Mechanisms of action of aloe polysaccharides and xanthan gum for control of black rot in cauliflower

Black rot is the main bacterial disease of crucifers. The establishment of this disease in the field can result in significant yield losses. The objective of this study was to evaluate the potential of xanthan gum (GUM) and polysaccharides extracted from Aloe barbadensis (aloe polysaccharides-AP) for controlling black rot and eliciting defense mechanisms as well as revealing changes in the physiological behavior of cauliflower. Cauliflower plants were sprayed with distilled water, AP (0.75-6.0 mg mL(-1)) or GUM (0.25-1.5 mg mL(-1)), inoculated with Xanthomonas campestris pv. campestris 4 days later and evaluated for disease severity at 14 days after inoculation. In vitro bacterial growth in a culture medium containing AP or GUM (0.0 to 3.0 mg mL(-1)) was evaluated for checking the antimicrobial activity of the polymers. Defense mechanisms (hypersensitivity reaction-HR, enzyme activities, content of phenolic compounds and flavonoids) and physiological changes (photosynthetic rate, stomatal conductance and transpiration) were quantified from cauliflower plants treated with distilled water, AP (1.5 mg mL(-1)) or GUM (0.5 mg mL(-1)), inoculated or not with X. campestris. On average, AP reduced bacterial blight symptoms by 68.1% compared to the control. At 0.5 mg mL(-1), GUM controlled 74.65% of the disease; however, it caused high levels of phytotoxicity on the leaf surface at 1.5 mg mL(-1). There was no direct effect of polysaccharides on the in vitro growth of X campestris. Peroxidase activity was increased significantly at 2 and 4 days after GUM application, while AP did not change the activity of this enzyme. There were no cells with HR, and no changes in polyphenol oxidase activity, phenolic compound content, flavonoid content or in the antioxidant activity in plants treated with polysaccharides. The photosynthetic rate in plants sprayed with GUM or AP was 22.55% and 39.10% lower, respectively, than the rate of plants treated with distilled water. On average, the polymers reduced conductance by 54.8%. A similar behavior was observed in the transpiration of the plants. Although GUM decreased black rot in cauliflower, it caused signs of stress and phytotoxicity on leaves. By contrast, the application of Aloe barbadensis polysaccharides can be considered as an effective alternative for controlling black rot. This paper also discusses how these polysaccharides reduced the severity of the disease. (C) 2016 Elsevier B.V. All rights reserved.