Light- and temperature-regulated BjAPY2 may have a role in stem expansion of Brassica juncea.

Tuber mustard (Brassica juncea (L.) Czern. et Coss. var. tumida Tsen et Lee) is an important vegetable crop with a characteristic of expanded stem that is edible. The underlying molecular mechanism of the stem expansion is not well understood. Here, we reported that a total of 51 differentially expressed fragments (DEFs) with three expression patterns during stem expansion of tuber mustard were identified by cDNA-AFLP analysis. Among the DEFs, DEF11 with high homology to Arabidopsis thaliana apyrase 2 (AtAPY2) that encodes an enzyme with ATPase and ADPase activity was development- and tissue-specific. DEF11 was thus renamed as BjAPY2. The expression levels of BjAPY2 increased with the stem expression and were the highest at stage IV, a developmental stage at which the stem expanded most rapidly. In contrast, the BjAPY2 expression levels in leaves were much lower and remained unchanged during leaf development and expansion, suggesting that BjAPY2 was closely associated with the expansion of stems but not of leaves in the tuber mustard. Interestingly, the expression of BjAPY2 was higher in the mustard under short-day (SD) photoperiod (8 h/16 h) than that under long-day (LD) photoperiod (16 h/8 h); similarly, the transcript levels of BjAPY2 were higher in the mustard grown at low temperature (14 °C/12 °C) than that at high temperature (26 °C /24 °C). The SD photoperiod and low temperature were two environmental conditions that favored the mustard stem expansion. Further cloning and analysis of the promoter region of BjAPY2 revealed that there were indeed several types of motifs in the promoter region, including the light and temperature responsive elements. These results suggested that BjAPY2 might play an important role during the stem expansion of the tuber mustard.