Abstract
Directional transport of auxin is essential for plant development, with PIN auxin transport proteins representing an integral part of the machinery that controls hormone distribution. However, unlike the rapidly emerging framework of molecular determinants regulating PIN protein abundance and subcellular localization, insights into mechanisms controlling PIN transcription are still limited. Here we describe PIN2 PROMOTER BINDING PROTEIN 1 (PPP1), an evolutionary conserved plant-specific DNA binding protein that acts on transcription of PIN genes. Consistent with PPP1 DNA-binding activity, PPP1 reporter proteins are nuclear localized and analysis of PPP1 null alleles and knockdown lines indicated a function as a positive regulator of PIN expression. Furthermore, we show that ppp1 pleiotropic mutant phenotypes are partially reverted by PIN overexpression, and results are presented that underline a role of PPP1-PIN promoter interaction in PIN expression control. Collectively, our findings identify an elementary, thus far unknown, plant-specific DNA-binding protein required for post-embryonic plant development, in general, and correct expression of PIN genes, in particular.
Highlights
T Directional transport of auxin is essential for plant development, with PIN auxin transport proteins representing an integral part of the machinery that controls hormone distribution
-1002 with respect to the PIN2 ATG start codon, we identified three independent cDNA clones, each corresponding to locus At5g08720
We found irregular distribution of PIN2 reporter signals, frequently resulting in stochastic formation of PIN2 reporter signal gradients that appeared uncoupled from the direction of the gravity vector (Fig. 3e)
Summary
T Identification of a PIN2 promoter interacting protein. A few regulators of gene expression have so far been demonstrated to influence PIN transcription via PIN promoter binding. We observed PPP1-dependent activation of yeast reporter genes, expressed by minimal promoters lacking this motif, overall arguing for limited PPP1 DNA-binding specificity At this moment, we can only speculate about the biological significance of these observations, since cis-acting requirements for TPPP1 DNA-binding and transcriptional control are not entirely explained. Only incomplete conservation of DNA motifs appears to be required for PPP1 binding, mutations within this PIN2 promoter fragment resulted in stochastic variations or even a total loss of gene expression, at a frequency considerably higher than in controls. Patchy PIN2-VENUS signals that we observed are a characteristic feature of reporter genes that underwent somatic gene silencing events This might argue for an involvement of the PPP1 DNA-binding motif in controlling the epigenetic status of the PIN2 promoter region. Studies that will characterize PPP1 START-domains and its interaction partners should help to obtain insights into the function of this plant-specific DNA-binding protein
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