crop rotation. A crop rotation program employing cereal
crops followed by green manuring helps reduce the
population size of R. radiobacter.
 Biological control. R. radiobacter are sensitive to the
antibiotic agrocin produced by , a closely related soil-borne
bacterium that does not infect plants. An example of an
antibiotic produced is Agrocin-84, which is an analog of
the opine agrocinopine A.
 Inside the R. radiobacter cell, the antibiotic Agrocin-84
inhibits DNA replication and cellular growth.

14. .genetic engineering :
 Genes encoding products that degrade or inactivate the T-
DNA strand complex when it enters the host cell
 Biotechnology companies, such as DNA Plant
Technology (Oakland, CA), are applying sense strand
messenger RNA or small-interfering RNAs to develop
crown gall resistant fruit and nut crops.
 Removal and destruction of early stage galls or complete
plant.

15. Hairy root
 Symptoms :
 In India, occurrence of hairy root caused by A.
rhizogenes (Riker et al.) was first reported by Singh in
1943 from Kumaon Hills of Uttar Pardesh.
 Hairy root disease is characterized by an overabundant
growth of adventitious roots at the site of infection
(usually on the stems of plants near and immediately
below the soil surface). Symptoms of hairy root disease
may also include wrinkled leaves.
 The roots are unable to absorb proper amount of water
and nutrients plant remains stunted and weak.

16.  In sever cases plant is unable to produce flowers and fruits.
 Hairy root disease symptoms have been divided into 2
categories, ‘simple or fibrous hairy-root’ and ‘woolly-knot
form of hairy root.’ Simple hairy root disease is identified
as stated above where as the woolly-knot form is
characterized by the initial formation of a smooth tumour
on the plant stem followed by an outgrowth of adventitious
roots from the tumour surface.

18.  Causal organism :
 Previously known as Agrobacterium rhizogenes
Now known as Rhizobium rhizogenes .
Host :
 The plant families Solanaceae, Rosaceae, Fabaceae, Crassulaceae,
Caesalpinaceae, Brassicaceae, Polygonaceae, and Asteraceae are
susceptible to hairy root caused by R. rhizogenes.
 Laboratory experiments have shown that many plants can serve as hosts
including monocots and primitive dicots . Hosts which are phenol
accumulators tend to be more sensitive to infection by R. rhizogenes .
 R. rhizogenes has a world-wide distribution but it cannot grow above 30°C
in culture.

19.  Biology :
 Agrobacterium rhizogenes had small rod, capsulate non-spore
forming, motile by one polar flagellum. However, on certain
media, Agrobacterium formed star shaped arrangements which
have been considered as stages in sexual processes of these
bacteria (Agrios, 1997). on nutrient agar and reported small
circular and smooth, glistening opaque to translucent convex
white colonies with smooth margins
 Optimum growth occurs at pH above 4 and 20-28°C. Many live
their entire lives as saprophytes (non-pathogenic) and do not
cause damage to plants.
 They can exist as biofilms on plant root surfaces. The strains
which contain Ri-plasmids with virulence genes are considered
plant pathogens.

21.  Dissemination :
 It is a soil born bacteria and can remain in soil for a long
time in absence of host on dead plant debris.
 Pathogen inters in host through injuries and wounds.
 Pathogenic strains of R. rhizogenes are capable of
inducing hairy root growth on their hosts. When a plant
is wounded it releases compounds which are sensed by
the bacterium in the soil. R. rhizogenes is attracted
towards the plant wound and it can transfer its DNA into
the host cell via transfer of a portion of the root-inducing
(Ri) plasmid. The transferred DNA (T-DNA) is
integrated into the plant cell genome.

22.  After integration the plant produces an abundance of
growth hormones and opines which are beneficial for
growth of R. rhizogenes. It is thought that the virulence
genes of R. rhizogenes are activated by the lignin forming
compounds in some plant cell walls .
 Pathogen can travel to a long distance in runoff water.

23.  Control measures :
 Field sanitation.
 Removal of plant debris from field and weed control.
 Crop rotation.
 Crop rotation with non host crops.
 Removal and destruction of infected plant.
 Destruction of infected parts by burning
 Soil solarisation.
 Usually applied before planting of crop.
 Using chemical control.
 Drenching the soil with streptomycin 500-1000 ppm can
effectively reduce disease incidence.
 Using biological control.

24. References…
 Wikipedia.
 www.nbd.com
 Google.

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