Characterization of drought and freezing tolerance of three representative cultivars of pak choi (Brassica rapa ssp. chinensis)

Abstract

Pak choi (Brassica rapa ssp. chinensis) is one of the most important cultivated vegetables in China, and its growth and development are often affected by drought and freezing stresses which often lead to yield reduction. However, the mechanisms by which pak choi responds to drought and freezing stresses are still largely unknown. In this study, we compared the tolerance of three representative Chinese cultivars of pak choi, Suzhouqing (BrL.1), Huangxinwucai (BrL.2) and Heixinwucai (BrL.3), to drought and freezing stresses. We found that BrL.3 showed increased tolerance to both drought and freezing stresses when compared with BrL.1 and BrL.2. Under drought stress, BrL.3 accumulated a higher level of proline, but not soluble sugar, by enhancing the expression of BrP5CS1 and BrP5CS2 genes and thus the activity of P5CS enzyme in proline synthesis pathway. Consistent with this result, exogenous addition of proline can almost diminish the phenotypic difference between BrL.3 and BrL.1/2 under drought stress, suggesting that enhanced drought tolerance in BrL.3 is associated with increased proline accumulation. In addition, under cold stress, there was no significant difference in the contents of proline and soluble sugar among BrL.1, BrL.2 and BrL.3. However, the expression of BrCBF1 gene, which encodes a key transcriptional factor in the cold stress pathway, and its downstream gene BrCOR15a was significantly increased in BrL.3. Further study showed that the heterologous overexpression of BrCBF1 gene in Arabidopsis could enhance freezing tolerance, suggesting that enhanced freezing tolerance in BrL.3 might be related to increased expression of the BrCBF1 gene. Taken together, these results indicate that the enhanced tolerance of BrL.3 to drought and freezing stresses is associated with enhanced proline accumulation and increased BrCBF1 expression, respectively.