Objective This study aims to investigate the relationship between soil microbial communities and the abnormal hairy root growth of the carrot cultivar 'SK4-316' (also known as “Sakata 7-inch”), identify the key factors contributing to this phenomenon and establish a basis for further research into its pathogenesis.

Method Pot experiments were conducted to compare SK4-316 carrot fibrous root hyperplasia between soil sterilization treatment (T) and unsterilized control (CK). The number of fully expanded leaves per plant was counted, and plant height and primary root length were measured to 0.1 cm precision using a ruler. Cultivable microorganisms were isolated by the dilution plating method. High-throughput sequencing was performed to analyze soil bacterial 16S rRNA genes and fungal ITS region sequences, characterizing microbial community diversity and thereby revealing key factors driving carrot fibrous root hyperplasia.

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.