Root border cell development is a temperature-insensitive and Al-sensitive process in barley

In vivo and in vitro experiments showed that border cell (BC) survival was dependent on root tip mucigel in barley (Hordeum vulgare L. cv. Hang 981). In aeroponic culture, BC development was an induced process in barley, whereas in hydroponic culture, it was a kinetic equilibrium process during which 300-400 BCs were released into water daily. The response of root elongation to temperatures (10-35 degrees C) was very sensitive but temperature changes had no great effect on barley BC development. At 35 degrees C, the root elongation ceased whereas BC production still continued, indicating that the two processes might be regulated independently under high temperature (35 degrees C) stress. Fifty microM Al could inhibit significantly BC development by inhibiting pectin methylesterase activity in the root cap of cv. 2000-2 (Al-sensitive) and cv. Humai 16 (Al-tolerant), but 20 microM Al could not block BC development in cv. Humai 16. BCs and their mucigel of barley had a limited role in the protection of Al-induced inhibition of root elongation, but played a significant role in the prevention of Al from diffusing into the meristems of the root tip and the root cap. Together, these results suggested that BC development was a temperature-insensitive but Al-sensitive process, and that BCs and their mucigel played an important role in the protection of root tip and root cap meristems from Al toxicity.