Hydrotropism Interacts with Gravitropism by Degrading Amyloplasts in Seedling Roots of Arabidopsis and Radish

Abstract

In response to a moisture gradient, roots exhibit hydrotropism to control the orientation of their growth. To exhibit hydrotropism, however, they must overcome the gravitropism that is dominant on Earth. We found that moisture gradient or water stress caused immediate degradation of the starch anchors, amyloplasts, in root columella cells of Arabidopsis and radish (Raphanus sativus). Namely, development of hydrotropic response was accompanied by a simultaneous reduction in starch content in columella cells. Rapid degradation of amyloplasts in columella cells also occurred in the water-stressed roots with sorbitol or mannitol. Both hydrotropically stimulated and water-stressed roots showed a reduced responsiveness to gravity. Roots of a starchless mutant, pgm1-1, showed an enhanced hydrotropism compared with that of the wild type. These results suggest that the reduced responsiveness to gravity is, at least in part, attributable to the degradation of amyloplasts in columella cells. Thus, the reduction in gravitropism allows the roots to exhibit hydrotropism.

Salt Modulates Gravity Signaling Pathway to Regulate Growth Direction of Primary Roots in Arabidopsis

“…4, A and B). This is consistent with the proposed role of columella cell amyloplasts in gravisensing (Takahashi et al, 2003;Ma and Hasenstein, 2005). However, our results indicate that salt-induced rapid degradation of amyloplasts, the proposed gravisensor, in the columella cells is not likely the main reason for a negative gravitropic response, because sos mutant roots exhibited profound negative gravitropic growth without apparent rapid digestion of amyloplasts (Fig.…”

“…Observation and measurement of the amyloplasts in the columella cells of the root cap were made according to the method described by Takahashi et al (2003).…”

“…Increased root length with strong root growth resulting from hydrotropism is one of the most important traits of drought-tolerant crops and has been used in breeding for crops with higher drought tolerance. A recent study showed that a moisture gradient generated by water stress caused immediate degradation of the amyloplasts in root columella cells of plants, resulting in reduced responsiveness to gravity and subsequent hydrotropism (Takahashi et al, 2003). However, how a gravity signal interacts with other environmental cues to modulate the direction of root growth is still largely unknown.…”

“…For example, roots show hydrotropism in response to moisture gradients, which represents an example of cross talk between the gravity signal and another environmental cue. Hydrotropism plays a very important role in plant growth and development to tolerate drought stress (Jaffe et al, 1985;Takahashi, 1997;Mizuno et al, 2002;Takahashi et al, 2003). Perception and subsequent response to interaction between gravitropism and hydrotropism may enable plants to direct root growth in the direction that maximizes the acquisition of water, thereby providing a fitness advantage during root development under drought conditions.…”

Plant tropisms: providing the power of movement to a sessile organism

“…Given the embryonic arrest of homozygous nhr1 mutants, it is difficult to assign a potential function to nhr1, although the authors argue for a role in cell proliferation (Eapen et al, 2003). The rhy mutants all show varying degrees of loss of hydrotropic response, but most do not have drastically altered responses to other tropic stimulations, although rhy4 does seem to have a slight reduction in phototropic response, perhaps signaling an area of overlap between these two tropic responses Takahashi et al, 2003). To date, none of the twelve rhy mutants has been cloned.…”