Abstract
In cucumber seedlings, gravitropism interferes with hydrotropism, which results in the nearly complete inhibition of hydrotropism under stationary conditions. However, hydrotropic responses are induced when the gravitropic response in the root is nullified by clinorotation. Columella cells in the root cap sense gravity, which induces the gravitropic response. In this study, we found that removing the root tip induced hydrotropism in cucumber roots under stationary conditions. The application of auxin transport inhibitors to cucumber seedlings under stationary conditions suppressed the hydrotropic response induced by the removal of the root tip. To investigate the expression of genes related to hydrotropism in de-tipped cucumber roots, we conducted transcriptome analysis of gene expression by RNA-Seq using seedlings exhibiting hydrotropic and gravitropic responses. Of the 21 and 45 genes asymmetrically expressed during hydrotropic and gravitropic responses, respectively, five genes were identical. Gene ontology (GO) analysis indicated that the category auxin-inducible genes was significantly enriched among genes that were more highly expressed in the concave side of the root than the convex side during hydrotropic or gravitropic responses. Reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR) analysis revealed that root hydrotropism induced under stationary conditions (by removing the root tip) was accompanied by the asymmetric expression of several auxin-inducible genes. However, intact roots did not exhibit the asymmetric expression patterns of auxin-inducible genes under stationary conditions, even in the presence of a moisture gradient. These results suggest that the root tip inhibits hydrotropism by suppressing the induction of asymmetric auxin distribution. Auxin transport and distribution not mediated by the root tip might play a role in hydrotropism in cucumber roots.
Highlights
Plants are sessile organisms that have evolved tropisms to regulate the growth direction of their organs in response to environmental cues, such as gravity and moisture gradients, causing the organs to grow towards more suitable environmental conditions
Ablating root columella cells with a laser reduces gravitropism, but not hydrotropism, in Arabidopsis roots [17], and the surgical removal of a 0.2-mm-long segment of the root tip reduces gravitropism but not hydrotropism in rice roots [7]. These findings piqued our interest in the effects of surgically removing the cucumber root tip on root hydrotropism, because gravitropism interferes with hydrotropism in cucumber roots and because the root tip plays a crucial role in root gravitropism
We showed that auxin transport and redistribution are required for the induction of hydrotropism in cucumber roots, but these processes are independent of root-tip-mediated auxin transport
Summary
Plants are sessile organisms that have evolved tropisms to regulate the growth direction of their organs in response to environmental cues, such as gravity and moisture gradients, causing the organs to grow towards more suitable environmental conditions. The lateral roots were slanted downward towards the ground, and when the seedlings were rotated using a 3D clinostat, the lateral roots slanted towards the side containing water-absorbent plastic foam, indicating that the lateral roots of cucumber seedlings can exhibit hydrotropism and that the hydrotropism of the lateral roots is usually suppressed by gravity responses [4]. The 3D clinostat experiment suggested that the primary roots of cucumber seedlings can induce hydrotropism when they are placed perpendicularly, but not in parallel, to the direction of the moisture gradient [5]. These findings suggest that gravity responses suppress root hydrotropism in pea and cucumber seedlings. The strength of the hydrotropic response and its interaction with gravitropism differ among plant species
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