Minisymposium 10: Tropisms

Abs # 23002: Genetic approach for studying hydrotropism in Arabidopsis

Presenter:	Cassab, Gladys I, gladys@ibt.unam.mx
Authors	Cassab, Gladys I (A)   Eapen, Delfeena  (A)   Barroso, María L (A)   Campos, María E (A)   Ponce, Georgina  (A)   Saucedo, Manuel  (A)
Affiliations:	(A): Department of Plant Molecular Biology, Institute of Biotechnology, National University of Mexico

Roots are capable to construct a three-dimensional perspective of their local space by sensing and responding to a variety of stimuli. By doing this, roots actively forage resources from their environment. Roots have positive hydrotropic response and modify their growth direction in search of water overcoming their positive gravitropic response. Using a screening system with a water potential gradient, we isolated a no hydrotropic response (nhr1) semi-dominant mutant of Arabidopsis that continued to grow downwardly into the medium with the lowest water potential contrary to the positive hydrotropic and negative gravitropic response seen in wild type-roots. The lack of hydrotropic response of nhr1 roots was confirmed in a system with a gradient in air moisture. The root gravitropic response of nhr1 seedlings was significantly faster in comparison with those of wild type. The enhanced gravitropic of nhr1 roots might suggest that the reduction in the strength of hydrotropism increases the net effectiveness of this growth response. Recently, it has been reported that hydrotropism interacts with gravitropism by degrading amyloplasts in roots of Arabidopiss (Takahashi et al., 2003). That is, the hydrotropic response of wild type Arabidopsis roots in the screening system occurred with a concurrent drop in both starch content in columella cells and in gravitropic downward growth response. However, hydrotropicaly stimulated nhr1 roots contained very large amyloplasts that persisted throughout the 8-day treatment. Amyloplasts constitute the susceptors for gravity perception and because nhr1 roots maintain their large amyloplasts during hydrostimulation, their perception and gravitropic response is not affected as in wild type roots. Furthermore, nhr1 roots had abnormal root cap morphogenesis and displayed decreases in DR5-GUS activity (“auxin maximum”), which may be required for determining cell fate in the root cap. The growth of nhr1 roots was less sensitive to the polar auxin transport inhibitor NPA. It remains to be determined whether the altered auxin gradient regulation in nhr1 roots inhibits degradation of starch upon hydrostimulation; or alternatively, whether the lack of sensing water gradients of nhr1 root caps prevents starch degradation and consequently affects auxin efflux transport regulation. Takahashi et al., 2003, Plant Physiol 132, 805