Investigations on the possible use of abiotic and biotic elicitors in defence-related responses in plants

Experiments were conducted to determine the protective effect triggered by the plant growth retardant prohexadione-Ca (Regalis®), bacterial lipopolysaccharides (LPS) and oligosaccharides (OLS) in different host-pathogen systems. These molecules did not show any direct antibacterial activity in vitro. On tobacco leaves, LPS infiltration (100 μg ml-1) prevented hypersensitive reaction (HR), whereas OLS at different concentrations did not give constant and reliable responses. On micropropagated pear plantlets in vials, only Regalis® and LPS (100 μg ml-1) gave a significant defence-related response against fire blight (Erwinia amylovora): in particular, plant-derived antimicrobial compounds were related to flavan-3-ols and appeared to be newly synthesized after LPS application. Regalis®, applied at different concentrations, significantly lowered progression of bacterial wilt (Ralstonia solanacearum) and reduced leaf lesions of bacterial speck (Pseudomonas syringae pv. tomato) and spot (Xanthomonas vesicatoria) on tomato plants in the greenhouse. OLS application (100 μg ml-1) significantly protected tomato leaves from bacterial speck, but not from bacterial spot. Two treatments with Regalis®, at a twelve-day interval and at different concentrations (75+50 ppm), gave a valuable protection of grapevine leaves to downy mildew (Plasmopara viticola) in the greenhouse. When Regalis® was sprayed at different concentrations and at different times on potted plum trees kept outdoors it was apparently ineffective against bacterial spot/canker (Xanthomonas arboricola pv. pruni). These findings further support the potential of prohexadione-Ca and of other biotic molecules as tools for plant defence responses against different pathogens, in the perspective of integrated, eco-friendly and sustainable control programmes.