Abstract
In plants, active transport of auxin plays an essential role in root development. Localization of the PIN1 auxin transporters to the basal membrane of cells directs auxin flow and depends on the trafficking mediator GNOM. GNOM-dependent auxin transport is vital for root development and thus offers a useful tool for the investigation of a possible tissue-specific response to dynamic auxin transport. To avoid pleiotropic effects, DEX-inducible expression of GNOM antisense RNA was used to disrupt GNOM expression transiently or persistently during embryonic root development. It was found that the elongation zone and the pericycle layer are the most sensitive to GNOM-dependent auxin transport variations, which is shown by the phenotypes in cell elongation and the initiation of lateral root primordia, respectively. This suggests that auxin dynamics is critical to cell differentiation and cell fate transition, but not to cell division. The results also reveal that GNOM-dependent auxin transport could affect local auxin biosynthesis. This suggests that local auxin biosynthesis may also contribute to the establishment of GNOM-dependent auxin gradients in specific tissues, and that auxin transport and local auxin biosynthesis may function together in the regulatory network for initiation and development of lateral root primordia. Thus, the data reveal a tissue-specific response to auxin transport and modulation of local auxin biosynthesis by auxin transport.
Highlights
In Arabidopsis and other plants with taproot systems, the root forms during embryogenesis and develops during seed germination; lateral roots are generated from the main root
An inducible antisense GNOM expression system for manipulating polar auxin transport To induce knock-down of GNOM expression in Arabidopsis roots, transgenic plants (InAGN) containing an antisense GNOM fragment were generated in the glucocorticoid-inducible expression vector pTA211 (Supplementary Fig. S1 available at JXB online), which allows induction of the antisense mRNA in the presence of DEX (Sanchez and Chua, 2001)
When germinated on half-strength Murashige and Skoog (MS) medium supplemented with 30 μM DEX or 0.1% EtOH for 7 d, all antisense GNOM lines displayed a reduction of primary root length compared with the control lines and mock-treated antisense GNOM lines, both of which showed no sign of growth repression (Fig. 1A)
Summary
In Arabidopsis and other plants with taproot systems, the root forms during embryogenesis and develops during seed germination; lateral roots are generated from the main root. One of the key players regulating these processes is the plant hormone auxin, whose homeostasis is established and maintained by both polar transport (Benkova et al, 2003) and local biosynthesis (Zhao et al, 2001; Cheng et al, 2006, 2007; Stepanova et al, 2008; Tao et al, 2008). Active polar auxin transport moves auxin in the root tip. Experimental evidence and mathematical modelling have shown that auxin at the tip of the primary root is transported by PINFORMED (PIN) auxin efflux proteins, maintaining post-embryonic primary root growth (Blilou et al, 2005; Grieneisen et al, 2007).
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