The effect of gibberellin and abscisic acid on plant defense responses and on disease severity caused by Meloidogyne java']javanica on tomato plants

Stimulating plant innate resistance is a choice method for developing safe alternatives to harmful chemical nematicides to control root-knot nematodes. Plant hormones have a key role in inducing resistance. Here, the ability of gibberellin and abscisic acid to induce biochemical defense responses in tomato roots was investigated by monitoring the activity of cytoplasmic enzymes phenylalanine ammonia lyase (PAL), peroxidase (POX), polyphenol oxidase (PPO) and catalase (CAT) every other day for the first week after inoculation with Meloidogyne javanica, then weekly for 8 weeks. Two second-stage juveniles were added to 1 g of sandy loam soil in each pot. Tomato seedlings were sprayed with gibberellin (GA) or abscisic acid (ABA) when free of nematodes and in pots with M. javanica (GA + N and ABA + N). There were also healthy plants (C) as well as plants that were inoculated only with M. javanica (N). The highest PAL, POX and PPO activity was found in gibberellin-sprayed plants during the first week, demonstrating that gibberellin induced a plant defense response. The amount of enzymes then decreased during weeks 2 and 3 and rose at the fourth week, then decreased weekly till the end of the experiment. On the basis of plant growth and nematode reproduction, gibberellin stimulated plant innate defense responses and increase plant resistance, whereas abscisic acid decreased plant resistance. Therefore, spraying tomato plants with gibberellin is recommended to control M. javanica.