Abscisic acid : interactions with auxin in the regulation of root growth under water deficit conditions
Water stress has been shown to inhibit shoot growth more than root growth (Sharp et al., 1988). Abscisic acid (ABA) has been shown to be involved in this differential growth response in maize (Zea mays L.) seedlings (Saab et al., 1990; Sharp et al., 1994). ABA accumulates in the elongation zone of the primary root at low water potentials, and this response is required for root growth maintenance under water deficit conditions (Saab et al., 1990; Sharp et al., 1994). Previous studies have shown that under severe water deficit in the ABA-deficient mutant viviparous 14 (vp14), plants produce high levels of endogenous ethylene (Spollen et al., 2000) and intracellular reactive oxygen species (ROS) in the root growth zone (Cho, 2006). Excess ethylene and ROS have been shown to have negative effects on plant growth and development. Therefore, it was hypothesized that these two factors were interconnected in this system. However, evidence presented in this study demonstrates that using a specific inhibitor of ethylene perception, 1-methylcyclopropane (1-MCP), neither ethylene nor ROS are the primary cause of root growth inhibition caused by ABA-deficiency, suggesting an alternate function of ABA in the regulation of root growth maintenance under water deficit conditions. The hypothesis was tested that an interaction of ABA with auxin could potentially play a key role in regulating maize root growth maintenance under water deficit conditions by using the ABA-deficient mutant vp14 to determine whether altered auxin levels are associated with primary root growth inhibition. The results show that auxin levels are decreased in the growth zone of ABA-deficient roots under water stress. To test whether the decrease in auxin is casually related to root growth inhibition, inhibitors of auxin transport and addition of auxin were administered during the growth of ABA-deficient roots at low water potentials. These treatments demonstrated that both inhibition of auxin transport and addition of auxin can completely restore endogenous auxin levels and root elongation in ABA-deficient water-stressed seedlings. These findings indicate that there is an important interaction between ABA and auxin in the regulation of maize primary root growth under water stress conditions. Further analysis with this system will lead to a greater understanding of the primary mechanisms involved in the regulation of root growth by ABA under water deficit conditions.