Gibberellin inhibition of taproot formation by modulation of DELLA-NAC complex activity in turnip (Brassica rapa var. rapa).

Turnip is a member of the Brassica rapa species and is characterized by a swollen taproot that develops from the hypocotyl and part of the root. Gibberellins (GAs) are plant growth regulators involved in promoting cell elongation and play important roles in many aspects of plant growth and development. Interestingly, exogenous application of GA was found to significantly inhibit taproot formation in turnip. Moreover, endogenous GA contents decreased during the early developmental stages of taproot formation, suggesting that GA plays a negative role in taproot formation. We examined the anatomical structure of the taproot and found that lignification of the xylem cell wall was enhanced after treatment with GA. Yeast two-hybrid assays suggested the occurrence of protein interactions between DELLAs and NACs in turnip. We also found that the expression of NAC-targeted genes involved in lignification of the secondary cell wall was significantly upregulated upon GA treatment. Taken together, these results supported the hypothesis that GA induced DELLA proteins degradation to release NAC proteins and induced xylem lignification, therefore inhibiting taproot formation, providing new insight into the molecular mechanism underlying turnip taproot formation.