A large number of plant defense response genes encoding antimicrobial proteins have been cloned. Most of these are transcriptionally activated in response to infection or exposure to microbial elicitor macromolecules.
1. PATHOGENESIS – RELATED (PR) PROTEINS :
These are low molecular weight proteins which accumulate to significant levels in infected plant tissues. The ability of hydrolytic enzymes e.g. chitinases & 1 – 3 beta – D Gluconase, to break chitin & glucan in the cell walls of fungal pathogen has been exploited to develop crops resistance to pathogen.
Chitinase gene of soil bacterium serratia marcescens was introduced into tobacco. The tobacco is resistant to Alternaria longipes, causing brown spot disease.
A chitinase gene of bean (Phaseolus vulgaris) under the control of promoter of CaMV 35S has been introduced into tobacco. The transgenic tobacco plants are resistant to the fungal pathogen Rhizoctonia solani, causing damping off of seedling.
The 1 – 3 beta – D Gluconase together with chitinase gene showed fungal resistance in tobacco (fungal disease caused by Cercospora nicotinae) & tomato (fungal disease caused by Fusarium oxysporum lycopersci).
2. ANTI MICROBIAL PROTEINS :
Plants and other organisms may contain antimicrobial proteins that are not necessarily associated with induced defence response, but the presence of these proteins exhibit resistance to pathogen.
Barely α – thionin gene when transferred to tobacco showed resistance against pseudomonas syringae pv tobacco
Antimicrobial proteins of non plant origin include lytic peptides & lysozymes.
Lytic peptides form pores in bacterial membrane (e.g. attacin, ucropin etc.)
Ucropins been expressed in transgenic potato and tobacco & attacins in apple plants.
A ucropin gene when transferred to tobacco showed resistance against Pseudomonas syringae & bacterial pathogen.
Expression of the bacterial gene bacteriopsin (bo) encoding a proton pump in transgenic tobacco resulted in complete resistance to P.syringae pv tobacco.
Bacteriophage T4 lysozyme gene transferred to potato showed resistance to Erwinia carotovora, subspecies atrosepta.
When human lysozyme was introduced into tobacco, transgenic tobacco showed enhanced resistance against Erisiphe uehoracearum & P.syringe.
When bovine lysozyme gene was introduced into tobacco produced bovine lysozyme that destroys the cell wall of invading pathogenic bacteria.
As the plant cells destroy bacteria, the plants can resist the bacterial pathogens.
The fungal endo – α – 1,4 – D polygalacturonases are partly responsible for the dissolution of the plant cell wall.
The cloning of a bean PGIP (Poly Galacturonase Inhibiting Protein) results in over expression of the inhibitor in transgenic plants & there by fungal disease development in plants is inhibited.
3. ENGINEERING TOXIN INSENSITIVITY :
Enzymes inactivating the toxins produced by bacterial and fungal pathogens are used to engineer disease resistance in plants.
The bacterial haloblight pathogen of bean, Pseudomonas phaseolica, produces a peptide toxin, phaseolotoxin, which causes the chlorotic halos.
Phaseolotoxin inhibits the enzyme ornithine trans carbamylase (OC).
Bacteria have been selected which contain a phaseolotoxin – insensitive OC and the gene encoding this enzyme, has been transferred to tobacco, where its expression prevents the disease symptoms.
Transgenic sugarcane expressing an albicidin detoxifying gene (alb) from pantoea dispera (bacterium) provides control against leaf & cold disease (caused by Xanthomonas albilineans) under the control of maize ubiquitin promoter.
4. PHYTOALEXINS :
Phytoalexins are secondary metabolites synthesized by plants and contribute to disease resistance.
During infection, stored phytoalexins (usually present in special cells or in a conjugated inactive) are mobilized, while genes are induced for the synthesis of more phytoelixins.
Resveratol is one of the commonest synthesized.
The key enzyme in resveratol synthesis is resveratol synthase (often known as stilbene (phytoalexins) synthase or STS)
STS gene of peanut transferred to tobacco proved its fungitoxic action (againt botrytis cinerea).
5. MANIPULATION OD DISEASE RESISTANCE GENES :
A gene from maize, which confers resistance to Cochlibolus carbonum has been cloned by transposoon tagging.
This gene encodes that inactivates the fungal toxin.
The gene that confers resistance to a pathogen in a resistant crop variety is transferred to a susceptible variety.
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