PLANT GENETICS and MOLECULAR BIOLOGY
Department of Cell Biology & Molecular Genetics
BIOLOGY:
I. Definition: Disease is a disfunction of normal physiological processes in plant caused by microorganisms or an abiotic factor.
Types of Pathogens:
Bacteria: prokaryotes; enter through wounds or stomata, live between plant cells.
Fungi: eukaryotes; filamentous growth with specialized structures for penetration, feeding in cells
and sporulation. Can penetrate directly into plant and move intercellularly or through cells.
Nematodes: Microscopic segmented worms, use stylet for feeding. Can feed on outside cells of
root or burrow inside to set up feeding relationship with one cell.
Viruses
-Nucleic acid (+ RNA mostly) encapsulated in a protein coat
-Spread by transfer of sap to one cell, then trhrough plant
-Genes on Genome:RNA replicase, cell-cell movement protein, and the coat protein
-solutions to problem of translation starts only at first AUG
1) Replicase can make "subgenomic” RNAs
2) Multipartite genomes where 3-4 different RNA molecules make up the genome.
3) Polyprotein. One long protein is made of the virus genome,
which is then cleaved to make individual proteins.
Approaches to studying Resistance
I. Genetics of pre-existing variation in host resistance:
Two types of resistance, based on degree of resistance and the reaction to the different races of a
specific pathogen.
1)Horizontal resistance
-a cultivar is partially resistant to a pathogen, but is equally resistant to all races of that
pathogen
-In RXS crosses, progeny show intermediate levels of resistance-- consistent with multiple
genes which have an additive effect on resistance.
2)Vertical resistance
-a cultivar is totally resistant to one or a few races of the pathogen, but is suseptible to
other races of that same pathogen.
-In R x S crosses, progeny are either totally resistant or suseptible, indicating a single gene
conferres resistance to that race of pathogen.
Genetics of Vertical Resistance
-Resistance to each race of pathogen is conferred by a single gene(R)
-Multiple resistance genes can be found in various cultivars or in one cultivar, each conferring
resistance to a different race.
-eg. there are 35 different R genes in wheat to wheat stem rust.
-In plant crosses R x S, resistance is dominant (resistant:suseptible progeny is 3:1).
R=resistant allele, r=suseptible allele
-In pathogen crosses, ability to cause disease(virulence) is recessive (virulent: avirulent is 1:3), or
stated differently, avirulence (lack of ability to cause disease on that R gene containing cultivar is dominant).
A=avirulence allele, a=virulence allele
PLANT
PATHOGEN RR, Rr rr
AA, Aa Resistant Disease
aa Disease Disease
Model from genetics:
-Avirulent pathogens have product (from avirulence gene) which is recognized by the Resistance
gene in the plant.
-Each avirulence gene has a corresponding resistance gene which recognizes it and triggers a
defense reaction
-so vertical resistance a recognition reaction which triggers general defenses of the plant.
Cloning vertical resistance genes
gene plant resistance to pathogen
HM1 corn fungus Helminithisporium carbonum(toxin resistance)
Cf-9,Cf-4 tomato fungus Cladosporium fulvum
L6 flax fungus- flax rust
PTO tomato bacteria Peudomonas syringae
RPS2, RPS4 arabidopsis bacteria Pseudomonas syringae
Xa21,Xa1 rice bacteria Xanthomonas oryzae
N tobacco Tobacco Mosaic Virus (TMV)
I2C tomato fusarium oxysporum
Rpp5 arabidopsis Powdery mildew
Hs1 pro-1 sugar beet Nematode
-many R genes occur in clusters of related genes- gene family for evolution of Resistance Genes?
Avirulence genes in pathogen: Most unknown
-eg. syringolides made from avrB from Pseudomonas
II. Biochemical
A. Identify antimicrobial compounds produced by plants, and their gene products. Expression IS OFTEN induced by pathogen.
-phytoalexin synthesis-antibiotic compound made by plants in response to pathogen or stress.
-PAL(Phenylalanine ammonia lyase) and CHS (Chalcone synthase) two enzymes early in pathway. Cloned in parsley and then in many other plants as well.
-Antimicrobial Proteins: Defensins, Thionins
B. Identify proteins expressed only during pathogenesis or resistant reaction
-PR proteins(Pathogenesis-Related) identified from tobacco infected with TMV
-acidic proteins
-resistant to proteases
-secreted into intercellular spaces
-classed in 5 groups by cross-relatedness, many members of each group
-one group--chitinases, another-- b 1-3 glucanses
-digest cell wall of fungi
C. signalling of these genes- how above genes are turned on by potential pathogens?
Components in signal transduction pathway:
1) Elicitors-compounds derived from a pathogen which can turn on plant defense genes similar to the effect of the whole pathogen
-glucans from polysaccharides in fungal cell walls
-arachidonic acid
2) Signals travelling between parts of the plant
Systemic Induced Resistance-where wounding or treatment with pathogen or elicitor will induce resistance in some other part of the plant.
-salicylic acid (asprin precursor)
-applying salicylic acid to plant induces PR proteins
-see increase in amount of compound after infection
III. Mutational Genetics
Arabidopsis
Mutants with decreased disease resistance
-pad3, pad5- fails to make camalexin (antimicrobial of arabidopsis)
-npr1, pad4- don’t turn on PR genes
Mutants with high constituitive PR protein expression
-lesion-mimic mutants: acd2, cpr5, lsd 1-7
-high PR expression cpr1,5,6
IV. ENGINEERING DISEASE RESISTANCE
Making transgenic plants for testing the role of a defense gene and making disease resistant plants
1) Changing regulation of defense genes
-overexpressed PR genes (chitinase and B-glucanase) on 35S promoter
-overexpressed NPR1 gene- protection without causing stunting
-presumably control many induced traits
-application of Salicylic acid inducers
2) Introducing defense genes from other plants
-eg. genes for phytoalexin biosynthesis, sucessful pathogens on one plant have evolved to become resistant to its host plant phytoalexin compound(s) but most are sensitive to the compounds used as phytoalexins from other plants.
-therefore, making one plant make another's phytoalexin should produce resistance against the plant's
usual pathogens.
eg. Transfer of stilbene synthase (reseveritol synthethis) from grape into tobacco, barley, tomato etc.
3) Introducing non-plant genes into the plant for resistance
a) viral coat protein
-idea derived from early cross-protection studies
-cloned coat protein gene from TMV, fused onto CaMV 35S promoter
b) introduction of glucose oxidase- produces active oxygen
-glucose oxidase (GO)- oxidizes glucose but forms hydrogen peroxide (H2O2) as product
- protected tubers from bacterial rot
-protected leaves from foliar pathogen Phytophthora infestans
c) introduced dsRNAase from yeast into potato
-protects from virus (RNA viruses replicate through dsRNA)
Links:
News
summary and
original
article of using NPR1 gene to increase resistance to disease
Last revised: April 19, 1999- Straney