Detection of chitinolytic activity in the rhizosphere using image analysis

Rhizosphere colonization of cotton plants by Aeromonas caviae, an effective biocontrol agent of several soil-borne fungal plant pathogens, was studied. When intact cotton roots were placed in Petri dishes containing solid medium with chitin as a sole carbon source, clearing zones appeared around the roots as a result of root-associated chitinolytic activity. A method was developed for assessing the rhizosphere competence of this biocontrol agent by measuring chitinolytic activity along the intact roots or root segments using an image analyzer. Using this method, soil amendments with suspension of Aeromonas caviae was found to cause a significant increase in chitinolytic activity along the roots as compared with the untreated control. Dilution plate counts of root samples revealed that the biocontrol agent completely prevailed over the natural rhizosphere bacteria when applied by drench or by mixing with soil. A positive linear correlation was found between the number of chitinolytic bacteria present and the measured area of the clearing zone along the roots.