Ano hydrotropic responseRoot Mutant that Responds Positively to Gravitropism in Arabidopsis1,

Abstract

For most plants survival depends upon the capacity of root tips to sense and move towards water and other nutrients in the soil. Because land plants cannot escape environmental stress they use developmental solutions to remodel themselves in order to better adapt to the new conditions. The primary site for perception of underground signals is the root cap (RC). Plant roots have positive hydrotropic response and modify their growth direction in search of water. Using a screening system with a water potential gradient, we isolated ano hydrotropic response (nhr) 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 nhr1roots 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 frequency of the waving pattern in nhr1 roots was increased compared to those of wild type. nhr1 seedlings had abnormal root cap morphogenesis and reduced root growth sensitivity to abscisic acid (ABA) and the polar auxin transport inhibitor N-(1-naphtyl)phtalamic acid (NPA). These results showed that hydrotropism is amenable to genetic analysis and that an ABA signaling pathway participates in sensing water potential gradients through the root cap.

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

“…We also established a two-layer assay (Fig. 1B), which may create an ion gradient (Eapen et al, 2003). In this method, a normal nutrient medium was in the top of a growth container and a salt stress medium was in the bottom of the container.…”

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

“…nhr1 is a semi-dominant mutation that seems to increase root growth sensitivity to abscisic acid (ABA), a plant hormone known to be involved in drought response (Ishitani et al, 1997). 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, two large-scale screens have been initiated in Arabidopsis. The first screen yielded no hydrotropic response 1 (nhr1 ) (Eapen et al, 2003), while a second screen has yielded 12 putative mutants termed root hydrotropism (rhy ) . Both screens made use of differing water potentials to find mutants that did not show a preference for higher water potential.…”

Where's the water? Hydrotropism in plants

“…Modern assays to study hydrotropism have been developed by two groups, who also used these methods as screens for isolating novel mutants of hydrotropism in Arabidopsis roots (23,24). Eapen et al (24) used a Petri dish with a normal nutrient agar medium at the top of the dish and an agar with a water-stressed medium in the bottom of the dish.…”

“…Eapen et al (24) used a Petri dish with a normal nutrient agar medium at the top of the dish and an agar with a water-stressed medium in the bottom of the dish. The water-stressed medium was made by adding glycerol and alginic acid as osmolytes to the agar.…”