This document discusses abnormal plant structures that form due to various biotic and abiotic factors. It provides examples of abnormal structures like viral-induced motley leaves, crown gall tumors caused by Agrobacterium tumefaciens, and insect galls. The document then examines in more detail hormone-induced callus formation, crown gall disease caused by A. tumefaciens, hairy root disease caused by A. rhizogenes, and insect galls. It describes the mechanisms through which these abnormal structures are formed, including the role of bacterial plasmids and plant hormones. The document concludes by noting that not all abnormalities are undesirable and that their usefulness depends on the causative agent.

1. FORMATION OF ABNORMAL
STRUCTURES IN PLANTS
1
By:
Sriraksha A Bapat

2. CONTENTS
 Introduction 3
 Examples of abnormal plant structures 4
 Abnormal structures 11
 Conclusions 27
 References 28
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3. INTRODUCTION
 Deviating from what is normal or usual, typically in a way
that is undesirable is abnormal.
 It manifests as far-reaching permanent changes induced by
physical means such as wounding and irradiation, by
chemical means with growth hormones, and by external
agents such as viruses, bacteria, fungi, and insects.
 The cells are made to undergo a major change in their
direction of differentiation, resulting in dramatic
modifications in the growth patterns of plant organs.
 Teratology is the study of abnormalities of physiological
development.
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4. 4
EXAMPLES OF PLANT ABNORMAL
STRUCTURES
 The stunted growth
accompanied by the
formation of motley
leaves induced by
viruses on tobacco,
potato, cauliflower etc.
Motley leaves in Tobacco plant

5.  Crown gall tumors on
many dicots caused by
Agrobacterium
tumefaciens.
 Proliferation of adventitious
roots (hairy roots) caused by
Agrobacterium rhizogenes.
5

6.  Nodules on the roots of
leguminous plants caused
by Rhizobium .
 Callus production
induced by auxins on the
cut end of stems.
6

7.  Hyperelongation of rice
seedlings caused by the
fungus Gibberella
fujikuroi.
G. fujikuroi
elongated rice seedlings
7

8.  Fasciation disease of
seedlings caused by
Corynebacterium
fascians.
C. fascians
Fasciation
8

9.  Galls are formed on
different parts of the
dicot plants by
various insects.
 Eg. A & B - Leaf galls
A
B
9

10.  Formation of the wound callus-a mass of cells that
grows around the wound in response to physical
damage.
wound callus
10

11. ABNORMAL STRUCTURES
 There are different types of abnormal growth, where
many rewarding cellular and molecular studies have
been made.
 With my own interest, I will be sharing a few details
on the following topics:
1. The hormone-induced callus.
2. The crown gall disease.
3. The hairy root disease.
4. Insect galls.
11

12. The hormone-induced callus
 Tissue culture studies launched in the late 1930s showed that
isolated, differentiated tissues and organized parts of plants,
when cultured in a mineral salt medium supplemented with
vitamins, sucrose, and an auxin such as IAA or 2,4-D, proliferate
to produce an undifferentiated mass of cells or callus.
 Repeated subculture to a fresh medium, the callus could be
propagated as a proliferating tissue almost indefinitely.
 An elegant series of experiments by Skoog and Miller (1957)
showed that by manipulating the proportions of hormonal
ingredients of the medium (mainly auxins and cytokinins), callus
originating from the pith tissue of tobacco can be channeled to
develop organized structures.
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13. 13
Auxin Cytokinin Callus growth and
differentiation
= = Normal rate
Root primordia
Shoot buds
In a medium containing both auxin and cytokinin, the amount of
growth in terms of callus produced, is considerably higher than that
obtained with auxin or cytokinin alone.
Other than auxin and cytokinin, there are other hormones and
enzymes that act as limiting factors for root and shoot initiation in a
callus. Eg. Gibberellins , protein kinase etc.

14. 14

15. Crown gall disease
 Can be identified by
tumors or galls of varying
shape and size on the
lower and main roots of
the infected
plants(oncogenesis).
 Caused by Agrobacterium
tumifaciens – soil borne,
gram negative bacteria
with broad host range.
 Plants with several galls
may be unable to move
water and nutrients up the
trunk and become
weakened, stunted and
unproductive.
A. tumifaciens
tumors
15

16. Mechanism of oncogenesis
 The bacterium has a pathogenic, mega, double stranded,
circular plasmid called Ti(tumor inducing) plasmid[210kb].
 The plasmid has a T-DNA fragment within, which is responsible
for the induction of tumors in the host.
 The soil bacterium enters the host in response to the phenolic
signals from wounded parts.
 Various other regions of the plasmid work hand-in-hand and
bring about the transfer of T-DNA into the host which causes
changes in the levels of phytohormones in the host bringing
about hypertrophy.
 The transformed cells also produce enzymes for opine
catabolism that accounts for the nutrition of the bacteria.
16

17. Mechanism of oncogenesis17

18. Hairy root disease
18
 Characterised by the
formation of proliferative
multibranched
adventitious roots at the
site of infection called
hairy roots.
 Caused by Agrobacterium
rhizogenes- a gram
negative, soil dwelling
bacteria.
 Also known as Crazy root
disease , it causes over
vegetative growth in
plants.

19. Mechanism of oncogenesis
 The bacterium has a double stranded, circular , mega, virulence
plasmid known as Ri (root inducing) plasmid [200kb].
 Ri plasmid shares extensive functional homology with the Ti
plasmid.
 The bacterium enters the host cell by recognising phenolic signals
released by the wounds.
 The virulent T-DNA gets integrated into the host genome with the
aid of associated genes within the plasmid.
 The transformed cells bring in changes in the phytohormones and
cause extensive growth of hairy roots.
 These transformed cells also produce enzymes for opine
catabolism that accounts for the nutrition of the bacteria.
19

20. 20

21. Insect galls
21
 Various arthropods are involved in formation of galls on
different parts of a plant.
 The two major orders being: Hymenoptera (insects with
jointed wings comprising the sawflies, wasps, bees, and
ants) and Diptera (true flies or two-winged flies that
includes mosquitoes, black flies, midges, fruit flies, and
house flies). Nonetheless great diversity can also be
found in galls formed by thrips, aphids, and insects from
other orders.
 Galls can have an unsightly appearance. However, most
do not seriously affect the health of a plant or
tree. Heavy infestations may distort leaves or cause an
early leaf drop

22. 22
Sawfly galls
young wasp larvae
emerging out of a gall
midge larvae
emanating from a gall
aphids inside a gall

23. Cont’d
23
 Gall-inducing insects, also called gall inducers, gall
makers, or simply gallers, live within the plant tissue.
 The host plant supplies food, protection against
unfavorable environmental factors and shelter
against natural enemies to the gallers and their
family.
 The host plant also produces certain defensive
chemicals such as tannins and waxes in response to
the unexpected intruder.

24. General structure of a gall
24
 The innermost region of a
gall is occupied by the
developing larvae.
 The nutritive zone
surrounds it ,which stays
connected with the
vasculature of the host plant.
 In some galls, to protect the
developing larvae, a layer of
sclerenchymatous cells
surround the nutritive zone.
 The outermost region is of
meristematic cells that keep
dividing, forming a gall.

25. Mechanism of gall formation
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 Due to extensive variety and complexity in inesct galls, it is quite
controversial to come up with a general mechanism for the
formation of the same till date. But, a few thought provoking
hypotheses have been made. Some of them are as follows:
1. Chemical hypothesis: The gallers produce certain chemicals
that trigger the gall formation in the host. Certain amino acids
present in the salivary secretions of gall-inducing insects,
essentially lysine, histidine, and tryptophan, could function as
“preconditioners” for gall induction. Enzymes like pectinase
and polyphenol oxidase(PPO) might sensitize the host tissue for
the gall formation. The plant hormones also work synergetically
to promote the induction and maturation of these structures.

26. Cont’d
26
2. Presence of endosymbiotic microorganisms: These microbes
are symbiotically associated with the gallers and contribute to
the viability of their hosts. It has been proposed that galling
insects acquired genes from symbiotic microorganisms
through horizontal gene transfer. Bartlett and Connor
(2014) hypothesized that the inducing insects obtained their
ability to induce galls via endosymbiotic microbes, which have
acquired the biosynthetic pathways to produce and manipulate
phytohormones.
3. Molecular basis of gall formation: The possibility of a molecular
induction mechanism in insect-induced plant galls involves the
transference of genetic elements, mutualistic viroid, or virus
into the plant genome, which would regulate and control the
process of gall formation.

27. CONCLUSION
 Not every abnormality is a misery.
 The usefulness or uselessness of the abnormal structure
depends on the causative agent.
27

28. REFERENCES
 Raghavan, V.2000. Developmental biology of flowering plants,
Springer-Verlag New York, pp 323-334.
 Shull, H.G.1901. Some plant abnormalities. Vol. 32, Botanical
Gazette, Pp 343-355.
 Wordsell, C.W. The Principles of Plant Teratology. Vol. 2, The
Ray Society, London, pp 2-17.
 Gätjens-Boniche,O. 2019. The mechanism of plant gall induction
by insects: revealing clues, facts, and consequences in a cross-
kingdom complex interaction, Laboratory of Molecular Biology,
School of Natural and Exact Sciences, Technological Institute of
Costa Rica, Santa Clara, San Carlos, Alajuela, Costa Rica.
ogatjens@itcr.ac.cr
 Bartlett, L., & Connor, E. F. 2014. Exogenous phytohormones
and the induction of plant galls by insects. Arthropod-Plant
Interactions, pp 339-348.
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29. THANK YOU!
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