Poster: Tropisms

Abs # 415: Gravitropism in akt1-1 roots lacks a significant DEZ contribution

Presenter:	Wolverton, Chris
Authors	Polter, Abigail M. (A)   Wibley, Amanda L. (A)   Mullen, Jack L. (B)   Wolverton, Chris  (A)
Affiliations:	(A): Dept. of Botany-Microbiology, Ohio Wesleyan University (B): Dept. of Biology, Indiana University

Plant roots are responsive to a number of environmental stimuli, including light and gravity. Upon reorientation in the gravity field, primary roots begin differential growth within 10 – 20 min in the distal elongation zone (DEZ). Differential growth in the DEZ is composed primarily of an increased rate of cell expansion on the upper flank. The plant hormone IAA plays a role in root gravitropism, but it is thought to act by inhibiting cell expansion on the lower flank of the central elongation zone, leaving open the question of DEZ regulation. Previous work has shown that DEZ cells are stimulated to elongate by the application of an exogenous electric field, raising the possibility that ionic signaling participates in DEZ regulation. To test this, we analyzed gravitropism in roots of an Arabidopsis potassium channel mutant, akt1-1, using high-resolution image analysis. We found that roots of akt1-1 have a greatly reduced gravitropic response, taking over 70 min to initiate curvature compared to 20 min for wild-type. In wild-type roots, the DEZ contributes the majority of curvature in the 1 h following the latent period. In akt1-1 roots, the DEZ contribution was almost completely lacking over the same period. Conversely, akt1-1 roots appear to have an intact phototropic response. Thus, the akt1-1 mutation appears to specifically disrupt the participation of the DEZ in gravitropism. We tested whether this disruption in gravitropism was due to an inability of the mutant to sense or respond to IAA, and found that mutant roots have a wild-type response to applied IAA. Future work aimed at the interaction between potassium uptake and auxin responses may shed light on the regulation of the DEZ.