ABSTRACT
Benzothiadiazole or BTH [benzo (1, 2, 3) thiadiazole-7-carbothioic acid S-methyl ester: CGA 245704] is probably the best known synthetic elicitor to possess host defenceinducing activity in a number of plant species against different pathogens. It was first marketed commercially as Bion® by Novartis (now marketed by Syngenta) for effective management of powdery mildew in wheat and was found to be effective up to ten weeks of its spray (Ruess et al., 1996). BTH does not possess any in-vitro anti-microbial activity; rather, it induces systemic host resistance to pathogens in wheat and several other plant species, even when applied locally (Gorlach et al., 1996). Increased activity of PAL and POX was observed by Stadnik and Buchenauer (2000) in BTH-treated powdery mildewsusceptible wheat variety. Subsequently, induced resistance was observed in susceptible variety against Blumeria graminis f. sp. tritici (Bgt). Upregulated PR-4 genes expressions as a means of SAR observed in wheat plants treated with various chemical inducers such as SA, BTH and MeJA (Bertini et al., 2003). Reignault et al. (2004) tested the effect of chitosan, Iodus 40®, Milsana®, salicylyl heptanoate, trehalose and pectic oligosaccharides on wheat for controlling powdery mildew and showed a reduction in the level of infection with Milsana®, salicylyl heptanoate and trehalose. Chitin (b-1,4 linked N-acetylglucosamine), which is a common constituent of fungal cell walls, has been reported to enhance host defence responses in a number of plant genera, including rice, wheat, barley, soybean, tomato parsley and melon (Zhang et al., 2002). Similarly, lipopolysaccharides (LPS) and lipooligosaccharides from the outer surface of Gram negative bacteria induce several disease-resistant components in plants. The culture filtrate of Pantoea agglomerans, releasing LPS and lipooligosaccharides in growth medium, when sprayed on susceptible wheat leaves, reduced wheat leaf rust disease (Kempf and Wolf, 1989). A carbohydrate
elicitor purified from the conidia of Blumeria graminis f. sp. tritici (Bgt), when sprayed on wheat leaves, activated most of the genes involved in SAR. This elicitor has been confirmed to induce resistance to subsequent challenge with Bgt spores (Schweizer et al., 2000). The induced resistance was not related to host cell death unlike race-specific R-genes-triggered resistance. Induced resistance against wheat pathogens due to prior inoculation of non-pathogens is also reported in wheat. Inoculation of wheat with the nonhost pathogen Erysiphe graminis f. sp. hordei resulted in overexpression of SAR genes and induced resistance to subsequent infection by Erysiphe graminis f. sp. tritici (Kmecl et al., 1995). Glycerol-3-phosphate (G3P), a conserved signalling molecule in dicots and monocots, contributes to SAR in wheat against stripe rust (Yang et al., 2013).
