Articles
HARPIN FROM ERWINIA AMYLOVORA INDUCES PLANT RESISTANCE
A common form of plant resistance is the restriction of pathogen proliferation to a small zone surrounding the site of infection.
Typically this restriction is accompanied by localized necrosis.
In addition to local defense response, plants also respond to infection by activating defenses in uninfected parts of the plant, which result in resistance of the plant to secondary infection (Dean and Kuc, 1985). Collectively, this phenomenon of induced resistance is called systemic acquired resistance (SAR). SAR reduces the severity of disease caused by all classes of pathogens and it can persist for several weeks or longer.
SAR can be induced by abiotic agents, such as salicylic acid as well as biotic agents, such as virulent and avirulent pathogens (Dean and Kuc, 1985; Malamy et al., 1990). Salicylic acid is believed to play a signal function in the induction of SAR since endogenous levels of salicylic acid increase after "immunization" with an incompatible pathogen.
However at present, little is known about the signal transduction pathways activated during responses of a plant to attack by a pathogen, although this knowledge is central to understanding disease susceptibility and resistance.
Erwinia amylovora is an often devastating plant pathogenic bacterium that causes the fire blight disease of pear, apple and many other rosaceous plants.
In non-host plants, E. amylovora elicits the hypersensitive response (HR), which is characterized by a rapid, localized death of tissues infiltrated with high concentrations of bacterial cells (>107 cfu/ml) (Klement, 1982). hrp genes are essential for E. amylovora to cause disease in host plants and to elicit the HR in non-host plants (Beer et al., 1991). Harpin is a heat-stable, glycine-rich, secreted protein with molecular mass of 37 kD. It is encoded by hrpN of E. amylovora (Wei et al., 1992). When infiltrated into intercellular spaces, harpin elicits the HR in many plants including tobacco, pepper, sunflower, tomato cabbage, arabidopsis, cucumber, geranium, watermelon and lettuce.
The HR is believed to be associated with plant defense against pathogens.
Hence, we reasoned that harpin-induced HR may induce plant resistance.
We tested harpin-induced resistance in more than seven different plants against eight diseases caused by fungi, bacteria and viruses.
All tested plants showed some resistance.
Here we report evidence of harpin-induced resistance to three diseases, southern bacterial wilt of tomato, tobacco mosaic virus and Gliocladium leaf spot of cucumber.