Polyamine regulation of growth and chilling tolerance of rice ( Oryza sativa L.) roots cultured in vitro

Abstract

Seminal rice ( Oryza sativa L. cv. Taichung Native 1) roots were grown in vitro to investigate the relationships among polyamine biosynthesis, root growth and chilling tolerance. At 25°C, the level of free putrescine and the activities of arginine decarboxylase (ADC; EC 4.1.1.19) and ornithine decarboxylase (ODC; EC 4.1.1.17) increased as growth progressed while the levels of free spermidine/spermine and the activity of S-adenosylmethionine decarboxylase (SAMDC, EC 4.1.1.50) decreased. Exogenously applied putrescine, ranging from 0.01 to 1 mM, enhanced the elongation of roots grown at 25°C whereas application of spermidine or spermine inhibited root elongation. α-Difluoromethylarginine (DFMA) at 5 μM or α-difluoromethylornithine (DFMO) at 10 μM inhibited the increase in root length and the levels of free putrescine at 25°C; these effects were reversed by the addition of 1 mM putrescine. Roots exposed to 5°C ceased growth and lost their re-growth ability after 9 days of chilling. The level of free putrescine and the activity of ADC in chilled roots increased with a peak at day 3 and then decreased to a plateau after 9 days. The levels of free spermidine and spermine increased after 9 days. When putrescine was applied at concentrations greater than 0.1 mM, chilled roots partially recovered their re-growth ability. Contrary to DFMO (10 μM), DFMA (5 μM) inhibited both the chilling-induced free putrescine increase and the re-growth ability of chilled roots, and these effects were reversed by 1 mM putrescine. These results suggest that polyamines are related to growth of rice roots cultured in vitro. The relationships between putrescine and chilling tolerance is discussed.