Extended Abstract
1. Introduction: Today, due to the indiscriminate use of chemical fertilizers and poisons during production, humans and agricultural products have suffered a lot of physical and environmental damage. With the advancement of technology, new findings from researchers and scientists, as well as the discovery of the positive effects of natural compounds, the approach of using medicinal plants has taken the place of chemicals and is progressing day by day. Plant metabolites in plants have various roles and functions. Generally, play a role in protecting the plant and contributing to some biological mechanisms such as pollination, resistance to pathogenic agents, and living enemies of the plant. With this point of view, researchers presented allelopathic plants for sustainable agricultural production, and their products, which show herbicidal, antifungal, insecticidal, and pesticide properties, have received much attention. These biological compounds or secondary metabolites have different types. Biochemically, these compounds are included in categories such as essential oils, phenols, alkaloids, steroids, glycosides, etc. These compounds, today, have a special place in the industry in various fields such as medicine, pesticides and insecticides, spices, and edible colors. These compounds also have important effects inside the plant. The role of these compounds on growth regulators, resistance against pests and diseases in plants, and defense against biological disturbance agents. In this regard, in recent decades, many plant extracts have been widely researched and investigated to examine their effect on the prevention and control of diseases. A large number of reports have documented that the use of plant extracts helps control certain plant and animal diseases. Based on this and considering the goal of organic production and green agriculture, extracts of six medicinal plants were investigated on the growth process and control of crown gall (Agrobacterium tumefaciens) disease in tomato plants.
2. Materials and Methods: The present research was conducted in two parts, in vitro and ex vitro, to investigate the effect of extracts of some medicinal plants on the control of crown gall disease (Agrobacterium tumefaciens) in a completely randomized design. For this purpose, in an ex-vitro (greenhouse) experiment, after contamination of the crown gall through wounding with T9 strain and the appearance of symptoms of contamination of the site of gall formation, were treated with the extracts of Juniperus sabina, Taraxacum officinale, Dorema aucheri, Rheum ribes, Conocarpus erectus, and Allium jesdianum were treated in three concentrations of 1000, 5000, and 8000 mg/L. The investigated traits in this experiment included gall weight, gall diameter, auxin and cytokinin regulator levels, total phenol, and total flavonoid. In-vitro experiment was conducted by two types of protocols. First protocol to investigate the effect of the extracts on the gall bacteria growth, directly, plant extracts were added to the LB culture medium, and the bacteria were cultivated to check the inhibitory effect of the extracts. Second protocol, Agrobacterium tumefaciens was cultured in LB medium, and then the effect of plant extracts was investigated by placing disks containing extracts in concentrations of 1000, 5000, and 8000 mg/L.
3. Results and Discussion: It was observed that under the effect of using plant extracts in the greenhouse, the weight of gall decreased by 23% and the best result was observed in the treatment of Taraxacum officinale with 8000 mg/L. In this regard, the growth diameter trend of gall decreased between 4-60% compared to the control. The best result was observed in the treatment of Dorema aucheri 5000 mg/L. The growth regulator of auxin had a downward trend, and cytokinin also decreased by 1%. In examining the direct effect of the extracts on the bacteria cultured in the LB environment, no significant difference was observed in both protocols. The results showed that plant extracts had a significant difference at the levels of 1 and 5%. Based on this, the effect of plant extracts on the diameter of crown gall showed a significant difference at 5% and in other traits such as weight of gall, total phenol, and flavonoid, 1% significant difference was observed compared to the control plant. Although this reduction process did not prevent gall formation, the growth process slowed down compared to the control. Also, the lowest amount of total phenol and flavonoid (2.123 and 1.077 mg/g FW) was observed in the control treatment. The effect of different concentrations of the extracts was different (all treatments), but in general, the application of plant extracts could increase plant metabolites in the gall mass compared to the control. Related to the difference between treatments, an increasing trend in the amount of total phenols and flavonoids was observed with the increase in the concentration of the extracts. The observed trend differs, and this difference is likely related to the variation in the internal levels of secondary metabolites among different plants. Now, by comparing the results of biochemical, morphological, and growth regulators in the plant, it is clear that this decrease in the amount of auxin and cytokinin levels has shown itself in the morphology of the collar gall.
4. Conclusion: Biochemical analysis of plant extracts showed that the used extracts are rich in phenolic, flavonoid, antioxidant, and auxin compounds. According to the obtained results, it was found that the pure extracts of the mentioned medicinal plants were able to have an inhibitory effect on the growth of crown gall under greenhouse conditions. As mentioned, plants activate their defense system when they are exposed to external compounds. Non-significant difference in in-vitro conditions and significant difference in plant conditions (ex-vivo) related to several issues. Probably, the observed antioxidant and phytohormone contents and the stimulation of the plant's defense system (SAR) are the main reasons for the observed results.