Objective Factors associated with the hyperplasia of hairy roots on carrots were investigated.

Method In a pot experiment, growth of hairy roots on the commonly known as “Sakata 7-inch carrots” of cultivar SK4-316 were compared before (CK) and after (T) the potting soil was sterilized. Fully extended leaves per plant were counted, plant height and carrot length measured with 0.1 mm precision, and culturable microbes isolated using the dilution-plate method with bacterial 16S rRNA genes and fungal ITS regions sequenced on a high throughput platform to characterize microbial community diversity. Factors driving excessive hairy root growth were analyzed.

Result After 90 d of cultivation under controlled conditions, significant morphological differentiations on the plants under CK and T were observed. Proliferated growth of hairy roots on the irregularly shaped, twisted or deformed carrots and etiolated stems and leaves on the aboveground plants were found with the CK group. In contrast, the T group bore fewer and finer hairy roots on the carrots and the plant development appeared normal. The plants in the CK group were significantly taller with more leaves and the carrots shorter in length than those in the T group (P＜0.05). The microbial community in the non-sterilized CK soil was rich and significantly more diverse than in the T counterparts, as sterilization invariably altered the structure and population. In total, 45 strains of bacteria and 2 of fungi were isolated from the soil. Among them, three Streptomyces species were found only in the CK soil. In addition, the high throughput sequencing showed a significantly greater relative abundance of 1.0272% on the CK soil as opposed to that of 0.0254% in the T soil.

Conclusion Sterilization invariably altered the microbial community in soil. By keeping climatic conditions, soil properties, nutrient contents, and pesticide application constant, the results obtained in this study appeared to support the preliminary conclusion that the hairy root hyperplasia on carrots, as well as the differentiated aboveground plant development, of SK4-316 to be directly associated with the presence of rhizosphere microbes. Streptomyces tanashiensis (CK4), S. rameus (CK10), and S. amritsarensis (CK22) were postulated to be the fungi possibly involved in the underline mechanism.