Ammonium-induced loss of root gravitropism is related to auxin distribution and TRH1 function, and is uncoupled from the inhibition of root elongation in Arabidopsis

Root gravitropism is affected by many environmental stresses, including salinity, drought, and nutrient deficiency. One significant environmental stress, excess ammonium (NH4+), is well documented to inhibit root elongation and lateral root formation, yet little is known about its effects on the direction of root growth. We show here that inhibition of root elongation upon elevation of external NH4+ is accompanied by a loss in root gravitropism (agravitropism) in Arabidopsis. Addition of potassium (K+) to the treatment medium partially rescued the inhibition of root elongation by high NH4+ but did not improve gravitropic root curvature. Expression analysis of the auxin-responsive reporter gene DR5::GUS revealed that NH4+ treatment delayed the development of gravity-induced auxin gradients across the root cap but extended their duration once initiated. Moreover, the beta-glucuronidase (GUS) signal intensity in root tip cells was significantly reduced under high NH4+ treatment over time. The potassium carrier mutant trh1 displayed different patterns of root gravitropism and DR5::GUS signal intensity in root apex cells compared with the wild type in response to NH4+. Together, the results demonstrate that the effects of NH4+ on root gravitropism are related to delayed lateral auxin redistribution and the TRH1 pathway, and are largely independent of inhibitory effects on root elongation.