Accumulation of High Levels of ABA Regulates the Pleiotropic Response of the nhr1 Arabidopsis Mutant

Plants have evolved a variety of mechanisms for responding to environmental cues, which allows them to survive in the presence of limited resources or environmental stresses. One of the most significant growth adaptations plants have attained is tropism, a growth response that involves bending of plant organs toward or away from a stimulus. Roots exhibit hydrotropism in response to moisture gradients, which is thought to be critical in acquiring water and establishing their stand in the soil. However, the mechanism underlying hydrotropism remains unsolved. Here, we report that the no hydrotropic response (nhr1) mutant of Arabidopsis, which is impaired in hydrotropism, is tolerant to drought. The no hydrotropic response phenotype of nhr1 was repressed by AbamineSG, an inhibitor of abscisic acid (ABA) biosynthesis, indicating that ABA negatively regulates hydrotropism. Furthermore, the content of ABA was higher in nhr1 compared to those of wild type (wt). However, the higher ABA levels in nhr1 plants were not due to higher transcript levels of 9-cis-epoxycarotenoid dioxygenase (NCED3), since these were diminished compared to those of wt. Our results indicated that the root hydrotropic response of the nhr1 mutant is modulated by ABA and that the higher ABA levels of the mutant might confer it drought resistance.