Growth retardation and differential regulation of expansin genes in chilling-stressed sweetpotato

We report here a first evaluation of chilling-responsive gene regulation in the sweetpotato. The growth of sweetpotato plants was severely retarded at 12°C; the lengths of the leaf, petiole, and root were markedly reduced and microscopic observation revealed that the elongation growth of the epidermal cells in each of these organs was significantly reduced. We examined the transcriptional regulation of three sweetpotato expansin genes (IbEXP1, IbEXP2 and IbEXPL1) in response to various chilling temperatures (12, 16, 22, and 28°C). In the leaf and petiole, the highest transcript levels were those of IbEXP1 at 28°C, whereas IbEXPL1 transcript levels were highest in the root. IbEXP1 mRNA levels in the 12°C-treated petiole showed a fluctuating pattern (transient decrease-recovery-stable decrease) for 48 h. In the leaf and petiole, IbEXP1 and IbEXPL1 exhibited a similar response to chilling in that their mRNA levels decreased at 22°C, increased at 16°C, and decreased dramatically at 12°C. In contrast, mRNA levels of IbEXP2 in the leaf fell gradually as the temperature fell from 28 to 12°C, while they remained unaltered in the petiole. In the root, mRNA levels of IbEXPL1 and IbEXP1 reached maximum levels at 16°C, and decreased significantly at 12°C. These data demonstrated that expression of these three expansin genes was ultimately down-regulated at 12°C; however, transcriptional regulation of each expansin gene exhibited its own distinctive pattern in response to various chilling temperatures.