Abstract
BackgroundAlthough cytokinins have been known for decades to play important roles in the regulation of plant growth and development, our knowledge of the regulatory mechanism of endogenous content of specific cytokinins remains limited.Methodology/Principal FindingsHere, we characterized two SOB five-like (SOFL) genes, AtSOFL1 and AtSOFL2, in Arabidopsis (Arabidopsis thaliana) and showed that they acted redundantly in regulating specific cytokinin levels. Analysis of the translational fusion AtSOFL1:AtSOFL1-GUS and AtSOFL2:AtSOFL2-GUS indicated that AtSOFL1 and AtSOFL2 exhibited similar expression patterns. Both proteins were predominantly expressed in the vascular tissues of developing leaves, flowers and siliques, but barely detectable in roots and stems. Overexpression of either AtSOFL1 or AtSOFL2 led to increased cytokinin content and obvious corresponding mutant phenotypes for both transgenic seedlings and adult plants. In addition, overexpression and site-directed mutagenesis experiments demonstrated that the SOFL domains are necessary for AtSOFL2's overexpression phenotypes. Silencing or disrupting either AtSOFL1 or AtSOFL2 caused no obvious developmental defects. Endogenous cytokinin analysis, however, revealed that compared to the wild type control, the SOFL1-RNAi62 sofl2-1 double mutant accumulated lower levels of trans-zeatin riboside monophosphate (tZRMP) and N6-(Δ2-isopentenyl)adenosine monophosphate (iPRMP), which are biosynthetic intermediates of bioactive cytokinins. The double mutant also displayed decreased response to exogenous cytokinin in both callus-formation and inhibition-of-hypocotyl-elongation assays.Conclusions/SignificanceTaken together, our data suggest that in plants AtSOFL1 and AtSOFL2 work redundantly as positive modulators in the fine-tuning of specific cytokinin levels as well as responsiveness.
Highlights
Cytokinins are classic phytohormones that play important roles in the regulation of plant growth and development [1,2,3,4]
Overexpression of Suppressor of phyB-4 #5 (SOB5) in Arabidopsis confers a dwarf phenotype [27].To investigate whether the two Arabidopsis SOB five-like (SOFL) genes, AtSOFL1 (At1g26210) and AtSOFL2 (At1g68870), have similar functions, each was overexpressed under the control of the cauliflower mosaic virus (CaMV) 35S promoter
When the AtSOFL1, AtSOFL2 and SOB5 genes were overexpressed at similar levels, 35S:AtSOFL1, 35S:AtSOFL2 transgenic plants still maintained some degree of apical dominance and were obviously much taller than sob5-D, which almost totally lost its apical dominance (Figure 1A, D) [27]. sob5-D plants have curled-down adult leaves [27], which were rarely observed for 35S:AtSOFL1 and 35S:AtSOFL2 transgenic plants (Figure 1A)
Summary
Cytokinins are classic phytohormones that play important roles in the regulation of plant growth and development [1,2,3,4]. IPTs are encoded by a small gene family in plants, with nine and eight members in Arabidopsis and rice (Oryza sativa), respectively [7,8,9]. Another cytokinin biosynthetic gene in petunia (Petunia hybrida), Shooting (Sho), was isolated in an activation-tagging mutant screen [10]. Cytokinins have been known for decades to play important roles in the regulation of plant growth and development, our knowledge of the regulatory mechanism of endogenous content of specific cytokinins remains limited
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