Plant growth-promoting ability and N-acyl-homoserine lactones production by siderophore-producing rhizobacteria

Eighty bacterial isolates were obtained from the rhizosphere of sugarcane, corn, chili and watercress. Approximately 18% (15 of 80) of the tested isolates produced sideroderphore units in the range 31.8-79.9% when tested using chrome azurol sulphonate assay. The siderophore-producing bacteria were also tested for their ability to produce plant growth-promoting factors including auxin, gibberellic acid and the ability to solubilize phosphate. All of these isolates were able to solubilize tricalcium phosphate (Ca3(PO4)2) in the range 56.8-270.3 μg.mL^-1 and were able to produce auxin and gibberellin in the range 2.4-22.9 μg.mL^-1 and 185.5-246.4 μg.mL^-1, respectively. Isolates Su04, Su09, and Wa65 produced the greatest amount of siderophore units. Sequence analysis of 16S rRNA gene from the three best bacterial isolates (Su04, Su09 and Wa65) indicated that the strains were Burkholder cepacia, Pseudomonas boreopolis and Agrobacterium tumefaciens, with 99, 98 and 98% sequence similarity, respectively. Fifteen siderophore-producing bacteria were able to produce N-acyl-homoserine lactones (AHLs) in the range 62.9-660.9 Miller units, with isolate Su04 producing the greatest amount of AHLs (660.9 Miller units). AHLs production by isolates Su04, Wa63 and Wa65 rapidly increased when the culture was grown to the late log phase, after about 18 hr of incubation, and increased until the late stationary phase after about 36 hr. The study identified a series of siderophore-producing rhizobacteria able to solubilize phosphate and produce plant growth-promoting factors and AHLs which have potential for application as plant growth-promoting agents in agriculture.