GSA-1/ ARG1 protects root gravitropism in Arabidopsis under ammonium stress.

Gravitropism plays a critical role in plant growth and development, plant stability and acclimation to changes in water and nutrient availability. Ammonium (NH₄⁺) is well known to have profound effects on root growth, but its impacts on gravitropism are poorly understood., To determine which genes are essential for the maintenance of root gravitropism under NH₄⁺ stress, we isolated and identified an NH₄⁺-sensitive mutant, gsa-1 ( gravitropism sensitive to ammonium- 1), in Arabidopsis thaliana, using an agar plate root reorientation assay., We found that, under NH₄⁺ stress, gsa-1 displayed increased loss of root gravitropism. Gene cloning and sequencing revealed that gsa-1 contains a G to C transversion mutation at the highly conserved 5′- GT splice position of intron 10 of ARG1 ( ALTERED RESPONSE TO GRAVITY 1), known to participate in the transduction of the root gravity signal. Genetic complement tests established the locus of GSA-1/ ARG1 and its role in resistance to NH₄⁺ inhibition on root gravitropism. GSA-1/ ARG1 is required for normal AUX1 expression and basipetal auxin transport in root apices. In addition, PIN-FORMED2 ( PIN2) is proposed as a target in the reduction of root gravitropism under NH₄⁺ stress, a response which can be antagonized by the GSA-1/ ARG1-dependent pathway., These results suggest that GSA-1/ ARG1 protects root gravitropism in Arabidopsis thaliana under ammonium stress. [ABSTRACT FROM AUTHOR]