VARIABILITY IN PLANT PATHOGENS

PRESENTED BY

Arushi Arora

A-2016-30-050

GENETICS AND VARIABILITY OF PLANT PATHOGENS

• Dr. E.C. Stakman (U.S. Deptt. Of Agriculture)defined need and use of variability in fungi

• Pathogen population mutates with time andenvironmental conditions like stress

• In nature variability in organisms may be dueto:

HYBRIDIZATION MUTATION

• When progenies show variations incharacterstics from parents it is called as aVARIANT.

• Physiological specialization: – with in thespecies of a pathogen there exist certainindividuals that are morphologically similarbut differs w.r.t their physiology, biochemicalcharacters and pathogenicity and aredifferentiated on the basis of their reaction oncertain host genera or cultivars.

• Physiological race: – a group of populationwithin a species which have ability of infectinga particular genotype and do not differ in theirmorphology but have physiological differencessuch as a specific host or food type orpathogenicity

• Variability: it is the property of an organism tochange its characters from one generation tothe other

• With in species or f.sp. There are furthersubgroups of individual that infect differentvarietes of the host– such subgroups arecalled RACES/ STRAINS

• A sub-group within a species or race,characterized by a common possesion ofsingle of few new characters and differ only infew minor characters from parents are calledBIOTYPES

BREAKDOWN OF RESISTANCE:

• This term is used when a previously resistantvariety suddenly develops disease.

• It implies that the host has changed, that theresistance mechanisms no longer work.

• New pathogen races have developed becauseof selection pressure that was put onpopulation by the host resistance mechanism

Races are defined by ability to develop on specific host genotypes (varieties, cultivars, hybrids, etc.).

Races are identified by ability to cause disease on members of a set of 10 differential varieties that contain specific

resistance genes.

V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 Race

x x x 14

x x x 27

x x 31

VARIABILITY IN FUNGI

MUTATION

HERTEROKARYOSIS

PARASEXUALISM

RECOMBINATION

CYTOPLASMIC ADAPTATION

HYBRIDIZATION

• Mating of dissimilar strains or species of fungi

• 2 haploid nuclei (1N) with different geneticmaterial combine to form a diploidnucleus(zygote)

• Eg. Basidiospores( haploid) from different racesoften infect the same leaf or plant

Chances of dikaryotization high

HERTEROKARYOSIS

Hyphae or parts of hyphae contain nuclei,which are genetically different, generally oftwo different kinds. This condition is known asheterokaryosis .

The phenomenon is commonly brought aboutby hyphal anastomosis between mycelia oftwo parental genotypes.

In Ascomycotina and Basidiomycotina

PARASEXUALISM

• First demonstrated by Pontecorvo (1956) inAspergillus nidulans.

Parasexualism is the process by which genetic recombination can occur with in fungal heterokaryon :

• Stage 1: heterokaryosis

• Stage 2: recombination

PARASEXUALISM

• Dissimilar nuclei to fuse and produce diploidsknown as mitotic recombination.

• Recombination without sexual cycle

• There is no fine coordination betweenplasmogamy, karyogamy and haplodisation

• This sequence of events has been described inthe parasexual cycle

PARASEXUALISM IMPORTANCE

• In rust fungi as P. graminis tritici, mitoticrecombination may represent a mostimportant method of generating new racesespecially in countries such as India wheresexual stage of the fungus is rare due toscarcity of the alternate host, the barberry.

• Common in deutromycotina

RECOMBINATION

• When two haploid nuclei (1N) containingdifferent gnentic maeterial unite to form diploid(2N) nucleus (Zygote,) when under go meioticdivision produce new haploid .

• Recombination of genes occurs during meioticdivision of zygote as a result of cross over inwhich part of chromatid of one chromosome of apair are expressed with that of the other

• Important in fungi Puccinia graminis.

Cereal rust fungus variability

• Races of rust pathogen Puccinia graminisdiffer from each other in shape and size ofuredospores but principle diffrence betweenthem is

• Preference for groups of host in differentmembers of graminae

Eg. Race tritici, secalis, avenae etc

MUTATIONSudden heritable change in genetic material of an

organism

Mutation represent change in sequences of thebases in DNA either by – substitution or bydeletion or addition

Mutations are spontaneous

May occur due to:

1. Improper cell division

2. Abnormalities during division

3. Physical radiations

4. Mutagenic chemicals

CYTOPLASMIC ADAPTATION

• Pathogens develop capicity to performbiochemical reactions which were not presentin them earlier

• Can utilize protoplasm of unfavourable host.

• Adaptation to new cytoplasm

Three types of cytoplasmicadaptatibility

I. Pathogen may aquire tolerance to toxic materials

II. Utilization of new types of cytoplasm

III. Change in virulence

VARIABILITY IN BACTERIA

Transformation transduction

Conjugation

CONJUGATION

• Transfer of DNA from one bacterial cell toanother Donor cell (F+) transfers DNA torecipient cell (F- )

• In this two compatible bacteria come incontact and exchange the portion of plasmidor chromosome through conjugation bridge orpilus

TRANSFORMATION

DNA taken up from external environment byabsorption

TRANSDUCTION

Transfer of bacterial genes with a bacteriophage

VARIABILITY IN VIRUSES

Recombination

• May results from mixed infection of twostrains of the virus

• Occurs mostly during replication

Mutation – Results from nucleotide changesin the coding regions due to addition ordeletion or replacement. – Ultimately leads tofunctional changes in the genes.

REFRENCES

• R.S. Singh, INTRODUCTION TO PRINCIPLES OF PLANT PATHOLOGY, Oxford & IBH Publishing CO. pg.200-208

• http://agriinfo.in/default.aspx?page=topic&superid=3&topicid=2131

• http://www.columbia.edu/itc/hs/medical/pathophys/id/2008/Notes.pdf