Systemic Acquired Resistance

Systemic Acquired Resistance

Resistance may be regarded as the ability of a plant to overcome completely or in some degree, the effect of a pathogen or other damaging factor. Systemic acquired resistance is one of the promising means of plant disease control. In this process pathogenic micro organisms or chemical compounds are applied in the plants. As a result of this process in the treated plant, appear and consequently the defense of the whole plant against subsequent infections by pathogens of same or different types is inactivated. Systemic acquired resistance spread systemically and develops in distal, untreated parts of the plant.

Introduction of SAR:-

Systemic acquired resistance become a subject of great importance and enquiry. It is a broad physiological immunity that develops from infection with a pathogen. In addition to this, certain natural and synthetic chemical compounds trigger similar plant responses. The carefully controlled laboratory study of SAR was published in 1961 when Ross used TMV on a host and found that subsequent infections were reduced in severe cases. Ross coined the term SAR for the resistance that developed in the distal untreated portions of TMV inoculated plants.

Characteristics of SAR:-

Some important characteristics of SAR is as follows-

(1) It is induced by agents or pathogens causing necrosis.

(2) There is a delay of several days between induction and full expression.

(3) Production in conferred on tissues not expose to induction.

(4) Production is long lasting.

(5) Production is not specific.

(6) Development of SAR is associated with expression of several gene families, for example- PR proteins.

(7) The signal for SAR is translocated and graph transmittable.

(8) The protein is not passed on the next progeny; the transmissible to clonal tissue is unresolved so far.

Mechanism of SAR:-

The mechanism of SAR is not clearly understood and it is still being studied. However it is thought to be induced by some chamicals like Salicylic acid, isonicotinic acid and also many microorganisms like virus, bacteria, fungi etc.

(a) Induction through Salicylic acid:-

Salicylic acid seems to be involved in the development of SAR. Salicylic acid Is a benzoic acid derivative and secondary metabolite of plants. It present in the phloem of plants after primary inoculation. As a results in increase levels of salicylic acid in the zone of infection that establishes SAR in distant regions of thye plant. According to van Bel and Gaupels, 2004, the transmission of SAR signal from infection site to other occurs through vascular tissues phloem is now considered to be the pathway of SAR signal.

Before the onset of acquired resistance its concentration level correlate with the induction of PR protein and its external application activate the SAR genes that are expressed after SAR induction by pathogens. Nevertheless other evidence suggests that a signal other than salicylic acid is responsible for systemic expression of SAR, but salicylic acid must be present for the real signal to be transduced induced into gene expression and acquired resistance3. It has been reported that salicylic acid reacts with catalase and generate reactive oxygen radicals, and this has been suggested by as a mechanism by which the plant cell reacts to salicylic acid signaling and induces SAR.

Flow chart

Salicylic acid is not the mobile SAR signal. Maldonado et al (2002) have shown that in Arabidopsis mutation in DIR 1 gene (defective in induced resistance-1 gene) inhibit SAR response. The gene is specifically exposed in phloem and encodes a lipid transfer protein and it has been suggested that the long distance SAR signal might be substance derived from a lipid.

Flow chart

Apart from phloem mobile signals, the plant may develop SAR through air borne signals. Salicylic acid may be converted into its methyl ester, methyl salicylate that is moderately volatile substance. Methyl salicylate may function as volatile air borne SAR inducing signal that is transmitted distant non infected neighbouring plants making them resistant to pathogens attack.

(b) Induction through isonicotinic acid:-

Inhibition of catalase activity by the salicylic acid produced active isonicotinic acid(INA) which results in the accumulation of oxygen radicals and thus in the hyper sensitive response, which showed in the following figure graphically.

Graph

A large number of microorganisms, have been tested for their ability to induce SAR in plants, but so far not have proved effective.

Some fungi stimulate changes in the metabolism of plant through an elicitor (a product of the pathogen). The binding of elicitor to a receptor in the plant plasma membrane, produces a hypersensitive response.