Molecular mechanisms responsible for interaction or differentiation between hydrotropism and gravitropism in roots
Abstract
Roots display hydrotropism in response to moisture gradient, but it is often interfered by gravitropic response on Earth. We demonstrated that roots of cucumber seedlings showed positive hydrotropism when exposed to moisture gradient and rotated on a two-axis clinostat. Under stationary conditions, however, gravitropic response overcame hydrotropic response. Using this experimental system, we examined the role of auxin in hydrotropism. Cucumber roots showed severely reduced hydrotropic response when treated with inhibitors of auxin transport (efflux) or auxin action. mRNA accumulation of auxin-inducible gene, CsIAA1, became more abundant in the concave side of the hydrotropically responding roots, compared with that of the convex side. To understand the auxin dynamics in cucumber roots, we isolated cDNAs of auxin efflux carriers, CsPINs, and examined the localization of their mRNAs and proteins. Of these CsPINs, CsPIN5 was localized peripherally in the region between lateral root cap and elongation zone of cucumber roots. In hydrotropically responding roots, CsPIN5 proteins decreased in the convex side while it was maintained in the concave side. These results suggest that auxin dynamics and action play important roles in inducing hydrotropism, similarly to the case of gravitropism in roots. In cucumber roots, therefore, hydrotropism interacts with gravitropism, possibly by competitive manner in auxin dynamics. We are currently preparing spaceflight experiment for separating the hydrotropic response mechanism from that of gravitropism to understand the regulatory mechanisms of root growth orientation and determine whether hydrotropic response can be used for controlling growth orientation of roots in microgravity. On the other hand, we identified MIZ1 gene essential for hydrotropism but not gravitropism in Arabidopsis roots. Thus, there exist molecular mechanisms shared and differed in the two tropisms.
- Publication:
-
37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.3125T