Abstract
Root elongation depends on the action of the gibberellin (GA) growth hormones, which promote cell production in the root meristem and cell expansion in the elongation zone. Sites of GA biosynthesis in the roots of 7-d-old Arabidopsis thaliana seedlings were investigated using tissue-specific GA inactivation in wild-type (Col-0) or rescue of GA-deficient dwarf mutants. Tissue-specific GA depletion was achieved by ectopic expression of the GA-inactivating enzyme AtGA2ox2, which is specific for C19 -GAs, and AtGA2ox7, which acts on C20 -GA precursors. In addition, tissue-specific rescue of ga20ox triple and ga3ox double mutants was shown. Furthermore, GUS reporter lines for major GA20ox, GA3ox and GA2ox genes were used to observe their expression domains in the root. The effects of expressing these constructs on the lengths of the root apical meristem and cortical cells in the elongation zone confirmed that roots are autonomous for GA biosynthesis, which occurs in multiple tissues, with the endodermis a major site of synthesis. The results are consistent with the early stages of GA biosynthesis within the root occurring in the meristematic region and indicate that the penultimate step of GA biosynthesis, GA 20-oxidation, is required in both the meristem and elongation zone.
Highlights
The action of the plant hormone gibberellin (GA) is necessary for normal root growth, lower GA concentrations are required to achieve maximal rates of root elongation than for the shoot (Tanimoto, 1994, 2012)
Tissue-specific suppression of GA signal transduction has demonstrated that this hormone determines both the size of the root apical meristem (RAM), that is the number of divisions before the cells exit into the transition zone (Achard et al, 2009; Ubeda-Tomas et al, 2009) and the final cell length achieved within the elongation zone (EZ) (Ubeda-Tomas et al, 2008)
GUS staining for the AtGA20ox1 reporter indicated expression in both the meristem and EZ with less expression in the transition zone (Fig. 2a)
Summary
The action of the plant hormone gibberellin (GA) is necessary for normal root growth, lower GA concentrations are required to achieve maximal rates of root elongation than for the shoot (Tanimoto, 1994, 2012). The RAM is the source of new cells within the root and plays a critical role in defining the lineages of the cell files that result from meristematic divisions It is located at the distal tip of the root, next to a nondividing group of cells known as the quiescent centre (QC) (Nawy et al, 2005; Dinneny & Benfey, 2008). Tissue-specific suppression of GA signal transduction has demonstrated that this hormone determines both the size of the RAM, that is the number of divisions before the cells exit into the transition zone (Achard et al, 2009; Ubeda-Tomas et al, 2009) and the final cell length achieved within the EZ (Ubeda-Tomas et al, 2008) In both cases, GA acts in the endodermis to allow the coordinated growth of the root cell files (Ubeda-Tomas et al, 2008, 2009)
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