GENETIC RESISTANCE AGAINST PATHOGENS IN WHEAT (Synopsis 2)

INTRODUCTION

Wheat is one of the major crop in Pakistan and occupies a position of paramount importance. It was planted over 7.2million ha annually during the period from 1981-82 to 1984-85(Shah, 1994) that yielded 11.6 million tonnes. The wheat crop in Pakistan is attacked by a large number of insects such as Aphids and Armyworm during spring after heading. the microbial pathogens that attacks wheat include Stem Rust, Leaf Rust and Stripe Rust caused by Puccinia graminis, P. recondita and P. striiformes, respectively(Akhtar, 1976). Leaf and stem rust occur all over country and become serious wherever and whenever weather conditions permit. moreover, root rot and leaf blight caused by Helminthosporium sativum occurs in parts of sind and Punjab plains. these disease epidemics cause heavy losses due to low yield and quality.
Reliance on heavy use of chemicals to control pests and microbial pathogens is no more desirable to the environment. It is becuase of their persistance and hazardous nature. Therefore, an urgent need to explore the available genetic resources for natural resistance with acceptable agronomic traits cannot be ignored. Plants are exposed to pathogens but do not have an immune system comparable to that of animals(Staskawicz, 1995). However, plants evolve defense mechanisms that involve several physical and chemical factors(Campbell, 1985). Physical defense mechanism includes tough epidermises, cuticular deposits, spines, thorns, prickles and stinging hairs whereas chemical defense mechanism includes toxins and repellents. Individual plant cells confront the disease by the combination of constitutive or induced defenses(Staskawicz, 1995). One of the earliest response is termed as Hypersensitive Response(HR), which is characterised by the oxidative burst. Release of Reactive Oxygen Species(ROS) renders toxicity to microbes and initiate cell death pathway. The second most common phenomenon is Systemic Acquired Resistance(SAR), which induces release of certain chemicals in the surrounding tissues. SAR occurs in response to necrotising infections caused by viruses, bacteria and fungi which persists for several weeks.
Disease resistance also occurs due to antimicrobial compounds such as Phytoalexins, Polypeptides and Proteins produced in response to infection. Some metabolites and plant hormones also increase in response to the fungal and viral infections in wheat(Metraux, 1990). Secondary metabolites sch as Hydroxamic acids(Niemeyer, 1988) has been reported in mediating plant defence against pathogens. Hydroxamic acid is present as an inactive Glucoside and upon mechanical injury came in contact with Glucosidase to produce 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one(DIMBOA). DIMBOA protect plants against aphids and microbial pathogen.

OBJECTIVES

1. To study the chemical defense system in Triticum aestivum L. cultivars against microbial pathgens,
2. To detect and isolate the factor responsible for resistance,
3. To study the linkage of disease resistance to the chemical defense system.
   

PLAN OF WORK

PLANT MATERIAL :
Pathogen resistance and susceptible cultivars of Triticum aestivum L.

Phase 1:-
In this phase local genetic resources will be analysed for the quantification of Hydroxamic acids and other molecules both in prescence and abscence of infection. This involves selection of suitable tissues for he extraction of Pathogenesis related(PR) compounds. This involves preparation of the protoplasts from leaf tissues. Respective metabolites may be identified using UV-TLC methods. For the quantification of Hydroxamic acids Spectrophotometric technique will be employed.

Phase 2:-
The second phase of this research involves correlation of the endogeous quantities of pathogen combating metabolites and plant resistance. For this purpose the symptomatology of the disease will be quantified in different genotypes. This may help in identifying the factors that enable a plant to resist a pathogenic attack more than a sensitive plant.