Breeding for resistance to biotic stresses

1. Breeding for Biotic Stress Resistance in forage crops
Introduction:
Stress:
Adverse conditions for crop growth and production caused by either environmental or
biological factors or both.
Biotic Stress:
Adverse conditions for crop growth and production caused by biological factors such as
disease, insects and parasitic weeds.
Need for biotic stress resistance in forage crops:
Biotic stress can cause serious loss in yield and quality of forage and turf grass and it’s
control is therefore important.The use of fungicide and insecticide is limited to forage crop because
of their cost because of their cost and consideration of safety to humans and environment.
One of the most efficient strategies for controlling disease and pest is development of
resistant varieties.
Mechanism of Disease Resistance:
A variety of mechanisms are involved in disease resistance. In some cases, the basis of
resistance is better known than in others. The various mechanisms of disease resistance are as
follows:
1) Mechanical,
2) Hypersensitivity and
3) Nutritional restriction.

2. 1) Mechanical:
Certain mechanical and or anatomical features of the host may prevent infection. For
example, closed flowering habit of wheat and barley prevents infection by the spores of ovary
infecting fungi.
2) Hypersensitivity:
In a large number of cases, immune reaction is due to the hypersensitive reaction of the
host. This mechanism is found in case of biotrophic organism or obligate parasites.
3) Nutritional restriction:
The reduction is growth and in spore production is generally supposed to be due to an
unfavourable physiological conditions within the host. Most likely, a resistant host does not fulfil
the nutritional requirements of the pathogen and thereby limits its growth and reproduction.
However, more precise information is not available on this aspect.
Gene for Gene Hypothesis
The concept of gene for gene hypothesis was first developed by Flor in 1956 based on his
studies of host pathogen interaction in flax, for rust caused by Malampsora lini. The gene for gene
hypothesis states that for ‘each gene controlling resistance in the host, there is corresponding gene
controlling pathogenicity in the pathogen’ The resistance of host is governed by dominant genes
and virulence of pathogen by recessive genes. The genotype of host and pathogen determine the
disease reaction. When genes in host and pathogen match for all loci, then only the host will show
susceptible reaction. If some gene loci remain unmatched, the host will show resistant reaction.
Now gene – for –gene relationship has been reported in several other crops like potato, sorghum,
wheat, etc. The gene for gene hypothesis is also known as “Flor Hypothesis.”
At molecular level, it is considered that gene for gene resistance usually involves production of
toxins antibiotic proteins by a resistance gene. The production of toxins is related to gene dosage.
The resistance controlled by domain gene is the most desirable. Gene for gene relationship are rare
or unknown for disease caused by viruses, bacteria, Fusarium, and other organisms that cause rot.

3. Disease resistance in forage crops:
Crown rust resistance in Italyian Rye Grass:
Crown rust is caused by Puccinia coronate f.sp.loii , it is a disease of rye grass such as
Italian rye grass and perennial rye grass.In this resistance is developed by gene
pyramiding.Development of new variety takes more than 10 years. New variety is overcome by
new races of diseases. It is nothing but the incorporation of two or more genes in the host for
specific resistance of the pathogen. New variety developed with two or three major genes namely
pc1 and pc2.
Leaf blight resistance in maize:
Nature of resistance may be due to normal cytoplasm . Taxas (T) type cytoplasm is
susceptible to leaf blight.One recessive gene is in this expression of resistance.
Disease resistance in alfalfa:
Plant secondary metabolites called phytoalexins have been associated with antifungal
activity in alfalfa. The nature of this activity tends to be broad-spectrum , and overexpression of
phytoalexin genes in alfalfa could provide defense against pathogens for which no specific source
of resistance is available. Alfalfa transformed with the phytoalexin resveratrol from peanut has
increased resistance to Phoma medicaginis. Likewise, alfalfa transformants overexpressing
isoflavone Omethyltransferase (IOMT) also show increased resistance to P. medicaginis . Reduced
severity of infection by Phytophthora megasperma was achieved in alfalfa overexpressing an
inducible beta-1, 3- glucanase, a pathogenesis-related (PR) protein.
Smut resistance in sorghum:
The genetics of disease resistance is due to oligogenic resistance in which the resistance is
goverened by one or few major genes.

4. Basis of Insect Resistance
There are three important basis of insect resistance, viz
1) morphological,
2) Physiological,
3) Biochemical features of host plant.
In other words, insect resistance is mainly governed by morphological, physiological and
biochemical features of host plant. These are briefly discusses as below:
A.Morphological Factors:
Various morphological traits, viz, hairiness, colour, thickness and toughness of tissue, and
several other characters are known to confer insect resistance in different crop plants.
1.Hairiness:
Hairiness of leaves is associated with resistance to many insect pests,
2. Colour of Plant:
Plant colour contributes to no preference in some cases.
3. Solid Stem:
In certain plants, soild stem confers resistance to flies.
4. Toughness of Tissue:
Tough and thick plant tissue cause mechanical obstruction to feeding and oviposition of
insect pests.

5. B. Physiological Factors:
Some physiological factors such as osmotic concentration of cell sap and leaf exudates are
associated with insect resistance. In Medicago disciformis, the secondary trichomes on the leaves
secrete antibiotics exudates. These exudates kill the alfalfa weevil at higher concentrations and
retard the growth at lower concentrations.
C. Biochemical Factors:
Biochemical factors are considered as more important than morphological and
physiological factors for insect resistance. A number of biochemical substances are known to be
associated with insect resistance in different crop plants. Some examples are given below:
In alfalfa high concentration of saponin in the leaves and stem confer, resistance to spotted alfalfa
aphid and pea aphid.
Certain biochemical substances act as feeding stunt for insect pests. Lack or low concentration of
such substances in host plant will lead to non-preference type of resistance. In sweet clover (
Mellilotus spp.) coumairn provides feeding stimulus to flying weevil. Low concentration of
coumairn confers resistance to this pest.
Mechanisms of Insect Resistance:
There are four mechanisms of insect resistance,
1) Non preference ,
2) Antibiotics,
3) Tolerance and
4) Avoidance or escape.
The first three mechanisms were given by Painter (1951) and the fourth one was added
subsequently. A resistant variety may have one, two or more of these mechanisms.

6. 1. Non – preference:
Non preference refers to various features of host plant that make the host undesirable for
unattractive to insects for food, shelter, or reproduction. This type of insect resistance is also known
as non acceptance and antixenosis. Non acceptance appears to be more accurate term, because in
most known examples of this type of resistance, insects will not accept a resistant host plant even
if there is no alternative source of food. Various plant character which are associated with non
preference include colour, light penetration, hairiness, leaf angle, odour and taste.
2. Antibiosis:
Antibiosis refers to the adverse effect of host plant on the development and reproduction
of insect pests which feed on resistant plant. Resistant plants retard the growth and rate of
reproduction of insect pest. In some cases, antibiotics may lead even to death of an insect. An
antibiotic is considered as the true form of resistance to insect pests.
3. Tolerance:
Tolerance refers to the ability of a variety to produce greater yield than susceptible variety
at the same level of insect attack. In other words, a tolerant variety will give higher yield than
susceptible one despite the insect attack. The tolerance is measured in terms of rejuvenation
potential, healthy leaf growth, flowering compensation potential and superior plant vigour. Hybrid
cottons, by virtue of their very high potential, show tolerance to the insect pest.
4. Avoidance or Escape:
Avoidance refers to escape of a variety from insect attack either due to earliness or its
cultivation in the season where insect population is very low. Avoidance is also an effective means
of protecting crop from the damage of insect pests.
Insect resistance in forage crops:
Resistant to thrips in alfalfa: Transgene mediated insect resistance has been reported in alfalfa

7. with the insertion of an insect proteinase inhibitor. Transformants expressing the anti-elastase
protease inhibitor (PI) from Maduca sexta exhibited reduced onset of thrips predation. Bt genes
code for crystalline (Cry) proteins, which breakdown in the gut of lepidopteran insects, and cause
paralysis of the digestive system, and death of the insect. Bt transgenes have been transferred to
alfalfa for the potential control of alfalfa weevil and clover root curculio.Alfalfa varieties like Cody
, Mopa ,Zia are resistant to spotted alfalfa aphid
Resistant to psyllid in subabul: In Subabul, the most probable control of the psyllid will occur
through the development of psyllid resistant hybrids such as L.pallida,L.diversifolia. Breeding
programmes to develop open pollinated and psyllid F1hybrid cultivars are well advanced.
Advantages of biotic stress resistance:
 Cheapest means of disease of control.
 Reduces the use of fungicide which is eco-friendly.
 Use of resistance varieties minimize year to year fluctuation in yield.
Disadvantages of biotic stress resistance:
 In resistance variety may have inferior quality in some sp.
 Insect resistance is available only in wild sp.
 It is long term programme.
 In this resistance to one plant may lead to susceptibility to another plant.

8. References
 Phundan Singh, Essentials Of Plant Breeding, sixth revised Edition ,2015,Kalyani
Publishers 4779/23,Ansari Road ,Darya Ganj,New Delhi-110002,Pp-300 to 323.
 link.springer.com/chapter/10.1007%2F1-4020-2591-2_2
 https://books.google.co.in/books?isbn=9401597006
 mellowmustard.envibipe.xyz/424528/plant-breeding-biotic-stress-resistance

9. Breeding for Biotic Stress Resistance in forage crops
Content
 Introduction
 Stress
 Biotic stress
 Need for biotic stress resistance in forage crops
 Mechanism of disease resistance
 Gene for Gene hypothesis
 Disease resistance in forage crops
 Basis of insect resistance
 Mechanism of insect resistance
 Insect resistance in forage crops
 Advantages of biotic stress resistance
 Disadvantages of biotic stress resistance
 References