Abstract
Plant root development is mediated by the concerted action of the auxin and cytokinin phytohormones, with cytokinin serving as an antagonist of auxin transport. Here, we identify the AUXIN UP-REGULATED F-BOX PROTEIN1 (AUF1) and its potential paralog AUF2 as important positive modifiers of root elongation that tether auxin movements to cytokinin signaling in Arabidopsis (Arabidopsis thaliana). The AUF1 mRNA level in roots is strongly up-regulated by auxin but not by other phytohormones. Whereas the auf1 single and auf1 auf2 double mutant roots grow normally without exogenous auxin and respond similarly to the wild type upon auxin application, their growth is hypersensitive to auxin transport inhibitors, with the mutant roots also having reduced basipetal and acropetal auxin transport. The effects of auf1 on auxin movements may be mediated in part by the misexpression of several PIN-FORMED (PIN) auxin efflux proteins, which for PIN2 reduces its abundance on the plasma membrane of root cells. auf1 roots are also hypersensitive to cytokinin and have increased expression of several components of cytokinin signaling. Kinematic analyses of root growth and localization of the cyclin B mitotic marker showed that AUF1 does not affect root cell division but promotes cytokinin-mediated cell expansion in the elongation/differentiation zone. Epistasis analyses implicate the cytokinin regulator ARR1 or its effector(s) as the target of the SKP1-Cullin1-F Box (SCF) ubiquitin ligases assembled with AUF1/2. Given the wide distribution of AUF1/2-type proteins among land plants, we propose that SCF(AUF1/2) provides additional cross talk between auxin and cytokinin, which modifies auxin distribution and ultimately root elongation.
Highlights
Plant root development is mediated by the concerted action of the auxin and cytokinin phytohormones, with cytokinin serving as an antagonist of auxin transport
During our attempts to define the functions of the nearly 900 FBX loci in Arabidopsis by various “omic” approaches (Gagne et al, 2002; Hua et al, 2011), we noticed that the expression of one FBX gene designated AUXIN UP-REGULATED F-BOX PROTEIN1 (AUF1) (At1g78100 in the C1 subclade) was shown in the Genevestigator and eFP DNA microarray data browsers to be strongly up-regulated by the natural auxin IAA but not by several other phytohormones or their precursors
When ARR5/15 transcript accumulation was tested in the auf backgrounds by semiquantitative reverse transcription (RT)-PCR and qRT-PCR, we found that the levels of both were substantially upregulated by 0.5 mM kinetin in auf1-1 and auf1-2 plants as compared with wild-type and auf2-1 plants (Fig. 6, C and D)
Summary
Plant root development is mediated by the concerted action of the auxin and cytokinin phytohormones, with cytokinin serving as an antagonist of auxin transport. AUF1 F-Box Protein Controls Root Growth of auxin influx proteins (Marchant et al, 1999), the PINFORMED (PIN) family of auxin efflux proteins (Chen et al, 1998; Galweiler et al, 1998; Friml et al, 2002; Blilou et al, 2005; Wisniewska et al, 2006), and the P-glycoprotein/multidrug resistance B family of ATP-binding cassette transporter B-type (ABCB) proteins (Noh et al, 2001; Lewis et al, 2007; Wu et al, 2007) Interference with these transport facilitators induces dramatic defects in shoot and root architecture
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